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We, the undersigned, hereby state that we expressly and unequivocally oppose the advertising of proprietary software products on government websites. Such advertising breaches impartiality and encourages citizens to employ technologies that unnecessarily restrict their freedom. The role of government is not to support certain market participants and not others, particularly when doing so works to maintain the monopolies of global software companies. In explanations of how to use digital resources that they provide, government agencies should clarify that multiple methods are available, and favour technologies which do not restrict users digital rights; by linking to PDFreaders.org, for example. Free Software guarantees the users right to use (for any purpose), study (without secrets), share (with anyone), and improve the software that they use. Public institutions should publish their documents in formats that can be read with Free Software. Indeed, many Free Software applications exist for reading such documents. Governments should lead citizens to freedom, and encourage them to make use of these applications. Sincerely,

Our co-op and 56 other businesses, 69 organisations and over 2200 individuals so far. How about adding your signature? Surely it s time for our governments to stop giving free adverts to Adobe? It was particularly annoying in the Digital Britain report, I thought.

phpMyAdmin is quite popular software (to give some numbers let's mention 10000 downloads daily on SourceForge.net or 122685 reports in Debian's popcon) and as such is quite attractive target for various scripted attacks. If you run phpMyAdmin installation somewhere you should really make sure it is enough secured, so that these script kiddies don't get through. In past month I've looked at what kind of attacks are these guys trying and in all cases these are pretty old vulnerabilities, some of them fixed years ago. So the first thing you should do is to update. It is always good to run latest stable version, but in case you can not for whatever reason, try at least taking the most important fixes and using them. In ideal world your distribution would do this job for you, but in case it did not, you can for example take patches from Debian, which is pretty good at taking our patches (surprisingly it is not much related to my involvement there). To check which patches they have applied you can use excellent patch-tracker tool, which exposes patches from all released packages. To give you overview of which issues are mostly being attempted to exploit by script kiddies right now, here is the list:

PMASA-2010-3 - yes, more than two years old, but still unpatched in some places
PMASA-2011-5 - "only" half year old
PMASA-2011-6 - only useful together with wrongly configured PHP

If you have fixed these, you should be pretty safe for now, but follow our security announcements for possible future issues (you can use RSS feed or subscribe to news mailing list, where all security issues are announced as well). However there are more things you can do to keep you safer:

remove setup directory from phpMyAdmin, you will probably not use it after initial setup
prevent access to libraries directory from browser, as it is not needed, supplied .htaccess file does this
properly choose authentication method - cookie is probably the best choice for shared hosting
in case you don't want all MySQL users to be able to access phpMyAdmin, you can use AllowDeny rules to limit them
consider hiding phpMyAdmin behind authentication proxy, so that MySQL credenticals are not all users need to login

So these are the basic steps which will help you against possible compromise, I might return to some of these in more details in future posts.

Filed under: Debian English Phpmyadmin Suse 0 comments Flattr this!

Now is the point in the Debian release cycle where I usually try to go through my packages and bring them generally up to date. In case other people are doing the same thing, here are a few new facilities or techniques that I'm rolling out across my packages. (This is apart from the obvious stuff, like multiarch where appropriate and debhelper 9, and the older stuff, like using dh.)

dh-autoreconf is a new helper tool that runs autoreconf on the package during the build and cleans up properly afterwards. I've started switching all of my packages that use Autoconf and Automake and can use autoreconf over to it. (Some upstreams have more complex scripts that have to be run to regenerate the build system.) It plugs in trivially as a dh add-on, and even adds support for --as-needed (see below). I'm doing this even for packages where I don't patch the build system, on the grounds that rebuilding everything from source, including the build system, is a good idea. It also means that I can patch the build system when I need to without having to add additional machinery at the time.
Linking with --as-needed. As I build new packages, I'm looking for anything that has warnings about generating unnecessary dependencies and adding --as-needed to the linker flags. The combination of dh-autoreconf and the new dpkg-buildflags support makes this trivial to do for most packages. Just add something like:
```
    export DEB_LDFLAGS_MAINT_APPEND = -Wl,--as-needed
```
to debian/rules. It's worth being aware that --as-needed can break some unusual uses of special cases around shared library loading, but I've not run into any of those cases with any of the software that I package.
dpkg-buildflags comes essentially for free with debhelper 9, but it's worth mentioning that, as mentioned above, it's a really easy way to add additional flags. And if you have to pass in flags via some other mechanism, use dpkg-buildflags to get the default flags.
Once you're up to debhelper 9 and are using dpkg-buildflags, adding hardening flags is easy. You get the default ones for free, and that's a pretty good start. (Install hardening-includes and use hardening-check to check the status of the binaries built by your package.) I always add at least hardening=+bindnow to DEB_BUILD_MAINT_OPTIONS (set with export in debian/rules), since the minor speed hit at startup doesn't matter for anything that I'm packaging. (It might for something like ls that runs all the time.) I usually try to also add +pie, but be careful of that. Libtool will cope correctly with it and switch it back to PIC for shared libraries, but other shared library build processes may not. And it doesn't always work; for example, gnubg (GNU Backgammon) just immediately dies if built PIE, for reasons that I didn't track down.
If, like me, you maintain your packaging in Git without using a separate tool that exports a patch series, and therefore use the single-debian-patch option, there is new support for applying that option only to your build as the maintainer but not to any other build that people do of your source package. This is good, since it means that any NMU diffs will be kept separate from your maintainer diff because they'll get the version of the NMU package added. To get this behavior, move debian/source/patch-header to debian/source/local-patch-header and debian/source/options to debian/source/local-options (assuming that's your only option; otherwise, you might need to split it). Then the patch header and options won't be included in the generated source package and hence won't apply to NMUs or other packaging changes based on the source package in the archive instead of on the packaging repository.

It's also worth mentioning that Ubuntu was responsible for breaking a lot of ground here. Due to bug reports and patches submitted from Ubuntu, several of my packages already had hardening build flags and --as-needed issues fixed before this round of packaging refresh, which made adding these features much easier than it would be otherwise.

I switched from Google Reader to Liferea a while back. I was mostly happy with the functionality of Reader but I don't really want my reading history to be recorded or my feed list to be vulnerable to the whim of Google's inscrutable abuse department. I'm still using Liferea but unfortunately it has a lot of problems. Yes I do have bug numbers:

Calls sync() many times while refreshing feeds (#623619). This is probably related to use of SQLite and not properly batching writes. I work around this by using 'eatmydata' to nullify those calls, but that's not ideal.
The UI thread also blocks on feed updates. Together with the previous bug, this can make it unresponsive for minutes while updating feeds (#542276).
It often leaks memory and other resources when I play embedded video (#660593).
There is no progress or 'busy' indicator for the browser (#660595).
Error/prompt callbacks are not handled (#660597) - for example, if I follow a link that happens to lead to a PDF, nothing happens.
There is no progress indicator for downloading 'enclosed' files (typically podcast episodes), and errors are simply ignored (#660599).
While updating feeds, it may reset the item pane back to the summary (#660602).

So, I'm looking for an alternative again. My requirements are:

MAY be a desktop or web application.
If it's a web application, it MUST be reasonably secure, e.g. it must not be written in PHP.
If it's a web application, it MUST allow for multiple independent users on the same server.
If it's a desktop application, it MUST embed a browser engine (presumably Gecko or WebKit) so I can follow links without having to switch windows.
MUST support organisation of feeds by folders or tags, including combined item lists.
MUST keep track of which items have been read.
MUST support a global 'unread items' list. SHOULD only remove items from this list when I refresh it, not as soon as I move away from an item.
SHOULD support a three-pane (folder/list/item) view or something similar. Google Reader's list view with expanding items is perhaps even better, though it means links must be opened in a separate tab.
SHOULD support folder and item navigation by keyboard.
SHOULD have some way to flag/bookmark items for later attention.
If it's a desktop application, it SHOULD have some sort of download manager to support podcasts.

A few people have been using the license-count tool that I wrote to analyze the breakdown of licenses in Debian and draw some conclusions. Unfortunately, some of those conclusions can't be drawn from the output of this specific tool (which doesn't make them wrong, just unproven). So here are a few cautions. license-count was written for a specific purpose: to see how many references there are to a particular license in Debian so that we had some facts behind our discussion of whether to include it in common-licenses. It was not written to classify packages into particular categories, only to find out how many copies of a license we could save by adding it to common-licenses. This means that using it to determine how much software in Debian is covered by a particular license fails in the following ways:

license-count numbers cannot be added. Since the goal is to find every reference to a given license, each package is not classified as under one and only one license. If a package has material under the GPL, the LGPL, and the GFDL, it will add one to each of those counts. This means that you cannot determine, from its output, questions like "how many packages are covered under one of the GPL family of licenses?" You will double- or treble-count packages when you add the numbers together.
license-count counts any reference, not just "important" ones. If the license is referenced in debian/copyright, that counts, since the goal is to see how many packages might benefit from being able to reference it in common-licenses. This may not be the license that the package is under, just some small portion of the package. For example, all of my packages that use Autotools add to the GPL count because I include all the source code licensing statements, including the statements for libtool and the Automake helper scripts, which say that the files can be released under either the GPL or the license of the including package. These packages are generally under a BSD-style license (or in some cases the Apache 2.0 license), not under the GPL, but since the reproduced license text has a reference to the GPL, license-count adds one to the GPL column for each of those binary packages. For most purposes for counting GPL-covered software, these are false positives. I don't know how many people are as complete in documenting license statements as I am, but just the binary packages that I maintain probably count for at least 50 false positives because of this alone.
license-count numbers include dual-licensing. Whether this matters for your analysis or not depends on what question you're asking, of course. If you're asking whether the package is covered under a copyleft license, including the thousands of packages that are dual-licensed under the Artistic license and the GPL may be okay, since the Artistic license is a sort of copyleft (although it allows you to avoid the copyleft requirement by renaming executables, providing the originals, and documenting your changes). But, for example, one of my packages is dual-licensed under a BSD-style license and the GPL (for reasons that aren't worth getting into). This package is not covered by copyleft under any reasonable definition of the term, but license-count will count it as a package using the GPL.

In order to draw interesting conclusions like "how much software in Debian is protected by copyleft," I'm afraid that you'd have to write a much more sophisticated tool than license-count.

The new RcppArmadillo release 0.2.35 now supports the Rcpp::Rcout output stream device. Based on a contributed Rcpp patch by Jelper Ypma, the Rcpp::Rcout output stream gets redirected to R's buffered output. In other words, R's own output and that eminating from C++ code using Rcpp::Rcout are now both in sync. This avoids a stern warning from Section 5.6 in the Writing R Extensions manual:

Using C++ iostreams, as in this example, is best avoided. There is no guarantee that the output will appear in the R console, and indeed it will not on the R for Windows console. Use R code or the C entry points (*note Printing) for all I/O if at all possible.

and does in fact provide exactly what is recommended: the same entry points R itself uses. Below is a sample program, once again using the wonderful inline package to compile, load and link C++ code into R from a simple text variable submitted to the cxxfunction. What is shown in R code to load the package, the definition of the C++ code as assigned to a variable src and the creation of the dynamically-loadaded R function called fun which contains the code from we compiled, link and load via a single call to cxxfunction() given src.

library
library(inline)
src <- '
  Rcpp::Rcout << "Armadillo version: " << arma::arma_version::as_string() << std::endl;
  // directly specify the matrix size (elements are uninitialised)
  arma::mat A(2,3);
  // .n_rows = number of rows    (read only)
  // .n_cols = number of columns (read only)
  Rcpp::Rcout << "A.n_rows = " << A.n_rows << std::endl;
  Rcpp::Rcout << "A.n_cols = " << A.n_cols << std::endl;
  // directly access an element (indexing starts at 0)
  A(1,2) = 456.0;
  A.print("A:");
  // scalars are treated as a 1x1 matrix,
  // hence the code below will set A to have a size of 1x1
  A = 5.0;
  A.print("A:");
  // if you want a matrix with all elements set to a particular value
  // the .fill() member function can be used
  A.set_size(3,3);
  A.fill(5.0);
  A.print("A:");
  arma::mat B;
  // endr indicates "end of row"
  B << 0.555950 << 0.274690 << 0.540605 << 0.798938 << arma::endr
    << 0.108929 << 0.830123 << 0.891726 << 0.895283 << arma::endr
    << 0.948014 << 0.973234 << 0.216504 << 0.883152 << arma::endr
    << 0.023787 << 0.675382 << 0.231751 << 0.450332 << arma::endr;
  // print to the cout stream
  // with an optional string before the contents of the matrix
  B.print("B:");
  // the << operator can also be used to print the matrix
  // to an arbitrary stream (cout in this case)
  Rcpp::Rcout << "B:" << std::endl << B << std::endl;
  // save to disk
  B.save("B.txt", arma::raw_ascii);
  // load from disk
  arma::mat C;
  C.load("B.txt");
  C += 2.0 * B;
  C.print("C:");
  // submatrix types:
  //
  // .submat(first_row, first_column, last_row, last_column)
  // .row(row_number)
  // .col(column_number)
  // .cols(first_column, last_column)
  // .rows(first_row, last_row)
  Rcpp::Rcout << "C.submat(0,0,3,1) =" << std::endl;
  Rcpp::Rcout << C.submat(0,0,3,1) << std::endl;
  // generate the identity matrix
  arma::mat D = arma::eye<arma::mat>(4,4);
  D.submat(0,0,3,1) = C.cols(1,2);
  D.print("D:");
  // transpose
  Rcpp::Rcout << "trans(B) =" << std::endl;
  Rcpp::Rcout << trans(B) << std::endl;
  // maximum from each column (traverse along rows)
  Rcpp::Rcout << "max(B) =" << std::endl;
  Rcpp::Rcout << max(B) << std::endl;
  // maximum from each row (traverse along columns)
  Rcpp::Rcout << "max(B,1) =" << std::endl;
  Rcpp::Rcout << max(B,1) << std::endl;
  // maximum value in B
  Rcpp::Rcout << "max(max(B)) = " << max(max(B)) << std::endl;
  // sum of each column (traverse along rows)
  Rcpp::Rcout << "sum(B) =" << std::endl;
  Rcpp::Rcout << sum(B) << std::endl;
  // sum of each row (traverse along columns)
  Rcpp::Rcout << "sum(B,1) =" << std::endl;
  Rcpp::Rcout << sum(B,1) << std::endl;
  // sum of all elements
  Rcpp::Rcout << "sum(sum(B)) = " << sum(sum(B)) << std::endl;
  Rcpp::Rcout << "accu(B)     = " << accu(B) << std::endl;
  // trace = sum along diagonal
  Rcpp::Rcout << "trace(B)    = " << trace(B) << std::endl;
  Rcpp::Rcout << std::endl;
'
fun <- cxxfunction(signature(), body=src, plugin="RcppArmadillo")
setwd("/tmp")                           # adjust on other OSs
fun()                                   # output to stdout
sink("rcpparma.log.txt")                # start 'sink' to output to file
fun()                                   # no output to screen
sink()                                  # stop 'sink'

We then switch to a temporary directory (as the example code, taken from one of the two examples in Conrad's Armadillo sources, creates a temporary file) and run the new function. To demontrate how it does in fact now mesh perfectly with R, we create an output 'sink' (which catches all output) and re-run. This simple example demonstrated how we can use the new Rcout output stream from Rcpp to have dynamically-loaded C++ code cooperate more cleanly with the (buffered) R output. It also demontrated some of the nice features in Armadillo which we bring to R via RcppArmadillo.

While improving GNOME s servers Nagios Notifications, I ended up working on a nice way to notify the relevant folks through GTalk in case something could go wrong on any of the hosted services. Looking around on the web, I found Seth Vidal s script, modified it to suit my needs and made it working with GTalk, here s the result:

#!/usr/bin/python -tt
import warnings
warnings.simplefilter("ignore")
import xmpp
from xmpp.protocol import Message
from optparse import OptionParser
import ConfigParser
import sys
import os
parser = OptionParser()
opts, args = parser.parse_args()
if len(args) < 1:
    print "xmppsend message [to whom, multiple args]"
    sys.exit(1)
msg = args[0]
msg = msg.replace('\\n', '\n')
# Connect to the server
c  =  xmpp.Client('gmail.com')
c.connect( ( 'talk.google.com', 5223 ) )
# Authenticate to the server
jid  =  xmpp.protocol.JID( 'example@gmail.com' )
c.auth( jid.getNode( ), 'yourgmailpassword' )
if len(args) < 2:
    r = c.getRoster()
    for user in r.keys():
        if user == username:
            continue
        c.send(Message(user, '%s' % msg))
else:
    for user in args[1:]:
        c.send(Message(user, '%s' % msg))

I, then, added the command definitions on the relevant Nagios configuration file:

define command 
        command_name    host-notify-by-xmpp
        command_line    /home/user/bin/xmppsend "Host '$HOSTALIAS$' is $HOSTSTATE$ - Info : $HOSTOUTPUT$" $CONTACTPAGER$
         
define command 
        command_name    notify-by-xmpp
        command_line    /home/user/bin/xmppsend "$NOTIFICATIONTYPE$ $HOSTNAME$ $SERVICEDESC$ $SERVICESTATE$ $SERVICEOUTPUT$ $LONGDATETIME$" $CONTACTPAGER$

And in the end on contacts.cfg:

define contact  
        contact_name    admin
        use             generic-contact
        alias           Full Name
        email           example@gmail.com
        pager           example@gmail.com
        service_notification_commands   notify-by-xmpp
        host_notification_commands      host-notify-by-xmpp

When done just reload the configuration files with:

sudo /etc/init.d/nagios3 reload

Enjoy your new XMPP Nagios notifications! Update: if you don t want the script to store your username or password, you can use the following modified script together with a nice config file like this one:

[xmpp_nagios]
username=example@gmail.com
password=yourgmailpassword

Then you can invoke xmppsend this way:

xmppsend -a config.ini

We needed a new package at work and once I'd written it, I realised that it could well be useful for others, so I did the ITP and the package is now in Debian unstable.

Ladder - Stepwise repository upgrade tool



Ladder creates a local repository of packages required

to upgrade a tarball of a chroot to a new milestone or

software release.

.

The repository can be signed and includes all specified

target packages and dependencies. The repository can then

be distributed and used to upgrade multiple devices in

sequence without needing network access.

The only dependencies are some simple perl modules, reprepro for the repository, apt and gnupg for the signing. None of those need to be particularly recent versions, so once ladder has had a little time in testing, I will be doing a backport to Squeeze. Lenny is a little less likely but eminently possible if someone specifically asks for it. Not that it particularly needs a backport mind, if you want to run ladder on some other Debian-based system, it's simply a case of installing the version in unstable. reprepro was in Lenny at a suitable version for what ladder requires, so any system remotely recent should be able to run ladder without changes.

Normally, whenever you need to upgrade a device, you need to get that device onto a network, get access over the serial port or some form of pre-configured network connection, run the commands and clean up afterwards. OK, now do that again... oh and here are another 40 or 100 devices which all need precisely the same thing done and they all have to be shipped to paying customers today. What does any geek do? Script it.

Ladder is part of that process. There would need to be some scripting / programming support on the devices concerned but ladder makes it easier to put that support in place at the design stage and then automate the actual update without the device needing to get onto a network and, potentially, allowing you to decide how to offer the upgrade (automatic, user involvement, engineers only etc.) Once that is implemented, the device is simply pointed at a locally mounted filesystem which contains the ladder step for the migration. Each step is a SecureApt signed repository which is available at a deterministic location which can be easily scripted and which contains only the packages necessary (with dependencies) to upgrade any device installed at Milestone A to Milestone B. Nice and small, only the stuff you need, whatever architecture you like and if you need to migrate from Milestone A to Milestone D, then you create a few steps on the ladder and automate the migration from A to B to C to D. All you need is enough space on whatever local media you need to use to plug into the devices. Then copy the media enough times, start the interface on each device and do something useful whilst the upgrade happens. Then, when the next lot need to be upgraded, you already have the media... Oh and ladder does not need to run on the same architecture as the devices - all package handling is entirely architecture-neutral. So create the ladder steps and media on your fast x86 machines and then let the slower SSD devices take their own sweet time via automation. Simple - maybe, hopefully.

I'm expressly talking about devices here because this is where the original requirement arose - embedded devices which don't have direct / automatic / accessible network ports or even a working network configuration preset. These aren't new images to flash onto the device either, these are apt repositories, so only the stuff which needs to be changed is changed which saves a lot of time writing to slow SSD storage.

If the mechanism sounds familiar, yes, it's using the same principles as xapt, multistrap and emdebian-grip but I thought it could be useful, so I generalised the interface (a bit) and uploaded.

Let me know via the BTS if you need tweaks to the support. Ladder isn't particularly about getting desktop machines from Debian Potato to Debian Wheezy because you need to identify a target package to be the top point of the dependency chain which gets into the step repository and all devices should start with the same packages at the same versions. Normally, the kind of devices I use at work have a single top level application which is the sole user interface and which directly or indirectly depends on all the other software required for that device. Whilst the target package value could take a few carefully selected packages instead of just one and there is support for an extra packages field, it's not intended / expected to be useful for desktops. This is for use with multiple devices which all have the same package selection, are at the same milestone at the start and which all need to migrate smoothly to whatever milestone is intended - one step at a time. Ladder could be used to migrate between Debian releases and I've included example config files to support that, but the principle usage is with internal / proprietary repositories based on a common Debian stable platform where the user interface software is managed via identifiable milestones and where users have no direct control over installing packages. It's a production / manufacturing support tool more than a user / admin support tool. If you want to manage servers and desktops which allow users to install (or request to be installed) arbitrary packages, there are plenty of tools which already support this and are used by DSA in Debian for exactly these tasks. Ladder is not puppet but Linp didn't sound like a good name...

Ladder step repositories only include the binary packages, not sources - so if the migration is not going to be done in-house and you're expecting to distribute these steps to users somehow, ensure that the original source repository is available online for the Debian / free software packages concerned. If you can't do that (because the source is proprietary), you possibly shouldn't be distributing the binaries as a user update mechanism in the first place because each step will include packages from the core Debian system which need to have the sources available when distributed.

Inevitably, the ladder source package already contains two POT files for translations of the runtime messages and the POD based manpage. I expect to need to expand / clarify the manpage in due course, so don't rush to translate it yet as it's likely to change.

const char *foo = "foo";

This was recently mentioned on bugzilla, and the problem is usually underestimated, so I thought I would give some details about what is wrong with the code above. The first common mistake here is to believe foo is a constant. It is a pointer to a constant. In practical ELF terms, this means the pointer lives in the .data section, and the string constant in .rodata. The following code defines a constant pointer to a constant:

const char * const foo = "foo";

The above code will put both the pointer and the string constant in .rodata. But keeping a constant pointer to a constant string is pointless. In the above examples, the string itself is 4 bytes (3 characters and a zero termination). On 32-bits architectures, a pointer is 4 bytes, so storing the pointer and the string takes 8 bytes. A 100% overhead. On 64-bits architectures, a pointer is 8 bytes, putting the total weight at 12 bytes, a 200% overhead. The overhead is always the same size, though, so the longer the string, the smaller the overhead, relatively to the string size. But there is another, not well known, hidden overhead: relocations. When loading a library in memory, its base address varies depending on how many other libraries were loaded beforehand, or depending on the use of address space layout randomization (ASLR). This also applies to programs built as position independent executables (PIE). For pointers embedded in the library or program image to point to the appropriate place, they need to be adjusted to the base address where the program or library is loaded. This process is called relocation. The relocation process requires information which is stored in .rel.* or .rela.* ELF sections. Each pointer needs one relocation. The relocation overhead varies depending on the relocation type and the architecture. REL relocations use 2 words, and RELA relocations use 3 words, where a word is 4 bytes on 32-bits architectures and 8 bytes on 64-bits architectures. On x86 and ARM, to mention the most popular 32-bits architectures nowadays, REL relocations are used, which makes a relocation weigh 8 bytes. This puts the pointer overhead for our example string to 12 bytes, or 300% of the string size. On x86-64, RELA relocations are used, making a relocation weigh 24 bytes! This puts the pointer overhead for our example string to 32 bytes, or 800% of the string size! Another hidden cost of using a pointer to a constant is that every time it is used in the code, there will be pointer dereference. A function as simple as

int bar() return foo;

weighs one instruction more when foo is defined const char *. On x86, that instruction weighs 2 bytes. On x86-64, 3 bytes. On ARM, 4 bytes (or 2 in Thumb). That weight can vary depending on the additional instructions required, but you get the idea: using a pointer to a constant also adds overhead to the code, both in time and space. Also, if the string is defined as a constant instead of being used as a literal in the code, chances are it s used several times, multiplying the number of such instructions. Update: Note that in the case of const char * const, the compiler will optimize these instruction and avoid reading the pointer, since it s never going to change. The symbol for foo is also exported, making it available from other libraries or programs, which might not be required, but also adds its own overhead: an entry in the symbols table (5 words), an entry in the string table for the symbol name (strlen("foo") + 1) and an entry in the symbols hash chain table (4 bytes if only one type of hash table (sysv or GNU) is present, 8 if both are present), and possibly an entry in the symbols hash bucket table, depending on the other exported symbols (4 or 8 bytes, as chain table). It can also affect the size of the bloom filter table in the GNU symbol hash table. So here we are, with a seemingly tiny 3 character string possibly taking 64 bytes or more! Now imagine what happens when you have an array of such tiny strings. This also doesn t only apply to strings, it applies to any kind of global pointer to constants. In conclusion, using a definition like

const char *foo = "foo";

is almost never what you want. Instead, you want to use one of the following forms:

For a string meant to be exported:

const char foo[] = "foo";
For a string meant to be used in the same source file:

static const char foo[] = "foo";
For a string meant to be used across several source files for the same library:

__attribute__((visibility("hidden"))) const char foo[] = "foo";

~$ dpkg --print-architecture
i386
~$ dpkg -s linux-image-$(uname -r)
Package  linux-image-3.2.0-1-amd64' is not installed and no info is available.
Use dpkg --info (= dpkg-deb --info) to examine archive files,
and dpkg --contents (= dpkg-deb --contents) to list their contents.
~$ dpkg -s linux-image-$(uname -r):amd64
Package: linux-image-3.2.0-1-amd64
Status: install ok installed
Priority: optional
Section: kernel
Installed-Size: 104822
Maintainer: Debian Kernel Team <debian-kernel@lists.debian.org>
Architecture: amd64
Source: linux-2.6
Version: 3.2.6-1

...

Under Linux, there are a number of related features around marking areas of a file, filesystem, or block device as no longer allocated . In the standard view, here s what happens if you fill a file to 500M and then truncate it to 100M, using the truncate syscall:

create the empty file, filesystem allocates an inode, writes accounting details to block device.
write data to file, filesystem allocates and fills data blocks, writes blocks to block device.
truncate the file to a smaller size, filesystem updates accounting details and releases blocks, writes accounting details to block device.

The important thing to note here is that in step 3 the block device has no idea about the released data blocks. The original contents of the file are actually still on the device. (And to a certain extent is why programs like shred exist.) While the recoverability of such released data is a whole other issue, the main problem about this lack of information for the block device is that some devices (like SSDs) could use this information to their benefit to help with extending their life, etc. To support this, the TRIM set of commands were created so that a block device could be informed when blocks were released. Under Linux, this is handled by the block device driver, and what the filesystem can pass down is discard intent, which is translated into the needed TRIM commands. So now, when discard notification is enabled for a filesystem (e.g. mount option discard for ext4), the earlier example looks like this:

create the empty file, filesystem allocates an inode, writes accounting details to block device.
write data to file, filesystem allocates and fills data blocks, writes blocks to block device.
truncate the file to a smaller size, filesystem updates accounting details and releases blocks, writes accounting details and sends discard intent to block device.

While SSDs can use discard to do fancy SSD things, there s another great use for discard, which is to restore sparseness to files. Normally, if you create a sparse file (open, seek to size, close), there was no way, after writing data to this file, to punch a hole back into it. The best that could be done was to just write zeros over the area, but that took up filesystem space. So, the ability to punch holes in files was added via the FALLOC_FL_PUNCH_HOLE option of fallocate. And when discard was enabled for a filesystem, these punched holes would get passed down to the block device as well. Take, for example, a qemu/KVM VM running on a disk image that was built from a sparse file. While inside the VM instance, the disk appears to be 10G. Externally, it might only have actually allocated 600M, since those are the only blocks that had been allocated so far. In the instance, if you wrote 8G worth of temporary data, and then deleted it, the underlying sparse file would have ballooned by 8G and stayed ballooned. With discard and hole punching, it s now possible for the filesystem in the VM to issue discards to the block driver, and then qemu could issue hole-punching requests to the sparse file backing the image, and all of that 8G would get freed again. The only down side is that each layer needs to correctly translate the requests into what the next layer needs. With Linux 3.1, dm-crypt supports passing discards from the filesystem above down to the block device under it (though this has cryptographic risks, so it is disabled by default). With Linux 3.2, the loopback block driver supports receiving discards and passing them down as hole-punches. That means that a stack like this works now: ext4, on dm-crypt, on loopback of a sparse file, on ext4, on SSD. If a file is deleted at the top, it ll pass all the way down, discarding allocated blocks all the way to the SSD: Set up a sparse backing file, loopback mount it, and create a dm-crypt device (with allow_discards ) on it:

# cd /root
# truncate -s10G test.block
# ls -lk test.block
-rw-r--r-- 1 root root 10485760 Feb 15 12:36 test.block
# du -sk test.block
0       test.block
# DEV=$(losetup -f --show /root/test.block)
# echo $DEV
/dev/loop0
# SIZE=$(blockdev --getsz $DEV)
# echo $SIZE
20971520
# KEY=$(echo -n "my secret passphrase"   sha256sum   awk ' print $1 ')
# echo $KEY
a7e845b0854294da9aa743b807cb67b19647c1195ea8120369f3d12c70468f29
# dmsetup create testenc --table "0 $SIZE crypt aes-cbc-essiv:sha256 $KEY 0 $DEV 0 1 allow_discards"

Now build an ext4 filesystem on it. This enables discard during mkfs, and disables lazy initialization so we can see the final size of the used space on the backing file without waiting for the background initialization at mount-time to finish, and mount it with the discard option:

# mkfs.ext4 -E discard,lazy_itable_init=0,lazy_journal_init=0 /dev/mapper/testenc
mke2fs 1.42-WIP (16-Oct-2011)
Discarding device blocks: done
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
655360 inodes, 2621440 blocks
131072 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=2684354560
80 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632
Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done 
# mount -o discard /dev/mapper/testenc /mnt
# sync; du -sk test.block
297708  test.block

Now, we create a 200M file, examine the backing file allocation, remove it, and compare the results:

# dd if=/dev/zero of=/mnt/blob bs=1M count=200
200+0 records in
200+0 records out
209715200 bytes (210 MB) copied, 9.92789 s, 21.1 MB/s
# sync; du -sk test.block
502524  test.block
# rm /mnt/blob
# sync; du -sk test.block
297720  test.block

Nearly all the space was reclaimed after the file was deleted. Yay! Note that the Linux tmpfs filesystem does not yet support hole punching, so the exampe above wouldn t work if you tried it in a tmpfs-backed filesystem (e.g. /tmp on many systems).

2012, Kees Cook. This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 License.

I've been wanting to write a blog post about running for a while now; things like that I finally registered for a few upcoming races in and around Z rich (which I hope I planned correctly, not like last year), or that I finally managed to find a schedule that allows me to run both in good and bad weather, or other boring things like this. However what I want to write now about is the morning run I had today. I woke up, planning to run at work during lunch or something like that, then I looked outside: snow! For the first time in about a month or so, real, serious snow falling, and white all around. I didn't want to miss this opportunity, so I first checked the temperature: 0 C ( 2 ), which is fine (I don't have gear to run at -10 C, for example), maybe a bit wet but acceptable. Got dressed, and went outside. Even the alley to the apartment building was not yet cleared . So I went on my usual 10K run to work and it was, by far, the best run ever or at least the best for the past 6 months, my memory is short . It was wonderful, snow falling down, the woods black and white, and a bit of wind. I managed to run about one thirds with both eyes open, about two thirds with just one eye open (heavy wind from one side), and some short streches with both eyes closed, due to heavy wind from ahead. I also had a better pace than expected; about 20 seconds slower per km, which is good, given that running in snow is harder than on clear ground. One surprising fact: usually I cross on average about one other runner each two-three hundres meters. However today, in the entire 10K, only one other runner. On top of that, a few people seemed surprised at work that I chose to run on such terrible weather. Why were people so scared of snow? To me it was wonderful, I felt completely like a child again, being able to plough through ankle-deep, pristine snow, ignoring (or rather enjoying) the cold wind, forgetting everything else. Too bad the snow half-melted by evening. I miss winters with lots of snow

As many would know, Debian GNU/Linux is one of the oldest, and the largest Linux distributions that is available for free. Since it was first released in 1993, several people have analysed the size and produced cost estimates for the project. In 2001, Jes s M. Gonz lez-Barahona et al produced an article entitled Counting Potatoes , an analysis of Debian 2.2 (code named Potato). When Potato was released in June 2003, it contained 2,800 source packages of software, totalling around 55 million lines of source code. When using David A. Wheeler s sloccount tool to apply the COCOMO model of development, and an average developer salary of US$56,000, the projected development cost that Gonz lez-Barahona calculated to start-from-scratch and build Debian 2.2 in 2003 was US$1.9 billion. In 2007 an analysis entitled Macro-level software evolution: a case study of a large software compilation by Jes s M. Gonz lez-Barahona, Gregorio Robles, Martin Michlmayr, Juan Jos Amor and Daniel M. German was released. It found that Debian 4.0 (codename Etch released April 2007) had just over 10,000 source packages of software and 288 million lines of source code. This analysis also delved into the dependencies of software packages, and the update flow between Debian release (not all packages are updated with each release). Today (February 2012) the current development version of Debian, codenamed Wheezy, contains some 17,141 source packages of software, but as it s still in development this number may change over the coming months. I analysied the source code in Wheezy, looking at the content from the original software that Debian distributes from its upstream authors without including the additional patches that Debian Developers apply to this software, or the package management scripts (used to install, configure and de-install packages). One might argue that these patches and configuration scripts are the added value of Debian, however the in my analysis I only examined the pristine upstream source code. By using David A Wheeler s sloccount tool and average wage of a developer of US$72,533 (using median estimates from Salary.com and PayScale.com for 2011) I summed the individual results to find a total of 419,776,604 source lines of code for the pristine upstream sources, in 31 programming languages including 429 lines of Cobol and 1933 lines of Modula3! In my analysis the projected cost of producing Debian Wheezy in February 2012 is US$19,070,177,727 (AU$17.7B, EUR 14.4B, GBP 12.11B), making each package s upstream source code wrth an average of US$1,112,547.56 (AU$837K) to produce. Impressively, this is all free (of cost). Zooming in on the Linux Kernel In 2004 David A. Wheeler did a cost analysis of the Linux Kernel project by itself. He found 4,000,000 source lines of code (SLOC), and a projected cost between US$175M and US$611M depending on the complexity rating of the software. Within my analysis above, I used the standard (default) complexity with the adjusted salary for 2011 (US$72K), and deducted that Kernel version 3.1.8 with almost 10,000,000 lines of source code would be worth US$540M at standard complexity, or US$1,877M when rated as complex . Another Kernel Costing in 2011 put this figure at US$3 billion, so perhaps there s some more variance in here to play with. Individual Projects Other highlights by project included:

Project	Version	Thousands of SLOC	Projected cost at US$72,533/developer/year
Samba	3.6.1	2,000	US$101 (AU$93M)
Apache	2.2.9	693	US$33.5M (AU$31M)
MySQL	5.5.17	1,200	US$64.2M (AU$59.7M)
Perl	5.14.2	669	US$32.3M (AU$30M)
PHP	5.3.9	693	US$33.5M (AU$31.1M)
Bind	9.7.3	319	US$14.8M (AU$13.8M)
Moodle	1.9.9	396	US$18.6M (AU$17.3M)
Dasher	4.11	109	US$4.8M (AU$4.4M)
DVSwitch	0.8.3.6	6	US$250K (AU$232K)

Debian Wheezy by Programming Language The upstream code that Debian distributes is written in many different languages. ANSI C with 168,536,758 is the dominant language (40% of all lines), followed by C++ at 83,187,329 (20%) and Java with 34,698,990 (8%).

Break down of Wheezy by Language

If you are intersted in finding the line count and cost projections for any of the 17,000+ projects, you will find them in the raw data CSV. Other Tools and Comparisons Ohcount is another source code cost analysis tool. In March 2011 Ohcount was run across Debian Sid: its results are here. In comparison, its results appear much lower than the sloccount tool. There s also the Ohloh.net Debian Estimate which only finds 55 Million source lines of code and a projected cost of US$1B. However Ohloh uses Ohcount for its estimates, and seems to be to be around 370 million SLOC missing compared to my recent analysis. Summary Over the last 10 years the cost to develop Debian has increased ten-fold. It s intersting to know that US$19 billion of software is available to use, review, extend, and share, for the bargain price of $0. If we were to add in Debian patches and install scripts then this projected figure would increase. If only more organisations would realise the potential they have before them. Need help with Linux (including Debian), Perl, or AWS? See www.jamesbromberger.com.

Hello In Debian package, the debian/control file has a specific syntax where each line must begin with a space and paragraphs are separated by a single . . If you want to follow DEP-5 syntax for debian/copyright files, you have a similar problem when comes the time to fill the license text for creative-common license: it s easy to miss a space or a dot. Let s say I want to add a new section in the description of my package. Here s the current description:

Description: module to automate definition of a DBIx::Class::Schema
 DBIx::Class::Schema::Loader is an extension to DBIx::Class that automates the
 definition of a DBIx::Class::Schema by scanning table schemas and setting up
 columns and primary keys appropriately. It supports MySQL, PostgreSQL, SQLite
 and DB2.
 .
 Bare table definitions are fairly straightforward, but relationship creation
 is somewhat heuristic, especially with respect to choosing relationship types
 and names, as well as join types. The relationships generated by this module
 will probably never be as well-defined as hand-generated ones.

I want to add This package is awesome but I don t want to be bothered by leading dots and white spaces. First, I go in the package directory, then check the syntax with cme:

$ cme check dpkg

Then create a loop directory, where the dpkg content will be mapped to a fuse directory:

$ mkdir loop
$ cme fusefs dpkg -fuse-dir loop/
Mounting config on loop/ in background.
Use command 'fusermount -u loop/' to unmount
$ cat loop/control/binary/libdbix-class-schema-loader-perl/Description
DBIx::Class::Schema::Loader is an extension to DBIx::Class that automates the
definition of a DBIx::Class::Schema by scanning table schemas and setting up
columns and primary keys appropriately. It supports MySQL, PostgreSQL, SQLite
and DB2.
Bare table definitions are fairly straightforward, but relationship creation
is somewhat heuristic, especially with respect to choosing relationship types
and names, as well as join types. The relationships generated by this module
will probably never be as well-defined as hand-generated ones.

Then edit loop/control/binary/libdbix-class-schema-loader-perl/Description with your favorite editor. For the sake of this blog, I will use echo to edit this file:

$ echo -e "\nThis module is awesome" >> loop/control/binary/libdbix-class-schema-loader-perl/Description

Now umount the loop (note that the actual debian/control is not changed before this step):

fusermount -u loop

Now the result:

$ cat debian/control
[...]
Description: module to automate definition of a DBIx::Class::Schema
 DBIx::Class::Schema::Loader is an extension to DBIx::Class that automates the
 definition of a DBIx::Class::Schema by scanning table schemas and setting up
 columns and primary keys appropriately. It supports MySQL, PostgreSQL, SQLite
 and DB2.
 .
 Bare table definitions are fairly straightforward, but relationship creation
 is somewhat heuristic, especially with respect to choosing relationship types
 and names, as well as join types. The relationships generated by this module
 will probably never be as well-defined as hand-generated ones.
 .
 This module is awesome

Here s another example to fill the license text for a license while respecting DEP-5 syntax, (done after the cme command):

# creating this dir will create a new DEP-5
# standalone license section with name CC-BY-30
mkdir loop/copyright/License/CC-BY-30
# fill the text part of the  license
wget http://creativecommons.org/licenses/by/3.0/legalcode -O - \
   html2text -style pretty > loop/copyright/License/CC-BY-30/text
$ fusermount -u loop
$ cat debian/copyright
[...]
License: CC-BY-30
 .
 .
 .
                                Creative_Commons
 .
 .
 Creative Commons Legal Code
 .
 .
 Attribution 3.0 Unported
[...]

And that s it. (note that cme command is new and is available only from version 2.004 of Config::Model) Of course, there s more than a awy to do it: you can also run cme edit dpkg and add or edit license text in the GUI (or in an editor launched by the GUI). cme will take care of the syntax details. All the best

Readers who look at our blog itself (rather than one of the lovely sites that reprint our articles) may have noticed that you can now comment in either the usual WordPress way (Name/Email/Link) or by logging in with a social media profile from one of a large range of providers, including WordPress, Livejournal, Yahoo, Google and many more. This uses the broadly-cooperative openID system. If you run a website that accepts reader contributions, you should allow comments with openid because it helps people to use their existing social media membership without you having to surrender any control to facebook, twitter, or anyone else (unless you choose to). You also don t have to ask your readers to weaken their security settings like with disqus (which requires javascript and third-party cookies). The comment form on our site is powered by the openid plugin, together with our co-op s version of the comments-with-openid plugin which can be downloaded from our site. Please download them if you d find them useful for your WordPress site. (I d love to adopt the official comments-with-openid at wordpress.org because the previous maintainer doesn t answer anyone know how to do that? I m surprised it s not in the FAQ.) Do you use some other platform? What tools have let you add openid logins to it? For example, Drupal has some openID support in its core distribution: what else is out there?

SNMP stands for Simple Network Management Protocol. It allows a manager to query information from an agent. A popular use of SNMP is to retrieve network interface counters to plot a bandwidth graph. While the S in SNMP stands for simple, SNMP can be quite difficult to deal with. However, it is still the de facto standard for retrieving metrics in an heterogeneous environment. Net-SNMP is a suite of SNMP applications including an agent. Out of the box, this agent exports a lot of information but if something is missing, there are several ways to extend it. Which ones? tl;dr: in my opinion, extending Net-SNMP should be done with either of those methods:

using extend directive;
using pass_persist directive;
using AgentX protocol.

SNMP in a nutshell The variables accessible with SNMP through the agent are organized in a tree. Each variable is typed and associated to an object identifier (or OID). For example, .1.3.6.1.2.1.31.1.1.1.1.2 is the OID of the name of the second network card. Usually, SNMP is operated over UDP, port 161. For the purpose of this article, we consider a manager can issue three kinds of requests:

GET: retrieve a variable.
GETNEXT: retrieve the next variable, in lexical order of their OID. This operation enables an SNMP manager to walk through all available variables, for example to discover the list of interfaces of an agent. This operation is what makes an SNMP agent difficult to implement but this is also a critical feature.
SET: request the modification of a variable.

Since such an OID may be difficult to handle by humans, available variables are described by a MIB module which is a file that should be parsed to get a mapping between OID and variable names and types. For example, the above OID can also be referred as IF-MIB::ifName.2. Those MIB modules are described using a subset of ASN.1 defined in RFC 2578. A MIB module allows to group several variables into a conceptual table. For example, IF-MIB::ifDescr, IF-MIB::ifType, IF-MIB::ifSpeed are columns of IF-MIB::ifTable. Each row is associated to an index. In the case of IF-MIB::ifTable, it is the interface index. Net-SNMP includes snmptable allowing one to display such a table in a natural manner¹:

$ snmptable localhost IF-MIB::ifTable
SNMP table: IF-MIB::ifTable
 ifIndex ifDescr           ifType ifMtu   ifSpeed
       1      lo softwareLoopback 16436  10000000
       2    eth0   ethernetCsmacd  1500 100000000
       3    eth1   ethernetCsmacd  1500  10000000
       4     br0   ethernetCsmacd  1500         0

Variables inside a table are called columnar objects while those outside are scalar objects. SNMP, as a protocol, does not care of this distinction since it does not know the concept of MIB modules. Other useful tools from Net-SNMP includes snmpget to get a specific variable, snmpwalk to discover available variables (with GETNEXT operation) from an OID and snmptranslate to translate between object names and OID.

$ snmpget localhost IF-MIB::ifDescr.2
IF-MIB::ifDescr.2 = STRING: eth0
$ snmpwalk localhost IF-MIB::ifDescr
IF-MIB::ifDescr.1 = STRING: lo
IF-MIB::ifDescr.2 = STRING: eth0
IF-MIB::ifDescr.3 = STRING: eth1
IF-MIB::ifDescr.4 = STRING: br0
$ snmpwalk -On public localhost .1.3.6.1.2.1.2.2.1.2
.1.3.6.1.2.1.2.2.1.2.1 = STRING: lo
.1.3.6.1.2.1.2.2.1.2.2 = STRING: eth0
.1.3.6.1.2.1.2.2.1.2.3 = STRING: eth1
.1.3.6.1.2.1.2.2.1.2.4 = STRING: br0
$ snmptranslate -On IF-MIB::ifDescr.3
.1.3.6.1.2.1.2.2.1.2.3

Extending Net-SNMP ethtool -S allows us to access various statistics from a network card, like the number of octets received and transmitted or the number of collisions. While some of those statistics are defined in IF-MIB, in RMON-MIB and in EtherLike-MIB, some of them are not available and very specific. Therefore, we would like to export those statistics, unmodified, through SNMP. To the best of my knowledge, no MIB module exists for such an usage. The first task is to write one. It can be called ETHTOOL-MIB. It only contains one table indexed by the interface index and the name of the statistic. Here is the expected output:

$ snmpwalk localhost ETHTOOL-MIB::ethtoolStat.2
ethtoolStat.2.'align_errors' = Counter64: 0
ethtoolStat.2.'broadcast' = Counter64: 25
ethtoolStat.2.'multicast' = Counter64: 345
ethtoolStat.2.'rx_errors' = Counter64: 0
ethtoolStat.2.'rx_missed' = Counter64: 0
ethtoolStat.2.'rx_packets' = Counter64: 262011
ethtoolStat.2.'tx_aborted' = Counter64: 0
ethtoolStat.2.'tx_errors' = Counter64: 0
ethtoolStat.2.'tx_multi_collisions' = Counter64: 0
ethtoolStat.2.'tx_packets' = Counter64: 296170
ethtoolStat.2.'tx_single_collisions' = Counter64: 0
ethtoolStat.2.'tx_underrun' = Counter64: 0
ethtoolStat.2.'unicast' = Counter64: 261641

I have coded various implementations of this MIB module and made them available in a GitHub repository.

With arbitrary commands The extend directive allows the agent to execute an arbitrary command and to provide its output and status through NET-SNMP-EXTEND-MIB module. If we want to export the statistics for eth0, we could add something like this in snmpd.conf, using a custom script to format the output of ethtool -S in a way that is more suitable for our use:

# Export eth0 statistics
extend eth0-names  /full/path/to/ethtool-stats eth0 names
extend eth0-values /full/path/to/ethtool-stats eth0 values

The result can be retrieved through NET-SNMP-EXTEND-MIB:: nsExtendOutput2Table:

$ snmpwalk localhost NET-SNMP-EXTEND-MIB::nsExtendOutput2Table
nsExtendOutLine."eth0-names".1 = STRING: tx_packets
nsExtendOutLine."eth0-names".2 = STRING: rx_packets
nsExtendOutLine."eth0-names".3 = STRING: tx_errors
nsExtendOutLine."eth0-names".4 = STRING: rx_errors
nsExtendOutLine."eth0-names".5 = STRING: rx_missed
nsExtendOutLine."eth0-names".6 = STRING: align_errors
nsExtendOutLine."eth0-names".7 = STRING: tx_single_collisions
nsExtendOutLine."eth0-names".8 = STRING: tx_multi_collisions
nsExtendOutLine."eth0-values".1 = STRING: 246309
nsExtendOutLine."eth0-values".2 = STRING: 223941
nsExtendOutLine."eth0-values".3 = STRING: 0
nsExtendOutLine."eth0-values".4 = STRING: 0
nsExtendOutLine."eth0-values".5 = STRING: 0
nsExtendOutLine."eth0-values".6 = STRING: 0
nsExtendOutLine."eth0-values".7 = STRING: 0
nsExtendOutLine."eth0-values".8 = STRING: 0

It is also possible to configure such a command using SET requests. For example, if we want to add statistics for eth2, we could issue the following command:

$ snmpset -m +NET-SNMP-EXTEND-MIB localhost \
>    'nsExtendStatus."eth2-names"'  = createAndGo \
>    'nsExtendCommand."eth2-names"' = /full/path/to/ethtool-stats \
>    'nsExtendArgs."eth2-names"'    = 'eth2 names' \
>    'nsExtendStatus."eth2-values"'  = createAndGo \
>    'nsExtendCommand."eth2-values"' = /full/path/to/ethtool-stats \
>    'nsExtendArgs."eth2-values"'    = 'eth2 values'
nsExtendStatus."eth2-names" = INTEGER: createAndGo(4)
nsExtendCommand."eth2-names" = STRING: /full/path/to/ethtool-stats
nsExtendArgs."eth2-names" = STRING: eth2 names
nsExtendStatus."eth2-values" = INTEGER: createAndGo(4)
nsExtendCommand."eth2-values" = STRING: /full/path/to/ethtool-stats
nsExtendArgs."eth2-values" = STRING: eth2 values
$ snmpgetnext -m +NET-SNMP-EXTEND-MIB localhost \
>    'nsExtendOutLine."eth2-names"'
nsExtendOutLine."eth2-names".1 = STRING: tx_packets

This can be very convenient but also a huge security risk. You should disable this functionality or enable it only for some SNMPv3 user. Using extend does not allow to implement an arbitrary MIB module. If the cache is not disabled, extend is a good solution for various simple needs, like providing the content of a one-line file or the output of some Nagios plugin. However, exporting anything tabular, like interface statistics, is not a good fit.

With pass-through scripts Net-SNMP allows one to extend the agent by delegating some sub-tree to a script defined by the pass_persist directive. Such a script will receive requests on its standard input with a very simple line-oriented protocol described in the manual page of snmpd.conf. Here is an example of interaction:

   PING
   PONG
   get
   .1.3.6.1.4.1.39178.100.1.1.1.2.2.109.117.108.116.105.99.97.115.116
   .1.3.6.1.4.1.39178.100.1.1.1.2.2.109.117.108.116.105.99.97.115.116
   counter64
   223941
   getnext
   .1.3.6.1.4.1.39178.100.1.1.1.2.2
   .1.3.6.1.4.1.39178.100.1.1.1.2.2.97.108.105.103.110.95.101.114
   counter64
   0

The protocol is simple enough to implement it from scratch in any scripting language. However, in Perl, you may prefer to use SNMP:: ExtensionSNMP:: PassPersist extension from S bastien Aperghis-Tramoni. You can find the complete example on Github.

use SNMP::Extension::PassPersist;
my $extsnmp = SNMP::Extension::PassPersist->new(
    backend_collect => \&update_tree
    );
$extsnmp->run;
sub update_tree  
    my @interfaces = </sys/class/net/*>;
    foreach my $interface (@interfaces)  
        $interface =~ s/^.*\///;
        open(ETHTOOL, "ethtool -S $interface 2>/dev/null  ") or next;
        while (<ETHTOOL>)  
            /^\s+(\w+): (\d+)$/ or next;
            my $name  = $1;
            my $value = int($2);
            my $oid   = oid_compute($interface, $name);
            $extsnmp->add_oid_entry($oid, "counter", $value);
         
        close(ETHTOOL);

With Python, snmp-passpersist module, from Nicolas Agius, provides a similar interface. Look at the complete example on GitHub. pass_persist allows you to implement an arbitrary MIB module with honest performances. Here are some important things to know about this directive:

Unless you have a recent version of Net-SNMP, 64bit types are converted to 32bit equivalents. The counters may overflow quickly.
While handling a request, you are not allowed to pause (for example, to fetch a remote information): the agent would become unresponsive. If you need slow-to-get data or data from the agent, grab them in a separate thread and ensure you have them beforehand.

With AgentX protocol AgentX is a protocol defined in RFC 2741 allowing a master agent to be extended by independent sub-agents. For `snmpd` to become a master agent, you just need to add `master agentx` in `snmpd.conf`. Here are some of the benefits of an AgentX sub-agent over `pass_persist`:

No configuration is needed for the master agent to accept an additional sub-agent. A sub-agent registers to the master agent the MIB modules (or part of them) it wants to take care of.

A sub-agent is decoupled from the master agent. It can run with a different identity or be integrated into another daemon to export its internal metrics, send traps or allow remote configuration through SNMP.

AgentX protocol can be carried over TCP. Sub-agents can therefore run on a foreign host or in a jailed environment.

64bit types are fully supported. Traps are also supported.

Perl and Python Net-SNMP provides a Perl module, `NetSNMP::agent`, to write a sub-agent using AgentX protocol. This module is not as convenient as the module for `pass_persist` described above since it does not provide a convenient layer to put required information into some datastore. Basically, it mimics the available C API. Here are a few examples of its use:

An implementation of ETHTOOL-MIB I have done in the same way as the `pass_persist` version. My Perl skills are a bit rusty and the code has many rough edges.

An implementation of a sub-agent for Redis servers.

An implementation of BRIDGE-MIB shipped with Net-SNMP.

There is no similar binding for Python but there is an agentx extension using `ctypes`. It provides a more high-level view. I did not write an example for this one but you can look at the example provided by the author.

C with new API To write a sub-agent in C, you have to choose between two API:

the API inherited from UCD-SNMP also known as the traditional API;
the new API developed for Net-SNMP 5.x.

With both API, an handler is registered to take care of one or several sub-trees. With the traditional API, the handler is invoked with very little information. It is then up to you to locate the appropriate variable by mapping the index part of the OID to the objects you want to export. It is quite simple for scalar objects but columnar objects with complex indexes are a pain with such an API. The new API provides various helpers. Some of them allow you to copy objects into an array or a list and let Net-SNMP library do the hard work of serving them. Another helper will provide methods to serve objects without putting them in a special structure but by providing an iterator. Some of those helpers can also help you to cache some variables. On top of this new API, Net-SNMP comes with mib2c, a tool that will help you convert a MIB module into C code: after a few questions, you will get a skeleton C code you will have to complete with the logic to retrieve real objects. A generated file also provides directions on what to do (with some magic command to have a step-by-step cookbook). I was able to implement ETHTOOL-MIB by just running it through mib2c, choosing the table container helper with cache and adding some code in ethtoolStatTable_data_access.c. You can look at the result on Github. Here is a stripped-down version² of what has been modified:

/* Socket for ioctl */
skfd = socket(AF_INET, SOCK_DGRAM, 0);
/* Iterate through all interfaces */
getifaddrs(&ifap);
for (ifa = ifap; ifa != NULL; ifa = ifa->ifa_next)  
    /* Grab statistics name and values */
    strcpy(ifr.ifr_name, ifa->ifa_name);
    drvinfo.cmd         = ETHTOOL_GDRVINFO;
    ifr.ifr_data        = (caddr_t) &drvinfo;
    ioctl(skfd, SIOCETHTOOL, &ifr);
    n_stats             = drvinfo.n_stats;
    strings->cmd        = ETHTOOL_GSTRINGS;
    strings->string_set = ETH_SS_STATS;
    strings->len        = n_stats;
    ifr.ifr_data        = (caddr_t) strings;
    ioctl(skfd, SIOCETHTOOL, &ifr);
    stats->cmd          = ETHTOOL_GSTATS;
    stats->n_stats      = n_stats;
    ifr.ifr_data        = (caddr_t) stats;
    ioctl(skfd, SIOCETHTOOL, &ifr);
    ifIndex = if_nametoindex(ifa->ifa_name);
    /* Iterate through statistics */
    for (i = 0; i < n_stats; i++)  
        /* Declare the appropriate index */
        strncpy(ethtoolStatName,
                (char *)&strings->data[i * ETH_GSTRING_LEN],
                ETH_GSTRING_LEN);
        ethtoolStatName[sizeof(ethtoolStatName) - 1] = '\0';
        ethtoolStatName_len = strlen(ethtoolStatName);
        /* Append this new variable to the container. */
        rowreq_ctx = ethtoolStatTable_allocate_rowreq_ctx();
        ethtoolStatTable_indexes_set(rowreq_ctx,
                    ifIndex,
                    ethtoolStatName, ethtoolStatName_len);
        rowreq_ctx->data.ethtoolStat.high = stats->data[i] >> 32;
        rowreq_ctx->data.ethtoolStat.low = (uint32_t)stats->data[i];
        CONTAINER_INSERT(container, rowreq_ctx);

See? It is pretty easy. I have only enumerated all the objects I wanted to expose. I didn t even have to convert statistics names to an OID, mib2c built a ethtoolStatTable_indexes_set() for this purpose. OpenHPI project provides an extensive documentation to write a sub-agent this way. Now, on the downside, to implement a single table, I have 14 generated files whose code style is usually unfit to integrate into an existing project : some people, me included, do not like automatically generated code, all the more when it is so verbose. The solution would be to use the new API without mib2c; unfortunately, while the documentation exists, all provided examples rely exclusively on mib2c tool. Unless you are able to grasp the new API without it, I would restrict its use to two cases:

Writing a new module for inclusion into Net-SNMP. Since most modules are now written with the help of mib2c, there is no coding style problem.
Writing a standalone sub-agent for some MIB. As a standalone project, you won t run into coding style problems either.

For other uses, let s have a look at the traditional API.

C with traditional API The traditional API is way simpler and even if it is quite old, it is easier to find associated examples. Here is how to initialize the agent, again, in a stripped-down version (the complete version is on GitHub):

static oid ethtool_oid[] =  1, 3, 6, 1, 4, 1, 39178, 100, 1 ;
static struct variable3 ethtool_vars[] =  
   1, ASN_COUNTER64, RONLY, ethtool_stat, 3,  1, 1, 2 
 ;
int main()
 
  netsnmp_enable_subagent();
  snmp_disable_log();
  snmp_enable_stderrlog();
  init_agent("ethtoolAgent");
  REGISTER_MIB("ethtoolStatTable", ethtool_vars,
               variable3, ethtool_oid);
  init_snmp("ethtoolAgent");
  while (1)
    agent_check_and_process(1);

The REGISTER_MIB() macro is used to register the provided root OID (ethtool_oid) to the master agent. It also registers handlers associated to various sub-OID through the ethtool_vars[] array. Each member of this array features the following fields:

magic is an integer allowing one to discriminate OID handled by the same handler. For a table, this allows to handle several columns with the same handler since they share a common index.
type is the type of the variable that will be returned.
acl tells if the object is read-only (RONLY) or read-write (RWRITE).
findVar is the handler associated to the object. Its task will be to find the appropriate object and return its value.
The two last fields are the relative OID this entry is responsible for along with its length.

The prototype of ethtool_stat() handler function is the following:

static u_char*
ethtool_stat(struct variable *vp, oid *name, size_t *length,
             int exact, size_t *var_len, WriteMethod **write_method);

This function should return the value of the requested variable or NULL if no appropriate object was found. The value can be statically stored since it will be copied. Here is the description of the arguments:

struct variable *vp is the structure the handle was associated with, except that vp->name is now a full OID, not a relative one. You can access vp->magic to determine which column or which scalar to handle if this handler is common to several objects.
oid *name and size_t *length describe the OID requested. If you want to return another OID (for a GETNEXT request for example), you can modify them. name points to an OID array large enough to fit any OID you want to return.
int exact tells if the requested OID should be exactly matched (case of GET or SET) or not (GETNEXT).
size_t *var_len is used to tell the size of the returned object.
WriteMethod **write_method is only used if the variable is modifiable. When a SET request is received, the API will call the handler with exact set to 1 and expects to get a pointer to the function that will handle the write operation³.

To implement this function, you are on your own. When relying on externally fetched data, you need to handle a cache yourself. I have used a red-black tree for this purpose (with code stolen from OpenBSD). This also enables an easy implementation of the GETNEXT operation. Here are three projects using this API to provide SNMP support:

Keepalived is a VRRP daemon and a monitoring daemon for LVS clusters. I have added a complete SNMP support. The most interesting files are core/snmp.c, check/check_snmp.c and vrrp/vrrp_snmp.c. It features a tight integration into Keepalived event loop that should be reusable by other projects. It includes read-only support of all data structures (configuration, state and statistics), ability to send traps on events and a write support for some values.
lldpd is an implementation of 802.1AB (LLDP) which is a protocol allowing an equipment to advertise itself to its neighbors on Ethernet level. 802.1AB comes with a huge MIB module and many extensions. lldpd provides a fairly complete read-only implementation of this MIB module. Some tables have complex indexes. Have a look at agent.c. There is currently no write support and no traps but lldpd features privilege separation and some code allows it to still use an Unix socket to speak with the master agent.
Asterisk, an open source PBX, includes an AgentX sub-agent allowing one to grab some metrics. Look at res/snmp/agent.c.

Other solutions The manual page for snmpd.conf explores other ways to extend Net-SNMP agent under the section Extending agent functionality :

`exec` and `sh` are deprecated. Use `extend` instead.

`pass` is a bit simpler than `pass_persist`. The command is run once for each request. Such a command can easily be turned into a `pass_persist` script. Moreover, Net-SNMP will not answer any request while a `pass` command is running.

`proxy` redirects requests to another SNMP agent. Unless you have some code that can only act as a complete SNMP agent, it is better to use AgentX instead. For programs using Net-SNMP API, turning an agent into an AgentX sub-agent is just one line of code. Since a sub-agent involves less code than a complete agent, it is more efficient and more reliable.

`smux` has been superseeded by AgentX.

`perl` and `dlmod` allow one to load and execute external code inside the running agent. Unless you have important performance requirements, it is better to use a sub-agent instead: if some bug happens, only the sub-agent will be affected, not the master agent. A sub-agent sticks more closely to the Unix philosophy: write programs that do one thing and do it well . However, Net-SNMP FAQ says:

Implementing the module in C within the main agent (directly or via `dlmod`) is probably the most efficient and reliable, closely followed by embedded perl (or python) extensions. These have the advantage of minimal overheads between the code implementing the MIB module, and the agent framework, and no inter-process communication issues.

Additional arguments for `snmp` are usually required to specify a version and a community (for SNMPv1 and SNMPv2c) or a user (for SNMPv3). In all examples, I do not specify anything. I assume that `~/.snmp/snmp.conf` contains the appropriate bits. Moreover, some outputs are truncated.

No allocations, no variable declarations and no error checkings. Do not do this at home!

If you want to look at an example of how to handle a SET* request, look at the
SNMP support for Keepalived. A SET operation is more complex because SNMP ensures that the
SET operation is atomic across all provided variables. It uses a three-staged commit. The traditional API is very inefficient for this kind of operation because the object has to be located again for each step.

Everyone agrees that backups are generally a good thing. But it seems that there is a lot less agreement about how backups should work. Here is a list of 5 principles of backup software that seem to get ignored most of the time: (1/5) Backups should not be Application Specific It s quite reasonable for people to want to extract data from a backup on a different platform. Maybe someone will want to extract data a few decades after the platform becomes obsolete. I believe that vendors of backup software have an ethical obligation to make it possible for customers to get their data out with minimal effort regardless of the circumstances. Often when writing a backup application there will be good reasons for not using the existing formats for data storage (tar, cpio, zip, etc). But ideally any data store which involves something conceptually similar to a collection of files in one larger file will use one of those formats. There have been backward compatible extensions to tar and zip for SE Linux contexts and for OS/2 EAs the possibility of extending archive file formats with no consequence other than warnings on extraction with an unpatched utility has been demonstrated. For a backup which doesn t involve source files (EG the contents of some sort of database) then it should be in a format that can be easily understood and parsed. Well designed XML is generally a reasonable option. Generally the format should involve plain text that is readable and easy to understand which is optionally compressed with a common compression utility (pkzip is a reasonable choice). (2/5) Data Store Formats should be Published For every data store there should be public documentation about it s format to allow future developers to write support for it. It really isn t difficult to release some commented header files so that people can easily determine the data structures. This includes all data stores including databases and filesystems. If I suddenly find myself with a 15yo image of a NTFS filesystem containing a proprietary database I should be able to find official header files for the version of NTFS and the database server in question so I can decode the data if it s important enough. When an application vendor hides the data formats it gives the risk of substantial data loss at some future time. Imposing such risk on customers to try and prevent them from migrating to a rival product is unethical. (3/5) Backups should be forward and backward compatible It is entirely unreasonable for a vendor to demand that all their users install the latest versions of their software. There are lots of good reasons for not upgrading which includes hardware not supporting new versions of the OS, lack of Internet access to perform the upgrade, application compatibility, and just liking the way the old version works. Even for the case of a critical security fix it should be possible to restore data without applying the fix. For any pair of versions of software that are only separated by a few versions it should be possible to backup data from one and restore to the other. Even if the data can t be used directly (EG a backup of AMD64 programs that is restored on an i386 system) it should still be accessible. If a new version of the software doesn t support the ancient file formats then it should be possible for the users to get a slightly older version which talks to both the old and new versions. Backups made on 64bit systems running the latest development version of Linux and on 10yo 32bit proprietary Unix systems are interchangeable. Admittedly Unix is really good at preserving file format compatibility, but there is no technical reason why other systems can t do the same. Source code to cpio, tar, and gnuzip, is freely available! Apple TimeMachine fails badly in this regard, even a slightly older version of Mac OS can t do a restore. It is however nice that most of the TimeMachine data is a tree of files which could be just copied to another system. (4/5) Backup Software should not be Dropped Sony Ericsson has made me hate them even more by putting the following message on their update web site: The Backup and Restore app will be overwritten and cannot be used to restore data. Check out Android Market for alternative apps to back up and restore your data, such as MyBackup. So if you own a Sony Ericsson phone and it is lost, stolen, or completely destroyed and all you have is a backup made by the Sony Ericsson tool then the one thing you absolutely can t do is to buy a new Sony Ericsson phone to restore the data. I believe that anyone who releases backup software has an ethical obligation to support restoring to all equivalent systems. How difficult would it be to put a new free app in the Google Market that has as it s sole purpose recovering old Sony Ericsson backups onto newer phones? It really can t be that difficult, so even if they don t want to waste critical ROM space by putting the feature in all new phones they can make it available to everyone who needs it. When compared to the cost of developing a new Android release for a series of phones the cost of writing such a restore program would be almost nothing. It is simply mind-boggling that Sony Ericsson go against their own commercial interests in this regard. Surely it would make good business sense to be able to sell replacements for all the lost and broken Sony Ericsson phones, but instead customers who get burned by broken backups are given an incentive to buy a product from any other vendor. (5/5) The greater the control over data the greater the obligation for protecting it If you have data stored in a simple and standard manner (EG the /DCIM directory containing MP4 and JPEG files that is on the USB accessible storage in every modern phone) then IMHO it s quite OK to leave customers to their own devices in terms of backups. Typical users can work out that if they don t backup their pictures then they risk losing them, and they can work out how to do it. My Sony Ericsson phones have data stored under /data (settings for Android applications) which is apparently only accessible as root. Sony Ericsson have denied me root access which prevents me running backup programs such as Titanium Backup, therefore I believe that they have a great obligation to provide a way of making a backup of this data and restoring it on a new phone or a phone that has been updated. To just provide phone upgrade instructions which tell me that my phone will be entirely wiped and that I should search the App Market for backup programs is unacceptable. I believe that there are two ethical options available to Sony Ericsson at this time, one is to make it easy to root phones so that Titanium Backup and similar programs can be used, and the other option is to release a suitable backup program for older phones. Based on experience I don t expect Sony Ericsson to choose either option. Now it is also a bad thing for the Android application developers to make it difficult or impossible to backup their data. For example the Wiki for one Android game gives instructions for moving the saved game files to a new phone which starts with root your phone . The developers of that game should have read the Wiki, realised that rooting a phone for the mundane task of transferring saved game files is totally unreasonable, and developed a better alternative. The best thing for developers to do is to allow the users to access their own data in the most convenient manner. Then it becomes the user s responsibility to manage it and they can concentrate on improving their application. Why Freedom is Important Installing CyanogenMod on my Galaxy S was painful, but having root access so I can do anything I want is a great benefit. If phone vendors would do the right thing then I could recommend that other people use the vendor release, but it seems that vendors can be expected to act unethically. So I can t recommend that anyone use an un-modded Android phone at any time. I also can t recommend ever buying a Sony Ericsson product, not even when it s really cheap. Google have done a great thing with their Data Liberation Front [1]. Not only are they providing access to the data they store on our behalf (which is a good thing) but they have a mission statement that demands the same behavior from other companies they make it an issue of competitive advantage! So while Sony Ericsson and other companies might not see a benefit in making people like me stop hating them, failing to be as effective in marketing as Google is a real issue. Data Liberation is something that should be discussed at board elections of IT companies. Keep in mind the fact that ethics are not just about doing nice things, they are about establishing expectations of conduct that will be used by people who deal with you in future. Sony Ericsson has shown that I should expect that they will treat the integrity of my data with contempt and I will keep this in mind every time I decline an opportunity to purchase their products. Google has shown that they consider the protection of my data as an important issue and therefore I can be confident when using and recommending their services that I won t get stuck with data that is locked away. While Google has demonstrated that corporations can do the right thing, the vast majority of evidence suggests that we should never trust a corporation with anything that we might want to retrieve when it s not immediately profitable for the corporation. Therefore avoiding commercial services for storing important data is the sensible thing to do.

[1] http://www.dataliberation.org/

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ZDNet has an insightful article by Robin Harris predicting the demise of RAID-6 due to the probability of read errors [1]. Basically as drives get larger the probability of hitting a read error during reconstruction increases and therefore you need to have more redundancy to deal with this. He suggests that as of 2009 drives were too big for a reasonable person to rely on correct reads from all remaining drives after one drive failed (in the case of RAID-5) and that in 2019 there will be a similar issue with RAID-6. Of course most systems in the field aren t using even RAID-6. All the most economical hosting options involve just RAID-1 and RAID-5 is still fairly popular with small servers. With RAID-1 and RAID-5 you have a serious problem when (not if) a disk returns random or outdated data and says that it is correct, you have no way of knowing which of the disks in the set has good data and which has bad data. For RAID-5 it will be theoretically possible to reconstruct the data in some situations by determining which disk should have it s data discarded to give a result that passes higher level checks (EG fsck or application data consistency), but this is probably only viable in extreme cases (EG one disk returns only corrupt data for all reads). For the common case of a RAID-1 array if one disk returns a few bad sectors then probably most people will just hope that it doesn t hit something important. The case of Linux software RAID-1 is of interest to me because that is used by many of my servers. Robin has also written about some NetApp research into the incidence of read errors which indicates that 8.5% of consumer disks had such errors during the 32 month study period [2]. This is a concern as I run enough RAID-1 systems with consumer disks that it is very improbable that I m not getting such errors. So the question is, how can I discover such errors and fix them? In Debian the mdadm package does a monthly scan of all software RAID devices to try and find such inconsistencies, but it doesn t send an email to alert the sysadmin! I have filed Debian bug #658701 with a patch to make mdadm send email about this. But this really isn t going to help a lot as the email will be sent AFTER the kernel has synchronised the data with a 50% chance of overwriting the last copy of good data with the bad data! Also the kernel code doesn t seem to tell userspace which disk had the wrong data in a 3-disk mirror (and presumably a RAID-6 works in the same way) so even if the data can be corrected I won t know which disk is failing. Another problem with RAID checking is the fact that it will inherently take a long time and in practice can take a lot longer than necessary. For example I run some systems with LVM on RAID-1 on which only a fraction of the VG capacity is used, in one case the kernel will check 2.7TB of RAID even when there s only 470G in use! The BTRFS Filesystem The btrfs Wiki is currently at btrfs.ipv5.de as the kernel.org wikis are apparently still read-only since the compromise [3]. BTRFS is noteworthy for doing checksums on data and metadata and for having internal support for RAID. So if two disks in a BTRFS RAID-1 disagree then the one with valid checksums will be taken as correct! I ve just done a quick test of this. I created a filesystem with the command mkfs.btrfs -m raid1 -d raid1 /dev/vg0/raid? and copied /dev/urandom to it until it was full. I then used dd to copy /dev/urandom to some parts of /dev/vg0/raidb while reading files from the mounted filesystem that worked correctly although I was disappointed that it didn t report any errors, I had hoped that it would read half the data from each device and fix some errors on the fly. Then I ran the command btrfs scrub start . and it gave lots of verbose errors in the kernel message log telling me which device had errors and where the errors are. I was a little disappointed that the command btrfs scrub status . just gave me a count of the corrected errors and didn t mention which device had the errors. It seems to me that BTRFS is going to be a much better option than Linux software RAID once it is stable enough to use in production. I am considering upgrading one of my less important servers to Debian/Unstable to test out BTRFS in this configuration. BTRFS is rumored to have performance problems, I will test this but don t have time to do so right now. Anyway I m not always particularly concerned about performance, I have some systems where reliability is important enough to justify a performance loss. BTRFS and Xen The system with the 2.7TB RAID-1 is a Xen server and LVM volumes on that RAID are used for the block devices of the Xen DomUs. It seems obvious that I could create a single BTRFS filesystem for such a machine that uses both disks in a RAID-1 configuration and then use files on the BTRFS filesystem for Xen block devices. But that would give a lot of overhead of having a filesystem within a filesystem. So I am considering using two LVM volume groups, one for each disk. Then for each DomU which does anything disk intensive I can export two LVs, one from each physical disk and then run BTRFS inside the DomU. The down-side of this is that each DomU will need to scrub the devices and monitor the kernel log for checksum errors. Among other things I will have to back-port the BTRFS tools to CentOS 4. This will be more difficult to manage than just having an LVM VG running on a RAID-1 array and giving each DomU a couple of LVs for storage. BTRFS and DRBD The combination of BTRFS RAID-1 and DRBD is going to be a difficult one. The obvious way of doing it would be to run DRBD over loopback devices that use large files on a BTRFS filesystem. That gives the overhead of a filesystem in a filesystem as well as the DRBD overhead. It would be nice if BTRFS supported more than two copies of mirrored data. Then instead of DRBD over RAID-1 I could have two servers that each have two devices exported via NBD and BTRFS could store the data on all four devices. With that configuration I could lose an entire server and get a read error without losing any data! Comparing Risks I don t want to use BTRFS in production now because of the risk of bugs. While it s unlikely to have really serious bugs it s theoretically possible that as bug could deny access to data until kernel code is fixed and it s also possible (although less likely) that a bug could result in data being overwritten such that it can never be recovered. But for the current configuration (Ext4 on Linux software RAID-1) it s almost certain that I will lose small amounts of data and it s most probable that I have silently lost data on many occasions without realising.

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For the past few years the anti-spam system I run has been based on a simplified version of something I previously ran commercially. Although the code is similar in intent there were both explicit feature removals, and simplifications made. Last month I re-implimented domain-blacklisting - because a single company keeps ignoring requests to remove me. So LinkedIn.com if you're reading this:

I've never had an account on your servers.
I find your junk mail annoying.
I suspect I'll join your site/service when hell freezes over.

I've also implemented TLD-blacklisting which has been useful.

TLD-blacklisting in my world is not about blocking mail from foo@bar.ph (whether in the envelope sender, or the from: header), instead it is about matching the reverse DNS of the connecting client. If I recieve a connection from 1.2.3.4 and the reverse DNS of that IP address matches, say, /\.sa$/i then I default to denying it. My real list is longer, and handled via files:
steve@steve:~$ ls /srv/_global_/blacklisted/tld/ -C
ar  br  cn  eg  hr  in  kr  lv  mn  np  ph  ro  sg  tg  ua  ve  zw
aw  cc  cy  gm  hu  is  kz  ma  my  nu  pk  rs  sk  th  ug  vn
be  ch  cz  gr  id  it  lk  md  mz  nz  pl  ru  su  tr  uy  ws
bg  cl  ec  hk  il  ke  lt  mk  no  om  pt  sa  sy  tw  uz  za

On average I'm rejecting about 2500 messagse a day at SMTP-time, and 30 messages, or so, hit my SPAM folder after being filtered with CRM114 after being accepted for delivery. (They are largely from @hotmail and @yahoo, along with random compromised machines. The amount of times I see a single mail from a host with RDNS mysql.example.org is staggering.). (Still looking forward to the development of Haraka, a node.js version of qpsmtpd.) ObQuote: "Mr. Mystery Guest? Are you still there? " - Die Hard

Quick howto for everyone having Asus UX31e aka Zenbook and wants the most stable vanilla kernel with long-time battery life and working touchpad made by Sentelic.
This should work for every Debian based distro, but has been tested only on the latest Linux Mint 12 (aka Lisa).
Tutorial is aimed mainly at beginners and should work in a copy&paste manner. Of course you have to change every occurence of linux-3.2.4 to whichever version you re going to install.
Just make sure that all commands are entered in the correct directories, prompts should look similar to these from examples.
You will need about 10GB of free disk space. Let s install some dependencies first:

fenio@zenbook ~ $ sudo apt-get install build-essential kernel-package fakeroot libncurses5-dev git iasl

Now let s download and unpack the latest kernel (3.2.4 at the time it was written):

fenio@fenio ~ $ mkdir kernel
fenio@fenio ~ $ cd kernel
fenio@fenio ~/kernel $ wget http://www.kernel.org/pub/linux/kernel/v3.0/linux-3.2.4.tar.bz2
fenio@fenio ~/kernel $ tar jxvf linux-3.2.4.tar.bz2

Now we take current kernel configuration and put in the kernel tree:

fenio@fenio ~/kernel $ cp /boot/config- uname -r  linux-3.2.4/.config

Now we have to download the latest driver for Sentelic touchpad (this is why we needed git as a dependency):

fenio@fenio ~/kernel $ git clone git://github.com/saaros/sentelic.git
fenio@fenio ~/kernel $ cp sentelic/src/sentelic.* linux-3.2.4/drivers/input/mouse/

It s time to fix broken DSDT table (part of ACPI). This is why we had iasl in dependencies.

fenio@fenio ~/kernel $ wget http://files.benesovi.eu/ux31e/ux31e_dsdt.dsl
fenio@fenio ~/kernel $ iasl -tc ux31e_dsdt.dsl

And include it in our kernel configuration:

fenio@fenio ~/kernel $ sed -ie 's/# CONFIG_ACPI_CUSTOM_DSDT is not set/CONFIG_ACPI_CUSTOM_DSDT=y/g' linux-3.2.4/.config 
fenio@fenio ~/kernel $ sed -ie "s@CONFIG_ACPI_CUSTOM_DSDT_FILE.*@CONFIG_ACPI_CUSTOM_DSDT_FILE=\" pwd /ux31e_dsdt.hex\"@g" linux-3.2.4/.config

Now ensure that all our options are set up correctly (you can change some other options if you want).
If you don t want to change anything then simply exit saving configuration.

fenio@fenio ~/kernel $ cd linux-3.2.4/
fenio@fenio ~/kernel/linux-3.2.4 $ make menuconfig

We re ready to start compilation now!

fenio@fenio ~/kernel/linux-3.2.4 $ fakeroot make-kpkg clean
fenio@fenio ~/kernel/linux-3.2.4 $ fakeroot make-kpkg --jobs=4 --initrd --append-to-version=-fenio --revision=20120204 kernel_image kernel_headers modules_image

Of course you can change append-to-version option for something own. In the meantime (kernel compilations takes about one hour) you can modify Grub options to enable powersaving features RC6:

fenio@zenbook ~ $ sudo sed -ie 's/GRUB_CMDLINE_LINUX=.*/GRUB_CMDLINE_LINUX="i915.powersave=1 i915.semaphores=1 i915.i915_enable_rc6=1"/g' /etc/default/grub

Be sure to do that before installation of kernel, otherwise you will have to run update-grub. After compilation you can finally install your new kernel:

fenio@zenbook ~/kernel/linux-3.2.4 $ sudo dpkg -i ../*.deb

Reboot and you re done! Feel free to comment this tutorial if something went wrong.

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