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Multi-Stream Transport 4k Monitors and X I'm sure you've seen a 4k monitor on a friends desk running Mac OS X or Windows and are all ready to go get one so that you can use it under Linux. Once you've managed to acquire one, I'm afraid you'll discover that when you plug it in, you're limited to 30Hz refresh rates at the full size, unless you're running a kernel that is version 3.17 or later. And then... Good Grief! What Is My Computer Doing! Ok, so now you're running version 3.17 and when X starts up, it's like you're using a gigantic version of Google Cardboard. Two copies of a very tall, but very narrow screen greets you. Welcome to MST island. In order to drive these giant new panels at full speed, there isn't enough bandwidth in the display hardware to individually paint each pixel once during each frame. So, like all good hardware engineers, they invented a clever hack. This clever hack paints the screen in parallel. I'm assuming that they've got two bits of display hardware, each one hooked up to half of the monitor. Now, each paints only half of the pixels, avoiding costly redesign of expensive silicon, at least that's my surmise. In the olden days, if you did this, you'd end up running two monitor cables to your computer, and potentially even having two video cards. Today, thanks to the magic of Display Port Multi-Stream Transport, we don't need all of that; instead, MST allows us to pack multiple cables-worth of data into a single cable. I doubt the inventors of MST intended it to be used to split a single LCD panel into multiple "monitors", but hardware engineers are clever folk and are more than capable of abusing standards like this when it serves to save a buck. Turning Two Back Into One We've got lots of APIs that expose monitor information in the system, and across which we might be able to wave our magic abstraction wand to fix this:

1. The KMS API. This is the kernel interface which is used by all graphics stuff, including user-space applications and the frame buffer console. Solve the problem here and it works everywhere automatically.
2. The libdrm API. This is just the KMS ioctls wrapped in a simple C library. Fixing things here wouldn't make fbcons work, but would at least get all of the window systems working.
3. Every 2D X driver. (Yeah, we're trying to replace all of these with the one true X driver). Fixing the problem here would mean that all X desktops would work. However, that's a lot of code to hack, so we'll skip this.
4. The X server RandR code. More plausible than fixing every driver, this also makes X desktops work.
5. The RandR library. If not in the X server itself, how about over in user space in the RandR protocol library? Well, the problem here is that we've now got two of them (Xlib and xcb), and the xcb one is auto-generated from the protocol descriptions. Not plausible.
6. The Xinerama code in the X server. Xinerama is how we did multi-monitor stuff before RandR existed. These days, RandR provides Xinerama emulation, but we've been telling people to switch to RandR directly.
7. Some new API. Awesome. Ok, so if we haven't fixed this in any existing API we control (kernel/libdrm/X.org), then we effectively dump the problem into the laps of the desktop and application developers. Given how long it's taken them to adopt current RandR stuff, providing yet another complication in their lives won't make them very happy.

All Our APIs Suck Dave Airlie merged MST support into the kernel for version 3.17 in the simplest possible fashion -- pushing the problem out to user space. I was initially vaguely tempted to go poke at it and try to fix things there, but he eventually convinced me that it just wasn't feasible. It turns out that all of our fancy new modesetting APIs describe the hardware in more detail than any application actually cares about. In particular, we expose a huge array of hardware objects:

• Subconnectors
• Connectors
• Outputs
• Video modes
• Crtcs
• Encoders

Each of these objects exposes intimate details about the underlying hardware -- which of them can work together, and which cannot; what kinds of limits are there on data rates and formats; and pixel-level timing details about blanking periods and refresh rates. To make things work, some piece of code needs to actually hook things up, and explain to the user why the configuration they want just isn't possible. The sticking point we reached was that when an MST monitor gets plugged in, it needs two CRTCs to drive it. If one of those is already in use by some other output, there's just no way you can steal it for MST mode. Another problem -- we expose EDID data and actual video mode timings. Our MST monitor has two EDID blocks, one for each half. They happen to describe how they're related, and how you should configure them, but if we want to hide that from the application, we'll have to pull those EDID blocks apart and construct a new one. The same goes for video modes; we'll have to construct ones for MST mode. Every single one of our APIs exposes enough of this information to be dangerous. Every one, except Xinerama. All it talks about is a list of rectangles, each of which represents a logical view into the desktop. Did I mention we've been encouraging people to stop using this? And that some of them listened to us? Foolishly? Dave's Tiling Property Dave hacked up the X server to parse the EDID strings and communicate the layout information to clients through an output property. Then he hacked up the gnome code to parse that property and build a RandR configuration that would work. Then, he changed to RandR Xinerama code to also parse the TILE properties and to fix up the data seen by application from that. This works well enough to get a desktop running correctly, assuming that desktop uses Xinerama to fetch this data. Alas, gtk has been "fixed" to use RandR if you have RandR version 1.3 or later. No biscuit for us today. Adding RandR Monitors RandR doesn't have enough data types yet, so I decided that what we wanted to do was create another one; maybe that would solve this problem. Ok, so what clients mostly want to know is which bits of the screen are going to be stuck together and should be treated as a single unit. With current RandR, that's some of the information included in a CRTC. You pull the pixel size out of the associated mode, physical size out of the associated outputs and the position from the CRTC itself. Most of that information is available through Xinerama too; it's just missing physical sizes and any kind of labeling to help the user understand which monitor you're talking about. The other problem with Xinerama is that it cannot be configured by clients; the existing RandR implementation constructs the Xinerama data directly from the RandR CRTC settings. Dave's Tiling property changes edit that data to reflect the union of associated monitors as a single Xinerama rectangle. Allowing the Xinerama data to be configured by clients would fix our 4k MST monitor problem as well as solving the longstanding video wall, WiDi and VNC troubles. All of those want to create logical monitor areas within the screen under client control What I've done is create a new RandR datatype, the "Monitor", which is a rectangular area of the screen which defines a rectangular region of the screen. Each monitor has the following data:

• Name. This provides some way to identify the Monitor to the user. I'm using X atoms for this as it made a bunch of things easier.
• Primary boolean. This indicates whether the monitor is to be considered the "primary" monitor, suitable for placing toolbars and menus.
• Pixel geometry (x, y, width, height). These locate the region within the screen and define the pixel size.
• Physical geometry (width-in-millimeters, height-in-millimeters). These let the user know how big the pixels will appear in this region.
• List of outputs. (I think this is the clever bit)

There are three requests to define, delete and list monitors. And that's it. Now, we want the list of monitors to completely describe the environment, and yet we don't want existing tools to break completely. So, we need some way to automatically construct monitors from the existing RandR state while still letting the user override portions of it as needed to explain virtual or tiled outputs. So, what I did was to let the client specify a list of outputs for each monitor. All of the CRTCs which aren't associated with an output in any client-defined monitor are then added to the list of monitors reported back to clients. That means that clients need only define monitors for things they understand, and they can leave the other bits alone and the server will do something sensible. The second tricky bit is that if you specify an empty rectangle at 0,0 for the pixel geometry, then the server will automatically compute the geometry using the list of outputs provided. That means that if any of those outputs get disabled or reconfigured, the Monitor associated with them will appear to change as well. Current Status Gtk+ has been switched to use RandR for RandR versions 1.3 or later. Locally, I hacked libXrandr to override the RandR version through an environment variable, set that to 1.2 and Gtk+ happily reverts back to Xinerama and things work fine. I suspect the plan here will be to have it use the new Monitors when present as those provide the same info that it was pulling out of RandR's CRTCs. KDE appears to still use Xinerama data for this, so it "just works". Where's the code As usual, all of the code for this is in a collection of git repositories in my home directory on fd.o:

git://people.freedesktop.org/~keithp/randrproto master
git://people.freedesktop.org/~keithp/libXrandr master
git://people.freedesktop.org/~keithp/xrandr master
git://people.freedesktop.org/~keithp/xserver randr-monitors

RandR protocol changes Here's the new sections added to randrproto.txt

                   
1.5. Introduction to version 1.5 of the extension
Version 1.5 adds monitors
   A 'Monitor' is a rectangular subset of the screen which represents
   a coherent collection of pixels presented to the user.
   Each Monitor is be associated with a list of outputs (which may be
   empty).
   When clients define monitors, the associated outputs are removed from
   existing Monitors. If removing the output causes the list for that
   monitor to become empty, that monitor will be deleted.
   For active CRTCs that have no output associated with any
   client-defined Monitor, one server-defined monitor will
   automatically be defined of the first Output associated with them.
   When defining a monitor, setting the geometry to all zeros will
   cause that monitor to dynamically track the bounding box of the
   active outputs associated with them
This new object separates the physical configuration of the hardware
from the logical subsets  the screen that applications should
consider as single viewable areas.
1.5.1. Relationship between Monitors and Xinerama
Xinerama's information now comes from the Monitors instead of directly
from the CRTCs. The Monitor marked as Primary will be listed first.
                   
5.6. Protocol Types added in version 1.5 of the extension
MONITORINFO   name: ATOM
          primary: BOOL
          automatic: BOOL
          x: INT16
          y: INT16
          width: CARD16
          height: CARD16
          width-in-millimeters: CARD32
          height-in-millimeters: CARD32
          outputs: LISTofOUTPUT  
                   
7.5. Extension Requests added in version 1.5 of the extension.
 
    RRGetMonitors
    window : WINDOW
      
    timestamp: TIMESTAMP
    monitors: LISTofMONITORINFO
 
    Errors: Window
    Returns the list of Monitors for the screen containing
    'window'.
    'timestamp' indicates the server time when the list of
    monitors last changed.
 
    RRSetMonitor
    window : WINDOW
    info: MONITORINFO
 
    Errors: Window, Output, Atom, Value
    Create a new monitor. Any existing Monitor of the same name is deleted.
    'name' must be a valid atom or an Atom error results.
    'name' must not match the name of any Output on the screen, or
    a Value error results.
    If 'info.outputs' is non-empty, and if x, y, width, height are all
    zero, then the Monitor geometry will be dynamically defined to
    be the bounding box of the geometry of the active CRTCs
    associated with them.
    If 'name' matches an existing Monitor on the screen, the
    existing one will be deleted as if RRDeleteMonitor were called.
    For each output in 'info.outputs, each one is removed from all
    pre-existing Monitors. If removing the output causes the list of
    outputs for that Monitor to become empty, then that Monitor will
    be deleted as if RRDeleteMonitor were called.
    Only one monitor per screen may be primary. If 'info.primary'
    is true, then the primary value will be set to false on all
    other monitors on the screen.
    RRSetMonitor generates a ConfigureNotify event on the root
    window of the screen.
 
    RRDeleteMonitor
    window : WINDOW
    name: ATOM
 
    Errors: Window, Atom, Value
    Deletes the named Monitor.
    'name' must be a valid atom or an Atom error results.
    'name' must match the name of a Monitor on the screen, or a
    Value error results.
    RRDeleteMonitor generates a ConfigureNotify event on the root
    window of the screen.
                   

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Continuing with his standard pace of approximately one new version per month, Conrad released a new minor release of Armadillo a few days ago. As before, I had created a GitHub-only pre-release which was tested against all eighty-seven (!!) CRAN dependents of our RcppArmadillo package and then uploaded RcppArmadillo 0.4.450.0 to CRAN. The CRAN maintainers pointed out that under the R-development release, a NOTE was issued concerning the C-library's rand() call. This is a pretty new NOTE, but it means using the (sometimes poor quality) rand() generator is now a no-no. Now, Armadillo being as robustly engineered as it is offers a new random number generator based on C++11 as well as a fallback generator for those unfortunate enough to live with an older C++98 compiler. (I would like to note here that I find Conrad's continued support for both C++11, offering very useful modern language idioms, as well as the fallback code for continued deployment and usage by those constrained in their choice of compilers rather exemplary --- because contrary to what some people may claim, it is not a matter of one or the other. C++ always was, and continues to be, a multi-paradigm language which can be supported easily by several standard. But I digress...) In any event, one cannot argue with CRAN about their prescription of a C++98 compiler. So Conrad and I discussed this over email, and came up with a scheme where a user-package (such as RcppArmadillo) can provide an alternate generator which Armadillo then deploys. I implemented a first solution which was then altered / reflected by Conrad in a revised version 4.450.1 of Armadillo. I packaged, and now uploaded, that version as RcppArmadillo 0.4.450.1.0 to both CRAN and into Debian. Besides the RNG change already discussed, this release brings a few smaller changes from the Armadillo side. These are detailed below in the extract from the NEWS file. On the RcppArmadillo side, we now have support for pkgKitten which is both very exciting and likely the topic of another blog post with an example of creating an RcppArmadillo package that purrs. In the process, I overhauled and polished how new packages are created by RcppArmadillo.package.skeleton(). An upcoming blog post may provide an example.

Changes in RcppArmadillo version 0.4.450.1.0 (2014-09-21)

• Upgraded to Armadillo release Version 4.450.1 (Spring Hill Fort)
  • faster handling of matrix transposes within compound expressions
  • expanded symmatu()/symmatl() to optionally disable taking the complex conjugate of elements
  • expanded sort_index() to handle complex vectors
  • expanded the gmm_diag class with functions to generate random samples
• A new random-number implementation for Armadillo uses the RNG from R as a fallback (when C++11 is not selected so the C++11-based RNG is unavailable) which avoids using the older C++98-based std::rand
• The RcppArmadillo.package.skeleton() function was updated to only set an "Imports:" for Rcpp, but not RcppArmadillo which (as a template library) needs only LinkingTo:
• The RcppArmadillo.package.skeleton() function will now prefer pkgKitten::kitten() over package.skeleton() in order to create a working package which passes R CMD check.
• The pkgKitten package is now a Suggests:
• A manual page was added to provide documentation for the functions provided by the skeleton package.
• A small update was made to the package manual page.

Courtesy of CRANberries, there is also a diffstat report for the most recent release. As always, more detailed information is on the RcppArmadillo page. Questions, comments etc should go to the rcpp-devel mailing list off the R-Forge page.

This post by Dirk Eddelbuettel originated on his Thinking inside the box blog. Please report excessive re-aggregation in third-party for-profit settings.

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I routinely use a Kinesis Advantage Pro keyboard, which is a split, ergonomic keyboard with thumb clusters that uses brown cherryMX switches. Over the thirteen years that I've been using it, I've become a huge fan of this style of keyboard. However, I have two major annoyances with the Kinesis. First, while the firmware is good, remapping the keys is complicated and producing more complicated keyboard layouts with layers and keycodes that are not present in the original layout is not possible. Secondly, the interconnect between the main key wells and the controller board in the middle occasionally fails, and requires disassembly and occasional re-tinning of the circuit board interconnect connector. 1 About a year ago, I became aware of the ErgoDox keyboard, which is a keyboard design which mimics the kinesis to some degree, but with completely separated key halves (useful, because I'm substantially bigger than the average human), programmable firmware (so I can finally have the layers and missing keys) and with slightly more elegant interconnects (TRRS cables). Unfortunately, at the time I first heard about it (and other custom keyboards), making it required sourcing circuit boards, parts, and finding someone to cut a case for the keyboard. Then, a few months ago, I learned about MassDrop, a company who puts together groups of people to do buys of products at near-wholesale level prices, and their offer of all of the parts to build an ErgoDox. After waiting for a group buy of the keyboard to become available, I put in an order, and received the parts two months later. Over a few hours yesterday, I learned how to do surface mount soldering of the 78 diodes (one for each key), and finished assembling and flashing the firmware. This morning, I fixed up the few key bindings that I needed to be productive, and viola, my laptop at home now has a brand new ergonomic keyboard.

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While researching some security-related stuff for a post I am currently writing, I found some interesting bits here and there that I though I should share, as they were new to me, and probably for many others.

DNSSEC The first thing is DNSSEC. I knew about it, of course, but never bothered to dig much into it. While reading about many interesting applications of DNS for key distribution, and thinking of ways to use them, it is clear that DNSSEC is a precondition for any of that to work. In case you don't know about it, it is an extension for the DNS service to make it safer, for example, to avoid the bad guys having you think that google.com points to sniffer.nsa.gov. Apart from these ber-cool applications I was thinking about, avoiding DNS-based attacks becomes more and more relevant these days. And I think Debian and the rest of the Free Software world should work on making this available to all end-users as easily as possible. While adoption still looks pretty low, there are some good news. First, Google claims its public DNS supports DNSSEC. Of course, you need to trust Google servers, and the path between your machine and them. But if your resolver supports DNSSEC, you can use their servers and validate the answers. On the other side, I am not too sure about their implementation, as half of the time, it would return a valid answer to a query for an invalid record: dig +dnssec sigfail.verteiltesysteme.net @8.8.8.8). Also, they have not published DNSSEC records for google.com, which seems crazy. Some packages included in Debian already take advantage of DNSSEC, if available (more on that later), but more importantly, there are a couple of DNSSEC-enabled recursive servers, including bind, unbound, and the more commonly-used dnsmasq (there is a wiki page summarising Debian's status). Sadly, the default configuration for dnsmasq does not enable DNSSEC, and most people will not use it, even if it installed, because DHCP-provided servers are usually preferred. It seems to me that it would be wise to have a package that would install dnsmasq with DNSSEC enabled, and make it the only valid resolver for the system. If you want to check if your resolver is correctly validating DNSSEC, you can use this test web page. Another good news is that many top-level domains already support DNSSEC, and in my case, Gandi.net has support in place to set it up. So I am going to look into enabling it for my own domain.

SSHFP One useful and simple advantage of using DNSSEC, is that you can store information there, and then trust it to be correct. One new DNS RR (resource record) that is useful in this context is the SSHFP RR, which allows the sysadmin of a host to publish the host SSH key fingerprint in the DNS zone. The ssh client, when enabling the VerifyHostKeyDNS option, will use that information to trust unknown hosts. One downside to this, is that either if you set the option to ask, or if your resolver does not support DNSSEC, you get the same message, which does not warn you about the extra risk. To help you create your DNS records, you can just run this command:

$ ssh-keygen -r brie.tincho.org
brie.tincho.org IN SSHFP 1 1 6ac93c63379828b5b75847bc37d8ab2b48983343
brie.tincho.org IN SSHFP 2 1 cf0d11515367e3aa7eeb37056688f11b53c8ef23

DANE, S/MIME and GPG Recently, while at FOSDEM, I attended talks that mentioned DANE. This proposed IETF standard introduces a mechanism to use DNS as a secure key distribution system, which could completely override the CA oligopoly, a very attractive proposition for many people. In short, it is very similar to the SSHFP mechanism, but it is not restricted to SSH host keys: it can be used to distribute public key information for any TLS-enabled service. So, instead (or in addition to) of having a CA sign your certificate, and relying on the chain of trust by means of having a local copy of all root CA certificates, you use the chain of trust embedded in DNSSEC to make sure that the DNS RRs you publish are valid. Then, the client application can trust the fingerprint published for the relevant service to verify that it is talking to the right server. This is a very exciting development, and I hope it gets widespread adoption. It is already supported in Postfix, there seem to be some work going on in Mozilla, as well as in Prosody which is a great start. Another exciting development of this, is the generalisation of DANE for other entities, like email addresses. There are two draft RFCs being worked on right now to deploy S/MIME and OpenPGP key material using DNSSEC. This could also change completely the way we manage the Web of Trust.

Today I ll be working out how to properly configure DNSSEC on a BIND9 installation, I ll also make sure to give you all the needed instructions to properly verify if a specific domain is being correctly covered by DNSSEC itself. In addition to that a few more details will be provided about adding the relevant SSHFP s entries on your DNS zone files to be able to automatically verify the authenticity of your domain when connecting to it with SSH avoiding any possible MITM attack. First of all, let s create the Zone Signing Key (ZSK) which is the key that will be responsible to sign any other record on the zone file which is not a DNSKEY record itself:

dnssec-keygen -a RSASHA1 -b 1024 -n ZONE gnome.org

Note: the dnssec-keygen binary file should be part of bind97 (RHEL 5) or bind (RHEL6) package according to yum whatprovides: RHEL 5:

32:bind97-9.7.0-17.P2.el5_9.2.x86_64 : The Berkeley Internet Name Domain (BIND) DNS (Domain Name System) server
Repo : rhel-x86_64-server-5
Matched from:
Filename : /usr/sbin/dnssec-keygen

RHEL 6:

32:bind-9.8.2-0.17.rc1.el6.3.x86_64 : The Berkeley Internet Name Domain (BIND) DNS (Domain Name System) server
Repo : rhel-x86_64-server-6
Matched from:
Filename : /usr/sbin/dnssec-keygen

Then, create the Key Signing Key (KSK), which will be used to sign all the DNSKEY records:

dnssec-keygen -a RSASHA1 -b 2048 -n ZONE -f KSK gnome.org

Creating the above keys can take several minutes, when done copy the public keys to the zone file this way:

cat Kgnome.org*.key >> gnome.org

When done you can clean out the useless bits from the zone file and just leave the DNSKEY records (which are not commented out as you will notice) An additional and cleaner way of accomplishing the above is to use the INCLUDE rule on the zone file itself as follows:

$INCLUDE /srv/dnssec-keys/Kgnome.org+005+12345.key
$INCLUDE /srv/dnssec-keys/Kgnome.org+005+67890.key

Choosing which method to use is really up to you. Once that is done you can go ahead and sign the zone file. As of myself I m making use of the do-domain script taken from the Fedora Infrastructure Team s repositories. If you are going to use it yourself, make sure to adjust all the relevant variables to match your setup, especially keyspath, region_zones, template_zone, signed_zones and AREA. The do-domain script also checks your zone file through named-checkzone before signing it.

/me while editing the do-domains script with the preview of gnome-code-assistance!

If instead you don t want to use the script above, you can sign the zone file manually in the following way:

dnssec-signzone -K /path/to/your/dnssec/keys -e +3024000 -N INCREMENT gnome.org

By default, the above command will append .signed to the file name, you can modify that behaviour by appending the -f flag to the dnssec-signzone call. The -N INCREMENT will increment the serial number automatically making use of the RFC 1982 arithmetics while the -e flag will extend the zone signature end date from the default 30 days to 35. (this way we can safely run a monthly cron job that will sign the zone file automatically) You can make use of the following script to achieve the above:

#!/bin/sh
SIGNZONE="/usr/sbin/dnssec-signzone"
DNSSEC_KEYS="/srv/dnssec-keys"
NAMEDCHROOT="/var/named/chroot"
ZONEFILES="gnome.org"
cd $NAMEDCHROOT
for ZONE in $ZONEFILES; do
$SIGNZONE -K $DNSSEC_KEYS -e +3024000 -f $ZONE.signed -N INCREMENT $ZONE
done
/sbin/service named reload

Once the zone file has been signed just make sure to include it on named.conf and restart named:

zone "gnome.org"  
file "gnome.org.signed";
 ;

When you re done with that we should be moving ahead adding a DS record for our domain at our domain registrar. My example is taken from the known gandi.net registrar.

Gandi s DNSSEC interface

Select KSK (257) and (RSA/SHA-1) on the dropdown list and paste your public key on the box. You will find the public key you need on one of the Kgnome.org*.key files, you should look for the DNSKEY 257 entry as dig DNSKEY gnome.org shows:

;; ANSWER SECTION:
gnome.org. 888 IN DNSKEY 257 3 5 AwEAAbRD7AymDFuKc2iXta7HXZMleMkUMwjOZTsn4f75ZUp0of8TJdlU DtFtqifEBnFcGJU5r+ZVvkBKQ0qDTTjayL54Nz56XGGoIBj6XxbG8Es+ VbZCg0RsetDk5EsxLst0egrvOXga27jbsJ+7Me3D5Xp1bkBnQMrXEXQ9 C43QfO2KUWJVljo1Bii3fTfnHSLRUsbRn8Puz+orK71qxs3G9mgGR6rm n91brkpfmHKr3S9Rbxq8iDRWDPiCaWkI7qfASdFk4TLV0gSVlA3OxyW9 TCkPZStZ5r/WRW2jhUY/kjHERQd4qX5dHAuYrjJSV99P6FfCFXoJ3ty5 s3fl1RZaTo8=

Once that is done you should have a fully covered DNSSEC domain, you can verify that this way:

dig . DNSKEY   grep -Ev '^($ ;)' > root.keys
dig +sigchase +trusted-key=./root.keys gnome.org. A   cat -n

The result:

105 ;; WE HAVE MATERIAL, WE NOW DO VALIDATION
106 ;; VERIFYING DS RRset for org. with DNSKEY:59085: success
107 ;; OK We found DNSKEY (or more) to validate the RRset
108 ;; Ok, find a Trusted Key in the DNSKEY RRset: 19036
109 ;; VERIFYING DNSKEY RRset for . with DNSKEY:19036: success
110
111 ;; Ok this DNSKEY is a Trusted Key, DNSSEC validation is ok: SUCCESS

Bonus content: Adding SSHFP entries for your domain and verifying them You can retrieve the SSHFP entries for a specific host with the following command:

ssh-keygen -r $(hostname --fqdn) -f /etc/ssh/ssh_host_rsa_key.pub

When retrieved just add the SSHFP entry on the zone file for your domain and verify it:

ssh -oVerifyHostKeyDNS=yes -v subdomain.gnome.org

Or directly add the above parameter into your /etc/ssh/ssh_config file this way:

VerifyHostKeyDNS=yes

And run ssh -v subdomain.gnome.org , the result you should receive:

debug1: Server host key: RSA 00:39:fd:1a:a4:2c:6b:28:b8:2e:95:31:c2:90:72:03
debug1: found 1 secure fingerprints in DNS
debug1: matching host key fingerprint found in DNS
debug1: ssh_rsa_verify: signature correct

That s it! Enjoy!