It has been nearly six years with a netbook and five since I last wrote about
wifi roaming from the bus to stay on irc without a costly celluar link during the daily commute. Since then, some readers have asked me to share my refinements to the method in a followup post. So here it is.
The software
On the server:
- openssh-server
- screen
- irssi
On the client:
- screen
- wpasupplicant
- isc-dhcp-client
- openssh-client
- openbox
- sudo & gksudo (optional)
- urxvt
- wavemon (optional)
- three shell scripts (provided below)
Putting it together: on the client
Make sure if you have a wireless manager installed (such as NetworkManager) it is configured to skip your wireless interface, disabled entirely, or if possible, removed. Set up
/etc/wpa_supplicant/wpa_supplicant.conf and
/etc/network/interfaces for roaming, as per the instructions in
/usr/share/doc/wpasupplicant/README.modes.gz. Don t forget to add yourself to the
netdev group if you are not in it already.
In
/etc/wpa_supplicant/wpa_supplicant.conf, list common names of open networks. Normally the catch-all network that associates with any essid, i.e. the first stanza below, works well. However, occasionally the strongest signal is neither one of the common networks nor an easily accessible network (e.g. web portals), so having a list of common open networks helps to quickly select from among those instead. The more you travel, the more of these will discover and add. Just use
reconfigure from
wpa_cli to reload your edited list each time you add a new one.
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
network=
key_mgmt=NONE
network=
ssid="default"
key_mgmt=NONE
network=
ssid="linksys"
key_mgmt=NONE
...
Since you ll be using
ssh repeatedly to connect and it has to be fast, make sure your server is set up to accept your key and use
ssh-add so that you only have to enter your ssh key password once.
You can tweak
isc-dhcp-client to make connections faster. In
/etc/dhcp/dhclient.conf, use:
backoff-cutoff 1;
initial-interval 1;
Here are a few scripts I wrote to facilitate quick roaming from one open AP to another and reconnect to
irssi running in
screen, to break a connection and try the next one, and to recover from occasional lockups (more about that later).
~/bin/screen_reconnect
This is a script to reconnect continuously via ssh to a screen session:
#!/bin/sh
reset
while ! ssh -t 10.9.8.7 'screen -UDr' 2>/dev/null ; do echo -n "." ; sleep .1 ; done
Just substitute the IP of your own server here. Using an IP instead of domain name makes the connection faster because a DNS lookup is not required.
~/bin/wifi_reassociate
This script closes any open
ssh sessions and informs
wpa_supplicant to attempt to connect again.
#/bin/sh
/sbin/wpa_cli rea
killall ssh >/dev/null 2>&1
~/bin/wifi_killall
This optional, somewhat ugly script addresses an issue I hope you never have. On my ASUS Eee PC 1001PX, occasionally scanning stops. When this happens, and I have never figured out why, apparently ACPI events are blocked. At this point wifi becomes unusable and ACPI sleep is inhibited. By trial and error I found that if you bring down the interface, kill all network-related processes, and bring it up again, ACPI events are unblocked and wifi is usable once more (and any pending request to sleep will finally happen). The script requires
sudo, and to use the
openbox key binding,
gksudo.
#/bin/sh
sudo ifdown wlan0
# in case any of these are hung
sudo killall dhclient3
sudo killall wpa_cli
sudo killall wpa_action
sudo killall wpa_supplicant
# in case any of these are *really* hung
sleep 1
sudo killall -9 dhclient3
sudo killall -9 wpa_cli
sudo killall -9 wpa_action
sudo killall -9 wpa_supplicant
sudo dhclient -r
sudo ifup wlan0
Openbox
Since certain actions need to be performed repeatedly and quickly, it is useful to have hotkeys bound in your window manager to the scripts. In
~/.config/openbox/rc.xml, key bindings for
<alt>-r to reassociate and
<alt>-d to disconnect a hung connection would look like:
<keyboard>
<!-- My keybindings -->
<keybind key="A-R">
<action name="Execute">
<execute>/home/synrg/bin/wifi_reassociate</execute>
</action>
</keybind>
<keybind key="A-D">
<action name="Execute">
<execute>gksudo /home/synrg/bin/wifi_killall</execute>
</action>
</keybind>
</keyboard>
Putting it together: on the server
There is very little to do here. Just start
screen, and start
irssi in screen. Running
screen on the client as well as the server means you should either bind the
screen meta keys to a different key sequence on each system, or else learn to press meta twice to pass through meta to the server
screen as needed. I use the latter approach. Alternatively, you could use a tabbed terminal on the client, or separate terminals per client process instead of
screen. This is a matter of personal taste.
Ready to roam
Here is a typical setup for roaming on the bus:
In a terminal (I use
urxvt), first
ssh-add, then start
screen with these three processes running in separate virtual terminals:
- /sbin/wpa_cli
- screen_reconnect
- wavemon (optional)
March of the dots
Most of the commute, just enjoy watching the dots march by, waiting for a new connection. If you estimate a connection is unusable, press
<alt>-r to reassociate immediately, giving the next network a chance. If the connection is already firmly established, this might not work on the first try. If the dots don t resume immediately, wait a bit and press it again. This might take a few tries.
Changing selected networks on the fly
Use
wpa_cli when you need to do some fine-tuning of network selections on the fly. While normally you can just watch the march of the dots until a connection is acquired, sometimes you can improve your chances of connecting to a good network by manually controlling the selected candidate networks here.
For example, by watching the speed of the bus relative to known good APs, you can predict which networks are more likely to succeed. Rather than connect to any arbitrary network, you might select a specific one by id, and then later when it goes out of range, revert to the original configuration, e.g.
> select_network 5
...
> reconfigure
You can use tab-completion in
wpa_cli to type these commands quickly or else just abbreviate the commands.
Another common scenario is when you pass through a business area with many captive portal hotspots. These rarely make good choices because they either require a password not known to you or else you can t click through I agree in time before the bus moves on. In this case, you might just disable the catch-all stanza and let the common open network stanzas you listed ( default , linksys , etc.) do the work:
> disable_network 1
Become a type ahead wizard
While running, a continuous stream of periods fills the screen, which provides you with a highly visible cue that no available APs are in range. When the movement stops, you know a connection is being attempted.
While waiting to connect, you can type ahead any comments you want to make in the current irssi window (taking care to remember which one you are in!) While having periods interspersed in what you type may be disorienting at first, you get used to it.
There is a point when a connection is first established and ssh is accepting input, but anything you type can no longer be seen while you re typing. Depending on whether the connection was completely successful or not, what you type now may or may not finally be sent. For best results, only type ahead before the dots stop moving.
Eventually you can become skilled enough at this to type ahead a comment in one channel, switch channels with
/win # and continue typing ahead in the new channel, all buffered until the next few seconds (or even fraction of a second) of connection time.
Fine-tune antenna direction with wavemon
When the bus has come to a standstill, you may find
wavemon useful to pull in a weak signal. Because
wavemon has continuously updated signal level and link quality bars, you can use it to fine-tune the antenna position. Just turn your laptop until the bars are at their maximum.
Captive portals
I have not figured out how to do any automation for this, so it really is a crapshoot, as it is likely the bus has moved on by the time you ve managed to manually navigate the login through a captive portal. But in rush hour, you may have the luxury of time to connect to these as you pass them. I have recently learned about the
CoovaFX Firefox plugin which automates logins to captive portals. I m going to give it a try to see if it helps. Update: I can t recommend this plugin, as it is not compatible with Iceweasel >= 23.0. Also, the standard it is based on,
WISPr, appears to have an uncertain future. That, coupled with the fact that the plugin appears to not be open source means I m still looking for alternatives.
Summary
If all of this sounds a bit nuts to you, well, it probably is. But after half a decade enjoying free access to irc from the bus, it all seems perfectly natural to me! If you try this method and like it, please let me know in the comments. Likewise, if you have any improvements to the process or scripts, please share them!
While I suspect that professional speakers have separate presentation
laptops that they use only to give talks, I don't do this often enough to
justify the hassle and cost of a separate machine. However, I do use a
separate user account to present from.
It allows me to focus on my presentation and not stress out about running
into configuration problems or exposing private information. But mostly, I
think it's about removing anything that could be distracting for the
audience.
The great thing of having a separate user for this is that you can do whatever
you want in your normal account and still know that the other account is ready
to go and configured for presenting on a big screen.
I love to say that Python is a nice subset of Lisp, and I discover that it's
getting even more true as time passes. Recently, I've stumbled upon the
It seems I'm catching up with the pace of this course I'm following and that is compelling me to go back to posting on my blog, 
EFI includes support for storing persistent 'variables': it
implements a key-value store where keys are (UUID, string) and
values are arbitrary blobs. (There are also some flags that
restrict when the variables can be accessed.) The physical storage
medium is normally flash. The Linux efivars module
provides access to variable storage, both through a
Two months to the day after the previous
Hi always wondered what the most popular options are, you usually find in 
One feature of 
Anything is possible, you could even end up 
