My laptop has had an interesting couple of days. The main filesystem went
read-only a couple of nights ago after a couple of random journal errors.
After being fsck'd and cleaned up it did it again, so I reinstalled it and
restored it from backup yesterday. Then last night it overheated, leading me
to open the case and clean the wall of dust out of the fan. Its back together
now, but a couple of lost of tiny parts means I have no indicator lights and
no trackpoint. Fortunately the trackpad still works, but its taking a little
getting used to. On the other hand, its not burning my lap or my hands
anymore, so its probably an overall victory though its not quite feeling that
way yet.
One of the things I did lose in the rebuild, due to it not living in one of my
backup locations (which is /etc/ and /home/rob) is my cute little mobile
roaming script. I rewrote it on the bus on the way home yesterday and thought
that perhaps its interesting enough to post here.
The basic idea is that every day I switch between at least two networks. My
home network has a PC in the hall cupboard which among other things runs a web
proxy and a mail server. Its also the firewall, so web and mail traffic can't
go out directly. Work on the other hand, implements transparent proxying (with
some network authentication) and has a SMTP server, but naturally it has a
different address. I also occassionally use other networks (friend's places,
coffee shops, etc) which usually have no facilities at all, requiring me to
fend for myself.
My laptop runs a SMTP server (Postfix) of course, because that's just what you
do on Unix. I also run a Squid proxy which I point all my local HTTP clients
at. This way, when I move networks, I only have to reconfigure the local proxy
rather than tweak every web client I have.
I spent a long time looking for a decent roaming reconfiguration package, but
I never managed to find one. Some would try to do network detection and too
often get it wrong. Some would have overly complicated and/or feature
deficient configuration languages. I vaguely recall that I really liked one of
them but it was tightly integrated with NetworkManager, which I don't use
because it could never seem to keep the network alive for more than a few
minutes (and it appears to be pretty much tied to the GUI, which is painful
when I need network on the console).
So, in the finest open source tradition, I rolled my own. The script itself is
trivial; its just a tiny template expander. I'll list the script in a moment,
but first I'll talk about its operation.
The script, which I call location, takes a location name on the command line
(like home or work), runs over a (hardcoded) list of config files, reads
them in, modifies them, and spits them out to the same file. It makes
modifications according to templates that may exist in the file. If the file
has no template, then location ends up emitting the unchanged file.
In any file you want it to modify, you add an appropriate template. This is
the template I have in my /etc/postfix/main.cf:
### START-LOCATION-TEMPLATE
##@ home relayhost = lookout.home
##@ work relayhost = smtp.monash.edu.au
##! /etc/init.d/postfix restart
##! sleep 1
##! /usr/bin/mailq -q
### END-LOCATION-TEMPLATE
When it finds itself inside a template, location stops its normal operation
of outputting the lines of the file as-is and instead starts parsing.
Interesting lines begin with ##, anything else is ignored. Its the third
character that determines how the line is interpreted. So far I have the
following functions:
#: do nothing, just output the line@: emit if at location. If the location specified on the command line matches the first argument to @, then the rest of the line is added to the file as-is.!: run command. Calls a shell to run the specified command after the file has been generated.>: interpolate line. Include the rest of the line in the file, but expand any %variable%-type markers. So far only %location% is defined, and is replaced with the location specified on the command line.
(I'll provide an example of that last one in a moment).
So in the case of main.cf, lets say we ran location with home as the
location. This would result in the template section being written to the
output file as:
### START-LOCATION-TEMPLATE
##@ home relayhost = lookout.home
relayhost = lookout.home
##@ work relayhost = smtp.monash.edu.au
##! /etc/init.d/postfix restart
##! sleep 1
##! /usr/bin/mailq -q
### END-LOCATION-TEMPLATE
The listed commands are then run, which cause Postfix to be restarted and the
mail queue to be flushed:
/etc/init.d/postfix restart
sleep 1
/usr/bin/mailq -q
Naturally Postfix interprets the template parts of the file as comments, so
nothing to worry about. The next time location is run, the "bare"
relayhost line is ignored, so it doesn't get in the way.
The config for Squid is similar. Because Squid's config file is huge, I don't
quite trust my script to handle the whole thing sanely, so at the bottom of
squid.conf I've added:
include /etc/squid/location.conf
And in location.conf I have:
### START-LOCATION-TEMPLATE
##@ home cache_peer lookout.home parent 8080 0 default
##@ home never_direct allow all
##! /etc/init.d/squid restart
### END-LOCATION-TEMPLATE
By default Squid will try and hit the internet directly, which is fine for
work and unknown locations. For home, i need to force it to always go to an
upstream proxy, which is what those the cache_peer and never_direct
directives will achieve.
The proxy at work used to be an authenticating proxy, so I had to specify both
a peer and a username/password combination. This made the required amount of
variable config a little unwieldy to be include in a template, which is where
the > function came from. location.conf used to have this:
##> include /etc/squid/upstream.%location%.conf
Which would arrange for upstream.home.conf, upstream.work.conf, etc to be
included depending on the location. There's every chance this will come in
useful again one day, so I've left the code in there for now.
Here's the script in its entirety:
#!/usr/bin/env perl
use 5.010;
use warnings;
use strict;
my @files = qw(
/etc/squid/location.conf
/etc/postfix/main.cf
);
use autodie qw(:default exec);
use FindBin;
if ($< != 0) {
exec "/usr/bin/sudo", "$FindBin::Bin/$FindBin::Script", @ARGV;
}
say "usage: location <where>" and exit 1 if @ARGV != 1;
my ($location) = @ARGV;
for my $file (@files) {
say "building: $file";
my @out;
my @cmd;
open my $in, "<", $file;
my $in_template = 0;
while (my $line = <$in>) {
chomp $line;
if ($line =~ m/^### START-LOCATION-TEMPLATE/) {
$in_template = 1;
push @out, $line;
next;
}
if ($line =~ m/^### END-LOCATION-TEMPLATE/) {
$in_template = 0;
push @out, $line;
next;
}
if (!$in_template) {
push @out, $line;
next;
}
my ($tag) = $line =~ m/^##([#@!>])/;
if (!$tag) {
next;
}
given ($tag) {
when ('#') {
push @out, $line;
next;
}
when ('@') {
push @out, $line;
my ($want, $rest) = $line =~ m/^##@ (\w+) (.*)/;
if ($want eq $location) {
push @out, $rest;
}
next;
}
when ('!') {
push @out, $line;
my ($cmd) = $line =~ m/^##! (.*)/;
push @cmd, $cmd;
next;
}
when ('>') {
push @out, $line;
my ($rest) = $line =~ m/^##> (.*)/;
$rest =~ s/%location%/$location/g;
push @out, $rest;
}
}
}
die "$file: unclosed location template" if $in_template;
close $in;
open my $out, ">", $file;
say $out $_ for @out;
close $out;
for my $cmd (@cmd) {
say "running: $cmd";
system $cmd;
}
}
Because its so trivial and I only run it a couple of times a day, I just run
it when I get to work (location work) or when I get home (location home).
If I felt inclined I could probably hook it up to my network stuff but I think
that would be more trouble than its worth.
On occassion I have to use Windows on the same machine. I have no idea how to
achieve something similar there, so I just reconfigure my browser. Fortunately
I don't go there often, and almost never from work. This is why I like open
source. I can make my system work in exactly the way I want and usually with a
minimum of fuss.
What can I say - sciatic pain is, well, a pain!
):
The Twitterverse was quick to respond. Thank you all for responding! A little summary:
#
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