Ubuntu, VLANs and Bridges

Bridge and VLAN support has improved dramatically under Ubuntu and probably Debian as well since I last looked into it. once upon a time to create a bridge linked to a VLAN interface you would have to do horrible things like.

auto eth0
ifconfig eth0 inet manual
    pre-up /sbin/vconfig set_name_type VLAN_PLUS_VID_NO_PAD || true

auto vlan7
iface vlan7 inet manual
    pre-up /sbin/vconfig add eth0 7 || true
    post-down /sbin/vconfig rem vlan7 || true

auto br0
    pre-up brctl addbr br0
    pre-up brctl addif br0 vlan7
    post-down brctl delbr br0

Now the bridge-utils and vlan packages provide hooks into the ifup and ifdown commands so you can simply do

auto br-vlan4
iface br-vlan4 inet static
    vlan-raw-device eth1
    bridge_ports vlan4
    bridge_maxwait 0
    bridge_fd 0
    bridge_stp off

Which will automagically

  • Bring up eth1
  • Create vlan4 bound to the eth1 interface
  • Bring up vlan4
  • Create the br0 with vlan4 attached
  • Give eth1 the same HW address as br0
  • Bring up br0 with the IP address


Mongrel, rails and the theory of relativity

Summary (E = mc²)

When using mongrel for rails and you want to deploy an app under /other_url then use

    ActionController::AbstractRequest.relative_url_root = "/other_url"

in config/environments/production.rb instead of

    ENV['RAILS_RELATIVE_URL_ROOT'] = "/other_url"

Proof (From first principals)

At Vquence we have a pretty standard rails setup

  • Apache with mod_proxy
  • pen
  • mongrel

Silvia recently wrote an application to allow us to edit the news articles posted to our corporate website. I wanted to do something I thought would be pretty simple, have the application appear at /news on our admin web server.

Step one was the obvious change to mod_proxy

    ProxyPass /news http://localhost:8000
    ProxyPassReverse /news http://localhost:8000

Of course the problem is that the rails app still thinks it is living on / so it returns URLs like /stylesheets/moo.css instead of /news/stylesheets/moo.css.

A bit of googling found a few email threads with a common solution. In your environment.rb set

    ENV['RAILS_RELATIVE_URL_ROOT'] = "/other_url"

This is where things fell apart fairly quickly. I could not get this to work no matter what I tried. After a few hours of following a HTTP request through the whole Mongrel and rails stack I discovered the following.

Setting RAILS_RELATIVE_ROOT will work fine if you are running rails using CGI. For the simple reason, which should have been more obvious to me sooner, that CGIs use environment variables to access their parameters. This can be seen in the
ruby CGI class


class CGI

def env_table

However mongrel overloads env_table and does the following instead


class CGIWrapper < ::CGI

    # Used to wrap the normal env_table variable used inside CGI.
    def env_table

This makes sense since the rails code is now running inside the web server so environment variables aren’t necessary. Upon investigation I found that the URL morphing magic is performed with rails as follows.


  class AbstractRequest
    cattr_accessor :relative_url_root
    # Returns the path minus the web server relative installation directory.
    # This can be set with the environment variable RAILS_RELATIVE_URL_ROOT.
    # It can be automatically extracted for Apache setups. If the server is not
    # Apache, this method returns an empty string.
    def relative_url_root
      @@relative_url_root ||= case
        when @env["RAILS_RELATIVE_URL_ROOT"]
        when server_software == 'apache'
          @env["SCRIPT_NAME"].to_s.sub(//dispatch.(fcgi|rb|cgi)$/, '')

What this all means is that you can solve the whole problem by placing the following in your config/environments/production.rb

    ActionController::AbstractRequest.relative_url_root = "/other_url"

Now if only Einstein had put his theories to good use and invented a time machine then maybe I could get the last 4 hours of my life back đŸ™‚

Update: Make sure /other_url isn’t the same name as one of your controllers or bad things happen.

linux.conf.au brings about another change

Being Technical Guru for linux.conf.au 2007 was one of the most amazing experiences I’ve had in recent years. It was a lot of hard work but it was totally worth it. Having a room burst into applause at the penguin dinner when you say your the network guy is pretty unbelievable.

I went up to the Hunter for a week to recover from the conference and as usual after linux.conf.au I did a lot of thinking as to whether it was time to try something new. This time change won out at the end of the day and after 6 years at Bulletproof I decided it was time to move on.

At the beginning of March I started as Director of Engineering at Vquence. Since we are a video company it was decided that we each needed to have our own video on the web.

The past three weeks have been so hectic that Bulletproof already seems a lifetime ago. I’ve been involved in everything from setting up the new office and the corporate infrastructure to product development.

Joining a startup right at the beginning is always an amazing experience. With just a few people on the ground you always get pulled in a few million directions and there is always a new challenge just another five minutes away. I definitely recommend anyone else to jump at the opportunity if it ever presents itself.