Intercepting traffic with IPFW on Linux

• by Brian Feeny

Outline

• NP: This configuration information is up-to-date as of Linux 2.0.33

ipfwadm Configuration (/etc/rc.d/rc.local)

• Replace SQUIDIP with the public IP squid may use to send traffic. Repeat the ipfwadm line for each such IP Squid uses.

# Accept all on loopback
ipfwadm -I -a accept -W lo

# Accept my own IP, to prevent loops (repeat for each interface/alias)
ipfwadm -I -a accept -P tcp -D SQUIDIP 80

# Send all traffic destined to port 80 to Squid on port 3127
ipfwadm -I -a accept -P tcp -D 0/0 80 -r 3127

it accepts packets on port 80, and redirects them to 3127 which is the port my squid process is sitting on.

Squid Configuration

First, compile and install Squid. It requires the following options:

./configure --enable-ipfw-transparent

You will need to configure squid to know the IP is being intercepted like so:

http_port 3127 transparent

• In Squid 3.1+ the transparent option has been split. Use 'intercept to catch IPFW packets.

http_port 3127 intercept

Testing

To test if it worked, use the nc utility. Stop squid and from the command line as root type in:

nc -l 3127

Then restart squid and try to navigate to a page.

You should now see an output like this:

> nc -l 3127
GET /mail/?ui=pb HTTP/1.1
User-Agent: Mozilla/5.0 (compatible; GNotify 1.0.25.0)
Host: mail.google.com
Connection: Keep-Alive
Cache-Control: no-cache
...

From there on out, just set your browsers up normally with no proxy server, and you should see the cache fill up and your browsing speed up.

CategoryConfigExample