Linux Networking
 
    Quick and dirty
  
   I will assume that you are using kernel 2.2.x, for which you will need
   ipchains to do some firewall stuff. Older kernels use a different
   utility called ipfwadm (I think).
 
   Okay, you've got your network card and got the modules loaded for it
   and have an eth0 device (right?). Next step is to give yourself an IP
   address. If this network is not accessible to the internet, you should
   pick a network ID that is reserved for private networks. I will use a
   reserved class C network: 192.168.1.x with [sub]net mask
   255.255.255.0. I will be assuming that YOUR IP node address is 1
   (192.168.1.1). This also assumes that your lo device is already setup
   and configured.
 
   To configure your IP address, use ifconfig:
ifconfig eth0 192.168.1.1 broadcast 192.168.1.255 netmask 255.255.255.0

   Next up is to configure your routing table: 
route add -net 192.168.1.0 netmask 255.255.255.0 dev eth0
route add -host 192.198.1.1 eth0
   
   Try pinging some computers on your network, it should work!
   
   Sharing an internet connection (do not forget the DENY line!!! It is
a SERIOUS security risk):
ipchains -P forward DENY
ipchains -A input -p TCP -s 192.168.1.0/24 -d 127.0.0.1/32 www -j ACCEPT
ipchains -A input -p TCP -s 192.168.1.0/24 -d 192.168.1.1/32 www -j ACCEPT
ipchains -A forward -s 192.168.1.0/24 -j MASQ
   
   Try pinging something on the internet. It should work!
   
   Here is a script I wrote to be run at start up. I'm not great with
   shell scripts, so don't make fun of me. I have two subnets
   (discluding the internet). This script configures the firewall to
   forward packets between these two networks so that my computer can act
   as a gateway server for the two subnets. It automatically forwards all
   data destined for port 80 (http/www) to go through a caching proxy
   server running on port 3128. Any data not destined for one of the
   subnets is masqueraded through my computer, picked up by the routing
   table and forwarded to the default route.
#! /bin/sh
# 19990501 by, Daniel Miller
# This configures IP masquerading for my network
   

# See how we were called from the command line
case "$1" in

 start)
  echo -n "socks5 proxy server: "
  /usr/local/bin/socks5 -t -s 2> /var/log/socks5
  echo done
   
   
  echo -n "Configuring IP masquerading: "
                                                                      (p4
of 6)
  ipchains -P forward DENY
  # For transparent proxy
   
  # ipchains -A input -p TCP -s 192.168.1.11/32 -d 0/0 www -j ACCEPT   
  ipchains -A input -p TCP -s 192.168.1.0/24 -d 127.0.0.1/32 www -j ACCEPT
  ipchains -A input -p TCP -s 192.168.1.0/24 -d 192.168.1.1/32 www -j
ACCEPT
  ipchains -A input -p TCP -s 192.168.1.0/24 -d 0/0 www -j REDIRECT 3128
  ipchains -A forward -s 192.168.1.0/24 -d 193.168.1.0/24 -j ACCEPT
  ipchains -A forward -s 192.168.1.0/24 -j MASQ
 
  ipchains -A input -p TCP -s 193.168.1.0/24 -d 127.0.0.1/32 www -j ACCEPT
  ipchains -A input -p TCP -s 193.168.1.0/24 -d 192.168.1.1/32 www -j
ACCEPT
  ipchains -A input -p TCP -s 193.168.1.0/24 -d 0/0 www -j REDIRECT 3128
  ipchains -A forward -s 193.168.1.0/24 -d 192.168.1.0/24 -j ACCEPT
  ipchains -A forward -s 193.168.1.0/24 -j MASQ
  
  echo -n "rules, "
  
  insmod ip_masq_ftp
  echo -n "ftp, "
  insmod ip_masq_irc
  echo -n "irc, "
  insmod ip_masq_raudio
  echo -n "raudio, "
  echo "done"
  
  
  ;;
  
 status)
  echo "blablabla - how's that for status???"
  ;;
  
 stop)
  echo -n "Turning off masquerading: "
  ipchains -F
  echo -n "rules, "
  echo "done"
  
  echo -n "Killing socks5: "
  killall socks5
  echo "done"
  
  ;;
  
 *)
  echo "Usage: masquerade {start|stop}"
  ;;
 esac
 
exit 0
    
   So, you want to get more advanced. Read the squid HOWTO and figure out
   how to set it up. To transparently redirect all http/www (port 80)
   requests through the a proxy server on port 3128, use this:
ipchains -A input -p TCP -s 192.168.1.0/24 -d 0/0 www -j REDIRECT 3128
  
   If you have multiple subnets then you can do more fun stuff with
   routing and things. ...but I'll save that info for another day.