I’ve just upgraded my home server from Ubuntu 8.10 to 9.04 and experienced that my ufw firewall (iptables) would not route traffic from my local network to the Internet. My IP masquerading was not working anymore and since I had not documented the process when I set it up I had to search the Ubuntu pages to find the solution and came up with this.

The purpose of IP Masquerading is to allow machines with private, non-routable IP addresses on your network to access the Internet through the machine doing the masquerading. Traffic from your private network destined for the Internet must be manipulated for replies to be routable back to the machine that made the request. To do this, the kernel must modify the source IP address of each packet so that replies will be routed back to it, rather than to the private IP address that made the request, which is impossible over the Internet. Linux uses Connection Tracking (conntrack) to keep track of which connections belong to which machines and reroute each return packet accordingly. Traffic leaving your private network is thus “masqueraded” as having originated from your Ubuntu gateway machine. This process is referred to in Microsoft documentation as Internet Connection Sharing.

ufw Masquerading

IP Masquerading can be achieved using custom ufw rules. This is possible because the current back-end for ufw is iptables-restore with the rules files located in

/etc/ufw/*.rules

These files are a great place to add legacy iptables rules used without ufw, and rules that are more network gateway or bridge related.

The rules are split into two different files, rules that should be executed before ufw command line rules, and rules that are executed after ufw command line rules.

• First, packet forwarding needs to be enabled in ufw. Two configuration files will need to be adjusted, in /etc/default/ufw change the

  DEFAULT_FORWARD_POLICY

  to “ACCEPT”:

  DEFAULT_FORWARD_POLICY="ACCEPT"

  Then edit /etc/ufw/sysctl.conf and uncomment:

  net.ipv4.ip_forward=1

  Similarly, for IPv6 forwarding uncomment:

  net.ipv6.conf.default.forwarding=1

• Now we will add rules to the /etc/ufw/before.rules file. The default rules only configure the filter table, and to enable masquerading the nat table will need to be configured. Add the following to the top of the file just after the header comments:

  # nat Table rules
  *nat
  :POSTROUTING ACCEPT [0:0]
  
  # Forward traffic from eth1 through eth0.
  -A POSTROUTING -s 192.168.0.0/24 -o eth0 -j MASQUERADE
  
  # don't delete the 'COMMIT' line or these nat table rules won't be processed
  COMMIT

  The comments are not strictly necessary, but it is considered good practice to document your configuration. Also, when modifying any of the rules files in /etc/ufw, make sure these lines are the last line for each table modified:

  # don't delete the 'COMMIT' line or these rules won't be processed
  COMMIT

  For each Table a corresponding COMMIT statement is required. In these examples only the nat and filter tables are shown, but you can also add rules for the raw and mangle tables.

  	In the above example replace eth0, eth1, and 192.168.0.0/24 with the appropriate interfaces and IP range for your network.

• Finally, disable and re-enable ufw to apply the changes:

  sudo ufw disable && sudo ufw enable

IP Masquerading should now be enabled. You can also add any additional FORWARD rules to the /etc/ufw/before.rules. It is recommended that these additional rules be added to the ufw-before-forward chain.

Source: https://help.ubuntu.com/9.04/serverguide/C/firewall.html