11.3. Configuring IP Masquerade

If you've already read the firewall and accounting chapters, it probably comes as no surprise that the ipfwadm, ipchains, and iptables commands are used to configure the IP masquerade rules as well.

Masquerade rules are a special class of filtering rule. You can masquerade only datagrams that are received on one interface that will be routed to another interface. To configure a masquerade rule you construct a rule very similar to a firewall forwarding rule, but with special options that tell the kernel to masquerade the datagram. The ipfwadm command uses the -m option, ipchains uses -j MASQ, and iptables uses -j MASQUERADE to indicate that datagrams matching the rule specification should be masqueraded.

Let's look at an example. A computing science student at Groucho Marx University has a number of computers at home internetworked onto a small Ethernet-based local area network. She has chosen to use one of the reserved private Internet network addresses for her network. She shares her accomodation with other students, all of whom have an interest in using the Internet. Because student living conditions are very frugal, they cannot afford to use a permanent Internet connection, so instead they use a simple dial-up PPP Internet connection. They would all like to be able to share the connection to chat on IRC, surf the Web, and retrieve files by FTP directly to each of their computers—IP masquerade is the answer.

The student first configures a Linux machine to support the dial-up link and to act as a router for the LAN. The IP address she is assigned when she dials up isn't important. She configures the Linux router with IP masquerade and uses one of the private network addresses for her LAN: 192.168.1.0. She ensures that each of the hosts on the LAN has a default route pointing at the Linux router.

The following ipfwadm commands are all that are required to make masquerading work in her configuration:
# ipfwadm -F -p deny
# ipfwadm -F -a accept -m -S 192.168.1.0/24 -D 0/0
or with ipchains:
# ipchains -P forward -j deny
# ipchains -A forward -s 192.168.1.0/24 -d 0/0 -j MASQ
or with iptables:
# iptables -t nat -P POSTROUTING DROP
# iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE 
Now whenever any of the LAN hosts try to connect to a service on a remote host, their datagrams will be automatically masqueraded by the Linux masquerade router. The first rule in each example prevents the Linux machine from routing any other datagrams and also adds some security.

To list the masquerade rules you have created, use the -l argument to the ipfwadm command, as we described in earlier while discussing firewalls.

To list the rule we created earlier we use:
# ipfwadm -F -l -e
which should display something like:
# ipfwadm -F -l -e
IP firewall forward rules, default policy: accept
 pkts bytes type  prot opt  tosa tosx ifname  ifaddress  …
    0     0 acc/m all  ---- 0xFF 0x00 any     any        …
The “/m” in the output indicates this is a masquerade rule.

To list the masquerade rules with the ipchains command, use the -L argument. If we list the rule we created earlier with ipchains, the output will look like:
# ipchains -L
Chain input (policy ACCEPT):
Chain forward (policy ACCEPT):
target     prot opt     source                destination           ports
MASQ       all  ------  192.168.1.0/24        anywhere              n/a

Chain output (policy ACCEPT):

Any rules with a target of MASQ are masquerade rules.

Finally, to list the rules using iptables you need to use:
# iptables -t nat -L
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination

Chain POSTROUTING (policy DROP)
target     prot opt source               destination
MASQUERADE  all  --  anywhere             anywhere           MASQUERADE

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         
Again, masquerade rules appear with a target of MASQUERADE.

11.3.1. Setting Timing Parameters for IP Masquerade

When each new connection is established, the IP masquerade software creates an association in memory between each of the hosts involved in the connection. You can view these associations at any time by looking at the /proc/net/ip_masquerade file. These associations will timeout after a period of inactivity, though.

You can set the timeout values using the ipfwadm command. The general syntax for this is:
ipfwadm -M -s <tcp> <tcpfin> <udp>

and for the ipchains command it is:
ipchains -M -S <tcp> <tcpfin> <udp>

The iptables implementation uses much longer default timers and does not allow you to set them.

Each of these values represents a timer used by the IP masquerade software and are in units of seconds. The following table summarizes the timers and their meanings:

NameDescription
tcp

TCP session timeout. How long a TCP connection may remain idle before the association for it is removed.

tcpfin

TCP timeout after FIN. How long an association will remain after a TCP connection has been disconnected.

udp

UDP session timeout. How long a UDP connection may remain idle before the association for it is removed.