The iptables-save command is, as we have already explained, a tool to save the current rule-set into a file that iptables-restore can use. This command is quite simple really, and takes only two arguments. Take a look at the following example to understand the syntax of the command.
iptables-save [-c] [-t table]
The -c argument tells iptables-save to keep the values specified in the byte and packet counters. This could for example be useful if we would like to reboot our main firewall, but not lose byte and packet counters which we may use for statistical purposes. Issuing a iptables-save command with the -c argument would then make it possible for us to reboot without breaking our statistical and accounting routines. The default value is, of course, to not keep the counters intact when issuing this command.
The -t argument tells the iptables-save command which tables to save. Without this argument the command will automatically save all tables available into the file. The following is an example on what output you can expect from the iptables-save command if you do not have any rule-set loaded.
# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:17 2002 *filter :INPUT ACCEPT [404:19766] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [530:43376] COMMIT # Completed on Wed Apr 24 10:19:17 2002 # Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:17 2002 *mangle :PREROUTING ACCEPT [451:22060] :INPUT ACCEPT [451:22060] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [594:47151] :POSTROUTING ACCEPT [594:47151] COMMIT # Completed on Wed Apr 24 10:19:17 2002 # Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:17 2002 *nat :PREROUTING ACCEPT [0:0] :POSTROUTING ACCEPT [3:450] :OUTPUT ACCEPT [3:450] COMMIT # Completed on Wed Apr 24 10:19:17 2002
This contains a few comments starting with a # sign. Each table is marked like *<table-name>, for example *mangle. Then within each table we have the chain specifications and rules. A chain specification looks like :<chain-name> <chain-policy> [<packet-counter>:<byte-counter>]. The chain-name may be for example PREROUTING, the policy is described previously and can, for example, be ACCEPT. Finally the packet-counter and byte-counters are the same counters as in the output from iptables -L -v. Finally, each table declaration ends in a COMMIT keyword. The COMMIT keyword tells us that at this point we should commit all rules currently in the pipeline to kernel.
The above example is pretty basic, and hence I believe it is nothing more than proper to show a brief example which contains a very small Iptables-save ruleset. If we would run iptables-save on this, it would look something like this in the output:
# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:55 2002 *filter :INPUT DROP [1:229] :FORWARD DROP [0:0] :OUTPUT DROP [0:0] -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT -A FORWARD -i eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT -A FORWARD -i eth1 -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT -A OUTPUT -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT COMMIT # Completed on Wed Apr 24 10:19:55 2002 # Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:55 2002 *mangle :PREROUTING ACCEPT [658:32445] :INPUT ACCEPT [658:32445] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [891:68234] :POSTROUTING ACCEPT [891:68234] COMMIT # Completed on Wed Apr 24 10:19:55 2002 # Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:55 2002 *nat :PREROUTING ACCEPT [1:229] :POSTROUTING ACCEPT [3:450] :OUTPUT ACCEPT [3:450] -A POSTROUTING -o eth0 -j SNAT --to-source 195.233.192.1 COMMIT # Completed on Wed Apr 24 10:19:55 2002
As you can see, each command has now been prefixed with the byte and packet counters since we used the -c argument. Except for this, the command-line is quite intact from the script. The only problem now, is how to save the output to a file. Quite simple, and you should already know how to do this if you have used linux at all before. It is only a matter of piping the command output on to the file that you would like to save it as. This could look like the following:
iptables-save -c > /etc/iptables-save
The above command will in other words save the whole rule-set to a file called /etc/iptables-save with byte and packet counters still intact.