Vincent Bernat blogged on using Netfilter rulesets, pointing out that inserting the rules one-by-one using iptables calls may leave your firewall temporarily incomplete, eventually half-working, and that this approach can be slow.

He’s right with that, but there are tools that do this properly. ;-)

Some years ago, for a multi-homed firewall, I wrote a tool called Pyroman. Using rules specified either in Python or XML syntax, it generates a firewall ruleset for you.

But it also adresses the points Vincent raised:

  • It uses iptables-restore to load the firewall more efficiently than by calling iptables a hundred times
  • It will backup the previous firewall, and roll-back on errors (or lack of confirmation, if you are remote and use --safe)

It also has a nice feature for the use in staging: it can generate firewall rule sets offline, to allow you reviewing them before use, or transfer them to a different host. Not all functionality is supported though (e.g. the Firewall.hostname constant usable in python conditionals will still be the name of the host you generate the rules on - you may want to add a --hostname parameter to pyroman)

pyroman --print-verbose will generate a script readable by iptables-restore except for one problem: it contains both the rules for IPv4 and for IPv6, separated by #### IPv6 rules. It will also annotate the origin of the rule, for example:

# /etc/pyroman/
-A rfc4890f -p icmpv6 --icmpv6-type 255 -j DROP

indicates that this particular line was produced due to line 82 in file /etc/pyroman/ This makes debugging easier. In particular it allows pyroman to produce a meaningful error message if the rules are rejected by the kernel: it will tell you which line caused the rule that was rejected.

For the next version, I will probably add --output-ipv4 and --output-ipv6 options to make this more convenient to use. So far, pyroman is meant to be used on the firewall itself.

Note: if you have configured a firewall that you are happy with, you can always use iptables-save to dump the current firewall. But it will not preserve comments, obviously.