Книга: Linux Network Administrator Guide, Second Edition

The ipchains support scripts

The ipchains support scripts

The ipchains software package is supplied with three support scripts. The first of these we've discussed briefly already, while the remaining two provide an easy and convenient means of saving and restoring your firewall configuration.

The ipfwadm-wrapper script emulates the command-line syntax of the ipfwadm command, but drives the ipchains command to build the firewall rules. This is a convenient way to migrate your existing firewall configuration to the kernel or an alternative to learning the ipchains syntax. The ipfwadm-wrapper script behaves differently from the ipfwadm command in two ways: firstly, because the ipchains command doesn't support specification of an interface by address, the ipfwadm-wrapper script accepts an argument of -V but attempts to convert it into the ipchains equivalent of a -W by searching for the interface name configured with the supplied address. The ipfwadm-wrapper script will always provide a warning when you use the -V option to remind you of this. Secondly, fragment accounting rules are not translated correctly.

The ipchains-save and ipchains-restore scripts make building and modifying a firewall configuration much simpler. The ipchains-save command reads the current firewall configuration and writes a simplified form to the standard output. The ipchains-restore command reads data in the output format of the ipchains-save command and configures the IP firewall with these rules. The advantage of using these scripts over directly modifying your firewall configuration script and testing the configuration is the ability to dynamically build your configuration once and then save it. You can then restore that configuration, modify it, and resave it as you please.

To use the scripts, you'd enter something like:

ipchains-save ›/var/state/ipchains/firewall.state

to save your current firewall configuration. You'd restore it, perhaps at boot time, with:

ipchains-restore ‹/var/state/ipchains/firewall.state

The ipchains-restore script checks if any user-defined chain listed in its input already exists. If you've supplied the -f argument, it will automatically flush the rules from the user-defined chain before configuring those in the input. The default behavior asks you whether to skip this chain or to flush it.

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