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FreeBSD 7 jail source address selection

I was browsing through the changesets of the upcoming 7.3 release and stumbled upon a nice addition which would have come handy a few months ago.

According to the default behaviour, when jails are trying to reach a host through one of the external interfaces, the source address used for communication is taken from the interface which has route to the destination.
In other words, if you have a single external IP address, and a couple of jails on the local interface, the source address for outbound connections will be the address on the external interface.
This is especially annoying if you’re trying to deploy per jail firewall rules for outgoing connections: you can’t use the jail ip address as source address for matching packets.
Until now there we had three different approaches to overcome this problem:

Multiple routing tables
The idea is that every process in a certain jail should use a routing table which has no default route, hereby restricting every outgoing connection. Note that incoming connections are assigned to the default (0) routing table, so you would still be able to access your services from the outside.

Add the following kernel configuration option and rebuild the kernel. The 2 is the number of FIB (Forward Information Base, synonym for a routing table here). The maximum value is 16.


This number can be modified on boot time. To do so, add the following to /boot/loader.conf and reboot the system:


Set a loader tunable net.my_fibnum if needed. This means the default number of routing tables. If not specified, 0 will be used. Set a loader tunable net.add_addr_allfibs if needed. This enables to add routes to all FIBs for new interfaces by default. When this is set to 0, it will only allocate routes on interface changes for the FIB of the caller when adding a new set of addresses to an interface. Note that this tunable is set to 1 by default.

Then add jail_name_fib=n to your rc.conf or ezjail configuration to assign the jail process to a certain routing table.

Jail match support
Find a firewall with jail match support. As we didn’t want to use anything else but pf, we looked into the source and found that an actual support for matching jails could be added easily. We even got to the stage where we had beta patches, but due to the fact that we needed a stable solution in our production environment we dropped the idea. Not to mention that it would have never been merged into pf, as OpenBSD has no jails and FreeBSD has no pf fork. (They treat pf as a port, so feature additions have to go through OpenBSD first. Uh good luck with that.)

Uid match support
Luckily pf has uid/gid match support, and by having all our jails use globally unique uids it became obvious that this method was the best so far.
pass out on $ext_if inet proto tcp from ($ext_if) to any port 25 user > 1000 keep state
The above pf rule blocks outgoing smtp access for each process that has a UID larger than 1000.

Disabling source address selection for jails
And finally the new addition of FreeBSD 7.3 allows you to disable the default source address selection method, always assigning the primary address of the jail to each outgoing packet. This can be done via the following sysctl flags:
security.jail.ip4_saddrsel=0 for IPv4
security.jail.ip6_saddrsel=0 for IPv6
Note that this is global and not a per jail setting.
Changeset: r202924

Posted 2010/03/05 11:35 by jos


  1. Jul 8, 08:16 AM

    kae Says:

    Freebsd over solaris..anyday!

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