tech stuff.

Forcing CARP failover using pfSense

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Anything that has a master/slave cluster should have some sort of mechanism for failing from the master to the slave, right?  We’ll see…

pfSense provides HA via OpenBSD’s (very excellent) CARP.  Of course I could just increment the advskew on all of the interfaces to fail them over individually, but that doesn’t really work for me.  That involves spending some amount of time in a partially failed-over state.  Having quite a bit of experience with OpenBSD, I expected there to be a more elegant solution and I certainly wasn’t disappointed.  It looks like OpenBSD provides interface groups and a setting called “carpdemote”.  Incrementing carpdemote on the master’s “carp” group would cause all carp interfaces to fail over to the next-in-line.  nice.

I guess I was just expecting FreeBSD to have the same functionality.  FreeBSD 7.0 does have interface groups, but I can’t find carpdemote in the documentation.  Still, maybe pfSense 1.2.1 will be worth the upgrade afterall.

All-in-all, forcing the failover one at a time wasn’t so bad.  Upping the advskew on all interfaces to 200 from a shell loop like the following got the job done, but that race condition just doesn’t sit well.

for i in 0 1 2 3; do 
    ifconfig carp$i advskew 200
done

Then I did my work and rebooted the master.  When the master came up, all of the interfaces were reset to an advskew of 0, which is less-than-ideal-but-still-expected.  Strangely, this didn’t cause the interfaces to fail back to the master.  For about 5 minutes the master interfaces remained BACKUP even though the master interfaces had an advskew of 0 and the backup had an advskew of 100.

After about 5 minutes all of the interfaces failed back to the master.  I expect this is just a relic of how the advskew parameter actually works by tuning carp announcement intervals.

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Written by Lee Verberne

2008/08/31 at 06:11

Posted in pfsense, Unix-type stuff

Tagged with , ,

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