Rule equivalence of pf uRPF check

Matthew Seaman m.seaman at
Sat Sep 12 13:10:26 UTC 2009

Maxim Khitrov wrote:

> block in quick on $int_if from !$int_if:network
> block in quick on !$int_if from $int_if:network
> block in quick from $int_if
> The OpenBSD pf faq states that urpf-check is equivalent to the
> antispoof rules, but the antispoof section lists only the last two
> rules in my example as being equivalent. So the question is does urpf
> imply the first rule as well?

Not if uRPF is intended as a general mechanism.  What would happen if
you applied that on $ext_if (the external interface you connect to the rest of
the internet with)?  It's perfectly valid for packets from other than directly
attached networks to be passed by your firewall -- not doing that would, in fact,
completely negate your web browsing experience...



Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                  Flat 3
PGP:     Ramsgate
                                                  Kent, CT11 9PW

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