packets to bridged interfaces bypass input filter

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packets to bridged interfaces bypass input filter

Sven M. Hallberg
hi all,

while playing around with pf, i noticed that some connections that i
thought should be blocked, were in fact not. here is my fairly standard
bridge setup between a wlan interface and a wired ethernet.

       # cat /etc/hostname.em0
       group lan

       # cat /etc/hostname.athn0
       mediaopt hostap
       nwid xxx wpakey abcd1234
       group lan

       # cat /etc/hostname.vether0
       group lan

       # cat /etc/hostname.bridge0
       add em0 add athn0 add vether0
       blocknonip athn0

i then set up pf rules that should let the (w)lan interfaces talk to
the world.

        # cat /etc/pf.conf
        set skip on lo
        block return
        pass out        # ingress filter
        pass in on lan from lan:network to any

now this all works as intended. but, when i remove athn0 from the lan
group, i would expect the above ruleset to block any incoming
connections from the wlan.

        # ifconfig athn0 -group lan

and indeed, i could not ping a wired host from the wlan. however, i
could still access the router ( itself from the wireless.
as far as i can tell, this is due to the following code in if_bridge.c,
bridge_process() -- comments by me:

        if (bridge_ourether(bif0->ifp, eh->ether_dhost)) {
                /* addressed to the iface it came in on */
                bif = bif0;
        } else {
                SMR_SLIST_FOREACH_LOCKED(bif, &sc->sc_iflist, bif_next) {
                        if (bif->ifp == ifp)
                        if (bridge_ourether(bif->ifp, eh->ether_dhost))
                                /* addressed to some other bridge member */
        if (bif != NULL) {
                /* ... */
                if (bridge_filterrule(&bif0->bif_brlin, eh, m) ==
                    BRL_ACTION_BLOCK) {
                        goto bad;
                /* ... */

a packet that comes in on a member of a bridge and is addressed to _any_
bridge member interface, is processed as input to (only) the destination
interface. if that destination interface differs from the one that the
packet actually came in on, pf rules for the actual input interface are
not evaluated.

this seems to contradict the following statement in the NOTES section of
bridge(4) -- in any case, it certainly contradicted my intuition:

     Bridged packets pass through pf(4) filters once as input on the receiving
     interface and once as output on all interfaces on which they are
     forwarded.  In order to pass through the bridge packets must pass any in
     rules on the input and any out rules on the output interface.  Packets
     may be blocked either entering or leaving the bridge.

now, to be clear, i don't mind that the packet passes through 'in' rules for
vether0 in my example, but i wonder if it should not also pass through
the 'in' rules for 'athn0'.

i would appreciate enlightenment on this. however, i am not subscribed
to this list, so please cc replies.


PS: i am using OpenBSD 6.6.