kern/40139: bridge(4) does not transmit DHCP replies to tap(4)

[Taylor R Campbell <campbell%mumble.net@localhost>]

>Number:         40139
>Category:       kern
>Synopsis:       bridge(4) does not transmit DHCP replies to tap(4)
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Dec 09 09:00:00 +0000 2008
>Originator:     Taylor R Campbell <campbell%mumble.net@localhost>
>Release:        NetBSD 4.0_STABLE
>Organization:
>Environment:
System: NetBSD slate.localdomain 4.0_STABLE NetBSD 4.0_STABLE (RIABWI) #35: Sun 
Nov 16 12:42:48 EST 2008 
riastradh%Oberon.local@localhost:/Users/riastradh/os/netbsd/4/obj/sys/arch/macppc/compile/RIABWI
 macppc
Architecture: powerpc
Machine: macppc
>Description:

        When a tap(4) interface is on a bridge(4), and a DHCP client
        such as dhclient(8) tries to configure the interface, DHCP
        requests make it over the bridge onto a physical ethernet
        network, but none of the replies on the physical network make
        it back over the bridge to the tap(4) interface.

>How-To-Repeat:

        If <if> is the name of an ethernet interface attached to a
        network with a DHCP server, run

        # ifconfig tap0 create
        # ifconfig bridge0 create
        # brconfig bridge0 add tap0 add <if> up
        # dhclient tap0

        Also run tcpdump(8) on <if>, and watch the outgoing DHCP
        requests and incoming DHCP replies, the latter of which don't
        make it to dhclient(8).

        (Make sure that no DHCP client is running to configure <if>, to
        avoid its interfering with the DHCP client configuring tap0.) 

>Fix:

        Currently the best I can manage is to browse the output of
        tcpdump(8) on the physical interface for DHCP replies, and to
        manually configure the interface from the information I thereby
        glean.  This is a pain.