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kern/39305: loops through gre(4) cause a hang or crash

>Number:         39305
>Category:       kern
>Synopsis:       loops through gre(4) cause a hang or crash
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Aug 06 19:30:00 +0000 2008
>Originator:     David Young
>Release:        NetBSD 4.99.70
OJC Technologies * Urbana, Illinois
System: NetBSD cuw 4.99.70 NetBSD 4.99.70 (elan) #0: Fri Jul 18 17:21:29 CDT 
Architecture: i386
Machine: i386
If a gre(4) is configured with inner destination address equal to
outer destination address, the kernel will loop forever trying
to deliver packets that pass through that gre(4) instance.
ifconfig gre0 create inet$SIX $SRC0 $DST tunnel inet$SIX $SRC1 $DST
ifconfig gre0 up
ping$SIX $DST
Short-term: as packets traverse a gre(4), tag them in gre_output()
        with an identifier for that gre instance.  But first check
        packets in gre_output() for that instance's tag.  If the
        instance tag is already present, send an ICMP/ICMPv6 error
        response to the sender to indicate the loop.

Long-term: in addition to the above, let us configure two routes
        to any destination---i.e., multipath routing.  Qualify a
        gre(4)'s outer destination address so that it resolves to
        a different route than the route to itself.  Use route
        metrics (say) to make the system prefer the route through

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