kern/115413: ipv6 pmtu not working

[Jacek Zapala]

>Number:         115413
>Category:       kern
>Synopsis:       ipv6 pmtu not working
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sat Aug 11 01:20:02 GMT 2007
>Closed-Date:
>Last-Modified:
>Originator:     Jacek Zapala
>Release:        6.2
>Organization:
>Environment:
FreeBSD xxxxx 6.2-RELEASE FreeBSD 6.2-RELEASE #0: Wed Apr 25 13:06:39 CEST 2007     root at xxxxxx:/usr/src/sys/i386/compile/DELL_2950  i386
>Description:
IPv6 path MTU discovery does not work. ICMP6 "packet too big" messages arrive, but seem to be ignored (tcp segment with unmodified mtu is retransmitted). Instead, tcp "discarded for bad checksums" counter gets incremented (could be a coincidence?)
Checksum for incoming ICMPv6 packet is ok (according to wireshark), checksum of tcp packet included in ICMPv6 message is of course wrong (as not whole original packet fits into 1280 bytes long ICMPv6 message).

Interface:
bce0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        options=3b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU>

Destination address is normal, unicast address (problem was noticed when sendmail could not send mail to some destinations).

Reboot didn't help ;-)
As a workaround I have set 1280 bytes mtu for default ipv6 route.

>How-To-Repeat:

>Fix:


>Release-Note:
>Audit-Trail:
>Unformatted: