kern/46668: wm reconfigures losing carrier when MTU is set.

[is%netbsd.org@localhost]

>Number:         46668
>Category:       kern
>Synopsis:       wm reconfigures losing carrier when MTU is set.
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Jul 07 16:15:00 +0000 2012
>Originator:     Ignatios Souvatzis
>Release:        NetBSD 6.0_BETA2
>Organization:
seal your e-mail: http://www.gnupg.org/
>Environment:
System: NetBSD henrietta 6.0_BETA2 NetBSD 6.0_BETA2 (GENERIC) #1: Wed Jun 27 
01:02:47 CEST 2012  
is@henrietta:/var/itch/sources/6/obj.amd64/sys/arch/amd64/compile/GENERIC amd64
Architecture: amd64
Machine: amd64

>Description:

Jul  7 17:59:40 henrietta dhcpcd[5541]: wm0: carrier acquired
Jul  7 17:59:40 henrietta dhcpcd[5541]: wm0: sending IPv6 Router Solicitation
Jul  7 17:59:40 henrietta dhcpcd[5541]: wm0: rebinding lease of 10.202.254.5
Jul  7 17:59:41 henrietta dhcpcd[5541]: wm0: Router Advertisement from 
fe80::200:24ff:fec0:d80
Jul  7 17:59:41 henrietta dhcpcd[5541]: wm0: acknowledged 10.202.254.5 from 
10.202.254.126 `marie'
Jul  7 17:59:41 henrietta dhcpcd[5541]: wm0: checking for 10.202.254.5
Jul  7 17:59:47 henrietta dhcpcd[5541]: wm0: leased 10.202.254.5 for 43200 
seconds
Jul  7 17:59:47 henrietta dhcpcd[8481]: wm0: wm0: MTU set to 1480
Jul  7 17:59:48 henrietta dhcpcd[5541]: wm0: carrier lost
Jul  7 17:59:48 henrietta dhcpcd[13880]: wm0: wm0: MTU restored to 1500
Jul  7 17:59:49 henrietta dhcpcd[5541]: wm0: carrier acquired

Result: ping6 gets a delay of about 700ms every 6 seconds, IPv4
connections (e.g. to the server's lpd) are impossible.

>How-To-Repeat:
        Add "options interface-mtu 1480;" to the DHCP server's dhcpd.conf

>Fix:
        Workaround: remove the "options interface-mtu 1480;" from the
        clients dhcpcd.conf, or make it not apply to that client on 
        dhcpd.conf