misc/60870: Downing a interface in the same subnet as the nfs-root freezes the system

["netbsd%lissners.de@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60870
>Category:       misc
>Synopsis:       Downing a interface in the same subnet as the nfs-root freezes the system
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    misc-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Oct 13 12:10:00 +0000 2026
>Originator:     Ekkehard
>Release:        netbsd11
>Organization:
>Environment:
NetBSD melone2 11.0 NetBSD 11.0 (GENERIC) #0: Thu Jul 30 15:23:12 UTC 2026  mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/i386/compile/GENERIC i386
>Description:
I have a machine with 2 network-interfaces: wm0 and wm1
The machine boots a nfs-root on wm0 on the ip 192.168.10.96
192.168.10.2 is the default gw, 192.168.10.6 is the server providing pxe, dhcp, nfs.


After bootup wm1 gets configured:
>>>
ifconfig wm1 192.168.10.97
<<<
(Remark: This is the same subnet as for wm0 running the nfs-root).
(Probably not related / critical: A parallel ping to wm0 / 192.168.10.96 misses ~7 packets during this operation; and this operation takes ~10 seconds)

>>>
ifconfig wm1 down
<<<
Now the system freezes.
The ping to wm0 / 192.168.10.96 didn't get responses anymore.
The console does not work anymore.


AI told me to append this information:
>>>
melone2# netstat -rn -f inet
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use    Mtu Interface
default            192.168.10.2       UGS         -        -      -  wm0
127/8              127.0.0.1          UGRS        -        -  33176  lo0
127.0.0.1          lo0                UHl         -        -  33176  lo0
192.168.0/20       link#1             UC          -        -      -  wm0
192.168.10.96      link#1             UHl         -        -      -  lo0
192.168.10.2       3c:a6:2f:3e:25:35  UHL         -        -      -  wm0
192.168.10.6       08:00:27:b3:81:f0  UHL         -        -      -  wm0
melone2# ifconfig wm1 192.168.10.97
melone2# netstat -rn -f inet
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use    Mtu Interface
default            192.168.10.2       UGS         -        -      -  wm0
127/8              127.0.0.1          UGRS        -        -  33176  lo0
127.0.0.1          lo0                UHl         -        -  33176  lo0
192.168.0/20       link#1             UC          -        -      -  wm0
192.168.10/24      link#2             UC          -        -      -  wm1
192.168.10.96      link#1             UHl         -        -      -  lo0
192.168.10.97      link#2             UHl         -        -      -  lo0
192.168.10.2       3c:a6:2f:3e:25:35  UHL         -        -      -  wm1
192.168.10.6       08:00:27:b3:81:f0  UHL         -        -      -  wm1
192.168.10.2       3c:a6:2f:3e:25:35  UHL         -        -      -  wm0
192.168.10.6       08:00:27:b3:81:f0  UHL         -        -      -  wm0
<<<

>How-To-Repeat:
Give the new interface a ip from the same subnet as the nfs-root-interface, then "down" the interface without explicitely removing the ip.
>Fix:
How it works:
Remove the ip from the interface before downing it:
>>>
ifconfig wm1 192.168.10.97
ifconfig wm1 192.168.10.97 delete
ifconfig wm1 down
<<<

What also works is giving the interface a ip in another network:
>>>
ifconfig wm1 192.168.100.97
ifconfig wm1 down
<<<

Expected behaviour:
The ip-address is automatically removed from wm1 before downing the interface.

My(!) expectation is not: A switchover to wm1 as the interface handling the nfs-root when downing wm0. Would be nice, though :-)