kern/60056: tap(4) loses carrier randomly

["martin%NetBSD.org@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60056
>Category:       kern
>Synopsis:       tap(4) loses carrier randomly
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Mar 04 14:45:00 +0000 2026
>Originator:     Martin Husemann
>Release:        NetBSD 11.99.5
>Organization:
The NetBSD Foundation, Inc.
>Environment:
System: NetBSD seven-days-to-the-wolves.aprisoft.de 11.99.5 NetBSD 11.99.5 (GENERIC) #849: Wed Mar 4 10:02:57 CET 2026 martin%seven-days-to-the-wolves.aprisoft.de@localhost:/work/src/sys/arch/amd64/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:

I have a version of the uae (amiga emulator) that runs my virtual NetBSD/amiga
machine.

The uae process opens /dev/tap0 to access my host network.

Before I start uae, I run this as root:

---8<---
ifconfig tap0 create up
ifconfig bridge0 create up
brconfig bridge0 add re0
brconfig bridge0 add tap0
chown martin:users /dev/tap0
--->8---

When the virtual amiga boots, I initially have working network. After a while
though tap0 starts flapping the carrier status (w/o any reason I see).

 root@seven-days-to-the-wolves # fstat -p 1678 ; ifconfig tap0
USER     CMD          PID   FD  MOUNT         INUM MODE         SZ|DV R/W
martin   uae         1678   wd  /         16642980 drwxr-xr-x     512 r 
martin   uae         1678    0  /dev/pts         9 crw--w----   pts/3 rw
martin   uae         1678    1  /dev/pts         9 crw--w----   pts/3 rw
martin   uae         1678    2  /dev/pts         9 crw--w----   pts/3 rw
martin   uae         1678    3  /          3750038 crw-------    tap0 rw
martin   uae         1678    4  /         16642994 -rw-r--r--  8589934592 rw
martin   uae         1678    5  /         16642995 -rw-r--r--  8589934592 rw
martin   uae         1678    6* internet stream tcp *:vnc-server
martin   uae         1678    7* internet6 stream tcp *:vnc-server
tap0: flags=0x8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
        ec_capabilities=0x5<VLAN_MTU,JUMBO_MTU>
        ec_enabled=0x1<VLAN_MTU>
        address: f2:0b:a4:f1:11:21
        status: no carrier
        inet6 fe80::f00b:a4ff:fef1:1121%tap0/64 flags 0x8<DETACHED> scopeid 0x21b


So despite /dev/tap0 still being open, carrier is lost.

I think this should not happen :-)

It used to work quite stable (but I am not sure when exactly this started
happening).

>How-To-Repeat:
s/a

>Fix:
n/a