Hi everyone,I’m seeing an intermittent issue on an RT-BE88U running Asuswrt-Merlin where dnsmasq stops running entirely, resulting in loss of DNS for the LAN. I’ve done some debugging and wanted to share the findings in case it’s helpful or if others are seeing the same behavior.
Router / Environment
Router: RT-BE88U
Firmware: Asuswrt-Merlin (current release)
Uptime when failure occurs: typically several days
Network setup includes:
- 2 AiMesh nodes
- RT-AX58U (Merlin)
- RT-AC68U (Merlin, though no longer officially supported)
- Debian 12 chroot environment running:
- Plex Media Server
- Transmission
- Entware installed
- Custom scripts for Transmission VPN routing
However none of the scripts interact with dnsmasq.
Additional WAN context:
I am bypassing the AT&T gateway using a WAS-110 fiber bridge / XGS-PON ONT directly on the BE88U. WAN connectivity remains up when the issue occurs, and the observed failure is that dnsmasq disappears entirely (no process, no port 53 listener) until service restart_dnsmasq is run. Because the most specific log clue so far is dnsmasq: failed to send packet: Bad file descriptor during IPv6 RA / DHCPv6 activity on br0, I’m not sure whether the WAN bypass is relevant or just incidental, but I wanted to disclose it.
Symptom
Internet suddenly stops working for LAN clients.
Observed behavior:
- DNS resolution fails
- Router itself still reachable via SSH
- WAN connectivity still appears normal
Running diagnostics shows dnsmasq is no longer running.
Example:
pidof dnsmasq
(no output)
ps | grep dnsmasq
grep only
Port 53 is also not bound by any process.
netstat -lntup | grep :53
(no dnsmasq entries)
Restarting dnsmasq immediately restores service:
service restart_dnsmasq
Relevant Log Entries
The most interesting log entry appears several minutes before the outage:
dnsmasq[25137]: failed to send packet: Bad file descriptor
Immediately prior to this, there are IPv6 RA / DHCPv6 related messages:
router advertisement on 2600:xxxx::, old prefix for br0
DHCPv6 stateless on 2600:xxxx:: constructed for br0
router advertisement on 2600:xxxx:: constructed for br0
dnsmasq[25137]: failed to send packet: Bad file descriptor
After that, dnsmasq continues logging DHCP activity for a while, then eventually disappears completely.
Confirmation During Failure
When the issue occurred I captured the following:
pidof dnsmasq
(no output)
netstat -lntup | grep :53
(no listener)
ps | grep -E 'dcd|dnsmasq|networkmap|wanduck|roamast'
wanduck
networkmap
roamast
dcd
Other router daemons were still running normally.
Additional Observations
After restarting dnsmasq, I consistently see:
dnsmasq-dhcp: Working around kernel bug: faulty source address scope for VRF slave br0
Bridge configuration:
br0
eth0
eth1
eth2
...
wl0
wl1
wl0.1
wl1.1
tap22
So dnsmasq is bound to a fairly busy bridge interface.
Memory / Resource Check
Memory does not appear to be exhausted.
Example:
Mem: ~2GB total
~57MB free
~736MB available including cache
No OOM killer messages in dmesg.
Other daemon instability observed
Occasionally the dcd process crashes earlier in the logs:
kernel: CPU: PID Comm: dcd
kernel: Tainted: P O
However dcd is still running during the dnsmasq outage, so it may be unrelated.
Current Workaround
Restarting dnsmasq restores service immediately:
service restart_dnsmasq
Questions
- Has anyone else seen dnsmasq disappear like this on the BE series routers?
- Could this be related to:
- IPv6 RA / DHCPv6 handling
- bridge interface (br0)
- kernel networking bug referenced in the dnsmasq log?
- Would disabling LAN IPv6 be a useful diagnostic test?
Thanks in advance for any insight.
Happy to capture additional diagnostics if there are recommended commands.
