[FS#3943] dnsmasq-full doesn't start when running in ujail

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FS#3943 - dnsmasq-full doesn't start when running in ujail
User who did this: Stijn Tintel (stintel)
Summary: dnsmasq-full running in ujail doesn't bind any socket after restarting the service -> dnsmasq-full doesn't start when running in ujail
Task details edited:
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When a device has been running for a while (hit it today with 28d uptime), restarting dnsmasq results in dnsmasq no longer being started, there is only the ujail process. There are no errors displayed on stdout/stderr while restarting, nor in syslog.


root at ar0:~# /etc/init.d/dnsmasq restart
udhcpc: started, v1.33.1
udhcpc: sending discover
udhcpc: no lease, failing
udhcpc: started, v1.33.1
udhcpc: sending discover
udhcpc: no lease, failing
udhcpc: started, v1.33.1
udhcpc: sending discover
udhcpc: no lease, failing
udhcpc: started, v1.33.1
udhcpc: sending discover
udhcpc: no lease, failing



Tue Jul 20 15:17:15 2021 user.notice dnsmasq: DNS rebinding protection is active, will discard upstream RFC1918 responses!
Tue Jul 20 15:17:15 2021 user.notice dnsmasq: Allowing 127.0.0.0/8 responses
Tue Jul 20 15:17:15 2021 user.notice dnsmasq: Allowing RFC1918 responses for domain plex.direct



root at ar0:~# ps aux | grep dnsmasq
root     21289  0.0  0.0   2088   872 ?        S    15:17   0:00 /sbin/ujail -n dnsmasq -u -l -r /dev/null -r /dev/urandom -r /etc/TZ -r /etc/dnsmasq.conf -r /etc/ethers -r /etc/group -r /etc/hosts -r /etc/passwd -r /sbin/hotplug-call -r /tftpboot -r /tmp/dnsmasq.d -r /tmp/etc/dnsmasq.conf.main -r /tmp/hosts/dhcp.main -r /usr/lib/dnsmasq/dhcp-script.sh -r /usr/share/dnsmasq/dhcpbogushostname.conf -r /usr/share/dnsmasq/rfc6761.conf -r /usr/share/dnsmasq/trust-anchors.conf -w /var/lib/dhcp.leases -w /var/run/dnsmasq/ -- /usr/sbin/dnsmasq -C /tmp/etc/dnsmasq.conf.main -k -x /var/run/dnsmasq/dnsmasq.main.pid
root     21455  0.0  0.0   1132   468 pts/1    S+   15:19   0:00 grep dnsmasq
root at ar0:~# ss -anput | grep dnsmasq
root at ar0:~#


Commenting out the lines in the init script starting with procd_add_jail and then restarting the service solves the problem. The problem also does not occur when dnsmasq is started during boot.

I've seen this problem before, mentioned it a few times on IRC, the first time was in October 2020, so before 21.02 was branched, so it's very likely this problem exists there as well.

I didn't reboot the system where I'm currently experiencing this, I've commented out the procd_add_jail lines instead. Uncommenting those lines brings back the problem, so further investigation is possible.
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More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=3943

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