I’ve sometimes found that it would be useful to be able to forward DNS requests from one network into another.
In this article, the examples are for forwarding Docker’s internal DNS. My (potential) use case is to (hopefully) work around Softether VPN’s internal DNS server not being able to resolve the names of other Docker containers on the Docker network (when running Softether VPN in a Docker container).
I tried the following on a CentOS 7.5 machine, but this only worked for the first request.
mkfifo0 mkfifo1 nc -l -u 172.19.0.3 53 < fifo1 > fifo0 & nc -u 127.0.0.11 53 < fifo0 > fifo1
Checking netstat -lnp after sending the first request, we see that nc is no longer listening. The problem is that the -k option is missing, but adding the -k option gets us this message:
Ncat: UDP mode does not support the -k or --keep-open options, except with --exec or --sh-exec. QUITTING.
Wait, “–exec”? “–sh-exec”? What, we don’t have to do this whole mkfifo stuff at all?!
#!/bin/sh nc -u 127.0.0.11 53
nc -k -l -u 172.19.0.3 53 -e /root/nc.sh
Note, -e is short for –exec. This appears to work just fine. (Note: the corresponding –sh-exec (-c) option wouldn’t work immediately and I didn’t feel like spending too much time on this.)
Here’s a dnsmasq command to do something similar:
dnsmasq -u root -i eth0 --no-dhcp-interface=eth0 --port=5353
This will also allow you to resolve things using /etc/hosts on the container running dnsmasq, while disabling dnsmasq’s internal DHCP. (If you change the listening interface given in -i, you’ll also have to change the interface given in –no-dhcp-interface.)