Blueman NAP feeds bad DNS to clients on Ubuntu with dhcpd

[David D Miller]

I'm on Ubuntu 19.04
blueman-2.0.5-1ubuntu2

Symptom: When configuring blueman to offer a Bluetooth Network Access Point and instructing it to use dhcpd for the DHCP server, it feeds 127.0.0.53 to the clients as the DNS server. This does not work because 127.0.0.0/8 is only accessible from localhost, and the clients route that address to themselves.

The code which generates the DHCP config is in /usr/lib/python3/dist-packages/blueman/main/NetConf.py and the offending code can be found in the get_dns_servers function at line 112

It pulls the DNS server out of /etc/resolv.conf.
On modern Ubuntu systems, Ubuntu has a DNS cache server installed, which binds to 127.0.0.53, so that's what it puts in /etc/resolv.conf. This is not the real DNS server of course, and it's not reachable from NAP clients.

Short of pulling the DNS server from NetworkManager (which seems complicated), a good compromise would be to give me a configuration option to override the DNS server so I can manually specify one to feed to the NAP clients.

[David D Miller (justdave)]

The following bash line appears to get the default DNS server out of NetworkManager:

systemd-resolve --status $( netstat -r | grep default | head -1 | awk '{ print $8 }' ) | grep 'DNS Server:' | sed -e 's/.*DNS Server: \(\S\+\).*/\1/'

[David D Miller (justdave)]

I just filed an upstream bug report for this at https://github.com/blueman-project/blueman/issues/1544