Finding DNS servers provided by DHCP using network manager on Linux

Suppose you want to figure out which DNS server you're using and it is provided via DHCP. This can be surprisingly difficult. Suppose there's:

1. Nothing in /etc/resolv.conf
2. `dig google.com` shows your machine uses a local DNS server (local dnsmasq)
3. /var/lib/dhcp/dhclient.leases looks...wrong

I resorted to:

sudo tcpdump -i eth0 -s0 -n 'udp port 53'

Which is guaranteed to show you what's going on. It turns out network manager stashes leases under its own directory. If you use

ps aux | grep dhclient

you can see network manager running dhclient (perhaps multiple instances per interface) and the -lf (lease file) parameter will point you at something like

/var/lib/NetworkManager/dhclient-[guid].lease

Mystery solved!