How can I find my DHCP server?

To find out the DHCP server that's giving you the IP, just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command(s) below:

cat /var/lib/dhcp3/dhclient.leases

Or you can just use grep command to get DHCP server address.

grep dhcp-server-identifier /var/lib/dhcp3/dhclient.leases

OR

grep dhcp-server-identifier /var/lib/dhcp/dhclient.leases

For Ubuntu 14.04, 16.04, and 17.10 you can use:

dhclient -d -nw eth0

Sample output:

Internet Systems Consortium DHCP Client 4.2.4
Copyright 2004-2012 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/eth0/00:0c:29:49:3e:67
Sending on   LPF/eth0/00:0c:29:49:3e:67
Sending on   Socket/fallback
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 3 (xid=0x4f723f9)
DHCPREQUEST of 192.168.138.136 on eth0 to 255.255.255.255 port 67 (xid=0x4f723f9)
DHCPOFFER of 192.168.138.136 from 192.168.138.254
DHCPACK of 192.168.138.136 from 192.168.138.254
RTNETLINK answers: File exists
bound to 192.168.138.136 -- renewal in 892 seconds.

In Ubuntu 14.04, the /var/lib/dhcp/dhclient.leases is empty. The actual lease file can be found on the command line of dhclient via ps. Look for the -lf option. This command should work in Ubuntu 14.04 installations (still valid as of 17.10):

cat $(ps aux | grep -o '[/]var/lib/NetworkManager/\S*.lease') | grep dhcp-server-identifier

With:

• nmap (source; DHCPv6):

  sudo nmap --script broadcast-dhcp-discover -e eth0  # DHCPv4
  sudo nmap --script broadcast-dhcp6-discover -6      # DHCPv6
  

• dhdump (source):

  sudo dhcpdump -i eth0
  

• tcpdump (source):

  sudo tcpdump -i eth0 -nev udp port 68
  

Others:

• dhcp_probe (dhcp-probe package)

In Ubuntu 16.04 you can try

journalctl | grep DHCPACK