DHCP

When connecting a computer to most enterprise networks, if the Dynamic Host Configuration Protocol (DHCP) is enabled, it will assign an IP address to that computer, and send a lot of information. Nameservers and domain names are usually set through DHCP offer packets.

On UNIX-like systems, the /etc/resolv.conf file will store information for name resolution operations after the DHCP offer.

The nmap tool can be used with its broadcast-dhcp-discover.nse script to easily parse those packets.

nmap --script broadcast-dhcp-discover

In many cases, there will be MAC address filtering, static IP addressing, VLANs or NAC (Network Access Control) that can prevent testers from obtaining this information. In those situations, Wireshark can be used to manually inspect broadcast and multicast packets that travel on the network and find valuable information that could help bypass those mitigations.