Locating DHCP servers on a LAN from a Microsoft Windows system
If you need to determine the
IP addresses of
systems on a
local area network (LAN) that are functioning as
Dynamic Host Configuration Protocol (DHCP) servers, one tool that you
can use on systems running a Microsoft Windows operating system is
dhcptest, developed
by
Vladimir Panteleev, aka
CyberShadow, a
Moldovan developer.
He provides a compiled version of the program for Windows systems at
dhcptest — there are
32-bit and
64-bit versions
of the program there.
When you open the program, type d to have the program send
a DHCP discover packet, which should result in responses from DHCP servers
on the LAN. You can type Ctrl-C or q to quit
the program.
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Using dhclient to locate DHCP Servers on a LAN
On a Linux system, you can use the
dhclient
command to identify any
Dynamic Host
Configuration Protocol (DHCP) servers that are available on the
local area network (LAN).
First, you need to determine a relevant network interface on the system
over which an IP address might be acquired via DHCP. You can do so using
the
ip command. If the
-f inet option is given to the command, it will show only IPv4
addresses.
# ip -f inet address
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
2: enp4s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
inet 192.168.0.35/24 brd 192.168.0.255 scope global dynamic enp4s0
valid_lft 156092sec preferred_lft 156092sec
3: virbr0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN
inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0
valid_lft forever preferred_lft foreverAlteratively, you can use the
ifconfig command with the -a option to show all network
interfaces and any assigned IP addresses for them. I.e.,
ifconfig -a.
In the example above from a CentOS 7 system, I can see that an IP address is
assigned to the enp4s0 network interface and that is the interface
I will use for the DHCP query to locate any DHCP servers on the network, which
may be legitimate or rogue DNS servers.
To have the dhclient command search for DHCP servers, I'll use the
-d and -nw options.
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