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Sat, Sep 07, 2024 9:58 pm

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.

[ More Info ]

[/network/dhcp] permanent link

Mon, Aug 08, 2016 10:09 pm

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 forever

Alteratively, 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.

[ More Info ]

[/network/dhcp] permanent link

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