Command line program to test DHCP service

Question

Is there a simple ping-like command to test whether a DHCP service is running on a network? ...on Linux

mivk · Answer 1 · 2017-11-22T08:02:50.520

Based on this answer, assuming you have installed nmap (sudo apt install nmap):

sudo nmap --script broadcast-dhcp-discover

Add the -e $interface option if you have more than one network interface. (For example: nmap --script broadcast-dhcp-discover -e eth0)

Sample output:

Starting Nmap 7.01 ( https://nmap.org ) at 2017-09-27 17:40 CEST
Pre-scan script results:
| broadcast-dhcp-discover: 
|   Response 1 of 1: 
|     IP Offered: 192.168.81.94
|     DHCP Message Type: DHCPOFFER
|     Server Identifier: 192.168.81.2
|     IP Address Lease Time: 5m00s
|     Subnet Mask: 255.255.255.0
|     Router: 192.168.81.2
|     Domain Name Server: 192.168.81.2
|     Domain Name: example.lan
|     NTP Servers: 192.168.81.10, 192.168.81.2
|     NetBIOS Name Server: 192.168.81.10
|_    NetBIOS Node Type: 8
WARNING: No targets were specified, so 0 hosts scanned.
Nmap done: 0 IP addresses (0 hosts up) scanned in 0.66 seconds

Or this output on a different network, with nmap v. 6 :

Starting Nmap 6.00 ( http://nmap.org ) at 2017-09-27 17:42 CEST
Pre-scan script results:
| broadcast-dhcp-discover: 
|   IP Offered: 192.168.4.101
|   DHCP Message Type: DHCPOFFER
|   Server Identifier: 192.168.4.1
|   IP Address Lease Time: 7 days, 0:00:00
|   Subnet Mask: 255.255.255.0
|   Time Offset: 7200
|   Router: 192.168.4.1
|   Domain Name Server: 208.91.112.53, 208.91.112.52
|   Renewal Time Value: 3 days, 12:00:00
|_  Rebinding Time Value: 6 days, 3:00:00
WARNING: No targets were specified, so 0 hosts scanned.
Nmap done: 0 IP addresses (0 hosts up) scanned in 1.15 seconds

this didn't find my DHCP server.- must be something wrong with my pc... ethernet works, just not that. — Jasen, Nov 22 '17 at 23:27
Note that [this script](https://github.com/nmap/nmap/blob/master/scripts/broadcast-dhcp-discover.nse) uses a fixed MAC address (`DE:AD:CO:DE:CA:FE`) and therefore will not work if your DHCP server is configured with a whitelist. — Bob, Oct 08 '19 at 05:27
@Bob An option has been added. `--script-args mac=XX:XX:XX:XX:XX:XX`. It hasn't been released yet though. — Aleksandr Dubinsky, Aug 14 '20 at 08:25

score 17 · Accepted Answer · answered Sep 20 '11 at 13:16

17

Even if you don't use a Nagios monitoring server, you can grep the check_dhcp binary or compile it from source and use the check to test a DHCP-Server. E.g.

./check_dhcp -v -s <dhcp_server_address> -r <expected_ip_address> -m <mac_address_touse>

answered Sep 20 '11 at 13:16

sandoz

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score 17 · Answer 3 · edited Aug 18 '16 at 15:33

17

or you can try dhcpcd in test mode:

dhcpcd -T eth0

edited Aug 18 '16 at 15:33

chicks

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answered Apr 12 '13 at 14:15

Paul M

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Use `dhcpcd -T eth0 -t 2` to timeout after 2 seconds – Maarten Wolzak Sep 22 '17 at 11:14
Could you please expand on why should this command detect DHCP servers on your network and what kind of output should we expect when someone will be detected? – Sopalajo de Arrierez May 29 '20 at 19:30
a dhcp server needs to see a packet on the network from a host requesting a dhcp configuration message. therefore we need to mimic a dhcp request,, and this tool will do that and print the response, which will be an ip address, netwmask, default router, and optionally dns resolvers, ntp servers etc. – Paul M Jun 22 '20 at 14:28
not all distros seem to have the dhcpcd program, so you'll have to try the dhclient program in test mode, as discussed elsewhere on this page. – Paul M Dec 11 '20 at 14:03

score 13 · Answer 4 · answered Nov 27 '15 at 01:01

dhcpdump is good for sniffing and displaying DHCP packets on the network.

E.g.

# dhcpdump -i eth0
---------------------------------------------------------------------------

  TIME: 2015-11-27 11:41:37.379
    IP: 0.0.0.0 (0:11:b9:5:0:b8) > 255.255.255.255 (ff:ff:ff:ff:ff:ff)
    OP: 1 (BOOTPREQUEST)
 HTYPE: 1 (Ethernet)
  HLEN: 6
  HOPS: 0
   XID: abce9327
  SECS: 0
 FLAGS: 0
CIADDR: 0.0.0.0
YIADDR: 0.0.0.0
SIADDR: 0.0.0.0
GIADDR: 0.0.0.0
CHADDR: 00:11:b9:05:00:b8:00:00:00:00:00:00:00:00:00:00
 SNAME: .
 FNAME: .
OPTION:  53 (  1) DHCP message type         1 (DHCPDISCOVER)
OPTION:  57 (  2) Maximum DHCP message size 576
OPTION:  55 (  4) Parameter Request List      1 (Subnet mask)
                          3 (Routers)
                         28 (Broadcast address)
                          6 (DNS server)

---------------------------------------------------------------------------

You can use it in conjunction with another tool to do DHCP requests, or just use it to watch PCs/devices query DHCP.

This seems to only only work if there is actual dhcp traffic on the network. — mivk, Sep 27 '17 at 15:51
@KenSharp Why? Yes. dhcpdump will only show something when a client on the network requests a dhcp address. If you don't want to wait and don't want to send a dhcp request yourself but only see which machine would answer what IF there were a request, then this is not the solution. — mivk, Feb 26 '18 at 15:49

score 8 · Answer 5 · edited Mar 15 '18 at 19:08

8

Why not just make a DHCP request?

dhclient

For verbose output use:

dhclient -v

edited Mar 15 '18 at 19:08

Community

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answered Aug 18 '10 at 09:21

Sirex

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Probably because that will make your system *use* the IP assigned by the DHCP server which is not what one wants when e.g. trying to find out if there are rogue DHCP servers in his network. – ThiefMaster Jun 09 '12 at 18:54
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-n option helps here? – akostadinov Jan 24 '13 at 22:20
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Option `-n` seems to be broken on ISC DHCP Client v4.2.4 on Ubuntu 14 LTS? – Josip Rodin Apr 15 '16 at 14:33
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I don't think `-n` does what you think it does. – Jasen Nov 22 '17 at 23:19
https://explainshell.com/explain?cmd=dhclient+-n: "Do not configure any interfaces. This is most likely to be useful in combination with the -w flag." – mwfearnley Jan 28 '19 at 12:22

score 7 · Answer 6 · answered Jun 16 '15 at 02:13

7

I wrote a DHCP test client program a while ago, works on Windows and POSIX:

https://github.com/CyberShadow/dhcptest

answered Jun 16 '15 at 02:13

Vladimir Panteleev

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Written in D... It seems possible (with `ldc2`) to get static binaries with `ldc2 -link-defaultlib-shared=false -static -L=-lphobos2-ldc -L=-lz dhcptest.d` – Gert van den Berg Oct 22 '18 at 07:27

score 5 · Answer 7 · edited Jul 25 '16 at 01:09

5

There is a program called dhcping (with just one p)

edited Jul 25 '16 at 01:09

user3771184

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answered Aug 18 '10 at 09:09

topdog

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score 0 · Answer 8 · answered Aug 21 '10 at 13:11

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if you use nagios you could monitor it with check_dhcp

answered Aug 21 '10 at 13:11

natxo asenjo

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score 0 · Answer 9 · answered Jan 09 '13 at 18:38

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On Debian-Ubuntu systems:

dhcpd -f

gives good feedback and tests the configuration.

On RH systems try:

/etc/rc.d/init.d/dhcpd configtest

answered Jan 09 '13 at 18:38

John P. Fisher

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That test is run on the dhcp server to check its configuration, it doesn't tell you if a device on the network will actually see a working server – Paul M May 30 '20 at 21:44

Command line program to test DHCP service

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