Get ip address dhcp linux

Linux find DHCP server IP address using CLI

Linux find DHCP server IP address command line options

The procedure to find out your DHCP IP address in Linux is as follows:

1. Open the terminal application
2. Run less /var/lib/dhcp/dhclient.leases command to list your DHCP server IP address
3. Another option is to type grep dhcp-server-identifier /var/lib/dhcp/dhclient.leases command to find the IP address of a DHCP server
4. One can use ip r Linux command to list default route which act as the DHCP Server on most home networks

Let us see all commands and examples in details.

The dhclient.leases file

Under Linux, you need to use dhclient command to obtain and managing dhcp tasks. In order to keep track of leases across system reboots and server restarts, dhclient keeps a list of leases it has been assigned in the dhclient.leases file. On startup, after reading the dhclient.conf file, dhclient reads the dhclient.leases file to refresh its memory about what leases it has been assigned.

When a new lease is acquired, it is appended to the end of the dhclient.leases file. In order to prevent the file from becoming arbitrarily large, from time to time dhclient creates a new dhclient.leases file from its in-core lease database. The old version of the dhclient.leases file is retained under the name dhclient.leases

until the next time dhclient rewrites the database.

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Find DHCP Server IP Address on Linux Command Line

I recently received an email from a reader asking “How do I find the IP address of my DHCP server”? My initial reaction was “Hmmm, good question”. That lead me to investigate the fastest way to find your DHCP server IP address from the Linux command line.

What is a DHCP Server?

A DHCP Server is a system on a network that manages dynamic configuration of client systems on the network. DHCP (Dynamic Host Configuration Protocol) is a standard network protocol used on almost all IP networks. It allows for dynamic configuration of network parameters on systems connected to the network. Before DHCP an administrator would have to configure each system individually. For more information on DHCP, see the Resource and Links section below.

How to Find Your DHCP Server IP Address

There is no simple command to show the IP address of the DHCP server on the network. However, there are places it is recorded, if you know where to look. Here are a few ways you can find the IP address of a DHCP server on your network.

NOTE: You will need root or sudo access to run all of these commands.

Find DHCP Server IP Using grep to Find Log Entries

The first method I thought of was to check the logs for the DHCPOFFER. The DHCPOFFER packet would come from the DHCP server and should be written to your logs. Here we use grep with some options (-I to ignore binary files, -R for recursive) to find the log file entry that contains our DHCP server IP address.

CentOS 7 example:

Fedora 30 example:

Ubuntu 19.04 example:

NOTE: Notice that every distribution stores it in a different log file? That is why we recursively search the whole /var/log/ directory.

Using Journalctl and grep

You can do the same as above using journalctl.

To learn more about using journalctl read “Viewing logs with journalctl“.

Find DHCP Server IP Address using Leases File

The dhclient utility stores your lease information in leases file. These lease files are kept in different places for different distros, software loads, and different versions.

NOTE: Replace [interface] with the name of your network interface (i.e. eth0, eno1, etc..)

Now use grep to find the DHCP server.

If you are using NetworkManager, you can find the lease file location by looking at the running processes (ps aux).

Now that you know where the leases file is, you can get the DHCP server IP address from that file.

We can string both of these operations together to simplify the process.

Find DHCP Server IP Using dhclient Utility

Another way would be to use the dhclient utility with some options. This also requires root or elevated privileges. The drawback of this is that the dhclient will go through the whole DHCP D.O.R.A. process. This also means that your DHCP server, network and client system need to be configured correctly. This is not much help if you are troubleshooting DHCP issues.

Conclusion

There you have it, a few different methods to find the IP address of your DHCP server. I am sure there are other ways, as with anything in Linux. If you know of another way please leave it in the comments and I will update the article.

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Linux Force DHCP Client (dhclient) to Renew IP Address

I am using Ubuntu Linux. How to force Linux to reacquire a new IP address from the DHCP server? What is the command of Linux equivalent to Windows’s “ ipconfig /renew ” command?

You need to use Dynamic Host Configuration Protocol Client i.e., dhclient command. The client normally doesn’t release the current lease as it is not required by the DHCP protocol. Some cable ISPs require their clients to notify the server if they wish to release an assigned IP address. The dhclient command, provides a means for configuring one or more network interfaces using the Dynamic Host Configuration Protocol, BOOTP protocol, or if these protocols fail, by statically assigning an address. Let us see Linux command to force DHCP client release your IP address.

Tutorial details
Difficulty level	Easy
Root privileges	Yes
Requirements	Linux with a terminal app
Est. reading time	4 minutes

Linux renew ip command using dhcp

Warning : Releasing your IP address always brings down your network interface/WiFi. So be careful with remote systems.

The -r flag explicitly releases the current lease, and once the lease has been released, the client exits. For example, open terminal application and type the command:
$ sudo dhclient -r
Now obtain fresh IP address using DHCP on Linux:
$ sudo dhclient

How can I renew or release an IP in Linux for eth0?

To renew or release an IP address for the eth0 interface, enter:
$ sudo dhclient -r eth0
$ sudo dhclient eth0
In this example, I am renewing an IP address for my wireless interface:

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Fig.01: Renew DHCP address example

Other options in Linux to renew dhcp

There is no need to restart network service. Above command should work with any Linux distro such as RHEL, Fedora, CentOS, Ubuntu and others. On a related note you can also try out the following commands:
# ifdown eth0
# ifup eth0
### RHEL/CentOS/Fedora specific command ###
# /etc/init.d/network restart
OR
### Debian / Ubuntu Linux specific command ###
# /etc/init.d/networking restart

nmcli command (NetworkManager) to renew IP address in Linux

The NetworkManager daemon attempts to make networking configuration and operation as painless and automatic as possible by managing the primary network connection and other network interfaces, like Ethernet, WiFi, and Mobile Broadband devices command-line tool for controlling NetworkManager. The nmcli is a command-line tool for controlling NetworkManager and getting its status. To renew IP address using nmcli for connection named ‘nixcraft_5G’ (use ‘ nmcli con ‘ command to get list of all connections):

Fig.02: nmcli command in action

Linux Force dhclient to renew IP address on a CentOS 7/Ubuntu/Debian and other Linux-based server

Most modern Linux-based system uses the systemd as a init system and here is how to force Linux to renew IP address using DHCP. Use the ip command to find out the current IP address:
ip a
ip a s eth0
Run:
dhclient -v -r eth0
OR use the systemctl command to restart network service on a CentOS 7:
systemctl restart network.service
systemctl status network.service

How do I force Linux to reacquire a new IP address from the DHCP server using desktop tool?

1. Open the Activities overview (ou can also press the Super key on your keyboard) and start typing Network.
2. Click on Network to open the panel.
3. Choose which connection, Wi-Fi or Wired, from the left pane.
4. Click on the toggle button next to the connection name to turn it off and then on again to renew IP address:

How to renew or release a dynamic IP address in Linux using GUI method

Conclusion

Linux force DHCP client to release and renew an IP address

Command		Command to release/renew a DHCP IP address in Linux
ip a		Get ip address and interface information on Linux
ip a s eth0		Find the current ip address for the eth0 interface in Linux
Method #1
dhclient -v -r eth0		Force Linux to renew IP address using a DHCP for eth0 interface
Method #2
systemctl restart network.service		Restart networking service and obtain a new IP address via DHCP on Ubuntu/Debian Linux
systemctl restart networking.service		Restart networking service and obtain a new IP address via DHCP on a CentOS/RHEL/Fedora Linux
Method #3
nmcli con		Use NetworkManager to obtain info about Linux IP address and interfaces
nmcli con down id ‘enp6s0’		Take down Linux interface enp6s0 and release IP address in Linux
nmcli con up id ‘enp6s0’		Obtian a new IP address for Linux interface enp6s0 and release IP address using DHCP

Please see the following man pages using the man command:
man 8 dhclient
man 1 nmcli

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Comments on this entry are closed.

Resolution provided was short but very effective.
Helped me to resolve the issue.

This worked great, I was having a bit of problems thought worked them out in the end, thought I would share.

I unplugged the network cable, then plugged back in a couple minutes later. Couldn’t access network, tried dhclient -r command and didn’t work. Found that dhcpcd was still running, so done a killall -9 dhcpcd then executed dhclient…worked!

Thanks so much! it’s work ^^

This command worked me..thanks a lot

This does not work on Ubuntu 9.04

It seems that network manager runs it own copy of dhclient.

Works for me on Ubuntu 9.04. It seems to me, that the network-manager does not notice the change. After sudo dhclient -r the command ifconfig eth0 shows no ip address at the interface. After sudo dhclient the command ifconfig eth0 shows an ip address again.

Nope. After sudo dhclient -r the interface still has the IP address. See details: http://pastebin.com/f8d2a4e2

Sure it has, depending on your dhcp server configuration. You can define how long one IP will be served to the same MAC address. Since you did not “change” your MAC address, the dhcp server might give you the same ip.

Dear sir
Please Help Me
How t o Assign IP Address For DHCP Client Linux..

Thank you for the recommendation, it works good.

I set up static DHCP assignments and then needed to switch my server over to DHCP. To do this remotely I simply put the two commands on a single line:

dhclient -r; dhclient

If you’ve set up the static DHCP to use the IP address that you’ve already given it then you shouldn’t become disconnected from your SSH session 🙂

It wasn’t working for me for me with Ubuntu 10.04…[continued if this posts]

Thank for your solution, really help me.

Well, it releases them, but when I renew it, it gives me the same ip back. I wanted a new IP. Is there way to force a NEW one?

You need to tell the dhcp server to issue a different one, typically I dhcp server will if it can issue the same address.

You may be able to tell it to forget what it last issued, or you can usually make a mapping between the mac address of your nic and the ip you want, but generally unless you do something on the server it wil reissue the same address.

Usually a DHCP server maintains a list of the leases by assigned IP and the MAC (unique identifier) of the device who has the lease.

If you don’t have access to the DHCP server in question, then the simplest way to get a new IP would be to change your device’s MAC somehow, as a DHCP server will usually give you your IP back since its still leased to you.

If you have access to the server than you can simply delete the entry, however, if its the first IP unassiged on the list, you’ll get it again unless another device takes it or if you exlude the IP address.

Hope this helps.

With some wireless routers and network modems it is possible to assign a “soft” MAC address that is different from the actual hardware MAC address. This information is usually stored in nonvolatile Flash memory, so it will survive a power-down and restart of the device. The pool of MAC addresses is finite, and there’s a slim chance if you just make one up, you could duplicate another one somewhere on the network. The best way to avoid it is rummage through your junk pile for an old network interface card (NIC) or cast-off computer with built-in NIC and “borrow” its MAC address. Blocks of MAC addresses are assigned by manufacturer and a given manufacturer normally won’t use the same MAC address twice. (However, I ran across a post within the last year by a network administrator who discovered to his horror that ALL the cheap NICs his organization had bought, made by some nameless ChiCom manufacturer, used the same identical MAC address, so confusion reigned on his organization’s intranet and nothing worked! All the NICs had to be scrapped and replaced.)

If you’re having problems getting your Linux distro to get a new IP the easiest thing to do is to just reboot your router (assuming you can do that). If that doesn’t work you can add the old address as a statically assigned DHCP address to a bogus MAC address. This will lock the Linux box out of it’s old address and force it to get the new address.

More detail, about this not working on Ubuntu:
sudo dhclient -r -pf /var/run/dhclient-eth1.pid -lf /var/lib/dhcp3/dhclient-eth1.lease eth1

This kill the running dhcp daemon which is noticed by Network Manager which then immediately downs the interface, so the dhclient can not send the DHCPRELEASE packet.

The solution is to disable NM (right clik on its icon and uncheck the first option “Enable Netwroking” – this is so on Ubuntu 10.10, other version might look a bit different), kill existing dhclient processes, then establish the connection manually, run dhclient eth0 , then run the above dhclient command to release the IP.

For connection to WPA protected WLAN networks follow the description on http://linux.icydog.net/wpa.php

(In windows this is justa matter of running “ipconfig /release wirelless*” 😛 )

Oh, webmaster please fix the typos in my previous post… 🙂
(for network device names, use the same one of course, above I used ince eth0, then eth1)

Fantastic! Worked like a charm on Ubuntu 10.10! Thanx!

Just what i needed!

Hey, is there anyway to do this command at system start?

After typing sudo dhclient -r, I lost my SSH connection…

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