Обновить dhcp через командную строку linux

Force DHCP Client to Renew IP Address – How to do

Wondering how to force DHCP Client to renew IP address? We can help you.

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.

Here at Bobcares, we handle DHCP Client on servers of our customers as a part of our Server Management Services.

Today let’s see to force DHCP clients to renew IP addresses along with the steps that our Support Techs follow to do this.

Linux force DHCP client to release IP address and renew the IP address

The following are two methods to force DHCP client to release IP address:

Using the below command to force Linux to renew IP address using a DHCP for eth0 interface

For Ubuntu/Debian servers we can use the following command to restart the networking service and obtain an IP address via DHCP:

systemctl restart network.service

For CentOS/RHEL/Fedora we can use the following command to restart the networking service and obtain an IP address via DHCP:

systemctl restart networking.service

Use the following NetworkManager command to obtain info about Linux IP address and interfaces

To take down Linux interface enp6s0 and release IP address in Linux we can use the following command:

For obtaining a new IP address for Linux interface enp6s0 and release IP address using DHCP we can use the following command:

Some other basic commands to release IP address or renew the IP address

1. In Windows, we can release IP address using the command:

2. In Windows, we can renew IP address using the command:

3. To Linux renew IP command using DHCP

4. To obtain a fresh IP address using DHCP on Linux:

How to 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

Other options in Linux to renew DHCP

We can also execute the following commands:

# ifdown eth0 # ifup eth0 ### RHEL/CentOS/Fedora specific command ### # /etc/init.d/network restart

### Debian / Ubuntu Linux specific command ### # /etc/init.d/networking restart

nmcli command (NetworkManager) to renew IP address in Linux

The nmcli is a command-line tool for controlling NetworkManager and getting its status. To renew IP address using nmcli for connection named ‘bobcares_5G’:

nmcli con // to get list of all connections nmcli con down id 'bobcares_5G' nmcli con up id 'bobcares_5G'

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

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:

OR use the systemctl command to restart network service on a CentOS 7:

systemctl restart network.service systemctl status network.service

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Conclusion

In short, we saw the steps that our Support Engineers follow to force DHCP client to renew IP address

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How to Release old IP or force Renew DHCP lease IP in Linux

In this article, we will see how to release old IP or force renew DHCP lease IP in Linux. Dynamically allocated IP is an IP leased out from a DHCP server for a particular amount of time over a network. If you are using a lease IP then you always have to renew your ip before the lease period expires. Usually, when a lease reaches 50% of its validity time period, the DHCP client unicasts a DHCP Request message to the DHCP server to request lease renewal. The same can be forced manually through a dhcp client tool in Linux Systems.

What is DHCP

The Dynamic Host Configuration Protocol is a network management protocol used on Internet Protocol networks for automatically assigning IP addresses and other communication parameters to devices connected to the network using a client–server architecture.

How to Release old IP or force Renew DHCP Lease IP in Linux

If you have any DHCP lease IP allocated to an interface which you want to release due to certain reasons then you always have the option to release it by using DHCP client tool called dhclient. For example, in my case I have two interfaces attached to my system — enp0s3 and enp0s8 .

To release the IP attached to both the above interfaces, we need to use sudo dhclient -r -v command. But sometimes what happens is that the IP does not get released as you can see below.

Please don’t ever try to release the IP over SSH connection as it will disconnect you from the host system and you may not be able to reconnect unless a new IP is assigned which is known to you.

If we try to check again the IP got released or not using ip addr sh command then we can notice that nothing happened. The assigned IP did not got released.

If you try to release IP from specific interface by choosing the interface using sudo dhclient -r -v enp0s3 command then also you can notice that the IP did not got released. This is a very common scenario faced by lot of folks hence I purposefully chosen this one while trying to explain how to release the DHCP lease IP so that you will understand what needs to be done here.

Sometimes even if you try to kill the dhclient process using kill -9 command then you will notice that it is getting started every time you kill this process and the IP is not getting changed. To check the process ID of currently running dhclient program, you can use ps -ef | grep dhclient command as shown below.

In my case process ID(PID) is 2167 , so to kill this one we are using kill -9 2167 command as shown below.

Now if I try to check the process again, I can see that a new dhclient command is launched with process ID(PID) 2169 . So now you must be wondering how did this happen.

Well, this happened because the network interfaces are currently controlled by Network Manager. You can verify this on Debian/Ubuntu systems by checking one of the below netplan configuration file depending on Ubuntu installation you are currently having.

Please note if you are using RHEL/CentOS based systems, then you need to check /etc/sysconfig/network-scripts/ifcfg- configuration file.

You can check the running status of Network Manager by using sudo systemctl status NetworkManager command as shown below.

If you are also facing problems like above, then you can try below solutions that works for you.

Solution 1: Restart Network Manager

First solution you can try is that you can restart Network Manager by running sudo systemctl restart NetworkManager command as shown below.

Solution 2: Using nmcli command

If the first solution does not work then next you can try restarting your connection using nmcli command. Here you need to first check the current connection using nmcli con command as shown below.

Then first take down the connection for the interface where you need to renew the IP. In our case, let’s say we want to renew the IP of enp0s3 interface, then we need to run nmcli con down id «Wired Connection 1» command after releasing the IP as shown below.

Then bring up the connection using nmcli con up id «Wired connection 1» command as shown below.

Solution 3: Restart systemd-networkd

If second solution also did not worked out, then you can try restarting systemd-networkd once by using sudo systemctl restart systemd-networkd command as shown below.

After applying above solution, you can see that you will be able to release the IP as shown below.

You can verify the same by checking the status of ip assigned to network interfaces using ip addr sh command as shown below. From the output, you can see that none of the IP is assigned to both the interfaces — enp0s3 and enp0s8 . This confirms that IP release was successful.

Once the IP is released, you will be able to get the new ip by running sudo dhclient -v command as shown below.

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

Where interface is the device you want to get a new address for.

The -r flag forces dhclient to first release any leases you have, you can then use this command to request a new lease:

 -r Tell dhclient to release the current lease it has from the server. This is not required by the DHCP protocol, but some ISPs require their clients to notify the server if they wish to release an assigned IP address. 

After this command my Debian failed to get a new IP, but after a reboot I was assigned to a new one, so it work’s!

@EricNord — You can still do this via SSH. Use the ‘at’ command to schedule a one-time command, 1 minute in the future or less, disconnect your SSH session then reconnect. You could even automate it, and reconnect to the same host via Multicast DNS (assuming you have that running, most home networks do)

Either of the following should get it to renew.

/etc/init.d/networking restart 

I wouldn’t recommend running either over an SSH connection, although you’ll probably get away with the first one if it doesn’t come back with a new ip address.

SSH connections will tolerate a few seconds of «disconnect» provided that you get the connection back up in a few seconds at the same IP address. Under these conditions, I have never lost a connection during /etc/init.d/networking restart , even when it was taking more than 5 seconds to come back up.

Worst case you reconnect. If you’re trying to automate this, see mDNS and ‘at’ command, and use a wrapper script to orchestrate the change and the reconnect.

Would comment p.campbell, but I have only 1 reputation and therefore cannot, first I review installed interfaces:

release IP from selected interface (e.g. eth0, eth1, enp1s0, sit0, wlan0. ):

sudo dhclient -r *interface* 

request new IP from DHCP server (alert on error):

sudo dhclient -1 *interface* 

Ubuntu 16.04 LTS confirmed

If you’re using the dhcpcd tool then:

dhcpcd -k interface dhcpcd -n interface 

The first says to release and deconfigure the interface, and the second says to reload configuration and rebind the interface again.

If the MAC address of the interface isn’t changed, the DHCP server may assign it the same address when renewing. Therefore, a simple release and renew with dhclient may not acquire a new address. Change the MAC address and acquire a new IP address with the following commands. Don’t forget to write the original down if you need to revert back to it at a later time.

ifconfig down ifconfig hw ether ifconfig up 

This was so helpful, thank you! No number of dhclient -r or ifconfig down gave me a new IP address on my work network, until I changed the MAC address to something random using that command.

In systems where NetworkManager is on, running.

. gets you a connection list, and running.

nmcli con down id 'Connection Name' nmcli con up id 'Connection Name' 

takes the connection down and back up

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