Обновить ip адрес dhcp 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 Renew DHCP IP Address in Ubuntu

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1. Overview

There are several occasions when we need to renew the DHCP-assigned system IP address, for example:

• For resolving connection issues
• For resolving IP address conflict in a place where many computers use the same DHCP server
• For some reason, the administrator sets a short lease time for the assigned DHCP IP

Besides these reasons, when a computer switches to a different network, its IP also changes.

In this tutorial, we’ll discuss renewing the DHCP-assigned IP address on a Ubuntu 22.04 system. Let’s first see an overview of the DHCP protocol.

2. Dynamic Host Configuration Protocol (DHCP)

To identify a computer/host on a network, we use IP addresses. Using these IP addresses, computers communicate with each other on a network. Basically, there are two ways of configuring IP addresses on a client machine:

1. Static IP Allocation: Manually setting IP configuration on each individual client
2. Dynamic Allocation: Automatically assigning IP addresses through a DHCP server

A DHCP server automatically assigns IP addresses to connected hosts on a network. The client cannot interfere with the DHCP settings they receive. However, these settings are transparent to the client side. An IP address set using the above way is not a permanent one, and we refer to it as a DHCP lease. If we do not renew it, the DHCP lease may be allotted to another computer. Therefore, to renew the lease, the client has to periodically check back with the DHCP server.

3. DHCP Clients

Broadly speaking, there are three DHCP clients that are common on Ubuntu:

However, not all versions of Ubuntu support all three clients. This is true for other Linux-based distributions too. For example, some of them come shipped with only one of the three clients, whereas others may come shipped with two or all of the three. However, dhclient is a commonly available DHCP client on Ubuntu systems.

Ideally, every system has a default DHCP client. On Ubuntu 22.04, the dhclient is the default DHCP client. At the time of writing this article, the other clients pump and dhcpcd are not available on Ubuntu 22.04.

Since we are working on Ubuntu 22.04 system, we’ll have to use dhclient as the DHCP client.

4. Process of DHCP Client-Server Interaction

There are several messages that pass between the DHCP server and the client, as described below:

1. DHCPDISCOVER: the client broadcasts a DHCPDISCOVER message asking for an IP address
2. DHCPOFFER: in response to the DHCPDISCOVER message, a DHCP server may transmit a DHCPOFFER message
3. DHCPREQUEST: the client will respond with a DHCPREQUEST message for a DHCPOFFER message from the DHCP server it wants to be configured with. The client broadcasts this message so all DHCP servers on the network can be informed about the selected server
4. DHCPACK: When a client finalizes a DHCP server, it receives a DHCPACK message from this DHCP server. This message also has other required configuration details
5. DHCPRELEASE: The client can send a DHCPRELEASE message to terminate the lease

After the last DHCPACK message, the client can use the leased IP address and other settings till the end of the lease time.

When half the lease time is over, the client can request IP renewal. For this, the client sends a DHCPREQUEST message to the DHCP server. If the current DHCP server says yes to the request, it responds with a DHCPACK message.

However, if the client does not get a response, the client might go on with the current DHCP settings. It can continue using them until the lease time is over.

Once the lease time is over, it is mandatory for the client to again restart with the DHCPDISCOVER procedure.

5. Renewing DHCP IP Using dhclient

Let’s first check if the dhclient service is running on our Ubuntu 22.04 system:

$ ps fax | grep dhclient 4988 pts/0 S+ 0:00 | \_ grep --color=auto dhclient

The ps command shows the information corresponding to different services running on the system.

Let’s now check our current leased DHCP IP address on our target interface wlo1:

$ ip addr | grep wlo1 wlo1: mtu 1500 qdisc noqueue state UP group default qlen 1000 inet 192.168.35.150/24 brd 192.168.35.255 scope global dynamic noprefixroute wlo1

As we can see, the current assigned IP is 192.168.35.150. Let’s release our leased DHCP IP using the -r flag of the dhclient command:

The -r flag releases the current DHCP lease and halts the running DHCP client. Note that releasing an IP address can result in loss of connection, so carefully perform this step on remote systems. Next, we will now request a new DHCP IP from the server:

$ sudo dhclient -v Internet Systems Consortium DHCP Client 4.4.1 Copyright 2004-2018 Internet Systems Consortium. All rights reserved. For info, please visit https://www.isc.org/software/dhcp/ Listening on LPF/wlo1/d0:c5:d3:3d:22:cf Sending on LPF/wlo1/d0:c5:d3:3d:22:cf Listening on LPF/eno1/c8:d9:d2:ee:4f:f9 Sending on LPF/eno1/c8:d9:d2:ee:4f:f9 Sending on Socket/fallback DHCPDISCOVER on wlo1 to 255.255.255.255 port 67 interval 3 (xid=0xaee34c7f) DHCPDISCOVER on eno1 to 255.255.255.255 port 67 interval 3 (xid=0xebcb010d) DHCPOFFER of 192.168.35.151 from 192.168.35.47 DHCPREQUEST for 192.168.35.151 on wlo1 to 255.255.255.255 port 67 (xid=0x7f4ce3ae) DHCPACK of 192.168.35.151 from 192.168.35.47 (xid=0xaee34c7f) bound to 192.168.35.151 -- renewal in 1452 seconds.

Let’s again check back our IP on the wlo1 interface:

$ ip addr | grep wlo1 wlo1: mtu 1500 qdisc noqueue state UP group default qlen 1000 inet 192.168.35.151/24 brd 192.168.35.255 scope global dynamic wlo1

As we can see, the IP address has now changed to 192.168.35.151.

The syslog file stores the logs generated by the DHCP server. We can search through these logs for DHCP logs:

$ cat /var/log/syslog | grep dhcp Oct 30 05:10:01 LHB NetworkManager[840]: [1667086801.8104] dhcp4 (wlo1): activation: beginning transaction (timeout in 45 seconds) Oct 30 05:10:01 LHB NetworkManager[840]: [1667086801.8418] dhcp4 (wlo1): state changed new lease, address=192.168.35.150 Oct 30 05:40:01 LHB NetworkManager[840]: [1667088601.7480] dhcp4 (wlo1): state changed new lease, address=192.168.35.150 

These logs are very helpful while troubleshooting DHCP issues.

6. Conclusion

In this article, we have learned how we can acquire a new IP address on the Ubuntu system. Specifically, we have used the dhclient utility to renew our IP configuration.

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Как заставить Linux повторно получить новый IP-адрес с сервера DHCP?

Средой является Debian , хотя ответ будет распространяться на все дистрибутивы.

Вы также можете использовать эту команду:

Где interface находится устройство, для которого вы хотите получить новый адрес.

В -r флаге сила dhclient в первый выпуск любой аренды у вас есть, вы можете использовать эту команду , чтобы запросить новый:

 -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. 

После этой команды моему Debian не удалось получить новый IP, но после перезагрузки мне был назначен новый, так что он работает!

Любое из следующего должно заставить это возобновить.

/etc/init.d/networking restart 

Я бы не рекомендовал работать по SSH-соединению, хотя вам, вероятно, не удастся использовать первое, если оно не вернется с новым IP-адресом.

Соединения SSH будут терпеть несколько секунд «разъединения», если вы восстановите соединение через несколько секунд с того же IP-адреса. В этих условиях я никогда не терял соединение /etc/init.d/networking restart , даже когда на восстановление ушло более 5 секунд .

Я бы прокомментировал p.campbell, но у меня только 1 репутация, и поэтому я не могу, сначала я проверяю установленные интерфейсы:

освободить IP с выбранного интерфейса (например, eth0, eth1, enp1s0, sit0, wlan0, . ):

sudo dhclient -r *interface* 

запросить новый IP с сервера DHCP (предупреждение об ошибке):

sudo dhclient -1 *interface* 

Ubuntu 16.04 LTS подтвердил

Если вы используете dhcpcd инструмент, то:

dhcpcd -k interface dhcpcd interface 

Первый говорит, чтобы остановить демона, а второй говорит, чтобы запустить его снова.

Если MAC-адрес интерфейса не изменился, DHCP-сервер может назначить ему тот же адрес при обновлении. Поэтому простой выпуск и обновление с помощью dhclient может не получить новый адрес. Измените MAC-адрес и получите новый IP-адрес с помощью следующих команд. Не забудьте записать оригинал, если вам нужно вернуться к нему позже.

ifconfig down ifconfig hw ether ifconfig up 

Это было так полезно, спасибо! Ни один номер dhclient -r или не ifconfig down дал мне новый IP-адрес в моей рабочей сети, пока я не изменил MAC-адрес на случайный, используя эту команду.

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