Как настроить dhcp на линуксе

The Dynamic Host Configuration Protocol (DHCP) enables client systems to obtain network configuration information from a DHCP server each time they connect to the network. The DHCP server is configured with a range of IP addresses and other network configuration parameters that clients need.

When you configure an Oracle Linux system as a DHCP client, the client daemon, dhclient , contacts the DHCP server to obtain the networking parameters. As DHCP is broadcast-based, the client must be on the same subnet as either a server or a relay agent. If a client cannot be on the same subnet as the server, a DHCP relay agent can be used to pass DHCP messages between subnets.

The server provides a lease for the IP address that it assigns to a client. The client can request specific terms for the lease, such as the duration. You can configure a DHCP server to limit the terms that it can grant for a lease. Provided that a client remains connected to the network, dhclient automatically renews the lease before it expires. You can configure the DHCP server to provide the same IP address to a client, based on the MAC address of its network interface.

The advantages of using DHCP include the following:

Centralized management of IP addresses
Ease of adding new clients to a network
Reuse of IP addresses reducing the total number of IP addresses that are required
simple reconfiguration of the IP address space on the DHCP server without needing to reconfigure each client

For more information about DHCP, see RFC 2131. Likewise, refer to the following manual pages:

Setting Up the Server’s Network Interfaces

By default, the dhcpd service processes requests on those network interfaces that connect them to subnets that are defined in the DHCP configuration file.

Suppose that a DHCP server has mutliple interfaces. Through its interface enp0s1 , the server is connected to the same subnet as the clients that the server is configured to serve. In this case, enp0s1 must be set in the DHCP service to enable the server to monitor and process incoming requests on that interface.

Before proceeding to either of the following procedures, ensure that you meet the following requirements:

You have the proper administrative privileges to configure DHCP.
You have installed the dhcp-server package. If not, install the package with the following command:

sudo dnf install dhcp-server

Configure the network interfaces as follows:

sudo cp /usr/lib/systemd/system/dhcpd.service /etc/systemd/system/

ExecStart=/usr/sbin/dhcpd -f -cf /etc/dhcp/dhcpd.conf -user dhcpd -group dhcpd --no-pid $DHCPDARGS int1-name int2-name

sudo systemctl daemon-reload

sudo systemctl restart dhcpd.service

sudo systemctl restart dhcpd

sudo cp /usr/lib/systemd/system/dhcpd6.service /etc/systemd/system/

ExecStart=/usr/sbin/dhcpd -f -cf /etc/dhcp/dhcpd6.conf -user dhcpd -group dhcpd --no-pid $DHCPDARGS int1-name int2-name

sudo systemctl daemon-reload

sudo systemctl restart dhcpd6.service

sudo systemctl restart dhcpd6

Understanding DHCP Declarations

The way the DHCP provides services to its clients is defined through parameters and declarations in the /etc/dhcp/dhcpd.conf file for IPv4 networks and /etc/dhcp/dhcpd6.conf file for IPv6 networks. The file would contain details such as client networks, address leases, IP address pools, and so on.

cp /usr/share/doc/dhcp-server/dhcpd.conf.example /etc/dhcp/dhcpd.conf

cp /usr/share/doc/dhcp-server/dhcpd6.conf.example /etc/dhcp/dhcpd6.conf

Then when you open either file, examples and explanations are available for your reference.

The information in the configuration file consists of a combination of the following declarations:

Global parameters define settings that apply to all networks that are supported or serviced by the DHCP server.

Consider the following settings that would globally apply through out the entire network:

Domain name of the company network: example.com .d
Network’s DNS servers: dn1.example.com and dns2.example.com
Lease time assigned to all clients: 12 hours (43200 seconds)
Maximum lease time that can be assigned: 24 hours (86400 seconds)

In this case, you would configure the global settings in the configuration file as follows:

option domain-name "example.com"; default-lease-time 43200; max-lease-time 86400; authoritative;

The authoritative parameter identifies the server as an official or primary server for DHCP services. The parameter is typically used in a setup that has multiple DHCP servers. Servers with the authoritative parameter have priority to process requests over servers without the parameter.

A subnet declaration provides details about a subnet to which the DHCP server is directly connected and where the systems in that subnet are also being served as clients.

Consider the following configuration of a DHCP server:

The server’s enp0s1 interface is directly connected to the 192.0.2.0/24 network.
The systems in the 192.0.2.0/24 network are DHCP clients.
The topology of this client subnet is as follows:
- Subnet’s DNS server: 192.0.2.1.
- Subnet gateway: 192.0.2.1.
- Broadcast address: 192.0.2.255.
- Address range for clients: 192.0.2.10 through 192.0.2.100.
- Maximum lease time for each client: 86,400 seconds (1 day).
subnet 192.0.2.0 netmask 255.255.255.0

On an IPv6 network environment, a subnet declaration in the dhcpd6.conf file would resemble the following example:

You define a shared-network declaration if the DHCP server needs to provide servers to clients in other subnets that are not directly connected to the server.

Consider the following example, which expands but slightly differs from the scenario in the preceding section:
- The DHCP server belongs to the 192.0.2.0/24 network but does not provide services to the systems in this network.
- The server processes requests from clients in the following remote subnets:
  - 192.168.5.0/24.
  - 198.51.100.0/24.
  shared-network example < option domain-name-servers 192.168.2.1; . subnet 192.168.5.0 netmask 255.255.255.0 < range 192.168.5.10 192.168.5.100; option routers 192.168.5.1; >subnet 198.51.100.0 netmask 255.255.255.0 < range 198.51.100.10 198.51.100.100; option routers 198.51.100.1; >. > subnet 192.0.2.0 netmask 255.255.255.0
  
  In the preceding example, the final subnet declaration refers to the server’s own network and is outside the shared-network scope. The declaration is called an empty declaration because it simply defines the server’s subnet. Because the server does not provide services to this subnet, no additional entries are added, such as lease, address range, DNS information, and so on. Though empty, the declaration is required, otherwise, the dhcpd service does not start.
  
  On an IPv6 network environment, a shared-network declaration in the dhcpd6.conf file would resemble the following example:
  
  shared-network example < option domain-name-servers 2001:db8:0:1::1:1 . subnet6 2001:db8:0:1::1:0/120 < range6 2001:db8:0:1::1:20 2001:db8:0:1::1:100 >subnet6 2001:db8:0:1::2:0/120 < range6 2001:db8:0:1::2:20 2001:db8:0:1::2:100 >. > subnet6 2001:db8:0:1::50:0/120
  
  You define a host declaration if a client needs to have a static IP address.
  
  Important: A client’s fixed IP address must be outside the pool of dynamic IP addresses distributed to other clients. Otherwise, address conflicts might occur.
  
  Systems are identified by the hardware ethernet address, and not the name in the host declaration. Thus, the host name might change, but the client continues to receive services through the ethernet address.
  
  On an IPv6 network environment, a host declaration in the dhcpd6.conf file would resemble the following example:
  
  You define a group declaration to apply the same parameters to multiple shared networks, subnets, and hosts all at the same time.
  - The DHCP server belongs to and serves the subnet 192.0.2.0/24.
  - One client requires a fixed address, while the rest of the clients use dynamic IP addresses from the server.
  - All of the clients use the same DNS server.
  In this case, you would enter the following declaration in dhcp.conf :
```
group < option domain-name-servers 192.0.2.1; host server1.example.com < hardware ethernet 52:54:00:72:2f:6e; fixed-address 192.0.2.130; >subnet 192.0.2.0 netmask 255.255.255.0 < range 192.0.2.10 192.0.2.100; option routers 192.0.2.1; option broadcast-address 192.0.2.255; max-lease-time 86400; >>
```
  On an IPv6 network environment, a group declaration in the dhcpd6.conf file would resemble the following example:
  
  group < option dhcp6.domain-search "example.com"; host server1.example.com < hardware ethernet 52:54:00:72:2f:6e; fixed-address 2001:db8:0:1::200; >host server2.example.com < hardware ethernet 52:54:00:1b:f3:cf; fixed-address 2001:db8:0:1::ba3; >> subnet6 2001:db8:0:1::/64
  
  Activating the DHCP Services
  
  All of the DHCP services are defined in the server’s /etc/dhcp/dhcpd.conf or /etc/dhcp/dhcpd6.conf file. To configure and then activate the configured services, follow these steps:
  - For IPv4 networks:
  - Open the /etc/dhcp/dhcpd.conf file.
  - Add parameters and declarations to the file. For guidance, refer to Understanding DHCP Declarations or to the comments and notes in the /usr/share/doc/dhcp-server/dhcpd.conf.example template.
  - Optionally, set the dhcpd service to start automatically in case of a server reboot.
```
sudo systemctl enable dhcpd
```
```
sudo systemctl start dhcpd
```
```
sudo systemctl enable dhcpd6
```
```
sudo systemctl start dhcpd6
```
  Recovering From a Corrupted Lease Database
  1. The service renames the existing lease files:
    - /var/lib/dhcpd/dhcpd.leases is renamed to /var/lib/dhcpd/dhcpd.leases~
    - /var/lib/dhcpd/dhcpd6.leases is renamed to /var/lib/dhcpd/dhcpd6.leases~
  2. The service re-creates brand new dhcpd.leases and dhcpd6.leases files.
  In the event that a lease database file is corrupted, you need to restore the lease database from the last known backup of the database.
  
  Typically, the most recent backup of a lease database is the filename .leases~ file.
  
  A backup instance is a snapshot taken at a particular point in time, and therefore might not totally reflect the latest state of the system.
```
sudo systemctl stop dhcpd
```
```
sudo mv /var/lib/dhcpd/dhcpd.leases /var/lib/dhcpd/dhcpd.leases.corrupt
```
```
sudo cp -p /var/lib/dhcpd/dhcpd.leases~ /var/lib/dhcpd/dhcpd.leases
```
```
sudo systemctl start dhcpd
```
```
sudo systemctl stop dhcpd6
```
```
sudo mv /var/lib/dhcpd/dhcpd6.leases /var/lib/dhcpd/dhcpd6.leases.corrupt
```
```
sudo cp -p /var/lib/dhcpd/dhcpd6.leases~ /var/lib/dhcpd/dhcpd6.leases
```
```
sudo systemctl start dhcpd6
```
  Источник
  
  Установка и базовая настройка DHCP сервера на Ubuntu
  
  Опубликовано: 12.03.2023
  
  Установка и настройка
  
  Сервер DHCP в Ubuntu может быть реализован с помощью пакета isc-dhcp-server. Его можно установить из стандартных репозиториев системы. Выполняем обновления кэша пакетов и установку:
  
  subnet 192.168.0.0 netmask 255.255.255.0 <
  range 192.168.0.100 192.168.0.200;
  option domain-name-servers 192.168.0.10, 192.168.0.11;
  option domain-name «dmosk.local»;
  option routers 192.168.0.1;
  option broadcast-address 192.168.0.255;
  default-lease-time 600;
  max-lease-time 7200;
  >
  - subnet — сеть, для которой будет работать данная группа настроек.
  - range — диапазон, из которого будут браться IP-адреса.
  - option domain-name-servers — через запятую перечисленные DNS-сервера.
  - option domain-name — суффикс доменного имени.
  - option routers — шлюз по умолчанию.
  - option broadcast-address — адрес сети для широковещательных запросов.
  - default-lease-time и max-lease-time — время и максимальное время в секундах, на которое клиент получит адрес, по его истечению будет выполнено продление срока.
  Проверить корректность конфигурационного файла можно командой:
  
  dhcpd -t -cf /etc/dhcp/dhcpd.conf
  
  Разрешаем автозапуск сервиса:
  
  systemctl enable isc-dhcp-server
  
  systemctl restart isc-dhcp-server
  
  Добавляем правило в firewall:
  
  iptables -I INPUT -p udp —dport 67 -j ACCEPT
  
  Возможные проблемы
  
  Not configured to listen on any interfaces!
  
  Сервис dhcp не запускается, а в логе можно увидеть ошибки, на подобие:
  
  No subnet declaration for ens18 (192.168.1.10).
  .
  ** Ignoring requests on ens18. If this is not what
  you want, please write a subnet declaration
  in your dhcpd.conf file for the network segment
  to which interface ens18 is attached. **
  .
  Not configured to listen on any interfaces
  
  Причина: в конфигурационном файле описана подсеть, которая не настроена ни на одном из сетевых интерфейсов сервера.
  
  Решение: конфигурация subnet должна включать только те подсети, в которых настроен сам сервер DHCP. Посмотреть сетевые настройки можно командой:
  
  После чего необходимо проверить настройки в конфигурационном файле сервера dhcp.
  
  Читайте также
  
  Другие инструкции, связанные с DHCP:
  
  Источник
  
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Как настроить dhcp на линуксе