Linux from which dhcp

Настройка DHCP-сервера в Linux

Протокол динамической конфигурации узлов (Dynamic Host Configuration Protocol, DHCP) — это сетевой протокол, используемый для автоматического получения узлами IP-адресов и сетевой конфигурации с сервера.

IP-адрес, выделенный DHCP-клиенту DHCP-сервером, находится в «аренде», время аренды может отличаться в зависимости от того, сколько времени клиенту требуется соединение или конфигурация DHCP.

Как работает DHCP?

• После загрузки подключенный к сети клиент (на котором настроено использование DHCP) отправляет DHCP-серверу пакет DHCPDISCOVER.
• Получив пакет запроса DHCPDISCOVER, DHCP-сервер отвечает пакетом DHCPOFFER.
• Клиент принимает пакет DHCPOFFER и отправляет серверу пакет DHCPREQUEST, подтверждая готовность принять сетевую конфигурацию, предоставленную в пакете.
• Получив от клиента пакет DHCPREQUEST, сервер отправляет пакет DHCPACK, который разрешает клиенту использование выделенного IP-адреса.

В данном руководстве мы рассмотрим настройку DHCP-сервера в Ubuntu/Debian Linux, но большенство настроек будет работать и в других дистрибутивах. Для получения административных привилегий все команды будут выполняться через sudo.

Шаг 1: Установка DHCP-сервера

1. Для установки пакета DCHP-сервера, ранее известного как dhcp3-server, нужно выполнить следующую команду:

2. После завершения установки отредактируйте файл /etc/default/isc-dhcp-server для определения интерфейсов, которые будет использовать DHCPD для обработки DHCP-запросов, при помощи опции INTERFACES.

Например, если вам нужно, чтобы демон DHCPD прослушивал eth0, задайте следующее значение:

Для этого сетевого интерфейса нужно настроить статический IP-адрес.

Шаг 2: Настройка DHCP-сервера

3. Основной файл конфигурации DHCP — /etc/dhcp/dhcpd.conf, в него нужно внести всю информацию, отправляемую клиентам.
В файле конфигурации DHCP есть два типа данных:

• параметры – указывают, как выполнять задание (например, на какое время выделять адрес), выполнять ли его вообще (например, выделять ли адреса неизвестным клиентам) или какие параметры сетевой конфигурации отправлять DHCP-клиенту.
• декларации – определяют топологию сети, описывают клиентов и предоставляемые им адреса, или применяют группу параметров к группе деклараций.

4. Для определения настроек DHCP-сервера откроем и отредактируем файл конфигурации:

Установите в начале файла следующие глобальные параметры, они будут действовать для всех указанных ниже деклараций (измените их в соответствии с вашими задачами). Это имя домена domain-name, имена DNS-серверов domain-name-servers, время аренды по умолчанию в секундах default-lease-time (если клиент не запросил его сам), максимальное время аренды в секундах max-lease-time и параметр authoritative, означающий «авторитетность» сервера в сегменте сети. Данный параметр нужен на тот случай, если клиент запросит неправильный IP-адрес — в этом случае сервер ответит ему отказом и предложит получить новый адрес.

Обратите внимание, что перед некоторыми параметрами указано слово option, а перед некоторыми — нет. Это слово задает параметры, которые передаются клиенту в сетевой конфигурации.

5. Теперь нужно указать подсеть, в нашем примере мы будем использовать DHCP в локальной сети 192.168.10.0/24.

Здесь мы использовали следующие параметры:

routers — IP-адрес маршрутизатора по умолчанию
subnet-mask — маска подсети
domain-search — имя домена
domain-name-servers — имена DNS-серверов
range — диапазон IP-адресов, выделяемый клиентам (можно указывать несколько диапазонов, но обязательно указать хотя бы один)

Шаг 3: Настройка статических IP-адресов для машин клиентов

6. Для выделения конкретным клиентам фиксированного (статического) IP-адреса нужно добавить в файл конфигурации секции следующего вида, где явно указаны MAC-адрес и статически выделяемый IP-адрес:

7. Запустим службу DHCP и установим ее автоматический запуск при загрузке:

8. Далее нужно создать правило для службы DHCP в брандмауэре (Демон DHCPD прослушивает UDP-порт 67):

Шаг 4: Настройка машин клиентов

9. Теперь можно настроить клиентские компьютеры в сети для автоматического получения IP-адресов от DHCP-сервера.

Отредактируйте файл конфигурации интерфейса Ethernet на клиентской машине (обратите внимание на имя/номер интерфейса):

Укажите следующие опции:

Сохраните и закройте файл, а затем перезапустите сетевые службы или перезагрузите систему:

Если все настроено правильно, клиентская машина должна автоматически получать IP-адреса от DHCP-сервера.

Настройка DHCP-сервера завершена. Более подробную информацию о dhcpd и dhcpd.conf с описанием всех возможных опций можно получить в соответствующих man-страницах:

Если вы нашли ошибку, пожалуйста, выделите фрагмент текста и нажмите Ctrl+Enter.

Источник

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.

Источник

Linux from which dhcp

The purpose of the Dynamic Host Configuration Protocol (DHCP) is to assign network settings centrally (from a server) rather than configuring them locally on every workstation. A host configured to use DHCP does not have control over its own static address. It is enabled to configure itself completely and automatically according to directions from the server. If you use the NetworkManager on the client side, you do not need to configure the client. This is useful if you have changing environments and only one interface active at a time. Never use NetworkManager on a machine that runs a DHCP server.

Tip: IBMВ Z: DHCP Support

On IBMВ Z platforms, DHCP only works on interfaces using the OSA and OSA Express network cards. These cards are the only ones with a MAC, which is required for the DHCP autoconfiguration features.

One way to configure a DHCP server is to identify each client using the hardware address of its network card (which should usually be fixed), then supply that client with identical settings each time it connects to the server. DHCP can also be configured to assign addresses to each relevant client dynamically from an address pool set up for this purpose. In the latter case, the DHCP server tries to assign the same address to the client each time it receives a request, even over extended periods. This works only if the network does not have more clients than addresses.

DHCP makes life easier for system administrators. Any changes, even bigger ones, related to addresses and the network configuration in general can be implemented centrally by editing the server’s configuration file. This is much more convenient than reconfiguring numerous workstations. It is also much easier to integrate machines, particularly new machines, into the network, because they can be given an IP address from the pool. Retrieving the appropriate network settings from a DHCP server is especially useful in case of laptops regularly used in different networks.

In this chapter, the DHCP server will run in the same subnet as the workstations, 192.168.2.0/24 with 192.168.2.1 as gateway. It has the fixed IP address 192.168.2.254 and serves two address ranges, 192.168.2.10 to 192.168.2.20 and 192.168.2.100 to 192.168.2.200 .

A DHCP server supplies not only the IP address and the netmask, but also the host name, domain name, gateway, and name server addresses for the client to use. In addition to that, DHCP allows several parameters to be configured in a centralized way, for example, a time server from which clients may poll the current time or even a print server.

33.1 Configuring a DHCP Server with YaST #Edit source

To install a DHCP server, start YaST and select Software  › Software Management . Choose Filter  › Patterns and select DHCP and DNS Server . Confirm the installation of the dependent packages to finish the installation process.

Important: LDAP Support

The YaST DHCP module can be set up to store the server configuration locally (on the host that runs the DHCP server) or to have its configuration data managed by an LDAP server. To use LDAP, set up your LDAP environment before configuring the DHCP server.

The YaST DHCP module ( yast2-dhcp-server ) allows you to set up your own DHCP server for the local network. The module can run in wizard mode or expert configuration mode.

33.1.1 Initial Configuration (Wizard) #Edit source

When the module is started for the first time, a wizard starts, prompting you to make a few basic decisions concerning server administration. Completing this initial setup produces a very basic server configuration that should function in its essential aspects. The expert mode can be used to deal with more advanced configuration tasks. Proceed as follows:

Select the interface from the list to which the DHCP server should listen and click Select and then Next . See Figure 33.1, “DHCP Server: Card Selection”.

Note: DHCP and firewalld

Please note that the option Open Firewall for Selected Interfaces does not (yet) support firewalld in SUSE Linux Enterprise Server 15 SP1 . To manually open the DHCP port, run

FigureВ 33.1: DHCP Server: Card Selection #

Use the check box to determine whether your DHCP settings should be automatically stored by an LDAP server. In the text boxes, provide the network specifics for all clients the DHCP server should manage. These specifics are the domain name, address of a time server, addresses of the primary and secondary name server, addresses of a print and a WINS server (for a mixed network with both Windows and Linux clients), gateway address, and lease time. See Figure 33.2, “DHCP Server: Global Settings”.

FigureВ 33.2: DHCP Server: Global Settings #

Configure how dynamic IP addresses should be assigned to clients. To do so, specify an IP range from which the server can assign addresses to DHCP clients. All these addresses must be covered by the same netmask. Also specify the lease time during which a client may keep its IP address without needing to request an extension of the lease. Optionally, specify the maximum lease time—the period during which the server reserves an IP address for a particular client. See Figure 33.3, “DHCP Server: Dynamic DHCP”.

FigureВ 33.3: DHCP Server: Dynamic DHCP #

Define how the DHCP server should be started. Specify whether to start the DHCP server automatically when the system is booted or manually when needed (for example, for testing purposes). Click Finish to complete the configuration of the server. See Figure 33.4, “DHCP Server: Start-Up”.

FigureВ 33.4: DHCP Server: Start-Up #

Instead of using dynamic DHCP in the way described in the preceding steps, you can also configure the server to assign addresses in quasi-static fashion. Use the text boxes provided in the lower part to specify a list of the clients to manage in this way. Specifically, provide the Name and the IP Address to give to such a client, the Hardware Address , and the Network Type (token ring or Ethernet). Modify the list of clients, which is shown in the upper part with Add , Edit , and Delete from List . See Figure 33.5, “DHCP Server: Host Management”.

FigureВ 33.5: DHCP Server: Host Management #

33.1.2 DHCP Server Configuration (Expert) #Edit source

In addition to the configuration method discussed earlier, there is also an expert configuration mode that allows you to change the DHCP server setup in every detail. Start the expert configuration by clicking DHCP Server Expert Configuration in the Start-Up dialog (see Figure 33.4, “DHCP Server: Start-Up”).

In this first dialog, make the existing configuration editable by selecting Start DHCP Server . An important feature of the behavior of the DHCP server is its ability to run in a chroot environment, or chroot jail, to secure the server host. If the DHCP server should ever be compromised by an outside attack, the attacker will still be in the chroot jail, which prevents them from accessing the rest of the system. The lower part of the dialog displays a tree view with the declarations that have already been defined. Modify these with Add , Delete , and Edit . Selecting Advanced takes you to additional expert dialogs. See Figure 33.6, “DHCP Server: Chroot Jail and Declarations”. After selecting Add , define the type of declaration to add. With Advanced , view the log file of the server, configure TSIG key management, and adjust the configuration of the firewall according to the setup of the DHCP server.

FigureВ 33.6: DHCP Server: Chroot Jail and Declarations #

The Global Options of the DHCP server are made up of several declarations. This dialog lets you set the declaration types Subnet , Host , Shared Network , Group , Pool of Addresses , and Class . This example shows the selection of a new subnet (see Figure 33.7, “DHCP Server: Selecting a Declaration Type”).

FigureВ 33.7: DHCP Server: Selecting a Declaration Type #

This dialog allows you specify a new subnet with its IP address and netmask. In the middle part of the dialog, modify the DHCP server start options for the selected subnet using Add , Edit , and Delete . To set up dynamic DNS for the subnet, select Dynamic DNS .

FigureВ 33.8: DHCP Server: Configuring Subnets #

If you chose to configure dynamic DNS in the previous dialog, you can now configure the key management for a secure zone transfer. Selecting OK takes you to another dialog in which to configure the interface for dynamic DNS (see Figure 33.10, “DHCP Server: Interface Configuration for Dynamic DNS”).

FigureВ 33.9: DHCP Server: TSIG Configuration #

You can now activate dynamic DNS for the subnet by selecting Enable Dynamic DNS for This Subnet . After doing so, use the drop-down box to activate the TSIG keys for forward and reverse zones, making sure that the keys are the same for the DNS and the DHCP server. With Update Global Dynamic DNS Settings , enable the automatic update and adjustment of the global DHCP server settings according to the dynamic DNS environment. Finally, define which forward and reverse zones should be updated per dynamic DNS, specifying the name of the primary name server for each of the two zones. Selecting OK returns to the subnet configuration dialog (see Figure 33.8, “DHCP Server: Configuring Subnets”). Selecting OK again returns to the original expert configuration dialog.

FigureВ 33.10: DHCP Server: Interface Configuration for Dynamic DNS #

To define the interfaces the DHCP server should listen to and to adjust the firewall configuration, select Advanced  › Interface Configuration from the expert configuration dialog. From the list of interfaces displayed, select one or more that should be attended by the DHCP server. If clients in all subnets need to be able to communicate with the server and the server host also runs a firewall, adjust the firewall accordingly.

Note: DHCP and firewalld

Please note that the option Open Firewall for Selected Interfaces does not (yet) support firewalld in SUSE Linux Enterprise Server 15 SP1 . To manually open the DHCP port, run

FigureВ 33.11: DHCP Server: Network Interface and Firewall #

After completing all configuration steps, close the dialog with OK . The server is now started with its new configuration.

33.2 DHCP Software Packages #Edit source

Both the DHCP server and the DHCP clients are available for SUSE Linux Enterprise Server . The DHCP server available is dhcpd (published by the Internet Systems Consortium). On the client side, there is dhcp-client (also from ISC) and tools coming with the wicked package.

By default, the wicked tools are installed with the services wickedd-dhcp4 and wickedd-dhcp6 . Both are launched automatically on each system boot to watch for a DHCP server. They do not need a configuration file to do their job and work out of the box in most standard setups. For more complex situations, use the ISC dhcp-client , which is controlled by means of the configuration files /etc/dhclient.conf and /etc/dhclient6.conf .

33.3 The DHCP Server dhcpd #Edit source

The core of any DHCP system is the dynamic host configuration protocol daemon. This server leases addresses and watches how they are used, according to the settings defined in the configuration file /etc/dhcpd.conf . By changing the parameters and values in this file, a system administrator can influence the program’s behavior in numerous ways. Look at the basic sample /etc/dhcpd.conf file in ExampleВ 33.1, “The Configuration File /etc/dhcpd.conf”.

ExampleВ 33.1: The Configuration File /etc/dhcpd.conf #

This simple configuration file should be sufficient to get the DHCP server to assign IP addresses in the network. Make sure that a semicolon is inserted at the end of each line, because otherwise dhcpd is not started.

The sample file can be divided into three sections. The first one defines how many seconds an IP address is leased to a requesting client by default ( default-lease-time ) before it should apply for renewal. This section also includes a statement of the maximum period for which a machine may keep an IP address assigned by the DHCP server without applying for renewal ( max-lease-time ).

In the second part, some basic network parameters are defined on a global level:

The line option domain-name defines the default domain of your network.

With the entry option domain-name-servers , specify up to three values for the DNS servers used to resolve IP addresses into host names and vice versa. Ideally, configure a name server on your machine or somewhere else in your network before setting up DHCP. That name server should also define a host name for each dynamic address and vice versa. To learn how to configure your own name server, read ChapterВ 32, The Domain Name System.

The line option broadcast-address defines the broadcast address the requesting client should use.

With option routers , set where the server should send data packets that cannot be delivered to a host on the local network (according to the source and target host address and the subnet mask provided). Usually, especially in smaller networks, this router is identical to the Internet gateway.

With option subnet-mask , specify the netmask assigned to clients.

The last section of the file defines a network, including a subnet mask. To finish, specify the address range that the DHCP daemon should use to assign IP addresses to interested clients. In Example 33.1, “The Configuration File /etc/dhcpd.conf”, clients may be given any address between 192.168.2.10 and 192.168.2.20 or 192.168.2.100 and 192.168.2.200 .

After editing these few lines, you should be able to activate the DHCP daemon with the command systemctl start dhcpd . It will be ready for use immediately. Use the command rcdhcpd В check-syntax to perform a brief syntax check. If you encounter any unexpected problems with your configuration (the server aborts with an error or does not return done on start), you should be able to find out what has gone wrong by looking for information either in the main system log that can be queried with the command journalctl (see ChapterВ 17, journalctl : Query the systemd Journal for more information).

On a default SUSE Linux Enterprise Server system, the DHCP daemon is started in a chroot environment for security reasons. The configuration files must be copied to the chroot environment so the daemon can find them. Normally, there is no need to worry about this because the command systemctl start dhcpd automatically copies the files.

33.3.1 Clients with Fixed IP Addresses #Edit source

DHCP can also be used to assign a predefined, static address to a specific client. Addresses assigned explicitly always take priority over dynamic addresses from the pool. A static address never expires in the way a dynamic address would, for example, if there were not enough addresses available and the server needed to redistribute them among clients.

To identify a client configured with a static address, dhcpd uses the hardware address (which is a globally unique, fixed numerical code consisting of six octet pairs) for the identification of all network devices (for example, 00:30:6E:08:EC:80 ). If the respective lines, like the ones in Example 33.2, “Additions to the Configuration File”, are added to the configuration file of Example 33.1, “The Configuration File /etc/dhcpd.conf”, the DHCP daemon always assigns the same set of data to the corresponding client.

ExampleВ 33.2: Additions to the Configuration File #

The name of the respective client ( host HOSTNAME, here jupiter ) is entered in the first line and the MAC address in the second line. On Linux hosts, find the MAC address with the command ip link show followed by the network device (for example, eth0 ). The output should contain something like

In the preceding example, a client with a network card having the MAC address 00:30:6E:08:EC:80 is assigned the IP address 192.168.2.100 and the host name jupiter automatically. The type of hardware to enter is ethernet in nearly all cases, although token-ring , which is often found on IBM systems, is also supported.

33.3.2 The SUSE Linux Enterprise Server Version #Edit source

To improve security, the SUSE Linux Enterprise Server version of the ISC’s DHCP server comes with the non-root/chroot patch by Ari Edelkind applied. This enables dhcpd to run with the user ID nobody and run in a chroot environment ( /var/lib/dhcp ). To make this possible, the configuration file dhcpd.conf must be located in /var/lib/dhcp/etc . The init script automatically copies the file to this directory when starting.

Control the server’s behavior regarding this feature by means of entries in the file /etc/sysconfig/dhcpd . To run dhcpd without the chroot environment, set the variable DHCPD_RUN_CHROOTED in /etc/sysconfig/dhcpd to “ no ” .

To enable dhcpd to resolve host names even from within the chroot environment, some other configuration files must be copied as well:

These files are copied to /var/lib/dhcp/etc/ when starting the init script. Take these copies into account for any changes that they require if they are dynamically modified by scripts like /etc/ppp/ip-up . However, there should be no need to worry about this if the configuration file only specifies IP addresses (instead of host names).

If your configuration includes additional files that should be copied into the chroot environment, set these under the variable DHCPD_CONF_INCLUDE_FILES in the file /etc/sysconfig/dhcpd . To ensure that the DHCP logging facility keeps working even after a restart of the syslog daemon, there is an additional entry SYSLOGD_ADDITIONAL_SOCKET_DHCP in the file /etc/sysconfig/syslog .

Источник