DHCP Server

 

A Dynamic Host Configuration Protocol (DHCP) server provides a framework to pass configuration information to client hosts on a TCP/IP network. A device acting as a DHCP server can dynamically allocate IP addresses and other configuration parameters, minimizing the overhead that is required to add clients to the network. Read this topic for more information.

Understanding DHCP Server Operation

As a DHCP server, a Juniper Networks device can provide temporary IP addresses from an IP address pool to all clients on a specified subnet, a process known as dynamic binding. Juniper Networks devices can also perform static binding, assigning permanent IP addresses to specific clients based on their media access control (MAC) addresses. Static bindings take precedence over dynamic bindings.

This section contains the following topics:

DHCP Options

In addition to its primary DHCP server functions, you can also configure the device to send configuration settings like the following to clients through DHCP:

  • IP address of the DHCP server (Juniper Networks device)

  • List of Domain Name System (DNS) and NetBIOS servers

  • List of gateway routers

  • IP address of the boot server and the filename of the boot file to use

  • DHCP options defined in RFC 2132, DHCP Options and BOOTP Vendor Extensions

Compatibility with Autoinstallation

The functions of a Juniper Networks device acting as a DHCP server are compatible with the autoinstallation feature. The DHCP server automatically checks any autoinstallation settings for conflicts and gives the autoinstallation settings priority over corresponding DHCP settings. For example, an IP address set by autoinstallation takes precedence over an IP address set by the DHCP server.

Chassis Cluster Support

DHCP server operations are supported on all SRX Series devices in chassis cluster mode.

DHCP Server Configuration Overview

A typical DHCP server configuration provides the following configuration settings for a particular subnet on a device interface:

  • An IP address pool, with one address excluded from the pool.

  • Default and maximum lease times.

  • Domain search suffixes. These suffixes specify the domain search list used by a client when resolving hostnames with DNS.

  • A DNS name server.

  • Device solicitation address option (option 32). The IP address excluded from the IP address pool is reserved for this option.

In addition, the DHCP server might assign a static address to at least one client on the subnet. Table 1 provides the settings and values for the sample DHCP server configuration.

Table 1: Sample DHCP Server Configuration Settings

Setting

Sample Value

DHCP Subnet Configuration

Address pool subnet address

192.168.2.0/24

High address in the pool range

192.168.2.254

Low address in the pool range

192.168.2.2

Address pool default lease time, in seconds

1,209,600 (14 days)

Address pool maximum lease time, in seconds

2,419,200 (28 days)

Domain search suffixes

mycompany.net

mylab.net

Address to exclude from the pool

192.168.2.33

DNS server address

192.168.10.2

Identifier code for router solicitation address option

32

Type choice for router solicitation address option

Ip address

IP address for router solicitation address option

192.168.2.33

DHCP MAC Address Configuration

Static binding MAC address

01:03:05:07:09:0B

Fixed address

192.168.2.50

See also

Minimum DHCP Local Server Configuration

The following sample output shows the minimum configuration you must use to configure an SRX300, SRX320, SRX340, SRX345, SRX550M, or SRX1500 device as a DHCP local server. In this output, the server group is named mobileusers, and the DHCP local server is enabled on interface ge-1/0/1.0 within the group. The address pool is named acmenetwork from low range of 192.168.1.10/24 to a high range of 192.168.1.20/24.

Note

You can configure the DHCP local server in a routing instance by using the dhcp-local server, interface, and address-assignment statements in the [edit routing-instances] hierarchy level.

Enabling TCP/IP Propagation on a DHCP Local Server

This topic describes how to configure TCP/IP settings on a DHCP local server, which includes a DHCP client and a DHCP local server.

Note

This feature is supported on SRX300, SRX320, SRX340, SRX345, SRX550M, and SRX1500 devices.

To enable TCP/IP setting propagation on a DHCP local server:

  1. Configure the update-server option on the DHCP client.
  2. Configure the address pool to specify the interface (where update-server is configured) from which TCP/IP settings can be propagated.
  3. Configure the DHCP local server.

Example: Configuring the Device as a DHCP Server

This example shows how to configure the device as a DHCP server.

For information on how to configure JDHCP in a routing instance, see How to configure JDHCP in a routing instance.

Requirements

Before you begin:

  • Determine the IP address pools and the lease durations to use for each subnet.

  • Obtain the MAC addresses of the clients that require permanent IP addresses. Determine the IP addresses to use for these clients.

  • List the IP addresses that are available for the servers and devices on your network; for example, DNS, NetBIOS servers, boot servers, and gateway devices. See the Understanding Management Predefined Policy Applications.

  • Determine the DHCP options required by the subnets and clients in your network.

Overview

In this example, you configure the device as a DHCP server. You specify the IP address pool as 192.168.2.0/24 and from a low range of 192.168.2.2 to a high range of 192.168.2.254. You set the maximum-lease-time to 2,419,200. Then you specify the DNS server IP address as 192.168.10.2.

Warning

Starting with Junos OS Release 15.1X49-D60 and Junos OS Release 17.3R1, the legacy DHCPD (DHCP daemon) configuration on all SRX Series devices is being deprecated. and only the new JDHCP CLI is supported. When you upgrade to Junos OS Release 15.1X49-D60 and later releases on a device that already has the DHCPD configuration, the following warning messages are displayed:

WARNING: The DHCP configuration command used will be deprecated in future Junos releases.

WARNING: Please see documentation for updated commands.

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the set access hierarchy level, and then enter commit from configuration mode.

GUI Step-by-Step Procedure

To configure the device as a DHCP server, specify the DHCP pool information, server information, lease time, and option information:

  1. In the J-Web interface, select Configure > DHCP > DHCP Services.
  2. Select DHCP Pools. Click Add.
  3. Specify the IP address that is used as the source address the DHCP server includes in IP packets when communicating with clients. The address is included in the DHCP packet in option 54.
  4. Specify the subnet information for the IPv4 address-assignment pool. Type 192.168.2.0/24.
  5. In the Address Range Low, type 192.168.2.2.
  6. In the Address Range High, type 192.168.2.254.
  7. In the Exclude Addresses box, type the addresses you want excluded from a DHCP address pool. Type 192.168.2.0/24
  8. Specify the server identifier to assign to any DHCP clients in this address pool. The identifier can be used to identify a DHCP server in a DHCP message.
  9. Specify the domain name to assign to any DHCP clients in this address pool.
  10. Specify the next server that DHCP clients need to contact. Type 192.168.10.2
  11. Define the maximum amount of time (in seconds) that DHCP should lease an address. Type 2419200.
  12. Define DHCP option 32, the device solicitation address option. You must enter a numeric value for option code. Select the option type from the list that corresponds to the option code.
  13. Click OK.
  14. If you are done configuring the device, click Commit > Commit.

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For instructions on how to do that, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure the device as a DHCP server:

  1. Configure an interface with an IP address on which the DHCP server will be reachable.
  2. Configure the DHCP server.
  3. Create an address pool for IPv4 addresses that can be assigned to clients. The addresses in the pool must be on the subnet in which the DHCP clients reside. Do not include addresses that are already in use on the network.
  4. (Optional) Specify the IP address pool range. Define a range of addresses in the address-assignment pool. The range is a subset of addresses within the pool that can be assigned to clients. If no range is specified, then all addresses within the pool are available for assignment. Configure the name of the range and the lower and upper boundaries of the addresses in the range.
  5. (Optional) Configure one or more routers as the default gateway on the client’s subnet.
  6. (Optional) Configure the IP address that is used as the source address for the DHCP server in messages exchanged with the client. Clients use this information to distinguish between lease offers.
  7. (Optional) Specify the maximum time period, in seconds, that a client holds the lease for an assigned IP address if the client does not renew the lease.
  8. (Optional) Specify user-defined options to be included in DHCP packets
  9. Assign a fixed IP address with the MAC address of the client.

Results

  • From configuration mode, confirm your configuration by entering the show access address-assignment command. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it.

  • From configuration mode, confirm your configuration by entering the show system services dhcp-local-server command. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it.

If you are done configuring the device, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

Verifying the DHCP Binding Database

Purpose

Verify that the DHCP binding database reflects the DHCP server configuration.

Action

From operational mode, enter these commands:

  • show dhcp server binding command to display all active bindings in the database.

  • show dhcp server binding address detail command (where address is the IP address of the client) to display more information about a client.

These commands produce following sample output:

user@host> show dhcp server binding
user@host> show dhcp server binding address detail

Verifying DHCP Server Operation

Purpose

Verify that the DHCP server operation has been configured.

Action

From operational mode, enter the following command:

  • show dhcp server statistics command to verify the DHCP server statistics.

user@host> show dhcp server statistics

Verifying and Managing DHCP Local Server Configuration

Purpose

View or clear information about client address bindings and statistics for the extended DHCP local server.

Note

If you delete the DHCP server configuration, DHCP server bindings might still remain. To ensure that DHCP bindings are removed, issue the clear dhcp server binding command before you delete the DHCP server configuration.

Action

  • To display the address bindings in the client table on the extended DHCP local server:

  • To display extended DHCP local server statistics:

  • To clear the binding state of a DHCP client from the client table on the extended DHCP local server:

  • To clear all extended DHCP local server statistics:

Release History Table
Release
Description
Starting with Junos OS Release 15.1X49-D60 and Junos OS Release 17.3R1, the legacy DHCPD (DHCP daemon) configuration on all SRX Series devices is being deprecated. and only the new JDHCP CLI is supported.