DHCP Options (Linux Server) Guide

  • Updated on August 31, 2022

Dynamic Host Configuration Protocol (DHCP) is a standardized network protocol used on Internet Protocol (IP) networks for dynamically distributing network configuration parameters, such as IP addresses for interfaces and services. With DHCP, network devices request IP addresses and networking parameters automatically from a DHCP server, reducing the need for a network administrator or a user to configure these settings manually.

DHCP servers can be configured to provide optional data that fully configures TCP/IP on a client. Some of the most common DHCP option types configured and distributed by the DHCP server during leases include default gateway, router, DNS, and WINS parameters.

This guide describes advanced DHCP options supported on Grandstream products. Administrators can use these DHCP options for easy setup, to provide specific configuration per device model, synchronize time with NTP servers, configure ACS server URL on devices and more…

ENVIRONMENT SETUP

This chapter provides steps to setup a minimal test environment to run DHCP options described in this guide. In this guide, we will use isc-dhcp-server on an Ubuntu 12 machine with static IP 192.168.1.11.

Administrators can use other Windows or Linux based DHCP servers at their convenience.

This chapter can be skipped if a DHCP server supporting customizing options is already setup.

Step 1: Install DHCP Server

  1. Launch a Linux terminal.
  2. Login as root by typing sudo su
  3. Download and install DHCP server using following command: apt-get install isc-dhcp-server
Figure 1: Installing isc-dhcp-server

Step 2: DHCP Server Basic Configuration

There are two main files to be configured:

/etc/default/isc-dhcp-server (specify network interface to use for DHCP server)

/etc/dhcp/dhcpd.conf (all DHCP Options in this guide can be defined in dhcpd.conf file)

  1. Enter nano /etc/default/isc-dhcp-server to edit isc-dhcp-server file
Figure 2: isc-dhcp-server file
  1. Replace eth0 with the network interface to use for DHCP server.
Figure 3: Edit isc-dhcp-server file
  1. Save and exit the file using Ctrl+X
  2. Enter nano /etc/dhcp/dhcpd.conf to edit dhcpd.conf file
Figure 4: “dhcpd.conf” file
  1. Configure DHCP server with basic options including subnet¸ netmask¸ range

Screenshot below shows an example of configuration; in this example DHCP server will provide clients IP addresses from range 192.168.1.10 to 192.168.1.200. The server will lease an IP address for 180 seconds if no specific time frame requested by the client; otherwise, maximum (allowed) lease is 7200 seconds. DHCP server will also advise clients to use 255.255.255.0 as subnet mask, 192.168.1.10 as router/gateway, 192.168.1.1 and 192.168.1.2 as DNS servers.

Figure 5: “dhcpd.conf”
  1. Restart DHCP server

Start and Stop DHCP service

Figure 6: Restart/Start/Stop Services

DHCP OPTIONS

DHCP Option 42 (NTP Server)

Description

DHCP option 42 specifies a list of NTP servers available to the client by IP address.

Please refer to RFC2132 for more details.

Example

Figure 7: DHCP Option 42

Screenshots

Below screenshots of DHCP Discover/Offer for Option 42.

C:\Users\Soukaina\Desktop\555.png
Figure 8: DHCP Discover Request for Option 42
C:\Users\Soukaina\Desktop\777.png
Figure 9: DHCP Offer Reply for the Option 42

DHCP Option 2 (Time Offset)

Description

DHCP option 2 informs the client about the time zone offset (in seconds).

A positive offset indicates a location east of the zero meridian and a negative offset indicates a location west of the zero meridian.

Please refer to RFC2132 for more details.

Example

Figure 10: DHCP Option 2

In above example, GMT+1 was set as an offset value

(One hour * 60 minutes/hour * 60 seconds/minute) = 3600.

To set Pacific Standard Time (GMT-8). This field would be filled with “-28800”.  

(Eight hours * 60 minutes/hour * 60 seconds/minute).

Screenshots

Below screenshots of DHCP Discover/Offer for Option 2

C:\Users\Soukaina\Desktop\555.png
Figure 11: DHCP Discover Request for Option 2
C:\Users\Soukaina\Desktop\777.png
Figure 12: DHCP Offer Reply for the Option 2

DHCP Option 66 (TFTP Server Name)

Description

DHCP option 66 provides the IP address or the hostname of a single provisioning server where devices will be redirected to get their configuration files. Without this DHCP option, a manual configuration is requested on each phone the first time it boots.

Please refer to RFC2132/RFC5859 for more details.

Example

Figure 13: DHCP Option 66

If http:// is not specified, default TFTP protocol is used for configured server.

Screenshots

C:\Users\Soukaina\Desktop\555.png
Figure 14: DHCP Discover Request for Option 66
C:\Users\Soukaina\Desktop\777.png
Figure 15: DHCP Offer Reply for the Option 66

DHCP Option 43 (Vendor Specific Information)

Description

This option is used by clients and servers to exchange vendor-specific information.

DHCP server can send one or more vendor specific parameters to clients, encoded in the form option_code/value_length/value in hexadecimal format.

Please refer to RFC2132 for more details.

Example

In following example, DHCP server is configured to send CWMP information (ACS URL http://192.168.1.18) encapsulated in option 43.

Figure 16: DHCP Option 43

Above DHCP option 43 contains the following:

0x01 (CWMP option for ACS URL)

0x13 (hex of decimal 19 = length of the URL)

19 bytes forming the URL in hexadecimal format (http://192.168.1.18)

Screenshots

C:\Users\Soukaina\Desktop\555.png
Figure 17: DHCP Discover Request for Option 43
C:\Users\Soukaina\Desktop\777.png
Figure 18: DHCP Offer Reply for the Option 43

DHCP Option 12 (Host Name)

Description

This option specifies the name of the client.

Option 12 is used to identify the client’s name against the DHCP server in order to make special configuration from the server side, this is similar to option 60 and 125.

Please refer to RFC1533/RFC2132 for more details

Screenshots

Below screenshot is taken from GXP2170, the value of Option 12 can be modified from the WebGUI under Network Settings > Basic Settings.

C:\Users\Soukaina\Desktop\555.png
Figure 19: DHCP Discover Advertisement for Option 12

DHCP Option 60 (Vendor Class Identifier)

Description

Option 60 is used by clients to optionally identify the vendor type and configuration of a DHCP client.

When using multiple devices from different vendors, DHCP server can provide specific configuration for each client based on received Option 60.

Please refer to RFC1533/RFC2132 for more details.

Example

In following example, option 60 is configured to identify GXP2170 with its value “Grandstream GXP2170 dslforum.org”.

If option 60 received matches the one configured, GXP2170 phones will get option 66 (tftp-server-name) with value 192.168.1.20. For all other clients, they will get option 66 with value 192.168.1.18

Figure 20: DHCP Option 60
Note:

32 is the number of digits that “Grandstream GXP2170 dslforum.org” contains.

Screenshots

C:\Users\Soukaina\Desktop\555.png
Figure 21: DHCP Discover Advertisement for Option 60
C:\Users\Soukaina\Desktop\777.png
Figure 22: DHCP Offer Reply for Option 60

DHCP Option 120 (SIP Server)

Description

The option is used to provide SIP server IP address or FQDN to SIP clients.

Please refer to RFC3361 for more details.

Example

Figure 23: DHCP Option 120

Screenshots

C:\Users\Soukaina\Desktop\555.png
Figure 24: DHCP Discover Request for Option 120
C:\Users\Soukaina\Desktop\777.png
Figure 25: DHCP Offer Reply for Option 120

DHCP Option 125 (Vendor-Identifying Vendor Options)

Description

DHCP clients may use this option to identify the vendor that manufactured the hardware on which the client is running the software in use in a unique way.

Option 125 is similar to option 12 & 60 but advertising more parameters of a device:

  • DeviceManufacturerOUI
  • DeviceSerialNumber (Grandstream products set DeviceSerialNumber with MAC address)
  • DeviceProductClass

Please refer to RFC3925 for more details.

Screenshots

During DHCP initiation, DHCP Discover/DHCP Request including option 125 are sent from client, the server checks V-I Vendor-specific information, if matching configured values, specific configuration will be provided to client, otherwise, common configuration is provided to client.

Figure 26: DHCP Discover Advertisement for Option 125

Advertised information in above option 125 are:

  • DeviceManufacturerOUI = 000b82
  • DeviceSerialNumber = DeviceMACaddress = 000b82XXXXXX
  • DeviceProductClass = GXV3370

DHCP Option 132 (Vlan ID)

Description

This option allows to assign a VLAN ID tag to devices during booting stage/DHCP renewal.

Please refer to RFC4578 / IEEE_802.1Q for more details.

Example

C:\Users\Soukaina\Desktop\111.png
Figure 27: DHCP Option 132

Screenshots

C:\Users\Soukaina\Desktop\555.png
Figure 28: DHCP Discover Request for Option 132
C:\Users\Soukaina\Desktop\777.png
Figure 29: DHCP Offer Reply for Option 132

In above screenshot, value 3230 is 20 (vlan-id) converted from text to hexadecimal.

After getting VLAN ID from DHCP server and finishing DHCP process, the device will send a second DHCP discover its new assigned VLAN tag to get an IP address on the VLAN range.

DHCP Option 133 (QoS priority level)

Description

This option assigns the priority within an Ethernet frame header when using VLAN tag, it specifies a priority value between 0 and 7 to differentiate the traffic priority.

Please refer to RFC4578 / IEEE_P802.1p for more details

Example

C:\Users\Soukaina\Desktop\444.png
Figure 30: DHCP Option 133

Screenshots

C:\Users\Soukaina\Desktop\555.png
Figure 31: DHCP Discover Request for Option 133
C:\Users\Soukaina\Desktop\777.png
Figure 32: DHCP Offer Reply for Option 133

In above screenshot, value 35 is 5 (priority level) converted from text to hexadecimal.

DHCP Option 150 (TFTP Servers List)

Description

DHCP option 150 provides one or more IP addresses of TFTP server(s) where devices will be redirected to download their configuration files. Without this DHCP option, a manual configuration is requested on each phone the first time it boots.

Please refer to RFC5859 for more details.

Example

Figure 33: DHCP Option 150

Screenshots

C:\Users\Soukaina\Desktop\555.png
Figure 34: DHCP Discover Request for Option 150
C:\Users\Soukaina\Desktop\777.png
Figure 35: DHCP Offer Reply for Option 150

DHCP Option 160 (Configuration Server Address)

Description

Similar to option 66, DHCP option 160 can provide one or more configuration server(s) to clients to get automatically provisioned. Without this DHCP option, a manual configuration is requested on each phone the first time it boots.

Example

Figure 36: DHCP Option 160

Screenshots

C:\Users\Soukaina\Desktop\555.png
Figure 37: DHCP Discover Request for Option 160
C:\Users\Soukaina\Desktop\777.png
Figure 38: DHCP Offer Reply for Option 160

In above screenshot, the value of the TFTP server was converted to hexadecimal. The phone contacts this IP address to get provisioned after receiving TFTP server value.

DHCP Option 242 (Avaya IP Phones)

Description

Once this option enabled, the phone will use configuration info issued by DHCP sever.

Option 242 can include following parameters:

  • MC IP address
  • VLAN configuration
  • HTTP server, Proxy
  • Transport Protocol

Example

Figure 39: DHCP Option 242

Screenshots

C:\Users\Soukaina\Desktop\555.png
Figure 40: DHCP Discover Request for Option 242
C:\Users\Soukaina\Desktop\777.png
Figure 41: DHCP Offer Reply for Option 242

In above screenshot, MCIPADD and HTTPSRVR are converted to hexadecimal.

SUPPORTED DEVICES

Following table shows Grandstream products supporting DHCP Options:

DHCP Options

Grandstream Models

GXP16XX

GXP17XX

GXP21XX

GRP261X

GRP260X

GVC320X

GAC2500

GXV33XX

GXW42XX

HT8XX

DP75X

Option 2

Option 12

Option 42

Option 43

Option 60

Option 66

Option 120

Option 125

Option 132

Option 133

Option 150

Option 160

Option 242

The GVC3212 does not support DHCP option 120.

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