DHCP Options in Plain English

Every day, different forms of configuration information passes between devices via a number of different mechanisms and protocols, and it’s easy to feel overwhelmed by the multiple protocols that govern this communication exchange. In networking, the main forms of configuration data transfers occur using DHCPv4 options, DHCPv6 options, and Type/Length/Value (TLV) objects, the latter of which is usually passed through a downloaded configuration file. Administrators and technical staff working in networking environments need to understand these terms and it’s useful to have a quick reference guide to translate this technical information into plain English.

The following tables list common, configurable DHCP options. Non-configurable options or TLVs have not been included, even though these may be present in a file or on the wire. The following tables also do not include options that are only necessary for the operation of the DHCP protocol. For example, DHCPv4 option 53 is the DHCP message type option that declares whether a particular message is a discovery, request, or other message type. However, the administrator can’t configure this option, so therefore it isn’t included in the lists below.
Of course, there will be some DHCPv4, DHCPv6, and internal DHCP options that are not listed at all. This is because this document aims to act as a guide for the most commonly-used options only. For further information on other DHCP options, refer to the IETF website.

Table 1 – DHCPv4 Options

The table below describes DHCP options commonly used in IPv4 networks. Please note: Certain classes of devices will use particular subsets, as listed below:

  • Cable Modems: 1, 2, 3, 4, 6, 7, 51, 66, 67, 122.1, 122.2
  • MTAs: 1, 2, 3, 4, 6, 7, 51, 122.3, 122.6.  If in BASIC mode, also 66, 67
  • Generic devices: 1, 3, 6, 51
Option Number Option Name Source Description
1 Subnet Mask RFC 2132 The subnet mask to apply to the address that is assigned to the client.
2 Time Zone Offset RFC 2132 Informs the client about the time zone offset, in seconds.  For example, Pacific Standard Time is GMT – 8 hours.  This field would be filled with “- 28800”.  (Eight hours * 60 minutes/hour * 60 seconds/minute)
3 Gateway RFC 2132 Tells the client which router is the default router.
4 Time Server RFC 2132 Tells the client the IP address of a time server that can determine the client’s current time. This is related to the Time Zone Offset option.
6 Domain Name Server RFC 2132 Carries the IP address(es) of the DNS servers that the client uses for name resolution.
7 Log Server RFC 2132 Carries the IP address of the syslog server that receives the client’s log messages.
12 Hostname RFC 2132 Carries the hostname portion of a client’s fully qualified domain name (FQDN). For example, the “www” part of “www.example.com”.
15 Domain Name RFC 2132 Carries the domain name portion of a client’s fully qualified domain name (FQDN).  For example, the “example.com” portion of “www.example.com”.
43 Vendor Specific Information RFC 2132 Carries some configuration data that is not defined in the standard DHCP RFCs.  Individual vendors can define different pieces of data that may be found in this option. CableLabs’ eDOCSIS specification, for example, defines certain sub-options that appear in this option.
51 Lease Time Option RFC 2132 This defines the maximum amount of time that the client may use the IP address.
60 Vendor Class Identifier RFC 2132 Carries some identifier from the client that may identify the client’s device class. The DHCP specification does not define what values go into this field, but other specifications do. For example, the DOCSIS specifications state that if the device is DOCSIS-compliant, option 60 must start with “docsis” (and also include other details).
66 TFTP Server Name RFC 2132 Carries the FQDN or IP address (or cluster identifier) that the device should use to download the file specified in option 67.  Note that often the data put into option 66 does not actually appear in the DHCP packet as option 66, but may have been moved into the “sname” field of the DHCP packet.  Additionally, the FQDN may have been resolved to an IP address and also placed in the “siaddr” field of the DHCP packet.
67 Filename RFC 2132 Carries the filename that is to be downloaded from the server specified in option 66. Note that often data put into option 67 does not actually appear in the DHCP packet as option 67, but may be moved into the “file” field of the DHCP packet.  (Note: Address Commander can do some dynamic file name generation for this name.)
82 Relay Agent Information Option RFC 3046 This option carries many other sub-options that are added by relay agents and not the clients themselves.  Some of the common options are listed below.
82.1 Circuit ID RFC 3046 This is an identifier added by the relay agent to indicate which interface the client is on. This is commonly used as a subscriber identifier in DSL-based networks.
82.2 Remote ID RFC 3046 This is an identifier added by the relay agent to identify the client. It is commonly used as a subscriber identifier in DOCSIS-based networks.  DOCSIS mandates that this option is filled with the MAC address of the cable modem and applies to all traffic from the cable modem, or any device appearing from behind that cable modem.
122 CableLabs Client Configuration Option RFC 3495 This is an option defined by CableLabs to carry the information necessary to configure PacketCable devices. The sub-options are listed below.
122.1 TSP Primary DHCP Server RFC 3495 The multimedia telephony adapter (MTA) portion of the device listens to this IP address of the DHCP server. This has two special values, 0.0.0.0 (which will disable the MTA), and 255.255.255.255 (which will instruct the MTA to accept replies from any DHCP server).
122.2 TSP Secondary DHCP Server RFC 3495 A second DHCP server’s IP address.  Otherwise the same as option 122.1
122.3 TSP Provisioning Server’s Address RFC 3495 The MTA contacts the server listed to complete its provisioning process. This is the FQDN of a multimedia provisioning service (MPS), whether a specific MPS, or a cluster identifier.
122.6 TSP Kerberos Realm Name RFC 3495 For PacketCable secure mode provisioning, the name of the Kerberos Realm. There are four more special values that may be in this field: BASIC.1, BASIC.2, HYBRID.1, and HYBRID.2.  This indicates whether the device is in basic, hybrid, or secure mode provisioning. The “.1” and “.2” parts indicate whether the MTA notified the provisioning server upon completion of the provisioning process.

 

Table 2: DHCPv6 Options

The table below describes DHCP options commonly used in IPv6 networks.

Option Number Option Name Source Description
17 Vendor Specific Information Option RFC 3315 This option contains vendor-specific option data, much like DHCPv4 option 43.  There is an extra difference in that in DHCPv6, this option carries a vendor ID as well, which allows for data from multiple vendors to be provided to the device.
17[4491] CableLabs Vendor-Specific Information Option CL-SP-CANN-DHCP-Reg-I08-111117 CableLabs has specified a set of sub-options for use with DOCSIS-compliant DHCPv6 devices.  The vendor ID for CableLabs is 4491.
17[4491].32 TFTP Server Addresses option CL-SP-CANN-DHCP-Reg-I08-111117 This is a list of IPv6 addresses that the client will use to try to download its boot file (specified in 17[4491].33) using TFTP.
17[4491].33 Configuration File Name CL-SP-CANN-DHCP-Reg-I08-111117 The name of the file to download using the TFTP servers specified in 17[4491].32.
17[4491].34 Syslog Server Addresses CL-SP-CANN-DHCP-Reg-I08-111117 This is the DHCPv6 analogue to the DHCPv4 option 7.
17[4491].37 RFC868 Servers CL-SP-CANN-DHCP-Reg-I08-111117 This is the DHCPv6 analogue to the DHCPv4 option 4
17[4491].38 Time Offset CL-SP-CANN-DHCP-Reg-I08-111117 This is the DHCPv6 analogue to the DHCPv4 option 2.
23 DNS Recursive Name Server Option RFC 3646 This is the DHCPv6 analogue to the DHCPv4 option 6.
24 Domain Search List RFC 3646 This option specifies the list of domain names that the client should use when attempting to resolve basic host names.
39 DHCPv6 FQDN option RFC 4704 This option specifies the clients host and domain names. This is the analogue of both DHCPv4 option 12 and DHCPv4 option 15 combined. 

References

RFC 2132: http://datatracker.ietf.org/doc/rfc2132/
RFC 3046: http://datatracker.ietf.org/doc/rfc3046/
RFC 3495: http://datatracker.ietf.org/doc/rfc3495/
RFC 3646: http://datatracker.ietf.org/doc/rfc3646/
RFC 4704: http://datatracker.ietf.org/doc/rfc4704/
CL-SP-CANN-DHCP-Reg-I08-111117: http://www.cablelabs.com/specifications/CL-SP-CANN-DHCP-Reg-I08-111117.pdf

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