DHCP options lookup
Dynamic Host Configuration Protocol (DHCP) has many configuration options that can be sent to clients when they request information from the DHCP server. You can use a lookup tool to see all of the DHCP options.
DHCP options define additional configuration data that the DHCP server passes along to clients in addition to an IP address. Typical options include subnet mask, domain name, router IP addresses, domain name server IP addresses, and static routes.
| Option number | Option | Description |
|---|---|---|
| 1 | Subnet mask | The subnet mask option specifies the client's subnet mask as per Request for Comments (RFC) 950. If both the subnet mask and the router option are specified in a DHCP reply, the subnet mask option must be specified first. The code for the subnet mask option is 1, and its length is 4 octets.  |
| 2 | Time offset | The time offset field specifies the offset of
the client's subnet in seconds from Coordinated Universal Time (UTC).
The offset is expressed as a two's complement 32-bit integer. A positive
offset indicates a location east of the zero meridian and a negative
offset indicates a location west of the zero meridian. The code for the time offset option is 2, and its length is 4 octets.  |
| 3 | Router | The router option specifies a list of IP addresses
for routers on the client's subnet. Routers should be listed in order
of preference. The code for the router option is 3. The minimum length for the router option is 4 octets, and the length must always be a multiple of 4.  |
| 4 | Time server | The time server option specifies a list of RFC
868 time servers available to the client. Servers should be listed
in order of preference. The code for the time server option is 4. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 5 | Name server | The name server option specifies a list of IEN
116 name servers available to the client. Servers should be listed
in order of preference. The code for the name server option is 5. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 6 | Domain Name Server | The domain name server option specifies a list
of Domain Name System (STD 13, RFC 1035) name servers available to
the client. Servers should be listed in order of preference. The code for the domain name server option is 6. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 7 | Log server | The log server option specifies a list of MIT-LCS
UDP log servers available to the client. Servers should be listed
in order of preference. The code for the log server option is 7. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 8 | Cookie server | The cookie server option specifies a list of
RFC 865 cookie servers available to the client. Servers should be
listed in order of preference. The code for the cookie server option is 8. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 9 | LPR server | The LPR server option specifies a list of RFC
1179 line printer servers available to the client. Servers should
be listed in order of preference. The code for the LPR server option is 9. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 10 | Impress server | The Impress server option specifies a list of
Imagen Impress servers available to the client. Servers should be
listed in order of preference. The code for the Impress server option is 10. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 11 | Resource location server | This option specifies a list of RFC 887 Resource
Location servers available to the client. Servers should be listed
in order of preference. The code for this option is 11. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 12 | Host name | This option specifies the name of the client.
The name might or might not be qualified with the local domain name
(see section 3.17 for the preferred way to retrieve the domain name).
See RFC 1035 for character set restrictions. The code for this option is 12, and its minimum length is 1.  |
| 13 | Boot file size | This option specifies the length in 512-octet
blocks of the default boot image for the client. The file length is
specified as an unsigned 16-bit integer. The code for this option is 13, and its length is 2.  |
| 14 | Merit dump file | This option specifies the path-name of a file
to which the client's core image should be dumped in the event the
client crashes. The path is formatted as a character string consisting
of characters from the NVT ASCII character set. The code for this option is 14. Its minimum length is 1.  |
| 15 | Domain name | This option specifies the domain name that client
should use when resolving hostnames through the Domain Name System. The code for this option is 15. Its minimum length is 1.  |
| 16 | Swap server | This specifies the IP address of the client's
swap server. The code for this option is 16, and its length is 4.  |
| 17 | Root path | This option specifies the path-name that contains
the client's root disk. The path is formatted as a character string
consisting of characters from the NVT ASCII character set. The code for this option is 17. Its minimum length is 1.  |
| 18 | Extensions path | A string to specify a file, retrievable via
TFTP, which contains information that can be interpreted in the same
way as the 64-octet vendor-extension field within the BOOTP response,
with the following exceptions:
The code for this option is 18. Its minimum length is 1.  |
| 19 | IP forwarding | This option specifies whether the client should
configure its IP layer for packet forwarding. A value of 0 means disable
IP forwarding, and a value of 1 means enable IP forwarding. The code for this option is 19, and its length is 1.  |
| 20 | Non-Local source routing | This option specifies whether the client should
configure its IP layer to allow forwarding of datagrams with non-local
source routes. A value of 0 means disallow forwarding of such datagrams,
and a value of 1 means allow forwarding. The code for this option is 20, and its length is 1.  |
| 21 | Policy filter | This option specifies policy filters for non-local
source routing. The filters consist of a list of IP addresses and
masks which specify destination/mask pairs with which to filter incoming
source routes. Any source routed datagram whose next-hop address does not match one of the filters should be discarded by the client. The code for this option is 21. The minimum length of this option is 8, and the length must be a multiple of 8.  |
| 22 | Maximum datagram reassembly size | This option specifies the maximum size datagram
that the client should be prepared to reassemble. The size is specified
as a 16-bit unsigned integer. The minimum legal value is 576. The code for this option is 22, and its length is 2.  |
| 23 | Default IP time to live | This option specifies the default time-to-live
that the client should use on outgoing datagrams. The TTL is specified
as an octet with a value between 1 and 255. The code for this option is 23, and its length is 1.  |
| 24 | Path MTU aging timeout | This option specifies the timeout (in seconds)
to use when aging Path MTU values discovered by the mechanism defined
in RFC 1191. The timeout is specified as a 32-bit unsigned integer. The code for this option is 24, and its length is 4.  |
| 25 | Path MTU plateau table | This option specifies a table of MTU sizes to
use when performing Path MTU Discovery as defined in RFC 1191. The
table is formatted as a list of 16-bit unsigned integers, ordered
from smallest to largest. The minimum MTU value cannot be smaller
than 68. The code for this option is 25. Its minimum length is 2, and the length must be a multiple of 2.  |
| 26 | Interface MTU | This option specifies the MTU to use on this
interface. The MTU is specified as a 16-bit unsigned integer. The
minimum legal value for the MTU is 68. The code for this option is 26, and its length is 2.  |
| 27 | All subnets are local | This option specifies whether the client can
assume that all subnets of the IP network to which the client is connected
use the same MTU as the subnet of that network to which the client
is directly connected. A value of 1 indicates that all subnets share
the same MTU. A value of 0 means that the client should assume that
some subnets of the directly connected network might have smaller
MTUs. The code for this option is 27, and its length is 1.  |
| 28 | Broadcast address | This option specifies the broadcast address
in use on the client's subnet. Legal values for broadcast addresses
are specified in section 3.2.1.3 of RFC 2132. The code for this option is 28, and its length is 4.  |
| 29 | Perform mask discovery | This option specifies whether the client should
perform subnet mask discovery using ICMP. A value of 0 indicates that
the client should not perform mask discovery. A value of 1 means that
the client should perform mask discovery. The code for this option is 29, and its length is 1.  |
| 30 | Mask supplier | This option specifies whether the client should
respond to subnet mask requests using ICMP. A value of 0 indicates
that the client should not respond. A value of 1 means that the client
should respond. The code for this option is 30, and its length is 1.  |
| 31 | Perform router discovery | This option specifies whether the client should
solicit routers using the Router Discovery mechanism defined in RFC
1256. A value of 0 indicates that the client should not perform router
discovery. A value of 1 means that the client should perform router
discovery. The code for this option is 31, and its length is 1.  |
| 32 | Router solicitation address option | This option specifies the address to which the
client should transmit router solicitation requests. The code for this option is 32, and its length is 4.  |
| 33 | Static route | This option specifies a list of static routes
that the client should install in its routing cache. If multiple routes
to the same destination are specified, they are listed in descending
order of priority. The routes consist of a list of IP address pairs. The first address is the destination address, and the second address is the router for the destination. The default route (0.0.0.0) is an illegal destination for a static route. The code for this option is 33. The minimum length of this option is 8, and the length must be a multiple of 8.  |
| 34 | Trailer encapsulation | This option specifies whether the client should
negotiate the use of trailers (RFC 893) when using the ARP protocol.
A value of 0 indicates that the client should not attempt to use trailers.
A value of 1 means that the client should attempt to use trailers. The code for this option is 34, and its length is 1.  |
| 35 | ARP cache timeout | This option specifies the timeout in seconds
for ARP cache entries. The time is specified as a 32-bit unsigned
integer. The code for this option is 35, and its length is 4.  |
| 36 | Ethernet encapsulation | This option specifies whether the client should
use Ethernet Version 2 (RFC 894) or IEEE 802.3 (RFC 1042) encapsulation
if the interface is an Ethernet. A value of 0 indicates that the client
should use RFC 894 encapsulation. A value of 1 means that the client
should use RFC 1042 encapsulation. The code for this option is 36, and its length is 1.  |
| 37 | TCP default TTL | This option specifies the default TTL that the
client should use when sending TCP segments. The value is represented
as an 8-bit unsigned integer. The minimum value is 1. The code for this option is 37, and its length is 1.  |
| 38 | TCP keep-alive interval | This option specifies the interval (in seconds)
that the client TCP should wait before sending a keepalive message
on a TCP connection. The time is specified as a 32-bit unsigned integer.
A value of zero indicates that the client should not generate keepalive
messages on connections unless specifically requested by an application. The code for this option is 38, and its length is 4.  |
| 39 | TCP keep-alive garbage | This option specifies whether the client should
send TCP keepalive messages with an octet of garbage for compatibility
with older implementations. A value of 0 indicates that a garbage
octet should not be sent. A value of 1 indicates that a garbage octet
should be sent. The code for this option is 39, and its length is 1.  |
| 40 | Network information service domain | This option specifies the name of the client's
NIS domain. The domain is formatted as a character string consisting
of characters from the NVT ASCII character set. The code for this option is 40. Its minimum length is 1.  |
| 41 | Network information servers | This option specifies a list of IP addresses
indicating NIS servers available to the client. Servers should be
listed in order of preference. The code for this option is 41. Its minimum length is 4, and the length must be a multiple of 4.  |
| 42 | Network time protocol servers option | This option specifies a list of IP addresses
indicating NTP servers available to the client. Servers should be
listed in order of preference. The code for this option is 42. Its minimum length is 4, and the length must be a multiple of 4.  |
| 44 | NetBIOS over TCP/IP name server | The NetBIOS name server (NBNS) option specifies
a list of RFC 1001/1002 NBNS name servers listed in order of preference. The code for this option is 44. The minimum length of the option is 4 octets, and the length must always be a multiple of 4.  |
| 45 | NetBIOS over TCP/IP datagram distribution server | The NetBIOS datagram distribution server (NBDD)
option specifies a list of RFC 1001/1002 NBDD servers listed in order
of preference. The code for this option is 45. The minimum length of the option is 4 octets, and the length must always be a multiple of 4.  |
| 46 | NetBIOS over TCP/IP node type | The NetBIOS node type option allows NetBIOS
over TCP/IP clients which are configurable to be configured as described
in RFC 1001/1002. The value is specified as a single octet which identifies
the client type as follows: In the above chart, the notation '0x' indicates a number in base-16 (hexadecimal). The code for this option is 46. The length of this option is always 1.  |
| 47 | NetBIOS over TCP/IP scope | The NetBIOS scope option specifies the NetBIOS
over TCP/IP scope parameter for the client as specified in RFC 1001/1002. The code for this option is 47. The minimum length of this option is 1.  |
| 48 | X Window System Font server | This option specifies a list of X Window System
Font servers available to the client. Servers should be listed in
order of preference. The code for this option is 48. The minimum length of this option is 4 octets, and the length must be a multiple of 4.  |
| 49 | X Window System display manager | This option specifies a list of IP addresses
of systems that are running the X Window System Display Manager and
are available to the client. Addresses should be listed in order of preference. The code for the this option is 49. The minimum length of this option is 4, and the length must be a multiple of 4.  |
| 51 | IP address lease time | This option is used in a client request (DHCPDISCOVER
or DHCPREQUEST) to allow the client to request a lease time for the
IP address. In a server reply (DHCPOFFER), a DHCP server uses this
option to specify the lease time it is willing to offer. The time is in units of seconds, and is specified as a 32-bit unsigned integer. The code for this option is 51, and its length is 4.  |
| 58 | Renewal (T1) time value | This option specifies the time interval from
address assignment until the client transitions to the RENEWING state. The value is in units of seconds, and is specified as a 32-bit unsigned integer. The code for this option is 58, and its length is 4.  |
| 59 | Rebinding (T2) time value | This option specifies the time interval from
address assignment until the client transitions to the REBINDING state. The value is in units of seconds, and is specified as a 32-bit unsigned integer. The code for this option is 59, and its length is 4.  |
| 62 | NetWare/IP domain name | Specifies the Netware/IP domain name. |
| 63 | NetWare/IP | Specifies the NetWare sub-options you want. The range is 1 to 255. Use option 62 to specify the NetWare/IP domain name. |
| 64 | NIS domain name | This option specifies the name of the client's
NIS+ domain. The domain is formatted as a character string consisting
of characters from the NVT ASCII character set. The code for this option is 64. Its minimum length is 1.  |
| 65 | NIS servers | This option specifies a list of IP addresses indicating NIS+ servers available to the client. Servers should be listed in order of preference. The code for this option is 65. Its minimum length is 4, and the length must be a multiple of 4.  |
| 66 | Server name | This option is used to identify a TFTP server
when the 'sname' field in the DHCP header has been used for DHCP options. The code for this option is 66, and its minimum length is 1.  |
| 67 | Boot file name | This option is used to identify a bootfile when
the 'file' field in the DHCP header has been used for DHCP options. The code for this option is 67, and its minimum length is 1.  |
| 68 | Home address | This option specifies a list of IP addresses
indicating mobile IP home agents available to the client. Agents should
be listed in order of preference. The code for this option is 68. Its minimum length is 0 (indicating no home agents are available) and the length must be a multiple of 4. It is expected that the usual length will be four octets, containing a single home agent's address.  |
| 69 | SMTP servers | The SMTP server option specifies a list of SMTP
servers available to the client. Servers should be listed in order
of preference. The code for the SMTP server option is 69. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 70 | POP3 server | The POP3 server option specifies a list of POP3 available to the client. Servers should be listed in order of preference. The code for the POP3 server option is 70. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 71 | NNTP server | The NNTP server option specifies a list of NNTP
available to the client. Servers should be listed in order of preference. The code for the NNTP server option is 71. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 72 | WWW server | The WWW server option specifies a list of WWW
available to the client. Servers should be listed in order of preference. The code for the WWW server option is 72. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 73 | Finger server | The Finger server option specifies a list of
Finger available to the client. Servers should be listed in order
of preference. The code for the Finger server option is 73. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 74 | IRC server | The IRC server option specifies a list of IRC
available to the client. Servers should be listed in order of preference. The code for the IRC server option is 74. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 75 | StreetTalk server | The StreetTalk server option specifies a list
of StreetTalk servers available to the client. Servers should be listed
in order of preference. The code for the StreetTalk server option is 75. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 76 | STDA server | The StreetTalk Directory Assistance (STDA) server
option specifies a list of STDA servers available to the client. Servers
should be listed in order of preference. The code for the StreetTalk Directory Assistance server option is 76. The minimum length for this option is 4 octets, and the length must always be a multiple of 4.  |
| 77 | User class | Specifies the class name of which the host is a member. You must have previously defined this class to the DHCP server during DHCP server configuration. |
| 78 | Directory agent | Specifies the IP address of the directory agent if clients use Service Location Protocol to transact messages. |
| 79 | Service scope | Specifies the scope of the directory agent that uses Service Location Protocol to respond to service request messages. |
| 80 | Naming authority | Specifies the naming authority for the directory agent if clients use Service Location Protocol to transact messages. The naming authority specifies the syntax for schemes that are used in URLs. |