Tip
This is the documentation for the 23.02 version. Looking for the documentation of the latest version? Have a look here.
DHCP Options¶
DHCP Options provide information to clients beyond the basic address assignment. These options give clients other aspects of the network configuration, tell clients how they should behave on the network, and give them information about services available on the network. Common examples are a default gateway, DNS Servers, Network Time Protocol servers, network booting behavior, and dozens of other possibilities.
See also
For a list of Standard IPv4 DHCP options, see Standard IPv4 DHCP Options. This list also includes the type of data expected and whether or not they take multiple values.
The general form of an option is:
tnsr(config-kea-dhcp4)# option <name>
tnsr(config-kea-dhcp4-opt)# data <comma-separated values>
tnsr(config-kea-dhcp4-opt)# exit
This example defines a global domain name for all clients in all subnets:
tnsr(config-kea-dhcp4)# option domain-name
tnsr(config-kea-dhcp4-opt)# data example.com
tnsr(config-kea-dhcp4-opt)# exit
This example defines a default gateway for a specific subnet:
tnsr(config-kea-subnet4)# option routers
tnsr(config-kea-subnet4-opt)# data 10.2.0.1
tnsr(config-kea-subnet4-opt)# exit
To see a list of option names, enter:
tnsr(config-kea-dhcp4)# option ?
When defining options the data can take different forms. The DHCP daemon uses comma-separated value (CSV) format by default and it will automatically convert the text representation of a value to the expected data in the daemon.
Inside the option
configuration mode, the following choices are available:
- always-send <boolean>:
Controls whether the DHCP server will always send this option in a response, or only when requested by a client. The default behavior varies by option and is documented in Standard IPv4 DHCP Options
- csv-format <boolean>:
Toggles between either CSV formatted data or raw binary data. This defaults to
true
unless an option does not have a default definition. In nearly all cases this option should be left at the default.- data <data>:
Arbitrary option data. Do not enclose in quotes. To see option data types and expected formats, see Standard IPv4 DHCP Options
- space <name>:
Option space in which this entry exists, defaults to
dhcp4
.
Standard IPv4 DHCP Options¶
This list contains information about the standard IPv4 DHCP options, sourced from the Kea Administrator Manual section on DHCP Options.
For a list of the Types and their possible values, see DHCP Option Types.
Name |
Code |
Type |
Array |
Always Return |
---|---|---|---|---|
time-offset |
2 |
int32 |
false |
false |
routers |
3 |
ipv4-address |
true |
true |
time-servers |
4 |
ipv4-address |
true |
false |
name-servers |
5 |
ipv4-address |
true |
false |
domain-name-servers |
6 |
ipv4-address |
true |
true |
log-servers |
7 |
ipv4-address |
true |
false |
cookie-servers |
8 |
ipv4-address |
true |
false |
lpr-servers |
9 |
ipv4-address |
true |
false |
impress-servers |
10 |
ipv4-address |
true |
false |
resource-location-servers |
11 |
ipv4-address |
true |
false |
boot-size |
13 |
uint16 |
false |
false |
merit-dump |
14 |
string |
false |
false |
domain-name |
15 |
fqdn |
false |
true |
swap-server |
16 |
ipv4-address |
false |
false |
root-path |
17 |
string |
false |
false |
extensions-path |
18 |
string |
false |
false |
ip-forwarding |
19 |
boolean |
false |
false |
non-local-source-routing |
20 |
boolean |
false |
false |
policy-filter |
21 |
ipv4-address |
true |
false |
max-dgram-reassembly |
22 |
uint16 |
false |
false |
default-ip-ttl |
23 |
uint8 |
false |
false |
path-mtu-aging-timeout |
24 |
uint32 |
false |
false |
path-mtu-plateau-table |
25 |
uint16 |
true |
false |
interface-mtu |
26 |
uint16 |
false |
false |
all-subnets-local |
27 |
boolean |
false |
false |
broadcast-address |
28 |
ipv4-address |
false |
false |
perform-mask-discovery |
29 |
boolean |
false |
false |
mask-supplier |
30 |
boolean |
false |
false |
router-discovery |
31 |
boolean |
false |
false |
router-solicitation-address |
32 |
ipv4-address |
false |
false |
static-routes |
33 |
ipv4-address |
true |
false |
trailer-encapsulation |
34 |
boolean |
false |
false |
arp-cache-timeout |
35 |
uint32 |
false |
false |
ieee802-3-encapsulation |
36 |
boolean |
false |
false |
default-tcp-ttl |
37 |
uint8 |
false |
false |
tcp-keepalive-interval |
38 |
uint32 |
false |
false |
tcp-keepalive-garbage |
39 |
boolean |
false |
false |
nis-domain |
40 |
string |
false |
false |
nis-servers |
41 |
ipv4-address |
true |
false |
ntp-servers |
42 |
ipv4-address |
true |
false |
vendor-encapsulated-options |
43 |
empty |
false |
false |
netbios-name-servers |
44 |
ipv4-address |
true |
false |
netbios-dd-server |
45 |
ipv4-address |
true |
false |
netbios-node-type |
46 |
uint8 |
false |
false |
netbios-scope |
47 |
string |
false |
false |
font-servers |
48 |
ipv4-address |
true |
false |
x-display-manager |
49 |
ipv4-address |
true |
false |
dhcp-option-overload |
52 |
uint8 |
false |
false |
dhcp-message |
56 |
string |
false |
false |
dhcp-max-message-size |
57 |
uint16 |
false |
false |
vendor-class-identifier |
60 |
binary |
false |
false |
nwip-domain-name |
62 |
string |
false |
false |
nwip-suboptions |
63 |
binary |
false |
false |
tftp-server-name |
66 |
string |
false |
false |
boot-file-name |
67 |
string |
false |
false |
user-class |
77 |
binary |
false |
false |
client-system |
93 |
uint16 |
true |
false |
client-ndi |
94 |
record (uint8, uint8, uint8) |
false |
false |
uuid-guid |
97 |
record (uint8, binary) |
false |
false |
subnet-selection |
118 |
ipv4-address |
false |
false |
domain-search |
119 |
binary |
false |
false |
vivco-suboptions |
124 |
binary |
false |
false |
vivso-suboptions |
125 |
binary |
false |
false |
DHCP Option Types¶
- binary:
An arbitrary string of bytes, specified as a set of hexadecimal digits.
- boolean:
Boolean value with allowed values
true
orfalse
.- empty:
No value, data is carried in suboptions.
- fqdn:
Fully qualified domain name (e.g.
www.example.com
).- ipv4-address:
IPv4 address in dotted-decimal notation (e.g.
192.0.2.1
).- ipv6-address:
IPv6 address in compressed colon notation (e.g.
2001:db8::1
).- ipv6-prefix:
An IPv6 address and prefix length
- record:
Structured data of other types (except
record
andempty
).- string:
Any arbitrary text.
- int8:
8-bit signed integer with values between
-128
to127
.- int16:
16-bit signed integer with values between
-32768
to32767
.- int32:
32 bit signed integer with values between
-2147483648
and2147483647.
- psid:
Port Set ID and length with values in the format
<id>/<length>
. Defines a port set for use by the client. The value of<id>
is0-65535
, value of<length>
is0-16
.- tuple:
A length encoded as an 8-bit unsigned integer followed by a string of this length.
- uint8:
8 bit unsigned integer with values between
0
and255
.- uint16:
16 bit unsigned integer with values between
0
and65535
.- uint32:
32 bit unsigned integer with values between
0
and4294967295
.
IPv4 DHCP Option Definitions¶
TNSR also supports custom DHCP option definitions. These allow new options not listed in Standard IPv4 DHCP Options to be defined and they may also be used to change the standard definitions. Once created, these entries may be used as any other option, in the manner covered earlier in this document.
To create a new option definition, start in config-kea-dhcp4
mode and use
the option-def <name>
command:
tnsr(config-kea-dhcp4)# option-def <name>
tnsr(config-kea-dhcp4-optdef)#
Note
To create a custom option, use a unique custom name; to redefine a standard option, enter its name from Standard IPv4 DHCP Options.
This command enters config-kea-dhcp4-optdef
mode where the following
additional commands are available:
- array <true|false>:
When set to
true
, the data for this option is an array of values.- code <code-val>:
The code number for the DHCP option.
- encapsulate <encap>:
Encapsulated option space name.
- record-types <types>:
Record field type list.
- space <space-name>:
Option space name.
- type <type>:
Type of the option data, see DHCP Option Types for a list of types and allowed values.