DHCP Options

Sub-menu: /ip dhcp-server option


With help of DHCP Option list, it is possible to define additional custom options for DHCP Server to advertise. Option precedence is as follows:


  • radius,

  • lease,

  • server,

  • network.

This is the order in which client option request will be filled in.


According to the DHCP protocol, a parameter is returned to the DHCP client only if it requests this parameter, specifying the respective code in DHCP request Parameter-List (code 55) attribute. If the code is not included in Parameter-List attribute, DHCP server will not send it to the DHCP client.

Properties

Property Description
code (integer:1..254; Default: ) dhcp option code. All codes are available at http://www.iana.org/assignments/bootp-dhcp-parameters
name (string; Default: ) Descriptive name of the option
value (string; Default: ) Parameter's value.


Starting from v6.8 available data types for options are:


  • 0xXXXX - hex string (works also in v5)

  • 'XXXXX' - string (works also in v5 but without ' ' around the text)

  • $(XXXXX) - variable (currently there are no variables for server)

  • '10.10.10.10' - IP address

  • s'10.10.10.10' - IP address converted to string

  • '10' - decimal number

  • s'10' - decimal number converted to string

RouterOS has predefined variables that can be used:

  • HOSTNAME - client hostname

  • RADIUS_MT_STR1 - from radius MT attr nr. 24

  • RADIUS_MT_STR2 - from radius MT attr nr. 25

  • REMOTE_ID - agent remote id

  • NETWORK_GATEWAY - first gateway from '/ip dhcp-server network', note that this option won't work if used from lease


Now it is also possible to combine data types into one, for example: "0x01'vards'$(HOSTNAME)"

For example if HOSTNAME is 'kvm', then raw value will be 0x0176617264736b766d.
raw-value (HEX string ) Read only field which shows raw dhcp option value (the format actually sent out)

Example

Classless Route

A classless route adds specified route in clients routing table. In our example, it will add

  • dst-address=160.0.0.0/24 gateway=10.1.101.1

  • dst-address=0.0.0.0/0 gateway=10.1.101.1


According to RFC 3442: The first part is the netmask ("18" = netmask /24). Second part is significant part of destination network ("A00000" = 160.0.0). Third part is IP address of gateway ("0A016501" = 10.1.101.1). Then There are parts of the default route, destination netmask (0x00 = 0.0.0.0/0) followed by default route (0x0A016501 = 10.1.101.1)

/ip dhcp-server option
add code=121 name=classless value=0x18A000000A016501000A016501
/ip dhcp-server network
set 0 dhcp-option=classless

Result:

[admin@MikroTik] /ip route> print
Flags: X - disabled, A - active, D - dynamic, C - connect, S - static, r - rip, b - bgp, o - ospf,
m - mme, B - blackhole, U - unreachable, P - prohibit
 #      DST-ADDRESS        PREF-SRC        GATEWAY            DISTANCE
 0 ADS  0.0.0.0/0                          10.1.101.1         0
 1 ADS  160.0.0.0/24                       10.1.101.1         0

Much more robust way would be to use built in variables, previous example can be rewritten as:

/ip dhcp-server option 
add name=classless code=121 value="0x18A00000\$(NETWORK_GATEWAY)0x00\$(NETWORK_GATEWAY)"


Auto Proxy Config

/ip dhcp-server option 
  add code=252 name=auto-proxy-config value="'http://autoconfig.something.lv/wpad.dat'"