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How to configure the DHCP server to distribute Static IP routes

Updated: Oct 3, 2021

Sometimes, we need to push a static route from the DHCP server to the client systems. And there are two DHCP option can be configured to achieve that.

  • RFC 3442 defines DHCP Option 121: Classless Static Route Option for DHCPv4

  • Microsoft introduced DHCP Option 249: Microsoft Classless Static Route Option.

We need to do some changes to our existing DHCP server.

Step:1 Add below additional entries to the DHCP configuration file.

# vi /etc/dhcp/dhcpd.conf
::::::::::::: CUT SOME OUTPUT ::::::::::::: 
# Specify Default Gateway
#       option routers        ;
# These options are used to add static routes
# Option Code 121 (0x79) - Classless Static Route Option
        option rfc3442-classless-static-routes code 121 = array of unsigned integer 8;
        option rfc3442-classless-static-routes 24,192,168,122, 192,168,122,1;
#  Option Code 249 (0xF9) - Microsoft Classless Static Route Option
#       option ms-classless-static-routes code 249 = array of unsigned integer 8;
#       option ms-classless-static-routes 24, 192, 168, 123, 192, 168, 123, 1;
# This distributes a route entry for network using the gateway
# The meaning of the bytes is (in brackets the value from the example above):
# NM, D1, D2, D3, R1, R2, R3, R4
# NM      = destination network mask width (24)
# D1..D3  = destination network address (192.168.1.*)
# R1..R4  = router address (
::::::::::::: CUT SOME OUTPUT ::::::::::::: 
Note: has put some note as well started with "#" symbol to understand more. And the default route can be disabled based on our requirements.

Step:2 To restart dhcpd service after any changes in the configuration file.

# systemctl restart dhcpd

Step:3 To verify the static route configuration from the client systems.


Though, there is an alternative DHCP option syntax has mentioned in the red hat documents, but for me below alternative DHCP option syntax doesn't work.

 option classless-static-routes;

Hope this post will help.

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