Hi,

I recently encountered a situation where I needed to configure DHCP clients to install static routes in order to reach their DNS servers, which are provided via DHCP but reside outside the client subnet.

Network Context:

• Client subnet: 10.35.0.1/24

• DNS servers: 10.51.0.15 and 10.51.0.22 (reachable via the subnet gateway at 10.35.0.1)

This setup works fine under normal circumstances. However, problems arise when some clients enable a full-tunnel VPN, which installs a broad static route like 10.0.0.0/8 and overrides more specific paths. Unfortunately, modifying this VPN behavior is not an option.

To address this, I attempted to advertise static routes for the DNS subnets using DHCP Option 121.

Config Using String Format (does not work):

• Option Code: 121

• Type: String

• Content: 24,10,51,0,10,35,0,1

This results in a malformed route. In Wireshark, I noticed that the third value (51) is misinterpreted as a subnet mask width, resulting in an invalid “mask width 51” error.

Config Using HEX Format (works):

• Option Code: 121

• Type: HEX

• Content: 18:0A:33:00:0A:23:00:01

This correctly represents the desired route:

• Destination: 10.51.0.0/24 → 0x18 0x0A 0x33 0x00

• Gateway: 10.35.0.1 → 0x0A 0x23 0x00 0x01

What I would like/expect:

I’ve managed to make it work using the hex format, but managing routes via the string format would be much more convenient and readable

Would appreciate any insights or workarounds. Thanks!