Forum Discussion

shoneycutt_8454's avatar
Icon for Nimbostratus rankNimbostratus
Nov 07, 2017

When using the radius monitor should the F5 accept the access-reject message as a successful response to the monitor?

I'm trying to use the radius monitor on an f5 running 11.4.1. I have configured the uname/password/secret information. In the debug of the monitor I see the access-request go out and the access-reject come back. I thought that this would be enough of a test to allow the monitor to mark the server as up. But it doesn't do that. Any thoughts?


4 Replies

  • documentation says: "Checks the status of Remote Access Dial-in User Service (RADIUS) servers. The check is successful if the server authenticates the requesting user. A RADIUS monitor requires a user name, a password, and a shared secret string for the code number."


    i read that as you need a successful authentication, but i can't find it clearer then here.


  • As my late father used to say: "Bugger off is also an answer". Here's a null radius request:

    Access-Request (1), id: 0x00, Authenticator: 00000000000000000000000000000000
      User-Name Attribute (1), length: 9, Value: 0000000
        0x0000:  3030 3030 3030 30
      User-Password Attribute (2), length: 18, Value: 
        0x0000:  3030 3030 3030 3030 3030 3030 3030 3030
      NAS-IP-Address Attribute (4), length: 6, Value:
        0x0000:  7f00 0001
      NAS-Identifier Attribute (32), length: 11, Value: localhost
        0x0000:  6c6f 6361 6c68 6f73 74 out slot1/tmm0 lis=

    And its (valid) 'bugger off' answer:

    Access-Reject (3), id: 0x00, Authenticator: xxxxxxxxxxxxxxxxxxxxxxxx in slot1/tmm0 lis= 

    Here's the UDP hex strings to send & expect: Send:




    Works a treat.

    BR Jan

  • Hi Jan,


    Awesome idea to construct the payload of a NULL-User Radius request. Thumps up for that one!


    Note: The expect-string seems to trigger on the (encrypted) Response Authenticator field (16bytes), so other peers would need to know your Shared-Key to adopt this solution. Looking for a sequence of \x03\x00\x00 (responce code, request id, first byte of the length field) should be a slightly more generic approach.


    Cheer, Kai


    • Bart's avatar
      Icon for Nimbostratus rankNimbostratus

      awesome , thanks for the idea