DHCP Options, Option 81 How to format value...

[rellek]

I want to implement DHCP Option 81 to relay to a local bind 9 DNS server that I have running to dynamically update it's database.

The documentation is horrific on how to set the value. Can anyone help?

[alemabrahao]

If it's an FQDN, you should probably use custom text.

https://documentation.meraki.com/General_Administration/Cross-Platform_Content/Setting_Custom_DHCP_O...

[rellek]

That doesn't really help. If you look up any of the documentation, it's boggling:

  The format of the Client FQDN option is:

        Code   Len    Flags  RCODE1 RCODE2   Domain Name
       +------+------+------+------+------+------+--
       |  81  |   n  |      |      |      |       ...
       +------+------+------+------+------+------+--

   The above figure follows the conventions of [12].

2.1.  The Flags Field

   The format of the 1-octet Flags field is:

        0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+
       |  MBZ  |N|E|O|S|
       +-+-+-+-+-+-+-+-+

Wireshark shows hex values similar to these, Microsoft used DWORD values. I'm lost on this one. 

[alemabrahao]

I got It. Have you tried the Custom Hex option?

http://www.unit-conversion.info/texttools/hexadecimal/

[rellek]

No matter what you actually format the value in... I don't know if I even have the correct value.

From a cached google page I was able to find something, but I don't know if it's right. I tried doing a packed capture, but I get no dhcp or dns results to the dns server ip.

IF the value I need is "23" the MX doesn't seem to care how it's typed. ALL of these examples equal the same. If you have any 2 of these in there, it gives an error that it's a duplicate:

[alemabrahao]

Take a look at this: https://jasonmurray.org/posts/2020/option81/

[PhilipDAth]

I don't know the answer.  I don't think this will work.

As I understand it, the client sends to the DHCP server DHCP option 81 informing the DHCP server of its FQDN.  If the MX is the DHCP server, it is not likely to use this option from the client, and even if it did receive the option there is no way to tell it to pass it onto a third-party DNS server.

DHCP 81 does not appear to be something that the DHCP server sends to the client.  It is the opposite way around.

[rellek]

Thanks Philip.

The info on the cached page I posted a link too makes it sound possible with this option.

My understanding is that when the DNS server saw the Option 81 data in the packets, it would see it and add it to the database. I could be wrong, I have been once before I think! 😉