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I am trying to configure options for a DHCP network and the proprietary software (that is layered on top of ISC's DHCP server) that I am using won't allow me to put in an IPv6 address. In other scenarios it will allow this, but not for DHCPv4 client option (6) DNS server.

Is it just "come on... you can't mix IPv6 with DHCPv4"? As in, there is some kind of inherent problem with the two? Or is it one of those arbitrary RFC definitions that doesn't really have to be there? Or maybe the authors just weren't thinking of IPv6 when they wrote it, so they didn't include that possibility (...nor did they retrofit it...)?

...Or is it likely something the proprietary software restricts, but DHCPd would be happy with?

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Watki02
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3 Answers3

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It's normal, I still see multiple enterprise network that just disable IPv6 on the client NIC and strictly use IPv4. Putting a IPv6 there in a scoop option for DHCPv4 would break the client to be able to contact the DNS server in such case.

It's like to tell your user to fill a web ticket to receive support, but their PC is broken.

yagmoth555
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  • I am familiar with the problem, and maybe that is why the "layered-on" software doesn't allow it, but can it be done with just ISC software? – Watki02 Apr 20 '20 at 15:03
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Is it just "come on... you can't mix IPv6 with DHCPv4"?

DHCP and DHCPv6 are different protocols with different options. OPTION_DNS_SERVERS (23) in DHCPv6, per RFC 3646 and the IANA list.

A screen telling you a v4 address is necessary, and hinting this is DHCP option 6, is not a DHCPv6 (or a dual stack) field. Ask the software's support about how it manages IPv6 DNS servers, and mention DHCPv6.

John Mahowald
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  • So that tells me the software doesn't like it... (of course I know that I am not using a DHCPv6 option) but you didn't say it is an inherent problem, right? I think it could make sense to apply an IPv6 address here. – Watki02 Apr 20 '20 at 15:01
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The RFC 2132 defining DHCP option 6 tells this in section 3.8:

3.8. Domain Name Server Option

The domain name server option specifies a list of Domain Name System (STD 13, RFC 1035 [8]) name servers available to the client. Servers SHOULD be listed in order of preference.

The code for the domain name server option is 6. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.

Code   Len         Address 1               Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
|  6  |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...
+-----+-----+-----+-----+-----+-----+-----+-----+--

Here multiple of 4 bytes clearly means it is an IPv4 address, as the legend above the chart tells. There's no provision in the specification of DHCP option 6 to provide or distinguish an IPv6 address (rather than four IPv4 addresses).

So the proprietary software dutifully follows the specification for the sake of interoperability with all other DHCP implementations: IPv4 only.

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