Re: [dhcwg] Option 119 (Domain Search Option) and Option 15 (Domain Name Option)

Alfred Hönes <ah@TR-Sys.de> Tue, 30 November 2010 20:52 UTC

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Subject: Re: [dhcwg] Option 119 (Domain Search Option) and Option 15 (Domain Name Option)
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Ted Lemon wrote:

> On Nov 30, 2010, at 1:07 AM, VithalPrasad Gaitonde wrote:
>> Is there any specification on the client behaviour when both
>> option 15 and option 119 are provided in the server response:
>
> Unfortunately, there is no specification.   Since the DSL option
> was documented after the Domain Name option, one could assume
> that it takes precedence.
>
> It would probably be worth updating RFC3397 to correct this
> problem--it's actually a pretty serious omission.

I guess everybody once saw RFC 3397 as a functional replacement
for option 15, since the DSL (#119) allows a compact representation
of multiple domain names by making use of domain name compression
as in the DNS protocol.  (This pretty much matches your previous
comment on space conservation.)
Option, OTOH, #15 encodes a single domain name; it cannot reasonably
be concatenated, and it doesn't need to, since a single non-FQDN will
always be shorter than 253 octets.

>From a DNS resolver implentation point of view, any system that
supports the configuration of a DSL would admit and prefer a _list_
of domain names, if it's the intent of the administrator to provide it.
Since, for a conceptual domain search, list order matters,
whereas the order of different DHCP options is arbitrary,
it does not make sense to have both options in a DHCP response.

Therefore, I propose the following rules:

 o  DHCP servers SHOULD implement option 119; it looks like good
    implementations should have the ability to send a single
    domain name supplied by their configuration for a particular
    client (or group of clients) in either #15 or #119 format;

 o  DHCP clients that would like to get domain search information
    SHOULD request option 119; if it is configured with a domain
    search list by other means, it SHOULD NOT request either
    (note that a configured DSL MUST NOT be overwritten via DHCP);

 o  DHCP clients SHOULD NOT request both options 119 and 15,
    unless they have an implementation-specific way to resolve
    ambiguities resulting from both being received;

 o  DHCP servers SHOULD supply options 15 and 119 only if they
    have been requested to do so; gratuitous supply is a waste
    of packet space (see note on preconfigured DSL above);

 o  DHCP servers SHOULD ignore requests to supply both #15 and #119
    and only send #119 in such case.


Do current DHCP server/client implementations already conform
to these rules?


Kind regards,
  Alfred Hönes.

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