[Dnsmasq-discuss] Getting dnsmasq to serve fqdns as the hostname

Simon Kelley simon at thekelleys.org.uk
Fri Oct 17 21:55:11 BST 2008

Alex Kiernan wrote:
> On Wed, Oct 15, 2008 at 9:38 PM, Simon Kelley <simon at thekelleys.org.uk> wrote:
>> Alex Kiernan wrote:
>>> On Tue, Oct 14, 2008 at 3:07 PM, Simon Kelley <simon at thekelleys.org.uk>
>>> wrote:
>>>> Alex Kiernan wrote:
>>>>> On Tue, Oct 14, 2008 at 1:58 PM, Simon Kelley <simon at thekelleys.org.uk>
>>>>> wrote:
>>>>>> Alex Kiernan wrote:
>>>>>>> I'm trying to use dnsmasq to serve DHCP/DNS for a Solaris network
>>>>>>> install server (jumpstart). It works perfectly and the DNS/DHCP
>>>>>>> integration means that lots of the traditional problems go away (the
>>>>>>> problem is that the Solaris installation procedure won't setup DNS as
>>>>>>> the name service unless you already have DNS for the machine you're
>>>>>>> building in place).
>>>>>>> dnsmasq works absolutely perfectly for this, with one minor problem -
>>>>>>> I need to serve an FQDN in the hostname option; if I feed this to
>>>>>>> dnsmasq in a dhcp-host then unless my domain setting matches it
>>>>>>> rejects it, and even then it strips it to the basename. If instead I
>>>>>>> force through an option 12, whilst it serves the FQDN, it doesn't
>>>>>>> update the DNS so the DNS checks fail later in the install.
>>>>>>> At the moment I'm working around it by removing the section of code
>>>>>>> from check_dhcp_hosts which enforces the domain/fqdn
>>>>>>> matching/stripping behaviour, but is there another way? Or another way
>>>>>>> which I could implement which is cleaner?
>>>>>> If you explicitly send the FQDN as option 12, set the basename (using
>>>>>> dhcp-host) _and_ set the domain in dnsmasq to the correct value, I
>>>>>> think
>>>>>> that will do what you want assuming I've understood correctly that you
>>>>>> want
>>>>>> to
>>>>>> 1) Send the FQDN as option 12
>>>>>> 2) have the FQDN resolve in DNS to the IP address allocated.
>>>>> Yeah, I should have mentioned that combo - it does exactly what I
>>>>> need, but only for one domain, whereas I need it to work for many
>>>>> domains (as I want this box to act as a jumpstart server across many
>>>>> different hosts). If domain could be scoped with a tag that'd do what
>>>>> I need (or even just support multiple maybe?)
>>>> That (domain scoped with a tag) is a long-standing feature request which
>>>> is
>>>> impossible to do in a backwards compatible way. See this list passim for
>>>> the
>>>> gory details.
>>>> HOWEVER, the next best thing (domain based on subnet or IP address range)
>>>> is
>>>> available at the bleeding edge. Try
>>>> http://thekelleys.org.uk/dnsmasq/test-releases/dnsmasq-2.46test15.tar.gz
>>> Thanks - I've had a play and it doesn't really do what I want as
>>> whilst I have some networks which have one domain on a subnet, we have
>>> others where we end up with a bunch of machines in differing domains.
>>> Is there a good way to go about adding FQDN hostname functionality[1]?
>>> If I could just add that one feature it'd do exactly what I need (when
>>> coupled with the dhcp-fqdn option).
>>> [1] which I'm happy to do and send you the diffs.
>> The problem is that the format of the leases file is fixed to contain a
>> non-fully qualified domain, and no domain field. To work, dnsmasq has to be
>> able to determine the FQDN of a host using just the information in the
>> leases file. Hence the new arrangement, which allows the domain to be
>> derived from the IP address.
>> To allow arbitrary domains means breaking the lease file format, which I
>> don't want to do.
>> Presumably, you add some configuration for each new host that you install,
>> if that consists of something like
>> dhcp-host=<mac address>,<IP address>,<bare name>,net:<FQDN>
>> dhcp-option=net:<FQDN>,12,<FQDN>
>> domain=<domain-fromFQDN>,<IP>,<IP>
> Pretty much - a complete (Sparc) example is this:
> dhcp-host=01-0:3:ba:12:c6:21,,net:010003BA12C621,"target"
> domain=example.com,
> dhcp-option=010003BA12C621,12,"target.example.com"
> dhcp-option=010003BA12C621,vendor:SUNW,11,"jumpstart.example.com"
> dhcp-option=010003BA12C621,vendor:SUNW,10,
> dhcp-option=010003BA12C621,vendor:SUNW,12,"/export/install/sol_10_508_sparc"
> dhcp-option=010003BA12C621,vendor:SUNW,3,"jumpstart.example.com"
> dhcp-option=010003BA12C621,vendor:SUNW,2,
> dhcp-option=010003BA12C621,vendor:SUNW,4,"/export/install/sol_10_508_sparc/Solaris_10/Tools/Boot"
> dhcp-option=010003BA12C621,vendor:SUNW,14,""
> dhcp-option=010003BA12C621,vendor:SUNW,13,""
> dhcp-boot=net:010003BA12C621,"010003BA12C621","jumpstart.example.com",
>> then that should work, domains don't have to be over whole subnets, just
>> defined by IP address.
> I was trying to avoid having to do static IP configuration at build as
> even though that's the final config I need for most boxes, there are
> some places where we have jumpstart servers on workstation networks
> which are largely DHCP based; but thinking about it, on those networks
> I'm constrained to using ISC DHCP anyway.
> So next question... is adding support for dhcp-bootfile (to supply
> multiple dhcp-boot options) and domain-file (to supply multiple domain
> entries) as easy as it looks it ought to be? I'm happy to hack it in,
> just wanted to make sure I wasn't opening a can of worms first!

You can supply multiple dhcp-boot options (with different net tags) is 
the standard configuration file, or another file included from it. The 
only extra that the dhcp-hostfile approach gives is the ability to 
re-read the file without re-starting dnsmasq. For most applications, 
re-starting dnsmasq is preferable (its always fast)

  The only difficult  part of hacking in this stuff is making sure that 
exiting records are garbage-collected and freed properly before re-reading.

> Sorry, one last question - is there a way to stop dnsmasq sending a
> router option for a network?

Yes, for options which are send automatically, specifying an empty 
option suppresses this :






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