[Dnsmasq-discuss] Option 120 (RFC 3361)

[Brian Candler]

On Mon, Nov 20, 2006 at 02:23:40PM +0100, Rune Kock wrote:
> A DHCP-server will only send those options that the client asks for.
> So maybe your client doesn't ask for option 120?

Ah, that'll be it :-)

OK, so I changed the client's /etc/dhclient.conf as follows:

  send host-name "wibble";
  option sip-server-address code 120 = { unsigned integer 8, ip-address };
  request subnet-mask,  broadcast-address,  time-offset, routers, domain-name, domain-name-servers, host-name, nis-domain,  nis-servers, ntp-servers, sip-server-address;

and dnsmasq still has:

  dhcp-option=120,1,192.168.1.1

Now, here's the end of the response I get back as seen by tcpdump:

        0x0140:  036c 616e 0c06 7769 6262 6c65 7808 ffff  .lan..wibblex...
        0x0150:  ffff c0a8 0101 ff                        .......

So there's option 120 (0x78), but for some reason the value '1' has been
encoded as ff ff ff ff, when I need it to be 01.

I tried changing to

  dhcp-option=120,1b,192.168.1.1

but then the response I got back was no good as the whole lot was intepreted
as an ASCII string:

        0x0140:  036c 616e 0c06 7769 6262 6c65 780e 3162  .lan..wibblex.1b
        0x0150:  2c31 3932 2e31 3638 2e31 2e31 ff         ,192.168.1.1.

Ah, but it seems I was misunderstanding the man page. OK, this seems to do
the trick:

  dhcp-option=120,01:C0:A8:01:01

Thanks for pointing me in the right direction.

It would be nice if I could mix'n'match integers and IP addresses though,
because then I could have

  dhcp-option=120,1,0.0.0.0

which would substitute the "correct" IP address automatically.

Regards,

Brian.