DHCP option 119

James Cameron quozl at laptop.org
Tue Aug 24 00:30:21 EDT 2010

Summary: the option is supported, but not requested by the DHCP client
in the current builds.  The search value is set to the domain name.

Tested using os852 on XO-1.  The result will be the same on XO-1.5.

1.  the DHCP Request emitted by the XO does not contain a domain-search
(119) request, therefore not unexpectedly there will be no reply that
will contain it, and no use of the data, (method to test was to capture
packet stream with tcpdump and analyse with wireshark),

2.  a manual request for the purpose of testing can be formed by turning
off Network Manager, and using "dhclient -R domain-search eth0", and
this correctly includes the domain-search (119) item in the DHCP
Request, but as my DHCP server does not provide this item there was no
response to the option,

3.  there will likely be some method by which Network Manager can be
directed to pass -R domain-search or an equivalent to dhclient via the
-cf option, but offhand I don't know what it is,

4.  /sbin/dhclient-script does contain logic for handling the
domain-search (119) reply, so if an XO is configured to request it we
know the reply should be handled, and the /etc/resolv.conf file properly

5.  in the absence of domain-search (119), the search tag in
/etc/resolv.conf is set to the domain-name option value if any.

6.  a deployment customised build could potentially be produced that
implemented domain-search support.

The domain-search option to DHCP is defined in RFC 3397.

(Opinion: it seems to be a crutch for badly designed namespaces or
failure to use reasonable alias names.  A large search list can have a
huge impact on clients and a network, since queries may be repeated
until satisfied.  I recommend against using it in a deployment of XOs,
due to the potential for increased DNS requests helping to saturate the
radio environment.)

James Cameron

More information about the Devel mailing list