Re: DHCP question: How to specify a static route _with_ netmask?
From: Derek Robert Price (derek_at_ximbiot.com)
Date: 07/29/04
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Date: Thu, 29 Jul 2004 11:04:12 -0400 To: W Cardwell <wrong@email.address>
W Cardwell wrote:
> RFC 3442 describes a classless static-routes option that encodes a prefix
> length within the 4-byte destination address. Not all clients support it and
> I haven't looked to see if Windows does, but here's a link:
> http://www.faqs.org/rfcs/rfc3442.html
Spent a lot of time researching this, so I thought I'd post an answer
despite the post's age. Hopefully Google will archive it.
Windows 2000 & XP, at least, support routes with netmasks. I'm not sure
about other clients. Instructions for configuring a Windows 2003 DHCP
Server are here:
<http://www.microsoft.com/technet/community/columns/cableguy/cg1003.mspx>.
There were several other Windows Tech Net articles I found via Google
on similar subjects as well.
The gist is, the RFC Mr. Cardwell sent above, and several others I
found, specify a DHCP code of 121 or "unassigned" for
classless-static-routes. Microsoft went with 249, maybe just to be
different or possibly to be first. Who knows.
Based on RFC 3442, RFC 2132, the code 249 from the Windows
documentation, and the "DEFINING NEW OPTIONS" section of my
"dhcp-options(5)" man page (from the ISC DHCPD 3.0p11 installation), I
added the following lines to my Linux DHCPD server's /etc/dhcpd.conf,
configuring my Linux dhcpd to send classless static routes suitable for
configuring Windows DHCP clients:
option new-static-routes code 249 = string;
option new-static-routes 18:0a:00:00:0a:02:00:01;
The first line defines a new option, "new-static-routes", sent as DHCP
code 249 (see the DHCP RFCs for an explanation of codes:
<http://www.faqs.org/rfcs/rfc2132.html>) and must be put in the global
section of the dhcpd.conf.
The second line above uses the new definition of new-static-routes and
may appear in global, subnet, or host sections of the dhcpd.conf file.
In this particular case, it defines 10.2.0.1 as the router for the
10.0.0.0/24 subnet, using a hex encoded data string based on the
specification in RFC 3442.
Unfortunately, my DHCPD does not yet support arrays of strings, so I
could only configure it to send a single subnet/router combination, but
that was all I needed. I'm not sure what happens if you specify
new-static-routes multiple times in dhcpd.conf.
In theory, if you had a client that used the RFC 3442's DHCP code of 121
for the option, you would need to define a second DHCP option and repeat
the classless static route definition line for that option number as
well for the other clients.
Cheers,
Derek
-- Derek R. Price CVS Solutions Architect Get CVS Support at Ximbiot <http://ximbiot.com>!
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