Subject: [dhcwg] Can a DHCPv4 server REPLY to non-standard port.

From: Senthil Kumar B <[email-masked]>
Date: Tue, 04 Feb 2003 15:13:00 +0530

I heard about cases where a DHCP server like ISC DHCP runs on a
different port (other than the standard port 67). In that Can I assume
the client also run on a selected non-standard port(other than the
standard port 68).

I just wanted to clarify one thing here. When a DHCP server runs on a
non-standard port (may be 1070) and the client is running in (may be
1080, instead of 1071 conventionally), the server should understand the
client's port from the client's request packet and reply to the
port(1071) accordingly.

Will there be any problem if the client runs on a non-standard dynamic
port(varies for different dhcpclients)? One of the problem i could see
is that, the client (may be customized) can request all the IP addresses
and eventually the server will be running out of addresses to assign
(Because, any normal user  can write up a program and send DHCP messages
to acquire all the IP address).

Is this scenario violating the standards and if so how to avoid the
problem ?

Appreciate your help in advance,

Thanks,
Senthil

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From: "Vasu, Vallabhaneni" <[email-masked]>
Date: Tue, 04 Feb 2003 07:39:57 -0600

> to acquire all the IP address).

This problem can happen even if the client uses the standard ports. All you
need to do is write c code to generate different client id's (MAC or option
61). This issue is  being handled in RFC 3118.

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From: Bud Millwood <[email-masked]>
Date: Tue, 4 Feb 2003 16:06:27 +0100

> 61). This issue is  being handled in RFC 3118.

We respond to the source port the client is using, but allow the administrator 
to override this at the server. It seemed like a bad idea to respond to a 
fixed port if no communication had been initiated from that port.

The best way to handle spoofing today, to my knowledge, is to use the trusted 
values found in option 82 for identifying the client. If your server supports 
resource limiting using option 82, and your relay agents support option 82, 
you should be fine.

Otherwise, some sort of pre-registration works, although that's a bit of a 
pain to manage.

Bud Millwood
Weird Solutions, Inc.
http://www.weird-solutions.com
tel: +46 8 758 3700
fax: +46 8 758 3687
mailto:[email-masked]

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From: Ted Lemon <[email-masked]>
Date: Tue, 4 Feb 2003 14:37:01 -0600

> fixed port if no communication had been initiated from that port.

The RFC is explicit about the port numbers to use when sending 
messages.   If you send to or from other ports, you are not doing DHCP. 
   Which is not to say that what you are doing is wrong for the 
particular case you're working on, but it isn't what's stated in the 
protocol specification.   You can't count on being able to interoperate 
successfully with conforming devices if you don't use the port numbers 
that the standard requires.