Subject: [dhcwg] Minimum length for inbound DHCP packets

From: Greg Kilfoyle <[email-masked]>
Date: 04 Mar 2002 12:22:04 -0800

Hi,

For a DHCP server or relay agent, I was wondering what is a reasonable
minimum length to check for on inbound packets. This is for DHCP support
only, no BOOTP support.

I was thinking an initial check should use the BOOTP minimum length and
check for a DHCP packet length of 300 bytes (which makes 328 bytes
including normal IP and UDP headers).

Using a test tool to test the DHCP server implementation showed that the
test tool was sending DHCP packets of less than 300 bytes (and the above
check was failing).

On one hand this is a problem with the test tool; on the other hand, how
many clients out there do not send the minimum BOOTP length? In other
words, if I don't allow less then the BOOTP minimum length, how many
clients will not work correctly?

Maybe a more forgiving approach would be a length that includes the
magic cookie and at least one option (an end option being 1 byte).

Any thoughts welcome.

Thanks, Greg.
-- 
Greg Kilfoyle ([email-masked])

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From: Tobin Coziahr <[email-masked]>
Date: Mon, 04 Mar 2002 14:27:10 -0800

Greg-

The generally accepted minimum length that I've seen in use is 240
bytes.  This is the bare minimum, the basic fields and the magic
cookie.  I actually had to update a server recently that was dropping
packets under 300 bytes, too.  Clients are more frequently using packets
under the BOOTP minimum size (Which is perfectly legal under RFC 2131).

-Tobin

-- 
Tobin Coziahr			650-786-7118 (x87118)
Solaris Networking		[email-masked]
Sun Microsystems