Re: [dhcwg] Minimum length for inbound DHCP packets

Tobin Coziahr <tobin.coziahr@sun.com> Tue, 05 March 2002 22:43 UTC

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Date: Mon, 04 Mar 2002 14:27:10 -0800
From: Tobin Coziahr <tobin.coziahr@sun.com>
Organization: Sun Microsystems
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To: Greg Kilfoyle <gregk@redback.com>
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Subject: Re: [dhcwg] Minimum length for inbound DHCP packets
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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

Greg Kilfoyle wrote:
> 
> 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 (gregk@redback.com)
> 
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-- 
Tobin Coziahr			650-786-7118 (x87118)
Solaris Networking		tobin.coziahr@sun.com
Sun Microsystems


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