[dhcwg] Re: DHCP behind NAT

Bernard Dugas <bernard.dugas@is-production.com> Wed, 29 August 2001 09:42 UTC

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Date: Wed, 29 Aug 2001 11:32:17 +0200
From: Bernard Dugas <bernard.dugas@is-production.com>
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Subject: [dhcwg] Re: DHCP behind NAT
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I've had no comment on that. Is it the right procedure ?

The patch is loadable at http://www.is-pronet.com/download/, with the
original version and the patched version of ISC dhcpd we were using.

Best regards, and thanks for help of mailing lists and open source
process ! 

Bernard Dugas a écrit :
> Hi,
> I have a comment to do on both RFC :
> - Dynamic Host Configuration Protocol (RFC 2131);
> - DHCP Relay Agent Information Option (RFC 3046).
> I couldn't find in these RFC's any mention of the possibility of a DHCP
> client being on a subnet behind some NAT device like a router.
> NAT is used quite often, specially in the kind of network represented in
> RFC3046 Figure 1 page 3, with network clients behind modems.
> Even more, it seems that the behaviour described by RFC2131
> is preventing a dhcp server to answer to a client behind a NAT.
> 3 problems have been identified :
> - PB1) In the case of a unicast DISCOVER, with a relay involved, the
> server is answering to the ip address of the relay (giaddr field). In a
> non-NAT context, the giaddr is the same that the source ip address of
> the DISCOVER packet. But with NAT, the DHCP server MUST answer to the IP
> source address of the DISCOVER packet. This problem may happen also in
> other requests.
> cf. "  If the 'giaddr' field in a DHCP message from a client is
> non-zero,
>    the server sends any return messages to the 'DHCP server' port on the
>    BOOTP relay agent whose address appears in 'giaddr'. "
> in :
> " Droms                       Standards Track                    [Page
> 23]
> RFC 2131          Dynamic Host Configuration Protocol         March 1997
> "
> - PB2) The same apply for to UDP port of the OFFER packet : it should be
> the udp port of the IP source address of the DISCOVER packet, because
> NAT may have changed the port too ;
> - PB3) The DHCP server is issuing a ICMP echo on client address before
> answering with the OFFER. In a NAT context, the echo can't go on the
> client subnet. The RELAY should do the ping before relaying the OFFER.
> To illustrate the PB1, I can send you a patch on ISC dhcp 3.0.RC11,
> which allow our DHCP server to answer to clients behind NAT.
> I would be very grateful to have an answer on these problems. Thanks a
> lot.


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