From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Craig Packard" Subject: UDP DNAT Problem Date: Thu, 3 Apr 2003 22:14:53 -0500 Sender: netfilter-admin@lists.netfilter.org Message-ID: <000f01c2fa58$5cd5b290$0b19a8c0@rock1> Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_000C_01C2FA2E.73D677B0" Return-path: Errors-To: netfilter-admin@lists.netfilter.org List-Help: List-Post: List-Subscribe: , List-Id: List-Unsubscribe: , List-Archive: To: netfilter@lists.netfilter.org This is a multi-part message in MIME format. ------=_NextPart_000_000C_01C2FA2E.73D677B0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable >From what I understand if I setup a DNAT rule, when a packet matching = the rule comes in it is sent to the specified host, obviously this is = done by changing the destination field to be the system 'behind' the = firewall. It was however my understanding--and what I've seen in = practice--that the source field would not be changed. So in other words = the source of the packet would still be the host out on the internet = that actually sent the original packet. However I have made a set of = DNAT rules that I couldn't get to work. So a setup a packet sniffer at = several points. Here is what I noticed and has me confused. The inside computer (A) sends out a UDP packet to the internet connected = computer (B), of course this packet goes through firewall (FW). The packet goes out as expect srcA dstB But the weird part is the response comes back in srcFW dst(A) Where I = would like it would be src(B) dst(A)!!! I think this is screwing up the communications. Can anyone help me = understand what is happening? - Craig ------=_NextPart_000_000C_01C2FA2E.73D677B0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
From what I understand if I setup a = DNAT rule, when=20 a packet matching the rule comes in it is sent to the specified host, = obviously=20 this is done by changing the destination field to be the system 'behind' = the=20 firewall.  It was however my understanding--and what I've seen in=20 practice--that the source field would not be changed.  So in other = words=20 the source of the packet would still be the host out on the internet = that=20 actually sent the original packet.  However I have made a set of = DNAT rules=20 that I couldn't get to work. So a setup a packet sniffer at several=20 points.  Here is what I noticed and has me confused.
 
The inside computer (A) sends out a UDP = packet to=20 the internet connected computer (B), of course this packet goes through = firewall=20 (FW).
 
The packet goes out as expect srcA=20 dstB
But the weird part is the = response comes=20 back in srcFW dst(A)   Where I would like it would be src(B)=20 dst(A)!!!
 
I think this is screwing up the=20 communications.  Can anyone help me understand what is=20 happening?
 
-    Craig
 
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