Re: FTP Server on Win XP Pro

From: Alun Jones (alun_at_texis.com)
Date: 04/30/03 

Date: Wed, 30 Apr 2003 14:29:05 GMT

In article <008e01c30ee5$88a41460$3401280a@phx.gbl>, "BigAl"
<bigallanrogers@hotmail.com> wrote:
>227 Entering Passive Mode (192,168,1,104,5,154).
>connecting to 192.168.1.104:1434
>- -
>connecting to 192.168.1.104:1434
>Connected to 192.168.1.104 port 1434
>LIST
>426 Connection closed; transfer aborted.
>! Retrieve of folder listing failed (4)
>
>Any suggestions? It looks like my computer freaks out=20
>with the whole passive thing, but if I don't use a=20
>passive connection in the FTP client, I can't connect at=20
>all. Please help! Thanks

This looks like you're running into a problem with your NAT router. Most
NAT routers are able to translate the PASV response, so that it gives an
external IP and port - but only if they know that you are running FTP.
Usually, they decide that you are running FTP by the fact that you are
operating the service on port 21, the well-known port for FTP. In your
case, however, you're running on a different port, 8181, and so the NAT has
no way to tell that this is FTP traffic, and that it should watch out for
PASV traffic. Some more expensive and more powerful NATs have the ability
to say "treat traffic on port X as if it were FTP", but I am not aware of
which particular models do this - most simply assume that FTP traffic is
only on port 21.

The key to note here is that the server is behind a NAT, so PASV is reliant
on the NAT monitoring FTP traffic for the PASV response. If the client is
behind a NAT, too, you'll be reliant on the client's NAT to modify any PORT
commands, and that NAT, too, will be likely to not think the traffic is FTP,
and therefore not modify the PORT commands. So, essentially, you're trying
to do something that isn't likely to work with your current configuration.

Possible solutions:
1. Move the server to port 21.
2. Buy a very expensive NAT router (or, if you have a very expensive NAT
router, configure it so that it recognises traffic on port 8181 as FTP, and
enables the FTP ALG - Application Level Gateway).
3. Give the server a public IP address - put it outside the NAT.
4. Give the client a public IP address - put it outside the NAT.

Merely opening the passive ports up, as BB suggested, will not help, because
the IP address needs to be translated, and only the NAT can do that
correctly. [Try and translate it in the application, as some FTP servers
do, and you run the real risk of providing an IP address and port number
that the NAT has already assigned to another mapping.]

Alun.
~~~~

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