Date: Sat, 19 Feb 2000 21:38:49 -0500 From: "Crist J. Clark" <cjc@cc942873-a.ewndsr1.nj.home.com> To: Charles Mott <cmott@scientech.com> Cc: questions@FreeBSD.ORG Subject: Re: Redirecting/mapping ports to a local machine... help! Message-ID: <20000219213848.H60348@cc942873-a.ewndsr1.nj.home.com> In-Reply-To: <Pine.LNX.4.10.10002191856540.22579-100000@if.scientech.com>; from cmott@scientech.com on Sat, Feb 19, 2000 at 07:05:27PM -0700 References: <20000219203204.G60348@cc942873-a.ewndsr1.nj.home.com> <Pine.LNX.4.10.10002191856540.22579-100000@if.scientech.com>
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On Sat, Feb 19, 2000 at 07:05:27PM -0700, Charles Mott wrote:
> On Sat, 19 Feb 2000, Crist J. Clark wrote:
> > This is a server issue not a client issue. An ftp client can do
> > passive ftp from behind a NAT box. However, active ftp would not
> > work.
>
> This is not correct. There is specific code in the
> packet aliasing library used by natd for handling "active"
> (i.e. non-passive) connections. Essentially, the packet
> aliasing code looks for a PORT command in the control
> stream and then sets up a back-channel to wait for the
> port 20 control connection from the ftp server.
>
> Many people use ftp in non-passive mode from behind natd
> without any problems.
Many people get lucky then. From the alias_ftp.c source,
For this routine to work, the PORT command must fit entirely
into a single TCP packet. This is typically the case, but exceptions
can easily be envisioned under the actual specifications.
> > Use of a control channel and a data channel is a basic part of the ftp
> > protocol. See RFC 959. Unimplemented RFC 2428 might be interesting too.
But we need to point out that the this special handling of FTP by NAT
is for _clients_ behind the NAT box only, not servers.
--
Crist J. Clark cjclark@home.com
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