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000175_jaltman2@nyc.rr.com_Sun Oct 23 14:19:16 2005.msg
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Path: newsmaster.cc.columbia.edu!newsfeed.nyu.edu!newsfeed.cw.net!news-FFM2.ecrc.de!informatik.uni-bremen.de!news.zfn.uni-bremen.de!news-ham1.dfn.de!news-lei1.dfn.de!newsfeed00.sul.t-online.de!t-online.de!border2.nntp.dca.giganews.com!nntp.giganews.com!cyclone1.gnilink.net!gnilink.net!cyclone.rdc-nyc.rr.com!news-out.nyc.rr.com!news-wrt-01.rdc-nyc.rr.com.POSTED!53ab2750!not-for-mail
From: Jeffrey Altman <jaltman2@nyc.rr.com>
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Newsgroups: comp.protocols.kermit.misc
Subject: Re: passive mode need to ignore IP returned from server
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Message-ID: <pad5f.3500$h25.517@news-wrt-01.rdc-nyc.rr.com>
Date: Tue, 18 Oct 2005 20:46:13 GMT
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LT wrote:
> I am connecting to a server that has 2 IP addresses (external and
> internal LAN). The server is configured to return the LAN IP.
>
> When I open a ftp session, I can connect and login to the server.
> > ftp open 32.97.xxx.xxx 21
>
> When I switch to passive mode and open a data connection, the server
> returns the LAN IP.
> > FTP RCVD [227 Entering Passive Mode (172,17,31,25,38,252)]
> ( IP = 172.17.31.25 , port = 38*256+252 = 9980 )
>
> Then LAN IP (172.17.31.25) uses a private IP network range and is not
> reachable from the outside world, but all I really need is the port
> number (9980).
>
> How can I get kermit to use the port number (9980), but continue to use
> the server name/IP that I originally used to start the FTP session.
>
> I found another FTP client that recognizes that the IP address returned
> from the server is different than the original IP and switches the IP
> back to the correct address. Is there an option
> to do this in kermit?
>
> Thanks, LT
That other FTP client would be broken. The idea in FTP is that there
can be more than two parties to each file transfer. It is perfectly
acceptable for the server to report an alternate address to have the
file set to. In this case, the server should be programmed to send
the correct IP address. If the server is multi-homed, then the server
should detect the address that is bound to the socket associated with
the command channel. If the connection is coming through a NAT, the
server should support SOCKS and obtain the correct IP address from the
NAT for the port that was opened.
Now, C-Kermit could of course be hacked to behave the same way as the
other client. The source code is publicly available. The source file
you want to look at is ckcftp.c
Jeffrey Altman
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