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Ftp high port

Posted on 2002-06-13
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What is the ftp high-port ? Is it about ftp active and passive mode ?
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Question by:rotaris357
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cincin77 earned 50 total points
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Different from other TCP/IP applications, FTP uses 2 ports instead of one; one for control data and the other for the data. In normal operation; you open a connection to the port 21 of the ftp server.(this is the control connection) For this connection your local machine also has a port #.

The commands you enter like GET, PUT go through this channel. Aa the time comes to send or receive some data, FTP server opens a connection from its port # 20 to a local port in your machine which you sent to the server in advance through the control channel. This local port should be the 'high-port'.

However today most client machines are behind a firewall so it is not possible for the ftp server to open a connection to your machine. So your ftp client asks the server if it supports passive mode in which case the data connection is again opened by your client machine.

regards...
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by:geoffryn
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Here is a great resource that explains FTP Active vs PASV in detail.

http://slacksite.com/other/ftp.html
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by:The--Captain
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>However today most client machines are behind a firewall
>so it is not possible for the ftp server to
>open a connection to your machine

What kind of firewall are you using?  Mine certainly does do this (supports non-passive FTP) - it's called a stateful firewall, and any firewall that can't do it (these days) is basically crap (although most older firewalls *were* crap, and couldn't do it, hence passive FTP)

Otherwise, all the info I see here seems good, particularly geoffryn's URL - I vote for his answer...

Cheers,
-Jon


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by:cincin77
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It is not a good idea to allow incoming tcp connection requests.
it can be compromised.
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by:The--Captain
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>It is not a good idea to allow incoming tcp connection
>requests. it can be compromised.

Please refer to the CERT ID that indicates such vulnerabilities in iptables (I would also be interested in *any* CERT advisory regarding a stateful firewall).

Cheers,
-Jon
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by:rotaris357
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Thanks Cincin77 and geoffryn that make me clear , but I choose Cincin77 because he answers the question with his intention , and I think he can make me clear in the short of the detail.
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by:The--Captain
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Whatever works...

Cheers,
-Jon
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