Ftp high port

Posted on 2002-06-13
Last Modified: 2010-04-11
What is the ftp high-port ? Is it about ftp active and passive mode ?
Question by:rotaris357
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Accepted Solution

cincin77 earned 50 total points
ID: 7075261
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.

LVL 11

Expert Comment

ID: 7075728
Here is a great resource that explains FTP Active vs PASV in detail.
LVL 16

Expert Comment

ID: 7084197
>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...


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Expert Comment

ID: 7085821
It is not a good idea to allow incoming tcp connection requests.
it can be compromised.
LVL 16

Expert Comment

ID: 7091048
>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).


Author Comment

ID: 7101463
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.
LVL 16

Expert Comment

ID: 7103275
Whatever works...


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