ipfw query

Martin Hopkins martin.hopkins at insignia.com
Wed Jun 20 08:43:14 BST 2001

>>>>> "Matthew" == Matthew Seaman <m.seaman at plasm.demon.co.uk> writes:

    Matthew> On Tue, Jun 19, 2001 at 10:04:29PM +0100, Jose Marques wrote:
    >> On Tue, 19 Jun 2001, Jose Marques wrote:
    >> > I've noticed that the timer gets reset back to 300 whenever there's
    >> > traffic so I guess it won't cause problems unless the connection is
    >> > dormant for over 300 seconds (a pain for ssh sessions).
    >> Please ignore this, I forgot that ftp uses two connections.  I assume that
    >> the control connection lies dormant during the transfer and gets caught by
    >> the 300 second limit.

    Matthew> Ah, but now you have penetrated to the thing that
    Matthew> perplexes me.  FTP traditionally has used two connections
    Matthew> by default, but because the second connection is made
    Matthew> from the remote server back to you, it's often considered
    Matthew> too risky to allow in many firewalls, including the
    Matthew> rulesets that were shown earlier in this thread.

    Matthew> Hence the alternative of passive mode FTP, where control
    Matthew> and data use the same channel.  This is typically the
    Matthew> default for Netscape accessing ftp:// URL's, and setting
    Matthew> FTP_PASSIVE_MODE=YES in your environment will enable it
    Matthew> for ftp(1) and fetch(1).

    Matthew> It's during long lasting passive mode FTP sessions where
    Matthew> I was seeing the problem.  The data comes down just fine,
    Matthew> but the immediately following exchange of packets to
    Matthew> shutdown the connection gets filtered, and fetch just
    Matthew> hangs.

I may be wrong, but I thought passive ftp still used two connections -
PASV asks the server to open a port for the data connection and tell
the client so that it can connect whereas with PORT the client opens a
port and sends the address to the server which then connects.


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