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List:       wuftpd-questions
Subject:    Re: FTP Server Welcome-Banner-Text?
From:       Gregory A Lundberg <lundberg+wuftpd () vr ! net>
Date:       2001-04-26 6:41:17
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On Wed, Apr 25, 2001 at 11:06:39PM -0400, Derman, Jon wrote:

> Looking at the data-packets with a network sniffer, I can see the the
> first packet is very short -- it consists of just that one first line
> (220-AGREEMENT ON TERMS OF USE).  The second packet is much longer (about
> 19 of those 220- lines).  What I'd like to know is this:
> 
> What is it that controls where one packet ends and the next packet
> begins?  Why is it that the first packet is just one line long, and the
> next packet is so much longer?  Might there be something embedded in that
> text that causes the packet to break at a certain point?  Is there
> something that the person who runs that FTP server could do to that
> welcome-text that would force packets to end at certain points within
> that text?

Several factors:

First, the write() pattern used in the daemon.  The first line is sent,
then all subsequent lines as a group.  This is because the code sending the
reply is not sure, when it sends the first line, that any more lines will
need to be sent .. they may not be there, or the user may have requested
they be suppressed by prepending a dash (-) to their password.

Second, the TCP implementation on the FTP server host.

Finally, current traffic and physical limitations of all routers in the
server-to-client (or, at least, server-to-sniffer) path.

If you think there is a problem with the packets as you're seeing them, you
need to correct your firewall.

-- 

Gregory A Lundberg              WU-FTPD Development Group
1441 Elmdale Drive              lundberg@wu-ftpd.org
Kettering, OH 45409-1615 USA    1-800-809-2195

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