[Zope-dev] ZServer and ftp port-1 data channel

[Shane Hathaway]

Here's what Sam Rushing told me regarding this issue.

Shane Hathaway writes:
 > I hear that you have found a way to get Medusa to use the correct
port
 > in active mode FTP.  This could be a great benefit to the Zope
 > community.  Anything you have (diffs, modified files, or any kind of
 > info) on the subject would be very much appreciated.

(Sam writes:)
Is this the 'L-1' thing?  Where the server-side port for the data
channel is supposed to be '20' (I think that's right).  I made some
changes to the server to support it, but it's disabled by default;
because

  1) most people don't care
  2) it has the potential to break the whole server
     in certain situations, depending on the OS.  Other ftp servers
     get all acrobatic trying to Do The Right Thing, IMHO not worth it
     in medusa's case.

See the 'bind_local_minus_one' variable of the ftp_channel class.
This should be checked into the medusa cvs, but I'll attach a copy as
well.

---

The next email:

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Shane Hathaway writes:
 > wu-ftpd solves this by temporarily restoring root privileges and
 > actually tries more than once to bind to the correct port, which is
 > ugly.

(Sam writes:)
It's even worse than that.  Here's a quote from proftpd-1.2/src/inet.c:

    /* According to one expert, the very nature of the FTP protocol,
     * and it's multiple data-connections creates problems with
     * "rapid-fire" connections (transfering lots of files) causing
     * an eventual "Address already in use" error.  As a result, this
     * nasty kludge retries ten times (once per second) if the
     * port being bound to is INPORT_ANY)
     */

    (...)

In other words, it's just bound to fail.  A forking server can afford
this sort of persistence, but in medusa I think it'd be overkill; I'd
have to set up event timers, an object to maintain the retry state,
etc.... [this alone it might add 50% to the size of ftp_server.py]

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I'd have to agree with Sam, it's just not worth the effort.

Shane

Kent Polk wrote:
> 
> It appears we still have a problem with ZServer's port 20 data
> channel response behavior with the latest released Zope (Zope 2.2.1
> (source release, python 1.5.2, linux2) and Active Mode ftp clients.
> 
> In particular, when you select the port-1 behavior, medusa can't
> seem to bind to port 20 when port 21 command channel is specified.
> 
> "ftp_server.py" line 278:
>         if self.bind_local_minus_one:
>                 cdc.bind (('', self.server.port - 1))
> 
> After verifying that self.server.port - 1 = 20, the current behavior
> is that medusa simply hangs at the bind statement. If there was a
> permissions problem my understanding (and experience) is that one
> would receive:
> 
>  socket.error: (13, 'Permission denied')
> 
> and not a process hang.  Any idea why medusa simply hangs on the
> attempted bind?