Hello Mats,
when I start the FTP client and connect to the FTP proxy I have no chance to specify the
FTP server I want to connect to prior to getting the input prompt from the FTP client and
entering the "user" command with the appropriate informations.
And since the FTP client immediately sends a SYST command to the FTP Proxy
(which is the FTP server from the point of view of the FTP client in this
moment) and the FTP proxy doesn't know what it should answer to this request
the FTP client was sending it closes the socket, and afterwards my only chance
is to leave the FTP client.
The answer to the SYST command should be as far as I know some informations about the
target system (FTP server). If the FTP proxy would answer this request with informations
about the system the proxy is running at this can be right or wrong. Let's assume the
proxy is running at a Windows Server and the target FTP server is running at a Unix or
Linux box. If the FTP proxy would answer the SYST command with the information "here
is a Windows Server" this answer will not match with the real FTP server after I
connected to it.
Due to that I think it's wrong in the case of using a FTP proxy that the FTP
client asks for the servers system informations. It might do so not earlier
than having connected to the target FTP server (via the FTP proxy server).
I think I've stated that at least the HP-UX implementation of the FTP client implements an
option usable in such cases: -c => neither SYST nor TYPE calls are made to the FTP server
or FTP proxy prior to establishing a connection. Note that the FTP proxy connects to the
real server by using informations given in the "user" command.
Regards,
Johannes
Mats Erik Andersson wrote on 5/12/2011 4:44 PM:
torsdag den 27 januari 2011 klockan 18:31 skrev Johannes Schmidt-Fischer detta:
When I use the ftp client command with option -n for connecting
to an FTP server via FTP proxy server the ftp client immediately
sends the SYST command to the FTP proxy who cannot answer this
request since it isn't connected yet to FTP server.
Transcript:
$ ftp -d -n ftp-proxy 2121
Connected to ftp-proxy.example.com
220 Ready
ftp: setsockopt: Bad file descriptor
---> SYST
421 Service not available, remote server has closed connection
ftp> bye
Is this behaviour allowed for an FTP proxy service? Should it not
transparently appear as a fully fledge FTP server, acting as such
seen from the view point of the FTP client?
That is my expectation, and if the assumption is legitimate, then
the fault lies with the FTP proxy, not in our client package.
Best regards,
MEA, co-maintainer
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