Let's look at libcurl trying to retrieve a file from FTP server with
default settings:
1) connection and login steps;
2) EPSV command, receiving server's open port and trying to connect;
3) If step 2 fails (due to firewall restrictions for example) trying
to send PASV command and retrieve server's port number in old
fashioned way, else step 4;
4) receiving file if data connection is established.

The point is that libcurl correctly handles (switching to PASV mode)
connection error in step 2 in the case of direct connection and
returns CURLE_COULDNT_CONNECT if we trying to connect via SOCKS proxy,
thus aborting file retrieval because of proxy connection error. In my
test setup I use local proxy and have no access to company's firewall
but the firewall (TMG) can analyze FTP traffic and open required ports
for PASV commands and doesn't support EPSV.
I think libcurl should correctly handle step 2 in the case of proxy
connection. Does this behavior should be considered as bug? Should I
file a bug report for this?
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Received on 2012-11-07