Bug 22855

It appears that libfetch will always send a SIZE and MDTM command to the
remote FTP server as part of _ftp_stat in /usr/src/lib/libfetch/ftp.c.
Sites (such as ours) which use an older proxy server that doesn't understand
how to forwared MDTM commands will always fail.  As a result, the fetch
will always fail.

Here's a cleaned-up tcpdump of the conversation that libfetch had with 
the proxy:

230 Guest login ok, access restrictions apply.
TYPE I
200 Type set to I.
CWD /pub/FreeBSD/ports/distfiles
250 CWD command successful.
SIZE qpopper3.0.2.tar.gz
213 1734453
MDTM qpopper3.0.2.tar.gz
500 command not understood

At this point, the whole fetch fails.

Fix: 

My temporary workaround is to commend out the MDTM section of ftp.c -- however
this is not an ideal solution.  Perhaps _ftp_stat should simply not fail
if one of these commands returns an error and replace the failure with some
safe data.
How-To-Repeat: 
1)  Trap yourself behind an old BSDi proxy server
2)  Make fetch anything in /usr/ports