ISA Clients, ISA Server Error 421

When users try to open a PDF on an FTP site they get the following error:
200 Type set to I.
200 PORT command successful
421 Server Error. (Trapped unexpected WarFileException). Connection closed.

ello,

The problem occurs because the target FTP server is failing to handle a
request
from the ISA server to retrieve a file, and is terminating the connection
with a
fatal error. The problem does not occur when ISA is not involved because a
direct
FTP client connection makes the request in a different, though equally
correct,
way. The primary difference is that ISA precedes the RETR command with a
CWD
against the filename. Apparently, the FTP server improperly handles this
and
abruptly ends to connection.

FTP is defined in RFC 959 (ftp://ftp.isi.edu/in-notes/rfc959.txt). The FTP
RFC
defines two verbs which are relevant to the issue you're currently
experiencing.
First, the CWD action which instructs the FTP server to change the current
working
directory to another directory. CWD is used to navigate through the
directory tree
hosted on the FTP server:

CHANGE WORKING DIRECTORY (CWD)

This command allows the user to work with a different
directory or dataset for file storage or retrieval without
altering his login or accounting information. Transfer
parameters are similarly unchanged. The argument is a
pathname specifying a directory or other system dependent
file group designator.


Second, the RETR verb instructs the FTP server to send the file requested
back to
the requesting client:

RETRIEVE (RETR)

This command causes the server-DTP to transfer a copy of the
file, specified in the pathname, to the server- or user-DTP
at the other end of the data connection. The status and
contents of the file at the server site shall be unaffected.


In a situation where the FTP client is not passing through ISA, the FTP
conversation looks like the following:

1. FTP client makes connection to server
2. server prompts for logon, client authenticates
3. client requests file by issuing RETR for the file name
4. server responds by sending the requested filename

In the scenario where ISA involved, the basic structure is the same, except
that
ISA, rather than initially sending a RETR for the filename, instead sends a
CWD for
it. ISA does this because it is acting as a proxy on the client's behalf
and
doesn't initially know whether the data requested by the client is a
directory or a
file. As such, ISA sends a CWD to attempt to change to a directory; if the
target
is not a directory, the FTP server should respond back saying that the
requested
directory name is invalid. At that point, ISA will realize that the
requested
target is an actual file and will issue a RETR to download it. So, the
conversation with ISA looks like the following:

1. client sends request to make connection to ISA, which then makes a
connection on
its behalf to the FTP server
2. server prompts for logon, ISA prompts client and returns credentials to
the FTP
server
3. client requests data, ISA proxies this request by making a CWD to the FTP
server
for the data requested
4. if the data requested is a file, the FTP server should respond with a
550,
requested directory is invalid
5. after seeing the 550, ISA knows the data is a file and requests it again
with a
RETR
6. FTP server sends the file to ISA, which then sends it to the client

In this case, the FTP server breaks down in step 6 where it fails to begin
sending
the