Can you please tell me specifically what kind of data flows through which port during an FTP connection?
To be specific, I'd like to know whether contents of the directory and the server response codes flow through port 20.
FTP uses two types of connections: (1) the control connection (default port 21), which is used to send commands to the server and receive status codes back, and (2) the data connection (default port 20), which is used to transfer the content requested from the server: the content of a file or a directory listing, for example. I recommend to use a network protocol analyzer to see it with your own eyes. FTP specification RFC959 is written in an easily understandable manner, don't afraid to have a look at it.
Related
I want to write a CLLE Program to transfer Files from the IFS to a Webserver.
The Webserver is registered in the Hosttable from the Iseries.
(commandline: CFGTCP -> Option 10 -> Scroll to the Webserver IP)
Current CLLE Program: (only the ftp-access)
PGM
OVRDBF FILE(INPUT) TOFILE(CMDDTAR7_2/US9FIP) OVRSCOPE(*JOB)
OVRDBF FILE(OUTPUT) TOFILE(CMDDTAR7_2/US9FOP) +
OVRSCOPE(*JOB)
FTP RMTSYS('10.1.2.99')
ENDPGM
After i call my Program i get some results in a PF-File.
The Results are in German but i translated shortly for you.
The Connection is unavalable to 10.1.2.99 on Port 21. Try it again later.
[...]
My Question at the end.
Do you have some solutions for me to transfer xml files to my Webserver from the iseries?
You're writing about a web server and you want to transfer files by ftp. Perhaps you're mixing up protocols here. I'll concentrate to resolve your apparent ftp connection problem.
Seems that the destination host 10.1.2.99 doesn't accept connections on Port 21.
Please make sure, there's no firewall or other stuff blocking your request to the destination host. Moreover, on the host 10.1.2.99 try telnet 10.1.2.99 ftp or ftp 10.1.2.99 in a command window, depending on OS and installed components.
Before automating, just test manually if you can successfully connect and log in via standard command line: ftp 10.1.2.99.
Transferring data by FTP is just fine in a local LAN. Beware that any data including username and password will be transferred in clear text and thus can be possibly read by others as the intended hosts.
Following up to your comment: Actually there are many possibilities to copy files. If you're running a recent version of IBM i, I'd opt for using scp. It's encrypted and thus safe for running outside of local LANs. Great to automate by utilizing Key-Authentication instead of passwords. And many web hosting companies offer scp/sftp access.
I am working on an FTP client for kicks and I am trying to understand the workflow of data connections.
As I understand, the initial (command) connection is permanent until you quit. However, I am unsure of the data connection - is it re-initiated per-command? So you call PORT ... or PASV, get a second connection, do a LIST, get the results, connection closes, start over?
Also, do you need to call PASV (or PORT ...) again after each connection closes? It seems that when I try to test some things out using a passive connection, I cannot re-connect to the same port after the first command has returned the results and closed the data connection. I can keep calling PASV -> Data Connect -> Run Command -> Get Results -> Data Connection closed -> PASV, but it seems like it's not how it's meant to run?
Also, if someone has a good material on FTP that is more terse than the RFC I really appreciate it.
You have to open a new connection every time. It's only the closing of the connection, how you (or the server) can tell that the transfer completed (at least in the common "stream mode").
You cannot even reuse the local/remote port number combination, as when a TCP connection is closed, it enters TIME_WAIT mode and the port number combination cannot be used for some time. So for two immediately consecutive transfers you have to use a different port number combination anyway.
Refer to RFC 959, section 3.3. Data management:
Reuse of the Data Connection: When using the stream mode of data
transfer the end of the file must be indicated by closing the
connection. This causes a problem if multiple files are to be
transfered in the session, due to need for TCP to hold the
connection record for a time out period to guarantee the reliable
communication. Thus the connection can not be reopened at once.
There are two solutions to this problem. The first is to
negotiate a non-default port. The second is to use another
transfer mode.
A comment on transfer modes. The stream transfer mode is
inherently unreliable, since one can not determine if the
connection closed prematurely or not. The other transfer modes
(Block, Compressed) do not close the connection to indicate the
end of file. They have enough FTP encoding that the data
connection can be parsed to determine the end of the file.
Thus using these modes one can leave the data connection open
for multiple file transfers.
See also:
Why does FTP passive mode require a port range as opposed to only one port?
How many data channel ports do I need for an FTPS server running behind NAT?
The Properties file of a file watcher contains many attributes. I was not able to find the use of certain attributes. The attributes are
FTPModeBinary and
FTPConnectionPASV
these take true or false as its value.
Can anyone please help me to understand the use of these attributes?
"Binary" (as opposed to "text") transfers the file exactly as-is without doing any end-of-line conversions (like LF to CR/LF).
"Passive" means that the FTP server always opens a listening socket for the data connection and the client connects to it. (FTP uses a separate TCP channel for the actual data transfer.) Traditionally, FTP would have the sender open a connection to the receiver but this began breaking when people started residing behind NAT firewalls as there's no easy way to open a connection back to the client in that case. Without passive, it's actually possible to instruct two FTP servers to transfer a file between themselves without ever passing through the client.
Why does Apache-Commons-Net's FTPClient sometimes make the wrong computation for the port number in the PORT command? This is in active mode. For example FTPClient it could send out
PORT <some>,<ip>,<address>,<here>,235,181 when in fact the port number used is 60340. What's the cause for this wrong computation?
This could happen on version 3.3.
I know ftpClient.enterLocalPassiveMode(); could solve this, but I want to know the part where the active mode doesn't work as expected.
From your comments, I assume you mistake an FTP control connection with a data connection.
I assume that the 60340 is local port of the FTP control connection. When opening data connection, 60341 is assigned (hence the PORT ...,235,181).
Reasoning: In an FTP active mode, the client opens listening port for the expected data connection, which it then reports to the server via PORT command over an existing control connection. If the server cannot connect to the port, no TCP/IP packet can ever come to that port. As you claim that the "two machines still communicate at port 60340", it must be the control connection. There cannot be any communication on port, if the connection failed ("Can't open data connection").
The actual cause of the "Can't open data connection" error is likely that you are behind a firewall, so the server cannot connect back to the client. What is a common nowadays. That's what passive mode is good for.
I want to add support for the PORT command to my FTP server. I'm reading RFC 959, but I can't figure out when it's safe to connect to the FTP client. For example, consider this sequence:
PORT 127,0,0,1,34,34
LIST
Does the FTP client start listening before issuing the PORT command, or after issuing the LIST command? Because if the server attempts to connect to the client immediately after receiving PORT, it might fail because the client might not have started listening yet.
What does the specification say? Can the server connect immediately, or should it wait until after it receives the command that will make use of the data connection?
The server shouldn't connect to the client until it gets a command that requests a data transfer, such as LIST or RETR. See section 7 of RFC 959, which shows a typical sequence of operations (RFC's didn't have the formal MUST/MAY/SHOULD specifications in those early days).
However, since the port used in the PORT command is typically an ephemeral port, the client needs to open a socket to get the OS to assign a port number. This implies that by the time the PORT command is sent, the port would have to be open. However, it's possible that it might not yet have called listen().