Wednesday, October 05, 2005

FTP tutorial

There are also some higher ports above 1024 that FTP uses to display
files and folders. FTP just grabs those ports whenever it needs them.

Following is a short FTP tutorial:

Active FTP vs. Passive FTP, a Definitive Explanation
Introduction
One of the most commonly seen questions when dealing with firewalls
and other Internet connectivity issues is the difference between
active and passive FTP and how best to support either or both of them.
Hopefully the following text will help to clear up some of the
confusion over how to support FTP in a firewalled environment.

The Basics

FTP is a TCP based service exclusively. There is no UDP component to
FTP. FTP is an unusual service in that it utilizes two ports, a 'data'
port and a 'command' port (also known as the control port).
Traditionally these are port 21 for the command port and port 20 for
the data port. The confusion begins however, when we find that
depending on the mode, the data port is not always on port 20.

Active FTP

In active mode FTP the client connects from a random unprivileged port
(N > 1024) to the FTP server's command port, port 21. Then, the client
starts listening to port N+1 and sends the FTP command PORT N+1 to the
FTP server. The server will then connect back to the client's
specified data port from its local data port, which is port 20.

From the server-side firewall's standpoint, to support active mode FTP
the following communication channels need to be opened:

FTP server's port 21 from anywhere (Client initiates connection)
FTP server's port 21 to ports > 1024 (Server responds to client's
control port)
FTP server's port 20 to ports > 1024 (Server initiates data connection
to client's data port)
FTP server's port 20 from ports > 1024 (Client sends ACKs to server's
data port)

When drawn out, the connection appears as follows:

In step 1, the client's command port contacts the server's command
port and sends the command PORT 1027. The server then sends an ACK
back to the client's command port in step 2. In step 3 the server
initiates a connection on its local data port to the data port the
client specified earlier. Finally, the client sends an ACK back as
shown in step 4.

The main problem with active mode FTP actually falls on the client
side. The FTP client doesn't make the actual connection to the data
port of the server--it simply tells the server what port it is
listening on and the server connects back to the specified port on the
client. From the client side firewall this appears to be an outside
system initiating a connection to an internal client--something that
is usually blocked.

Passive FTP

In order to resolve the issue of the server initiating the connection
to the client a different method for FTP connections was developed.
This was known as passive mode, or PASV, after the command used by the
client to tell the server it is in passive mode.

In passive mode FTP the client initiates both connections to the
server, solving the problem of firewalls filtering the incoming data
port connection to the client from the server. When opening an FTP
connection, the client opens two random unprivileged ports locally (N
> 1024 and N+1). The first port contacts the server on port 21, but
instead of then issuing a PORT command and allowing the server to
connect back to its data port, the client will issue the PASV command.
The result of this is that the server then opens a random unprivileged
port (P > 1024) and sends the PORT P command back to the client. The
client then initiates the connection from port N+1 to port P on the
server to transfer data.

From the server-side firewall's standpoint, to support passive mode
FTP the following communication channels need to be opened:
FTP server's port 21 from anywhere (Client initiates connection)
FTP server's port 21 to ports > 1024 (Server responds to client's
control port)
FTP server's ports > 1024 from anywhere (Client initiates data
connection to random port specified by server)
FTP server's ports > 1024 to remote ports > 1024 (Server sends ACKs
(and data) to client's data port)
When drawn, a passive mode FTP connection looks like this:

In step 1, the client contacts the server on the command port and
issues the PASV command. The server then replies in step 2 with PORT
2024, telling the client which port it is listening to for the data
connection. In step 3 the client then initiates the data connection
from its data port to the specified server data port. Finally, the
server sends back an ACK in step 4 to the client's data port.
While passive mode FTP solves many of the problems from the client
side, it opens up a whole range of problems on the server side. The
biggest issue is the need to allow any remote connection to high
numbered ports on the server. Fortunately, many FTP daemons, including
the popular WU-FTPD allow the administrator to specify a range of
ports which the FTP server will use.

The second issue involves supporting and troubleshooting clients which
do (or do not) support passive mode. As an example, the command line
FTP utility provided with Solaris does not support passive mode,
necessitating a third-party FTP client, such as ncftp.
With the massive popularity of the World Wide Web, many people prefer
to use their web browser as an FTP client. Most browsers only support
passive mode when accessing ftp:// URLs. This can either be good or
bad depending on what the servers and firewalls are configured to
support.

Summary
The following chart should help admins remember how each FTP mode
works:
Active FTP :
command : client >1024 -> server 21
data : client >1024 <- server 20

Passive FTP :
command : client >1024 -> server 21
data : client >1024 -> server >1024

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