FTP proxy concepts

The FortiGate explicit FTP proxy

You can use the FortiGate explicit FTP proxy to enable explicit FTP proxying on one or more FortiGate interfaces. The explicit web and FTP proxies can be operating at the same time on the same or on different FortiGate interfaces.

In most cases you would configure the explicit FTP proxy for users on a network by enabling the explicit FTP proxy on the FortiGate interface connected to that network. Users on the network would connect to and authenticate with the explicit FTP proxy before connecting to an FTP server. In this case the IP address of the explicit FTP proxy is the IP address of the FortiGate interface on which the explicit FTP proxy is enabled.

Enabling the explicit FTP proxy on an interface connected to the Internet is a security risk because anyone on the Internet who finds the proxy could use it to hide their source address.

If the FortiGate unit is operating in transparent mode, users would configure their browsers to use a proxy server with the FortiGate unit management IP address.

The FTP proxy receives FTP sessions to be proxied at FortiGate interfaces with the explicit FTP proxy enabled.

The FTP proxy uses FortiGate routing to route sessions through the FortiGate unit to a destination interface. Before a session leaves the exiting interface, the explicit FTP proxy changes the source addresses of the session packets to the IP address of the exiting interface. When the FortiGate unit is operating in transparent mode the explicit web proxy changes the source addresses to the management IP address.

Example explicit FTP proxy topology

To allow anyone to anonymously log into explicit FTP proxy and connect to any FTP server you can set the explicit FTP proxy default firewall proxy action to accept. When you do this, users can log into the explicit FTP proxy with any username and password.

In most cases you would want to use explicit proxy policies to control explicit FTP proxy traffic and apply security features, access control/authentication, and logging. You can do this by keeping the default explicit FTP proxy firewall policy action to deny and then adding explicit FTP proxy policies. In most cases you would also want users to authenticate with the explicit FTP proxy. By default an anonymous FTP login is required. Usually you would add authentication to explicit FTP proxy policies. Users can then authenticate with the explicit FTP proxy according to users or user groups added to the policies. User groups added to explicit FTP proxy policies can use any authentication method supported by FortiOS including the local user database and RADIUS and other remote servers.

If you leave the default firewall policy action set to deny and add explicit FTP proxy policies, all connections to the explicit FTP proxy must match an or else they will be dropped. Sessions that are accepted are processed according to the ftp-proxy security policy settings.

You can also change the explicit FTP proxy default firewall policy action to accept and add explicit FTP proxy policies. If you do this, sessions that match explicit FTP proxy policies are processed according to the policy settings. Connections to the explicit FTP proxy that do not match an explicit FTP proxy policy are allowed and the users can authenticate with the proxy anonymously.

There are some limitations to the security features that can be applied to explicit FTP proxy sessions. See The FortiGate explicit FTP proxy on page 411.

You cannot configure IPsec, SSL VPN, or Traffic shaping for explicit FTP proxy traffic. Explicit FTP proxy policies can only include firewall addresses not assigned to a FortiGate unit interface or with interface set to any. (On the web-based manager you must set the interface to Any. In the CLI you must unset the associatedinterface.)

How to use the explicit FTP proxy to connect to an FTP server

To connect to an FTP server using the explicit FTP proxy, users must run an FTP client and connect to the IP address of a FortiGate interface on which the explicit FTP proxy is enabled. This connection attempt must use the configured explicit FTP proxy port number (default 21).

The explicit FTP proxy is not compatible with using a web browser as an FTP client. To use web browsers as FTP clients configure the explicit web proxy to accept FTP sessions.

The following steps occur when a user starts an FTP client to connect to an FTP server using the explicit FTP proxy. Any RFC-compliant FTP client can be used. This example describes using a command-line FTP client. Some FTP clients may require a custom FTP proxy connection script.

1. The user enters a command on the FTP client to connect to the explicit FTP proxy.

For example, if the IP address of the FortiGate interface on which the explicit FTP proxy is enabled is 10.31.101.100, enter:

ftp 10.31.101.100

2. The explicit FTP proxy responds with a welcome message and requests the user’s FTP proxy user name and password and a username and address of the FTP server to connect to: Connected to 10.31.101.100. 220 Welcome to FortiGate FTP proxy Name (10.31.101.100:user):

You can change the message by editing the FTP Explicit Banner Message replacement message.

3. At the prompt the user enters their FTP proxy username and password and a username and address for the FTP server. The FTP server address can be a domain name or numeric IP address. This information is entered using the following syntax:

<proxy-user>:<proxy-password>:<server-user>@<server-address>

For example, if the proxy username and password are p-name and p-pass and a valid username for the FTP server is s-name and the server’s IP address is ftp.example.com the syntax would be:

p-name:p-pass:s-name@ftp.example.com

4. The FTP proxy forwards the connection request, including the user name, to the FTP server.
5. If the user name is valid for the FTP server it responds with a password request prompt.
6. The FTP proxy relays the password request to the FTP client.
7. The user enters the FTP server password and the client sends the password to the FTP proxy.
8. The FTP proxy relays the password to the FTP server.
9. The FTP server sends a login successful message to the FTP proxy.
10. The FTP proxy relays the login successful message to the FTP client.
11. The FTP client starts the FTP session.

All commands entered by the client are relayed by the proxy to the server. Replies from the server are relayed back to the FTP client.

Explicit FTP proxy session

From a simple command line FTP client connecting to an the previous sequence could appear as follows:

ftp 10.31.101.100 21 Connected to 10.31.101.100.

220 Welcome to FortiGate FTP proxy

Name (10.31.101.100:user): p-name:p-pass:s-name@ftp.example.com 331 Please specify the password. Password: s-pass 230 Login successful.

Remote system type is UNIX

Using binary mode to transfer files. ftp>

Security profiles, threat weight, device identification, and the explicit FTP proxy

You can apply antivirus, data leak prevention (DLP), and SSL/SSH inspection to explicit FTP proxy sessions.

Security profiles are applied by selecting them in an explicit FTP proxy policy or an authentication rule in an FTP proxy security policy.

Traffic accepted by explicit FTP proxy policies contributes to threat weight data.

The explicit FTP proxy is not compatible with device identification.

Explicit FTP proxy options and SSL/SSH inspection

Since the traffic accepted by the explicit FTP proxy is known to be FTP and since the ports are already known by the proxy, the explicit FTP proxy does not use the FTP port proxy options settings.

When adding UTM features to an FTP proxy security policy, you must select a proxy options profile. In most cases you can select the default proxy options profile. You could also create a custom proxy options profile.

The explicit FTP proxy supports the following proxy options:

l Block Oversized File and oversized file limit

The explicit FTP proxy does not support the following protocol options: l Client comforting

Explicit FTP proxy sessions and antivirus

For explicit FTP proxy sessions, the FortiGate unit applies antivirus scanning to FTP file GET and PUT requests. The FortiGate unit starts virus scanning a file in an FTP session when it receives a file in the body of an FTP request.

Flow-based virus scanning is not available for explicit FTP proxy sessions. Even if the FortiGate unit is configured to use flow-based antivirus, explicit FTP proxy sessions use the regular virus database.

Explicit FTP proxy sessions and user limits

FTP clients do not open large numbers of sessions with the explicit FTP proxy. Most sessions stay open for a short while depending on how long a user is connected to an FTP server and how large the file uploads or downloads are. So unless you have large numbers of FTP users, the explicit FTP proxy should not be adding large numbers of sessions to the session table.

Explicit FTP proxy sessions and user limits are combined with explicit web proxy session and user limits. For information about explicit proxy session and user limits, see Explicit proxy sessions and user limits on page 1.