It is common, that the FTP server is not configured properly and provides its internal IP address, that cannot be used from a client network. When the FTP server is behind a NAT, it needs to know it’s external IP address, so it can provide it to the client in a response to PASV command. And the same range has to be opened/routed on the firewall/NAT. Typically, the FTP server software has a configuration option to setup a range of the ports, the server will use. The firewall and NAT on the FTP server side have to be configured not only to allow/route the incoming connections on FTP port 21, 2 but also a range of ports for the incoming data connections. Notes for Uncommon Local Network Configurations.Use Passive mode session settings to toggle between the active and the passive mode. In the passive mode, the client uses the control connection to send a PASV command to the server and then receives a server IP address and server port number from the server, which the client then uses to open a data connection to the server IP address and server port number received. Using the passive mode is preferable because most of the complex configuration is done only once on the server side, by experienced administrator, rather than individually on a client side, by (possibly) inexperienced users. ADSL modem), unable to accept incoming TCP connections.įor this reason the passive mode was introduced and is mostly used nowadays. built-in Windows firewall) or NAT router (e.g. Nowadays, it is typical that the client is behind a firewall (e.g. The firewall must allow connections to the ephemeral ports used by the FTP application.Īdditional information about constructing firewall rules can be found here.In the active mode, the client starts listening on a random port for incoming data connections from the server (the client sends the FTP command PORT to inform the server on which port it is listening).The firewall must allow connections on port 21.Two firewall rules are necessary for passive FTP to function properly: This configuration will ensure that clients are able to make inbound c onnections on the passive FTP port provided by the server. With a Microsoft IIS server in the default configuration, firewall rules must allow inbound connections on ports through 65535. For example, Microsoft IIS uses ports 1024 through 65535 by default. The documentation about your particular FTP server software should contain information about the ephemeral ports used when passive FTP is requested by a client. Ephemeral ports are typically high numbered and outside the range of IANA registered ports. An ephemeral port is a temporary, non-registered port used for communication. Firewall rules must be constructed to allow inbound connections on port 21 and inbound connections on the ephemeral ports used by the client when connecting to the FTP server using a passive connection. The FTP session has now been establishedīecause the client initiates all connections, the client firewall will not block any traffic, as shown below:Ĭonfiguration for passive FTP on an MX appliance requires some additional knowledge of the FTP application. The client initiates a connection to the server on this ephemeral port. The port command specifies a random, high-numbered (ephemeral) port that the client can connect to. The server responds with the PORT command. The source port is a random, high-numbered port. The client sends the PASV command to an FTP server on port 21. A passive FTP connection follows the following process: This process is effective because most firewalls allow inbound traffic from sessions initiated by the client. When passive FTP is used, the client will initiate the connection to the server. Both the server and the client must support passive FTP for this process to work. Passive FTP is an FTP mode that can be requested by a client to alleviate the issues caused by client-side firewalls.
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