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A file transfer protocol that is “firewall friendly” typically has most or all of the following attributes:

1) Uses a single port
2) Connects in to a server from the Internet
3) Uses TCP (so session-aware firewalls can inspect it)
4) Can be terminated or proxied by widely available proxy servers

For example:

Active-mode FTP is NOT firewall friendly because it violates #1 and #2.
Most WAN acceleration protocols are NOT firewall friendly because they violate #3 (most use UDP) and #4.
SSH’s SFTP is QUITE firewall friendly because it conforms to #1,2 and 3.
HTTP/S is probably the MOST firewall friendly protocol because it conforms to #1, 2, 3 and 4.

As these examples suggest, the attribute file transfer protocols most often give up to enjoy firewall friendliness is transfer speed.

When proprietary file transfer “gateways” are deployed in a DMZ network segment for use with specific internal file transfer servers, the “firewall friendliness” of the proprietary protocol used to link gateway and internal server consists of the following attributes instead:

1) Internal server MUST connect to DMZ-resident server (connections directly from the DMZ segment to the internal segment are NOT firewall friendly)
1) SHOULD use a single port (less important than #1)
3) SHOULD uses TCP (less important than #2)