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Some SOHO NAT Routers Handle PASV Requests Improperly  -  KB Article #1749

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Some users have reported problems when connecting to Serv-U through Small Home/Office Routers such as the LinkSys router models WRT54G and W300G. The error manifests when using a manually configured PASV port range that is smaller (the 5 port range recommended by RhinoSoft).

When a client sends the PASV command to Serv-U, Serv-U opens a listening socket so that the client can connect to it for a data transfer. In some cases, there may already be a data transfer occurring on the same port, which will cause no conflict. However, on problem routers if Serv-U opened a listening socket that is also in use for an ongoing data transfer the router would interpret the data transfer as a listening socket. As a result, the data transfer would be interrupted and closed in order to establish a listening socket, when in reality there was never any conflict.

To work around this, set a larger PASV port range in Serv-U and configure the same PASV port range in the router. A PASV port range of 50 will almost always resolve the issue, and directions on configuring this are available at Knowledge Base Article #1044.