4.26.2 Selecting Internet protocol version for forwarded ports

This switch allows you to select a specific Internet protocol (IPv4 or IPv6) for the local end of a forwarded port. By default, it is set on ‘Auto’, which means that:

• for a local-to-remote port forwarding, PuTTY will listen for incoming connections in both IPv4 and (if available) IPv6
• for a remote-to-local port forwarding, PuTTY will choose a sensible protocol for the outgoing connection.

This overrides the general Internet protocol version preference on the Connection panel (see section 4.13.4).

Note that some operating systems may listen for incoming connections in IPv4 even if you specifically asked for IPv6, because their IPv4 and IPv6 protocol stacks are linked together. Apparently Linux does this, and Windows does not. So if you're running PuTTY on Windows and you tick ‘IPv6’ for a local or dynamic port forwarding, it will only be usable by connecting to it using IPv6; whereas if you do the same on Linux, you can also use it with IPv4. However, ticking ‘Auto’ should always give you a port which you can connect to using either protocol.