The Port used for incoming connections field specifies the port that BitTorrent should use to listen for incoming connections. This port must be unfirewalled for optimal usage of BitTorrent. Setting the port to 0 indicates to BitTorrent that you would like for it to select a random port after the changes are confirmed.
The Random port button selects a random port between 10000 and 65000 as the listening port.
Enable UPnP port mapping allows BitTorrent to communicate with the router to forward a port without your manual intervention. Some devices do not support Universal Plug and Play (UPnP), so you might still have to forward your ports manually. Disable UPnP if that is the case.
Enable NAT-PMP port mapping allows BitTorrent to attempt to forward a port with routers that support the NAT port mapping protocol (Apple products, for example).
Randomize port each start makes BitTorrent select a random port between 1000 and 65000 to use at startup. Note that this option might conflict with your port forwarding settings, so if you manually open ports in your router and firewall, it's best to leave this option disabled.
Add Windows Firewall exception allows BitTorrent to add an entry to the Windows Firewall exceptions list that lets it bypass the firewall. This is useful only if you actually have Windows Firewall enabled. Note that this option works only on operating systems released after (and including) Windows XP with at least Service Pack 2 (SP2) installed.
Use proxy for peer-to-peer connections forces BitTorrent to communicate and transfer data with peers through the proxy. By default, this option is disabled, and BitTorrent only uses the proxy to communicate with trackers. This option may not work with some HTTP proxies (not all HTTP proxies support HTTP CONNECT).
Note: BitTorrent can only proxy UDP-based communication through SOCKS5 proxies. Proxying of IPv6 traffic is currently not supported. By default, connection types that your proxy does not support will simply connect as normal.
Disable all local DNS lookups will disable reverse DNS and prevent hostname lookups from bypassing the proxy. This will not function properly without enabling "Use proxy for hostname lookups." If your proxy is misconfigured or not working, then DNS will simply fail. As a safeguard, BitTorrent's autoupdates and crash reports will bypass this option.
Disable features that leak identifying information will prevent BitTorrent from sharing your non-proxied IP through handshakes with other peers, as well as through DHT. It will also prevent it from handing out your IPv6 address to IPv4 peers and vice versa.
Disable connections unsupported by the proxy will disable connection types that your proxy type cannot handle. For HTTP, HTTPS and SOCKS4 proxies, this will disable all UDP-based communication (DHT, uTP, UDP trackers, IPv6). For SOCKS5, it will only disable IPv6, as IPv6 is currently not proxied. If your proxy is misconfigured or not working, all traffic in the client will stop.