Version 1 (modified by charles, 12 years ago) (diff)

moved from "Port Forwarding Guide" to better follow wiki naming conventions

Port Forwarding Guide

A detailed guide is also available in Transmission's built in help, which can be accessed from the Help menu.

If you didn't already know, BitTorrent is a system that requires you to share the file you're downloading.

For this to be possible, it is required to be accessible from the internet. However, this isn't always as straight forward as it may seem. Because of the nature of the internet and security reasons, routers create a local network that makes your computer invisible to the internet. This technology is called NAT.

Open your local Firewall

To allow other peers to communicate with Transmission, you have to open your local firewall. However, if you're behind a NAT (a router or DSL 'modem'), it is safe disable the firewall.

On Mac OS X 10.4 Tiger

  1. Open Transmission, go to Preferences >> Network and and note down the number for the port. Then quit Transmission.
  2. Open System Prefs >> Sharing >> Firewall. Click "New." In the "Port Name" pop-up menu, select Other, and fill in the settings as follows:
    • TCP Port Number(s): the port you chose in step 1 - (default is 51413).
    • Description: Transmission
  3. Click OK.

On Mac OS X 10.5 Leopard

Upon opening Transmission for the first time, a Mac OS X dialogue box should appear asking if you will allow Transmission to receive incoming connections. Click Accept.

If this doesn't happen, you can add Transmission to Leopard's firewall manually:

  1. Open System Prefs >> Security >> Firewall. Make sure "Set access for specific services and applications" is selected.
  2. Click the "+" button and select Transmission from you Applications folder.
  3. Make sure the pull down menu is set to "Allow incoming connections".

On Unix

  • For instructions on how to use it, open a Terminal open the man page of your Firewall. (eg. 'man ufw')
  • You need to ensure that Transmission's port (displayed in Preferences) is opened in the firewall.

Open ports & forwarding

To allow other peers to connect to you, you'll need to forward a port from the router to your computer.


By default Transmission will try to forward this port for you, using UPnP or NAT-PMP.

Most routers manufactured since 2001 have either the UPnP or NAT-PMP feature.

  • Open Transmission.
  • Go to Preferences >> Network >> Ports, and check 'Forward port from router'.
  • If Transmission reports that the 'Port is open' then you have successfully port forwarded!

Forward manually through a Router

  1. Find out what your IP address is.
    • On Mac OS X
      • Go to System Preferencess >> Network, double-clicking on your connection (for instance, Built-in Ethernet), and clicking the TCP/IP tab. The address is probably something like, or The IP of your router is here too.
    • On Unix
      • In Ubuntu, right click the Network Manager applet in the menu bar, and select 'Connection Information'. The address is probably something like, or
      • If you don't have Network Manager, open a Terminal and type 'ifconfig'. It will list information for each of your network devices. Find the one you are using, and use the number after 'inet addr:'.
  2. Open Transmission, go to preferences, and enter a number for the port. It is recommended you pick a random number between 49152 and 65535. The default is 51413. Then quit Transmission.
  3. Go into your router configuration screen. Normally this is done via your web browser using the address etc.
  4. Find the port forwarding (sometimes called port mapping) screen. While the page will be different for each router generally you will enter something similar to the following:
  5. For 'Application' type 'Trans'.
  6. For 'Start Port' and 'End port' type in the port you chose in Step 2. (eg. 51.413).
  7. For Protocol, choose Both.
  8. For IP address, type in your IP address you found in Step 1. (eg.
  9. Check Enable.
  10. Click save settings.

For more comprehensive instructions specific to your router, visit and choose your router from the list.


  1. Go to
  2. Enter the Port Transmission uses
  3. If Transmission reports that the 'Port is open' then you have successfully forwarded the port!

Common Problems


For UPnP/NAT-PMP compatible routers, make sure:

  • UPnP/NAT-PMP is enabled. Consult your router's documentation for instructions. If your router doesn't support UPnP/NAT-PMP, you will have to forward manually.
  • DMZ mode is disabled.
  • The port has not already been forwarded manually.

Note: NAT-PMP is only for Apple Airport routers.

Double NAT

Another possible reason your port remains closed could be because your router is not the only device on the network which needs to be configured.

For example, your network might resemble the following: ADSL modem/router --> Netgear Router --> Laptop.

If you have multiple routers in your home network (such as in the example above), you have two options. The easiest way is to turn one of the routers into 'Bridge mode' which means you then only have to configure one device rather than all of them. So, in our above example, we would set the Netgear router to 'Bridge'. See your router's help documentation for instructions.

The second way is to map Transmission's port on all of the devices on your network. Transmission can only automatically port map the router the computer is directly connected to. Any others in between this router and your modem will have to be forwarded manually. For detailed instructions, visit

Finally make sure your firewall is either disabled, or you have allowed Transmission's port. The firewall can cause the port to remain closed, even if it has been successfully mapped by the router(s).

ISP Blocking Port

Though initially this was done to "combat viruses and spam", it is sometimes used to keep out "bandwidth hogs". Normally the default (51413) port is fine. However, it might be that an ISP does decide to block that port. In such a case it is recommended to pick a random number between 49152 and 65535. If you can't find a port that's open (check with, you have a different issue.


eg. Universities, Wifi hotspots, RV parks and some 'true' ISPs (common in Rome, Italy).

Though these ISPs are often very interesting, offering high speeds, unlimited bandwidth (sometimes) and low prizes. This because they don't buy IP adresses for their clients and provide the (local) network themselves.

Basically the only thing you can do, is to politely ask for them to open (forward) a port for you.


Waldorf: I want to move "Common Problems" to Why is my port closed?

Waldorf: I'm totally confused. I thing the "Unix" stuff needs serious review.

jah: shouldn't this page just read 'consult T's help file'? All the info here has either been directly copy/pasted from the help file, or slightly modified but not materially better. I see no reason to have duplication.

theCrank: Well, maybe it's a good idea to let this Page evolve a bit and replace the help file?

jah: why? isn't one of the 'guidelines' on the wiki front page to avoid duplication? frankly, there isn't much (if anything) that could be added here on this topic.

theCrank: In the help files there are no examples which help to identify the problem (like log snipplets, ...). We can use the wiki to describe the most common problems (and their symptoms?) more detailed and maybe stop some users in the forums to post the same questions over and over again.

Waldorf: Let's look at it differently: This is, by far, the most asked question. And, apparently, most of the ppl screaming for help either didn't read the help or think they did, but didn't. So, if, because of this page here, ppl can resolve their issues (even if it's just forwarding to the build-in help), I think it's worth it.