The usual run of internet user has and wants no unsolicited traffic coming in. Everything that does come in is in reply to something he asked for.
In reality, there are those who probe any computer connected to the internet, hoping to find an unsuspecting machine that does have some application which, unbeknown to the owner, is listening to and responding to unsolicited requests. One example of that is the windows message app, which is intended for situations such as a network administrator informing everyone that the network will be going down in 15 minutes, and similar things. Someone figured out how to abuse this to place ads on the desktop.
The recommended response is to have a firewall. A firewall rejects all unsolicited traffic, unless it has been specifically told to allow the traffic through.
Bittorrent is quite different from most other applications, because it does want unsolicited connections. These are connections from other members of the swarm. They’re not in response to anything you sent. Instead, they’re basically offers to compare what pieces of the torrent you have, and what they have, and whether you both want to swap those pieces. This is the same communication that you yourself are sending out to various members of the swarm.
Bittorrent clients have a “listen port” on which they expect to receive such offers from other, previously unknown swarm members. When you send out a communication to another member, you send it on their listen port, where their client is expecting to find such communications. So also, your client is listening for other people to send such communications, on your listen port.
Firewalls block unsolicited communications, as said. That’s their job and why they exist. What you must do is make sure that no firewall is blocking incoming traffic to your listen port, as it will do by default unless specifically told otherwise.
Some people have more than one firewall. They have a firmware firewall in the router, and a software firewall running on their computer, and they may even have more than one software firewall. And they mya not even be aware of this.
It only takes one firewall to block all of the traffic on your listen port. If it is blocked, you won’t be able to make many of the best connections you could have made, because you don’t know they’re available. Your speed suffers as a result.
Your internet connection may, itself, be behind a firewall that you don’t control. That happens often when you get your connection as part of the facility – it comes with the apartment, or the dormitory, or business, or wireless connection. If that’s the case, you can’t do much about it. But that’s not most people’s connection. Most connections are directly to the ISP via DSL or cable, and these are usually not firewalled.
As far as firewalls you do or can control, they are your responsibility, and it is up to you to know what’s running on your hardware. If you have a software firewall you don’t know about, then you need to find out and take the appropriate steps.
If your router’s external firewall has been blocking your listen port, then you need to instruct the router to forward traffic which is on that particular port, and intended for your particular computer, to pass through unblocked. But your responsibility does not end there. You may have resolved the situation for that one firewall, but all of the others you have are still your responsibility, and you must unblock this port for each and every firewall that you are running.
It’s strongly discouraged to have multiple firewalls like this. They don’t add to your security, but they do add to your management headaches.
Test. Hook your computer directly to the modem, bypassing the router. Is BC’s listen port unblocked now, so that you are getting remote connections? If it is, then it was your router, which is still blocking those connections and you still haven’t configured it right. If not, then you don’t know whether your router is configured rcorrectly or not, but you do know that you still have something blocking the port, and it’s on your machine, or else it’s further upstream on your connection. You have to figure out which, and fix it if you can.
You can use the port test at www.canyouseeme.org for a quick test of whether some step has finally unblocked the port or not. While using it, remember that it only takes one firewall to block the port completely. If you’ve got three of them, and you’ve fixed the first and the third, it won’t matter because the second is still completely blocking the port.
