[TUTORIAL] How to check if you're able to Port Forward

[ianfontinell]

Not everyone knows that port-forwarding is crucial for maintaining a healthy ratio, even less people know that port-forwarding is impossible on a few circumstances.

Carrier Grade-NAT

Internet providers can assign an exclusive IP address to each client, but this is expensive, and as a result most home users are behind a Carrier Grade Network Address Translator (CG-NAT),

What it does: Multiple users are assigned the same IP address, and the NAT interface is responsible for figuring out where to direct each client's connections.

How it works: The CG-NAT keeps a translation table that maps all IP addresses, ports, protocols, and orchestrates all incoming/outgoing packets to their respective destinations. Any incoming connection that does not exist in that table is dropped before it can reach the client.

Here's a metaphor: Imagine you live in a floating island, and the only way for anyone to reach your house is through a tunnel that only you can open. When you open that tunnel, you can only open it for one specific person, but you can have multiple tunnels. When you close that tunnel, even though the person knows where you live and how to get there, if they try to create their own tunnel, when they get to the end there will be no door.

In shorter terms, any incoming packet will be dropped unless your machine have initiated the connection. Because of this, other peers cannot connect to your torrent client directly, instead they have to wait for your client to try connecting to them, which can delay the connection or prevent it altogether if the two of you are behind a CG-NAT.

How to check if you're behind a CG-NAT:
Open your terminal and run this command:
tracert -d 8.8.8.8 if you're on Windows or traceroute -n 8.8.8.8 if you're on Linux.

You'll see a list of all IP addresses that are part of the route to Google's DNS server.
You must look for any address falling in this range: 100.64.0.0 to 100.127.255.255.

If you see an IP that fits that range, you are behind a CG-NAT, and Port Forwarding is ineffective.

For example, this is my result:

1    <1 ms    <1 ms    <1 ms  192.168.18.1
2     2 ms     2 ms     2 ms  10.255.255.9
3     2 ms     2 ms     2 ms  100.127.249.217
4     4 ms     3 ms     4 ms  200.153.91.173
5     *        *        *     Request timed out. 
6     4 ms     4 ms     4 ms  72.14.220.222
7     5 ms     4 ms     4 ms  142.251.238.17
8     5 ms     4 ms     4 ms  74.125.37.135
9     4 ms     4 ms     4 ms  8.8.8.8
Trace complete.

As you can see on hop 3 the IP address 100.127.249.217 is inside the CG-NAT range. Therefore, I'm behind a CG-NAT and port-forwarding is not possible.

What to do in this case:
You can use a VPN with port-forwarding support, like ProtonVPN or PIA (there are a few others).
A regular VPN will not suffice, incoming connections will be dropped. It has to be a VPN provider that supports port-forwarding, they will assign a port exclusive to you that can be used in your torrent client to make it reachable to other peers.

No CG-NAT and still blocked:
Sometimes, even when you have an exclusive IP, the ISP may block all ports, or ports from a specific range. In this case, if you try multiple port numbers and they all fail, you need to contact your ISP and request them to open the specific port number that you want to use.

[Kevin4fm]

I'd quibble only very slightly, in that I wouldn't say port forwarding is "crucial".

I managed to maintain a respectable ratio of around 2:1 since I joined the site quite a while ago without it. I've also not really felt restricted as to what I can download. It did take a fair bit of careful husbandry though. I seed 24/7. I have a very large amount of torrents I'm seeding, which I split into "long term" (files I'll keep, regardless of whether I'm seeding them or not) and "short term" (a mixture of recently downloaded files that I genuinely wanted - but aren't worth the aggro to set up as 'long term' - and freeleech stuff grabbed mainly to seed onwards for ratio. My short term seeders are rotated to earn their keep. They get deleted once they've not had any traffic for a month, or sooner if I'm running short of space I've dedicated for that purpose.

However, I finally got round to switching to a VPN that offered port forwarding a couple of months ago. Once set up, which was extremely easy, I saw an immediate increase in both the number and volume of torrents seeded. I don't have a huge upload bandwidth, but it's now nearly always running near maximum and, week by week, my ratio is improving. I'm currently sitting at around 45TB up and 20TB down, so it takes a lot to swing the needle even a little, but it is slowly swinging, and in the right direction.

The other advantage of having port forwarding set up is that I can more often grab those torrents that have only one or two seeders, and seed the lone leecher who want one of my long term torrents where I'm the only seeder. Previously, we'd sit forever with me "available to seed" and the leecher "ready to grab", with nothing actually happening, because we couldn't connect to each other.

But port forwarding takes minutes to set up - once you're with the right VPN provider - and pays immediate dividends. Well worth any investment of both time and slightly higher VPN costs if necessary.