Bitcoin-Core P2P Client Has UPnP.

I use a Bitcoin client, which is called “Bitcoin Core“. This is a version of Bitcoin, based on Peer-To-Peer protocols. This version of the wallet-program should not be confused with the Android version, which some people have on their phones, and which is Not P2P.

Several people who use the P2P version of this wallet-program, which builds our local copy, of the global block-chain, on a zero-trust basis, have observed that they leave their client running 24/7, yet that they do not seem to be functioning as a full peer. As a full peer, their computer can act to facilitate Bitcoin transfers. Even when not being used as a full peer, this version of the program will connect with up to 8 other peers – using outbound connections – and will use these connections to keep its internal version of the block-chain – and therefore their wallet – synced with the rest of the network.

So after some time, what people may simply ask is, ‘Why don’t I receive incoming connections, from other wallets, expecting me to help complete their transactions?’ This would be a reasonable question, yet gurus elsewhere have given a wrong answer.

In general, this P2P Node needs to listen on TCP Port 8333. Therefore, what some people expect, is that they need to establish a port-forwarding rule on their router, which forwards TCP Port 8333 to whichever machine on the LAN is running Bitcoin Core. The advice has sometimes been given, that if you forward this port, you will start receiving massive numbers of inbound connections, and will become useful to the network.

There’s a slight problem with this version of an explanation. Bitcoin Core has the ability to use ‘UPnP’, which is also known as “Universal Plug-And-Play”. What UPnP does, is allow individual clients of our LAN to open a required port on the WAN-side of the router, such as TCP Port 8333 if need be. Because some users believe that enabling UPnP on their routers, makes their routers ineffective as a firewall, they disable this feature. This would be, because those users cannot even trust their LAN-clients, in which case the LAN-clients could trivially request forwarding rules, which the operators of such a LAN did not authorize.

The problem I see, is that I, personally, have UPnP enabled on my router, because I believe my actual LAN-clients to be secure, so that according to me, if they want a WAN port number, they can have it. Also, I have UPnP enabled on my Bitcoin Core P2P / wallet-program. Therefore, the LAN-client in question is requesting this Port 8333, and is obtaining it. Yet, I still don’t see a wealth of inbound connections asking for my CPU time.

bitcoin-core-upnp_1

There could be several reasons for this, one of which might have been, that a software-firewall on the client-machine in question could be blocking Port 8333. But I, personally have checked my software-firewall. It tells me that it is allowing all connections to and from my Bitcoin Core client a-okay. Maybe the firewall of some other participant is not?

Answer: The Windows computer my Bitcoin Core client is running on, had the LAN connection set to Public. According to Windows firewall rules, access to this program on the host machine is only granted when the network would be Private. This is to allow quick access to Public networks, which are not trusted, without reconfiguring the computer, while setting up a more-liberal set of rules for Private networks. Changing the network-type to Private seems to have solved this problem.

With certainty, my Bitcoin Core client will not show me any transactions it has facilitated, because those transactions do not affect my wallet. Bitcoin is designed to be anonymous, so that I will only see transactions which affect my own balance.

Dirk