I'm trying to build a peer-to-peer game, where each player is both a server and a client with tcp sockets. The sockets can connect fine when I'm using local ip:s, but of course fails when I'm trying to use external ip:s.
But I'm thinking that the players should be able to connect to each other if they just knew the external address + port that the router assigns to them.
Setting up port forwarding is out if the question since I don't have access to the players routers.
I'm thinking of having a server in between, just to be able to read the external address and port of the players, and tell the other player about it so that it can connect.
But I haven't found any info anywhere if that's how port forwarding works. If computer A makes a request from a local address and a port to the server, and the router assigns this address + port to an external address + port, and the server tell computer B which address + port to use. Can computer B use that external address + port to connect to the computer A and start a tcp socket with it? Is there any way to know that this external address + port stays the same when another computer makes a request against them?
The problem is that most people don't expose their PC directly to the internet. They have a router that has an external address. When you send a packet to their IP address, it is going to their router. The router doesn't know where to forward it to and what port to use without port forwarding.
So, getting everyone to enable port forwarding is out of the question, as it should be. A simpler mechanism is to have a server on the internet that you control. It has a firewall with port forwarding setup. The clients are just clients, they connect to the server on a port and send and receive info about the current status of the game. That way, everyone has real-time updates on their local game engine. Plus, this way is much easier to program and implement.
No, TCP doesn't work like that.
The source port that has been used to talk to the rendezvous server will be transient and specific to that particular initial TCP socket connection and can only be used as a destination for return traffic on the same connection from the rendezvous server, and can't be used by a third party to make new inbound connections.
The typical (only?) practical solution when NATs are involved and port forwarding is not available is to have that central server relay all messages bidirectionally between the peers.
Hole punching is what I was looking for.
https://en.wikipedia.org/wiki/Hole_punching_(networking)
Related
I'm currently working on my group voice chatting program. This is my plan:
Build up a server on 192.168.1.125. Listen to port 3490 and forward it to the router.
Clients connect to the server and get the user IP list(Whenever a client connects to the server, its IP address is recorded by the
server)
Clients communicate with each other using UDP multicast(is this ever possible across the router?).
But now I'm running into troubles. If a client inside the LAN, e.g. 192.168.1.120, connects to the server, it's taken down as 192.168.1.120, which won't work for clients from WAN.
I'm wondering if there is any way to convert the LAN IP to public IP? (this might have something to do with the router, I think)
Thanks.
You will have to be prepared for the case where direct client-to-client communication fails. Relaying through the server or NAT traversal/penetration may be necessary. Significant expertise is required to do this correctly.
What I am asking is if two computers listen to the same port and a packet of information enters the router through the WAN Ip and the same port. Would the packet go to both computers? Neither? One or the other?
IE
computer 1 -(internal IP)-> 192.168.1.3 -(listens to port)-> 4444
computer 2 -(internal IP)-> 192.168.1.2 -(listens to port)-> 4444
computer 3 -(connects and sends)-> 24.157.358.45:4444
packet -> computer 1 AND computer 2
The code in VB6 is:
LAN.LocalPort = 4444
LAN.Protocol = sckTCPProtocol
LAN.Listen
I am using a WinSock object in the Microsoft WinSock Control 6.0 in VB6 Professional
If there is something that needs to be clarified I would be more than happy to.
The router won't send an inbound packet to either machine unless communication has already been established.
If 192.168.1.3 calls out to some other machine (e.g. 4.5.6.7) from its port 4444, the router will assign an arbitrary port on its external address (say 24.157.358.45 [sic] :5555) and pass the packets on to 4.5.6.7. 4.5.6.7 will send reply packets to 24.157.358.45:5555 -- because that's the only address it knows about -- and the router will relay those to 192.168.1.3:4444.
That's the normal course of things, but there are a lot of additional details to this scheme that make it possible to establish communication with a machine behind a router via trickery.
The system of having machines with private IP addresses behind a router with a public address is called network address translation (NAT); it's a pretty deep topic.
From my knowledge of routers, unless port forwarding is setup, the router will discard any packets sent on that port.
If port forwarding is setup, only one of the computers could be setup to receive the packets.
If the packet is an inbound request to establish a new TCP connection with a server that is running behind the router, the router must have an explicit port-forwarding rule configured, either statically in the router's configuration or dynamically via uPNP or SNMP, that tells the router where to route inbound packets on 24.157.358.45:4444 to, either to 192.168.1.2:4444 or to 192.168.1.3:4444, otherwise the packet will be discarded. So no, both of your listening servers will not see the same packet.
Once a TCP connection is established, the router knows which specific LAN machines are associated with which connections and will route incoming packets belonging to those connections accordingly.
The previous answers are correct, you need to enable port forwarding. If it is not enabled port 4444 will be closed on the router.
It sounds like you have multiple servers and want to forward to whatever server is turned on at the moment. This is not possible (*), the router does not care whether or not PC1 or PC2 are listening on port 4444, it will simply forward everything to the address configured in the port forwarding.
(*): Ok it is possible but it takes some extra work.
Solution 1: Trick the router into thinking there is only one server. Give PC1 and PC2 a virtual network interface with the same IP address and forward to that address. Make sure only one of these interfaces is enabled, having duplicate IP addresses in your network can have unintentional behaviour.
Solution 2: Make the router care about which server is on. You will need to write a program to run on the router (or on another server) that can detect which server is on and forward the packets accordingly. If you are using Linux the program iptables can be worth looking at.
I've been investigating networking for use in a two-player game I'm writing, and I'm still not clear on when a device must have a port forwarded in order to communicate with the outside world.
From what I've seen in other games, port forwarding is always required in order to host a server, but is not required on the client. In addition, there are other situations, such as skype (which, to my understanding is ultimately client to client), where neither end must forward a port.
So my question is, in over-the-Internet communication, when is and isn't port forwarding necessary, and what steps can i take as a developer to make it so my users don't have to worry about it? Thanks in advance!
Port forwarding is needed when a machine on the Internet needs to initiate a connection to a machine that's behind a firewall or NAT router. If the connection is initiated by the machine behind the firewall, the firewall/router automatically recognizes the reply traffic and sends it to the machine that opened the connection.
But if a packet arrives on the external interface, and it's not a part of such a connection, the router needs to know what to do with it. By default, it will reject it. But if forwarding is configured for the port, that tells it what internal machine to send it to.
Put another way: you need port forwarding if you want to run a server behind the NAT firewall/router, you don't need it if you're just running a client.
There is reason why Skype don't (not always) need manual setting of port forwarding:
When you install Skype, a port above 1024 is chosen at random as the
port for incoming connections. You can configure Skype to use a
different port for incoming connections if you wish, but if you do,
you must open the alternative port manually.
If the port chosen for incoming connections becomes unavailable, by
default ports 80 and 443 will be used as alternatives. If another
application (such as Apache HTTP server or IIS) uses these ports, you
can either configure the application to use other ports, or you can
configure Skype to not use these ports.
Port forwarding is must if you host a server.
You can use same technique as Skype...
I am not sure if there is any other option...
Port forwarding (occurs) when a NAT, firewall or some other device blocks communication on all or some ports.
To answer your question as an example, most commercial routers use NAT to allow multiple people to use the same IP(As view from the outside world) provided by ISPs. Most ISP's use NAT to allow multiple customers to use the same IP(As viewed from the outside world). To get this to work, the NAT changes the internal IP and the port number of a communication to THE(there is only one for the entire sub network) external IP and a new port number. By doing this, the router/isp/ect can tell which internal IP and port each external communication goes to.
Anytime one of the computers communicating over the internet are behind a NAT, port forwarding is required. I'm sure there are way more situations than this, and the solution to each can be quite complicated. But this covers the vast majority.
So right now I'm using only TCP for my clients - they connect to the server, open socket and freely getting packets.
But what if I will decide to use also UDP in my game? Will they gonna have to open ports? For example, if they are using a regular WiFi, can I send UDP to the client without having opening ports problem?
Thanks.
TCP and UDP are just two examples of transport layer implementations. Both of them are using term 'port' to determine which app should receive incoming packet, but they could be routed/filtered differently by routers/switches/firewalls/etc.
So the answer is no. You will have similar problems with opening ports. Just except 'TCP port xxx should be opened' you have to demand 'UDP port xxx should be opened'.
In most home networks firewall rules allow outgoing packets (requests) to any remote port (on your server for example, where this port should be opened). And when such a packet goes through a router - it creates temporary rule to allow answers come back to the local port from which request packet.
So, normal scenario is like that:
Packet originated from home computer with IP 5.5.5.5. Lets say it has source UDP port 55555, source IP address 5.5.5.5 and destination port 8888.
Packet reaches home router. As it is going from inside - router allows it to pass through and creates rule say for 2 minutes to allow packets targeted to 5.5.5.5 to UDP port 55555.
Packet reaches corporate router before your server. It has rule to pass packets for port 8888 so packet is allowed to go.
Your server receives the packet and processes it. In response it creates packet for IP 5.5.5.5 and UDP port 55555.
Corporate router allows response to go.
Home router allows response to go according to temporary rule.
Your computer receives the response.
Corporate computers and routers often more restrictive to ensure security, so second point could restrict packet if your user (IP 5.5.5.5) is in corporate network.
It is very simplified as in reality there's almost always things like NAT and rules are more complex... But in general it gives the idea how it works internally.
If I have two internal computers connecting to the same external IP address through a NAT router, how is the router able to get the traffic to the correct internal computer? It is my understanding that NAT forwards incoming packets to the computer that recently sent outgoing packets to the [incoming packet's] sender's IP address. Since both computers are sending to the same address, does the router forward the packet to both? If that is the case, is it the responsibility of the client software to determine which packets are relevant?
Is it possible if both computers are attempting to connect to the same port?
When you open a socket, you need to address a port of the destination system and open a conjugate listening port on your own system to receive any response. You have to send the destination system your listening port.
Having more than one system using the same modem
When you start a web browser, and go to www.google.com:80, your browser obtains/searches for a free non-system conjugate port from the system for listening. Let us say, the conjugate port is 10000. The listener port is for receiving the http stream back from google.
Then your kid sitting next to you incidentally also browses www.google.com:80 and his/her google session of the play station or xbox-whatever also incidentally is assigned conjugate port 10000.
Both of you are sitting behind a cable modem, and behind the cable modem is your wireless router. And both of your systems are behind the wireless router - All sitting in that sequence, network topology-wise.
To prevent port address collision on the router/modem
Let us say that your cable company DHCP assigns your modem ip4 adress 72.72.72.72. But your wireless router DHCP assigns 192.168.0.10 to your system and to 192.168.0.11 to your kid's system.
When the frame carrying the information of your listener ports passes thro your NAT router, it would translate either one or both listening- ports. Let's say port 15000 for your page and port 16000 for your kid's page.
Your wireless router then sends your requests to google server as coming from 72.72.72.72:15000 and 72.72.72.72:16000.
The google server then responds individually to 72.72.72.72:15000 and 72.72.72.72:16000 and when you wireless router encounters the response, it reaches into the mapping that it has stored and translates 72.72.72.72:15000 to 192.168.0.10:10000 to reach your system but translates 72.72.72.72:16000 to 192.168.0.11:10000 to reach your kid's system.
Running web/game/ftp/etc servers
But what if you have a web server or an ftp server running on your system. What if you have two systems and both have a web server and both web servers are listening on port 80?
Let us say the local ip addresses registered/assigned with your wireless router of your first web server system is 192.168.0.30 and your second web server system is 192.168.0.40.
The wireless router would have a configuration web page usually by default 192.168.0.1:80, unless you changed it. There would be a tab to on the page where you could define/reserve application port mappings.
You could register with your wireless router to reserve the mapping
192.168.0.30:80 => outgoing port 8080
192.168.0.40:80 => outgoing port 8088
So that you have to phone your friends your web/game servers are addressable through
72.72.72.72:8080 and 72.72.72.72:8088 respectively,
where the wireless router would preclude its port 8080 and 8088 from its own dynamic NAT usage.
Of course, 72.72.72.72 is as good as only before your ISP DHCP decide to renew the ip4 address of your modem to say, 72.72.90.200. After which you would have to phone/email your friends and say
Hey, the servers' addresses have changed to 72.72.90.200:8080 and 72.72.90.200:8088 respectively. Or you could subscribe to dynamic dns (ddns) service to use a named domain where the ddns service will need you to install a simple heartbeat utility on your system to help them monitor the address variation. DDNS translation is a separate issue/strategy.
NAT modems
Newer ISP contracts supply you with a modem that has NAT. If so, you have to switch off either the one on your modem or the one on your wireless router. You should not use both - what's the point in translating twice because NAT is simply to prevent address collision. When you switch off NAT from your wireless router, it can operate as a hub switch and not a router anymore so that you could connect it to the modem using one of its LAN socket instead of thro its WAN socket.
The router manages "source" ports that are separate for each computer. While you may be connecting to port 80 on the "destination" the router may assign the source port to some high number port.
Wikipedia sums it up as
Network address translation involves
re-writing the source and/or
destination IP addresses and usually
also the TCP/UDP port numbers of IP
packets as they pass through the NAT.
Checksums (both IP and TCP/UDP) must
also be rewritten to take account of
the changes.
Already good answers are provided, but here is another example:
HOST A addr HOST B addr
10.1.0.2:4040 10.1.0.3:4040
-----------------------------------------
NAT 200.50.50.28:4040 200.50.50.28:4041 (what external host sees)
200.50.50.28 is router's global (internet) IP.
Every port number is unique in the NAT table. And of course the router does all the dirty job of modifying the source and destination addresses transparently.
It uses different ports for incoming external traffic, and the NAT then routes the packets on one port to one internal IP address, and the packets from the other port to the other internal IP address... The iniital request from each internal computer, when it goes through the NAT on the way out, establishes which port will be used for the incoming traffic from the external ip address, and it tells the external server what port to send it's traffic back on for that connection.
RFC3022 provides a lot of information on how this works
Since public facing or external IP Address that was given by Internet Service Provider (ISP) has been discussed, I would like to add on this.
You can ask your ISP to have your public IP Address not change. It will become static, so that you do not have to inform your friends to change the IP Address if they want to access your server inside your Network Address Translator (NAT).
As of this writing, static IP Address cost around 100 bucks. Most of the ISP they call it business account.
You can determine your public facing IP Address by googling "what is my ip address".