The scenario is the following. I have two machines A and B:
A: Client (behind NAT)
B: Server (behind NAT)
I want B to be able to listen on any given port, so that A can send packets to B through that specific TCP port and receive any response. If both machines are not behind a NAT it is pretty straight foward process. However how do I make it work so that it works even when B is behind a router, without him having to go change the router configuration enable some port forwarding etc...
For example, how do peer-to-peer programs like torrent clients work without the user having anything to configure?
To answer the example of Peer to Peer programs, and in general: There is a technology called Universal Plug and Play which NAT routers can use to allow clients behind them to expose ports to the outside. That's what bittorrent clients can use so the other clients can directly connect to them.
An alternative to a proxy server is a match-making server. Instead of proxying all of the traffic, the match maker just negotiates until the peers can talk to each other. This involves finding the external public IPs of the peers and talking to each one so that the firewall/router knows that the peers wish to communicate.
This is called hole punching and it often has to be done by the match maker rather than the peers themselves. Once the hole are punched though, the match maker can tell the peers about each other and they can communicate directly.
You will have to either:
Set up port forwarding from the nat
gateway in front the server into the machine your server software is running, and have the client
connect to the IP address of that
gateway.
Create a proxy server sitting
inbetween the 2 nat gatewys so both
your server and client can connect
to that. Both your server and client
have to set up a connection to that
proxy which will mediate the data
between those 2 connections.
Hole punching is moderately well-understood for UDP communication, but it can be reliably used to set up peer-to-peer TCP streams as well. Here is the well detailed article on both TCP and UDP:
http://www.brynosaurus.com/pub/net/p2pnat/
Related
I have two peer devices which are behind NAT. But the problem comes while establishing communication between them, as there are four different types of NAT and each observing a different behaviour. What can be the possible solution in this scenario?
Also I have a STUN server which is used to relay the IP addresses and port number of the peer device.
Without access to some interface with which you can configure the NAT'ing on the gateways, you can't establish a connection. Simple as that. NAT is designed to forward communication packets to the (hidden) host behind the NAT that is a) configured to get packets for that specific port or b) based on tracked connections.
Typically, this problem is overcome in SOHO routers by employing UPnP, which allows to set up port forwardings "on the fly". Knowing that this basically is the same as shutting down the firewall and exposing a PC, you won't get that in institutional networks. Your only choice then is to use a third party as kind of a broker/proxy or to use VPN technologies to "bridge" these networks (or to bring one party into the other party's network, to be more specific).
This paper has been very useful to me in order to learn about hole punching: https://www.usenix.org/legacy/event/usenix05/tech/general/full_papers/ford/ford.pdf
As last option, when you cannot do hole punching, what everybody does is the typical implementation through a server. Peer A and Peer B have a connection through a server that they can poll all the time.
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.
I'm struggle with what technique to choose for a server client aspect of my application.
Defining design
Windows, C# on .net 2
On many machines there is a .net 2 service. I call that the Client.
Machines can be in different networks behind NAT's (or not) connected to Internet.
Server services are public.
Requirements
To communicate with the Clients on demand.
Client must listen for incoming connections.
The server can be or not online.
Port forwarding is not possible.
What are my choices to do something like that?
Now I'm looking in the UDP Hole punching technique. The difference between the UDP hole punching technique setup and my setup is that instead of having 2 clients behind a NAT and a mediation Server, I got only the client behind the NAT that must communicate with the server. That must be easier but I'm having hard time to understand and implement.
I'm on the right way with the this kind of NAT traversal or may be some other methods much easier to implement?
Other methods that I've taken in consideration:
When the service sees the server online, creates a connection to the server using TCP. The problem is that I have something around 200 clients, and the number is rising and I was afraid that this is a resource killer.
When the service sees the server online, checks a database table for commands then at every 30 seconds checks again. This is also a resource killer for my server.
Bottom of line is, if the UDP Hole Punching tehnique is the right way for this scenario, please provide some code ideas for de UDPServer that will run on the service behind NAT.
Thank you.
Hole punching and p2p
You might be interested in a high level discussion of UDP hole punching. Hole punching is needed if you want clients (who both might be behind a firewall) to communicate directly without an relaying server. This is how many peer 2 peer (p2p) communications work.
With p2p, typically NAT'ed clients must use some external server to determine each other's "server reflexive address." When NAT translation occurs, behind the firewall ports can be mapped to some arbitrary port to the public. A client can use a STUN server to determine its "server reflexive address." Clients then will, through an intermediary server, exchange server reflexive addresses and can initiate communication (with hole punching to initiate the session).
Often, a NAT will not behave in a manner to allow direct communication as described above. Sending packets to different destinations will cause a NAT to map ports to entirely different values depending on the destination. In this case, a TURN server is needed.
Links
Nat traversal and different types of NAT behaviors
STUN RFC
TURN RFC
Server-client communication
If your client only needs to communicate with a server, hole punching is not needed. As long as the client can communicate with the public Internet, then you can use any C# socket API (I'm not familiar with C#) to make connections to the server's public IP/port combination. Typically, clients making socket connections don't specify a source port and let the underlying socket API make that decision since it really doesn't matter.
Your server should be listening to a specific port (you make this determination), and when it receives a packet from the client, the source address of the packet will be some NAT'ed address. In other words, the source address will be the public IP of whatever firewall your client is behind. If the NAT changed the source port of the client's packet, the server will see this NAT'ed port as the source port. It really doesn't matter, since when the server sends back a response packet, the NAT on the client machine will translate the destination port (it stores translations internally) and correctly send the packet back to the correct private host (the client).
I'm writing a P2P application. Peers regularly ping a main server to update their current IP/port, so when a peer wants to reach another one it can ask the server for that information. For now peers use UPnP to configure the NAT (for classic home setups) to be accessible from outside.
So everything works, except when a peer's client tries to reach another (or the same) peer's server and both are behind the same NAT. Since in that case the client is trying to reach its own "external" (public) IP address from behind the NAT, the NAT doesn't do the port forwarding and is unable to route the IP packet.
For now I'm thinking of two solutions:
query the NAT with UPnP to see to which local IP the port is forwarded
store on the main server the internal IPs of the peers
Can you think of other solutions? What strategies do mainstream P2P applications implement to solve this problem?
Since in that case the client is trying to reach its own "external"
(public) IP address from behind the NAT, the NAT doesn't do the port
forwarding and is unable to route the IP packet.
This is known as the hairpin condition. Not all router/NAT solve this properly. The solutions are:
a) Check whether your router/NAT can be configured to enable 'hairpining'. This solution works iff you control all router/NATs in your deployment.
b) Buy another router/node allowing this. Just like a), it works iff your control all router/NATs in your deployment.
c) If you can get obtain the port information for from UPnP, this is a solution too, but not all Router/NAT know or support UPnP. It does not cover for all cases in large deployment.
d) Using multicasting to 'discover' other nodes on the LAN and even communicate with them is a common solution to this problem. You need to agree on an IP address and have peers listen to it.
e) Storing the private IP address on the server is a solution too, but it requires more storage capacity on the server than solution d. There is a timeout (i.e., expiration of data validity) to handle too.
f) Use a TURN like communication between peers (i.e., communication between nodes pass through central server). This solution is rock solid, but not the most efficient in terms of bandwidth consumption.
Hope this helps.
Interactive Connectivity Establishment (ICE) was specifically developed for solving that type of NAT problems. It uses STUN and TURN to achieve the result and is used in modern p2p applications. (e.g. Voicechat)
The PJNATH library has a document explaining
unlike standalone STUN solution, it (ICE) solves the hairpinning issue, since it also offers host candidates.
I was wondering that how application like skype ( a popular chat client ) works in local network with one router, How it can listen on particular port?
for example:=
In one network A and B are two machines running skype , gateway of both is G1,
now how A and B will have same IP on internet that is of G1, but how can they ensure that they are listening on different ports? How can they ask to router G1 for unique port.
I want to make a simple text chat server on linux. How can I have connections between two different computers in two different networks?
Solution to your problem is to have a forwarding server somewhere in the net.
Different programs use different means to connect to each other. But every chat server, including Skype, has a server, which forwards data or information about subnet IP/port availability.
There are two types of clients: "listening" clients and "passive" ones. Listening clients have direct access to Internet via router port forwarding, and "passive" ones have to use additional tricks to get their hands on external data, line external servers or additional ports to listen.
The point is, not clients connect to each other, but they connect to a server, which then connects back to them to verify they are available, and, if at least one of them is not firewalled, direct another on to connect to the first one, excludint itself from further communication. And if both are firewalled, then is has to forward their messages through itself.
Host Discovery
Manual discovery, client A knowns who client B is
Discovery through broadcast UDP which is used by lot of games for LAN play. A client sends out a packet to the broadcast address for their subnet. The peers can choose to pick up this broadcast and respond. The downside is that this is limited to the current subnet. The more general INADDR_BROADCAST (255.255.255.255) works for all subnets on the local-link, but it cannot be routed, so won't work over internet (this is what DHCP auto-configuration uses).
Discovery through a central (Rendezvous) server. Each individual client knows the address of the server, and the latter informs them about each other. This technique is used by IRC, Voip, IMs and by most 'peer-to-peer' networks.
Communication
After the initial discovery is done you want to be able to talk to eachother. On the internet this can get tricky. Most people nowadays have their own router and sit behind a NAT, so direct connections are impossible.
Using a Rendezvous server, you can possibly talk to each other using the server itself. client A tells the server what to say, and it in turn tells client B, since both clients have an outbound connection to the server.
It is possible for the clients to talk to each other without the server proxying. This requires either DMZ, port forwarding or UPnP. DMZ will basically forward all incoming connections on all the ports to a given local IP. Port forwarding only forwards certain ports to local IPs. UPnP is a bit more advanced, the client requests that the router temporarily forwards a port to it, and you tell the other client via the rendezvous server where to connect.
Chatting app implementation
The easiest solution to your problem is most likely to use a central server, which is known by all the clients, that proxies host discovery and possibly the communication between the clients. If you want the clients to communicate directly, you can just proxy host discovery, and then let either DMz, manual port forwarding or UPnP do the rest.
Another solution would be to just have direct communication through NAT traversal techniques discussed above, and do manual host discovery.
Yet another solution would be to use a public webserver and 'abuse' its ability to insert content to chat with each other.
You need a central UDP Rendezvous Server.
After the initial connection from the client to the server the UDP clients can be redirected to talk to eachother directly even if firewalled.
The trick is to open an UDP connection from the inside.
Check out Real-Time Media Flow Protocol and how they use it.
Check out UDP Hole Punching
alt text http://labs.adobe.com/technologies/stratus/images/p2pvideo_250x215.jpg
Traditional NAT servers replace the source address and port with the address and a random port number of the external interface of the NAT server. This works well for simple protocols such as HTTP and SMTP, but it can create problems for more complex protocols that require multiple response ports on the external interface of the NAT server. NAT servers also aren’t aware of information stored in the data portion of the application layer header without the help of NAT editors and similar software fixes.
Windows XP’s answer to these problems is NAT Traversal, which can automatically allow the UPnP-enabled NAT client application to communicate with a UPnP NAT device. NAT Traversal provides methods to allow the UPnP client to learn the public IP address of the NAT server and to negotiate dynamically assigned port mappings for UPnP NAT client applications.
NAT Traversal features can be built into any hardware device or software application. Applications that commonly cause troubles for NAT devices but work well when UPnP-enabled include the following:
Multiplayer Internet games
Audio and video communications
Terminal Services clients and servers
Peer-to-peer file sharing applications
When these applications are UPnP-enabled, access through the Windows XP ICS allows them to work seamlessly.
Unless A and B are actually "listening" to the responses to outgoing requests, your router will need to be cofigured to forward the relevant port numbers to the relevant hosts. This isn't something that you can request in the code, it's something you need to configure on the router itself.