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Does the port change when a server accepts a TCP connection?
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Closed 4 years ago.
I understand the basics of how ports work. However, what I don't get is how multiple clients can simultaneously connect to say port 80. I know each client has a unique (for their machine) port. Does the server reply back from an available port to the client, and simply state the reply came from 80? How does this work?
First off, a "port" is just a number. All a "connection to a port" really represents is a packet which has that number specified in its "destination port" header field.
Now, there are two answers to your question, one for stateful protocols and one for stateless protocols.
For a stateless protocol (ie UDP), there is no problem because "connections" don't exist - multiple people can send packets to the same port, and their packets will arrive in whatever sequence. Nobody is ever in the "connected" state.
For a stateful protocol (like TCP), a connection is identified by a 4-tuple consisting of source and destination ports and source and destination IP addresses. So, if two different machines connect to the same port on a third machine, there are two distinct connections because the source IPs differ. If the same machine (or two behind NAT or otherwise sharing the same IP address) connects twice to a single remote end, the connections are differentiated by source port (which is generally a random high-numbered port).
Simply, if I connect to the same web server twice from my client, the two connections will have different source ports from my perspective and destination ports from the web server's. So there is no ambiguity, even though both connections have the same source and destination IP addresses.
Ports are a way to multiplex IP addresses so that different applications can listen on the same IP address/protocol pair. Unless an application defines its own higher-level protocol, there is no way to multiplex a port. If two connections using the same protocol simultaneously have identical source and destination IPs and identical source and destination ports, they must be the same connection.
Important:
I'm sorry to say that the response from "Borealid" is imprecise and somewhat incorrect - firstly there is no relation to statefulness or statelessness to answer this question, and most importantly the definition of the tuple for a socket is incorrect.
First remember below two rules:
Primary key of a socket: A socket is identified by {SRC-IP, SRC-PORT, DEST-IP, DEST-PORT, PROTOCOL} not by {SRC-IP, SRC-PORT, DEST-IP, DEST-PORT} - Protocol is an important part of a socket's definition.
OS Process & Socket mapping: A process can be associated with (can open/can listen to) multiple sockets which might be obvious to many readers.
Example 1: Two clients connecting to same server port means: socket1 {SRC-A, 100, DEST-X,80, TCP} and socket2{SRC-B, 100, DEST-X,80, TCP}. This means host A connects to server X's port 80 and another host B also connects to the same server X to the same port 80. Now, how the server handles these two sockets depends on if the server is single-threaded or multiple-threaded (I'll explain this later). What is important is that one server can listen to multiple sockets simultaneously.
To answer the original question of the post:
Irrespective of stateful or stateless protocols, two clients can connect to the same server port because for each client we can assign a different socket (as the client IP will definitely differ). The same client can also have two sockets connecting to the same server port - since such sockets differ by SRC-PORT. With all fairness, "Borealid" essentially mentioned the same correct answer but the reference to state-less/full was kind of unnecessary/confusing.
To answer the second part of the question on how a server knows which socket to answer. First understand that for a single server process that is listening to the same port, there could be more than one socket (maybe from the same client or from different clients). Now as long as a server knows which request is associated with which socket, it can always respond to the appropriate client using the same socket. Thus a server never needs to open another port in its own node than the original one on which the client initially tried to connect. If any server allocates different server ports after a socket is bound, then in my opinion the server is wasting its resource and it must be needing the client to connect again to the new port assigned.
A bit more for completeness:
Example 2: It's a very interesting question: "can two different processes on a server listen to the same port". If you do not consider protocol as one of the parameters defining sockets then the answer is no. This is so because we can say that in such a case, a single client trying to connect to a server port will not have any mechanism to mention which of the two listening processes the client intends to connect to. This is the same theme asserted by rule (2). However, this is the WRONG answer because 'protocol' is also a part of the socket definition. Thus two processes in the same node can listen to the same port only if they are using different protocols. For example, two unrelated clients (say one is using TCP and another is using UDP) can connect and communicate to the same server node and to the same port but they must be served by two different server processes.
Server Types - single & multiple:
When a server processes listening to a port that means multiple sockets can simultaneously connect and communicate with the same server process. If a server uses only a single child process to serve all the sockets then the server is called single-process/threaded and if the server uses many sub-processes to serve each socket by one sub-process then the server is called a multi-process/threaded server. Note that irrespective of the server's type a server can/should always use the same initial socket to respond back (no need to allocate another server port).
Suggested Books and the rest of the two volumes if you can.
A Note on Parent/Child Process (in response to query/comment of 'Ioan Alexandru Cucu')
Wherever I mentioned any concept in relation to two processes say A and B, consider that they are not related by the parent-child relationship. OS's (especially UNIX) by design allows a child process to inherit all File-descriptors (FD) from parents. Thus all the sockets (in UNIX like OS are also part of FD) that process A listening to can be listened to by many more processes A1, A2, .. as long as they are related by parent-child relation to A. But an independent process B (i.e. having no parent-child relation to A) cannot listen to the same socket. In addition, also note that this rule of disallowing two independent processes to listen to the same socket lies on an OS (or its network libraries), and by far it's obeyed by most OS's. However, one can create own OS which can very well violate this restriction.
TCP / HTTP Listening On Ports: How Can Many Users Share the Same Port
So, what happens when a server listen for incoming connections on a TCP port? For example, let's say you have a web-server on port 80. Let's assume that your computer has the public IP address of 24.14.181.229 and the person that tries to connect to you has IP address 10.1.2.3. This person can connect to you by opening a TCP socket to 24.14.181.229:80. Simple enough.
Intuitively (and wrongly), most people assume that it looks something like this:
Local Computer | Remote Computer
--------------------------------
<local_ip>:80 | <foreign_ip>:80
^^ not actually what happens, but this is the conceptual model a lot of people have in mind.
This is intuitive, because from the standpoint of the client, he has an IP address, and connects to a server at IP:PORT. Since the client connects to port 80, then his port must be 80 too? This is a sensible thing to think, but actually not what happens. If that were to be correct, we could only serve one user per foreign IP address. Once a remote computer connects, then he would hog the port 80 to port 80 connection, and no one else could connect.
Three things must be understood:
1.) On a server, a process is listening on a port. Once it gets a connection, it hands it off to another thread. The communication never hogs the listening port.
2.) Connections are uniquely identified by the OS by the following 5-tuple: (local-IP, local-port, remote-IP, remote-port, protocol). If any element in the tuple is different, then this is a completely independent connection.
3.) When a client connects to a server, it picks a random, unused high-order source port. This way, a single client can have up to ~64k connections to the server for the same destination port.
So, this is really what gets created when a client connects to a server:
Local Computer | Remote Computer | Role
-----------------------------------------------------------
0.0.0.0:80 | <none> | LISTENING
127.0.0.1:80 | 10.1.2.3:<random_port> | ESTABLISHED
Looking at What Actually Happens
First, let's use netstat to see what is happening on this computer. We will use port 500 instead of 80 (because a whole bunch of stuff is happening on port 80 as it is a common port, but functionally it does not make a difference).
netstat -atnp | grep -i ":500 "
As expected, the output is blank. Now let's start a web server:
sudo python3 -m http.server 500
Now, here is the output of running netstat again:
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 0.0.0.0:500 0.0.0.0:* LISTEN -
So now there is one process that is actively listening (State: LISTEN) on port 500. The local address is 0.0.0.0, which is code for "listening for all". An easy mistake to make is to listen on address 127.0.0.1, which will only accept connections from the current computer. So this is not a connection, this just means that a process requested to bind() to port IP, and that process is responsible for handling all connections to that port. This hints to the limitation that there can only be one process per computer listening on a port (there are ways to get around that using multiplexing, but this is a much more complicated topic). If a web-server is listening on port 80, it cannot share that port with other web-servers.
So now, let's connect a user to our machine:
quicknet -m tcp -t localhost:500 -p Test payload.
This is a simple script (https://github.com/grokit/dcore/tree/master/apps/quicknet) that opens a TCP socket, sends the payload ("Test payload." in this case), waits a few seconds and disconnects. Doing netstat again while this is happening displays the following:
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 0.0.0.0:500 0.0.0.0:* LISTEN -
tcp 0 0 192.168.1.10:500 192.168.1.13:54240 ESTABLISHED -
If you connect with another client and do netstat again, you will see the following:
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 0.0.0.0:500 0.0.0.0:* LISTEN -
tcp 0 0 192.168.1.10:500 192.168.1.13:26813 ESTABLISHED -
... that is, the client used another random port for the connection. So there is never confusion between the IP addresses.
Normally, for every connecting client the server forks a child process that communicates with the client (TCP). The parent server hands off to the child process an established socket that communicates back to the client.
When you send the data to a socket from your child server, the TCP stack in the OS creates a packet going back to the client and sets the "from port" to 80.
Multiple clients can connect to the same port (say 80) on the server because on the server side, after creating a socket and binding (setting local IP and port) listen is called on the socket which tells the OS to accept incoming connections.
When a client tries to connect to server on port 80, the accept call is invoked on the server socket. This creates a new socket for the client trying to connect and similarly new sockets will be created for subsequent clients using same port 80.
Words in italics are system calls.
Ref
http://www.scs.stanford.edu/07wi-cs244b/refs/net2.pdf
Specifically, I have a Windows server (Windows 7), but the netstat -an command only shows whether ports are TCP or UDP. I thought these were the only kinds of ports, but node.js seems to distinguish between HTTP ports and TCP ports (example at bottom of linked page). I'm currently using node.js in a program that will run on my server, and it opens HTTP ports by default. These appear as TCP ports under netstat -an.
Is there a command line trick for distinguishing whether an open port on this server is HTTP or TCP? I make requests to my Information Technology office about ports that I need on this server, and they need to know whether these ports will be TCP, UDP, etc.
If necessary to use a remote client, I have a Mac that can do the job.
HTTP is an application protocol. Its definition presumes an underlying and reliable transport layer protocol. The transmission Control Protocol is commonly used. However HTTP can use unreliable protocols too (example SSDP).
Now to answer to your question:
netstat -lt : List TCP Listening Ports
netstat -lu : List UDP Listening Ports
If you want to know wether a TCP Port is running HTTP or not, you can check the standard port on HTTP (grep :80). The standard HTTP port is 80. The standard HTTPS port is 443.
All HTTP traffic is transmitted over TCP Ports. I think what may be causing some confusion is that the first node.js example uses the http module and the second example uses the net module.
The http module is built on top of the net module (which handles the TCP layer underlying HTTP).
Refer to: nodejs - net or http module
EDIT:
Only one process can bind to a port. If you're having difficulties connecting, shut down any other applications that may be using the same port, such as your Java Hello World server. Run netstat -an to make sure you don't see the port listed that you're trying to listen on in you node.js TCP server (port 1337) in the example. If nothing is listening on the port, fire up your node.js TCP server and run netstat -an again to make sure it's actually listening on the intended port.
Another suggestion - temporarily turn off the software firewall then use the telnet client on the local server to attempt to connect to the port using the command telnet 127.0.0.1 1337 (replace 1337 with your port) from the command prompt. This will eliminate any network devices such as firewalls between the client (telnet in this case) and the server. If the screen goes blank, you've connected. If you get the message Could not open connection to the host, on port then it's still not listening on the TCP Port. If you can connect locally from Telnet but you cannot connect remotely then there is something blocking your connection.
I have a client which is intended to connect to a server. For the client, the remote TCP port number is fixed(i.e. 102). I can NOT change it(while I can change the remote IP address). However, the TCP Port number the server is listening on is fixed as well(i.e. 1024) and I can NOT change it too. These two port numbers are different. I want to make the client connect to the server smoothly.
At the first, I had a idea that I setup a proxy listening on localhost:102 and the client connect to 127.0.0.1:102. Then this proxy redirect these TCP traffic to the real address RemoteServerIP:1024. But I found on my windows, there was already a process which is listening on 0.0.0.0:102 and I can NOT change its listening port. So this idea can NOT work.
Thank you very much.
if you cannot do it on the same windows machine running client, why not try to do it on another (linux maybe) machine?
Can I establish more than 1 TCP connections to same server on same port ?
For example :
Connection 1: machine A to Machine B on port 445
Connection 2: machine A to Machine B on port 445
Without terminating previously established connection 1.
If yes, do i need to do any setting to enable such scenario ?
Yes, you can. Without any special setting.
That's exactly what happens when you start, let's say Internet Explorer and Mozilla Firefox and navigate to the same site with both.
Behind the scene to connect to the server, machine A opens a socket using whatever port number and tries to connect to machine B port 445. Machine B, who was listening on port 445 can accept this incoming connection, and uses another socket using whatever other port number to connect it to. In the end, the actual TCP connection is established between whatever port en machine A and whatever port on machine B. On machine B, port 445 is left for listening only (and can accept other connections).
Yes you can, a TCP/IP session is identified by the following 5-tuple:
(src-ip, src-port, dst-ip, dst-port, protocol).
In your case, 4 of the 5 values have to be the same: src-ip (A), dst-ip (B), dst-port (445), protocol (tcp). As such, the src-port still gives you a degree of freedom, by variating this, you can set up multiple sessions. This is what should happen by default, your OS will select a new, unique source port for each connection to the same server.
TCP/IP standard supports this with ephemeral ports. You connect to fixed port on server side, but port on client side is not under your control. TCP/IP stack will choose client port, and connection is then defined and distinguished from other connections with 4 bits of information: client IP address, client port, server IP address, and server port.
I am using Boost asio in Visual C++. I am trying to connect to a static IP which listens TCP on port 1222. unfortunately i can only connect with Lan and cannot connect from another Lan to the TCP listening PC(203.143..).
It connects perfectly with the EC2 server. Is it something to do with firewall. but when i run the app it ask for the user to allow firewall for the specific port. I can post my code but i guess it's not something to with the code.
Usually a firewall on the server side (the PC you are trying to connect to) have to be configured to allow incoming connections.
Firewall on your side should be OK, it usually allows any type of outgoing connections.
I don't know what you meant by "the app ask for the user to allow firewall for the specific port" (clarify).