I need to bind the client port number for the tcpclient when connecting to a server.
I have created the server with socket module in Python at port 1234 and need to connect a MATLAB script to it.
Currently, I have written the MATLAB script like this.
server = tcpclient(ip_address, 1234)
But this randomly selects a port number for the client (which is the MATLAB script). I need it to be a fixed port (For Example: 55000) to enable some functions requiring port forwarding.
Is there a way to bind this to a particular port number.
Related
I have set up a client and a server using sockets in python where my client sends data to the server, servers performs an operation, then returns some data to the client. Originally both the client and server were to have the same port number (9999). My issue currently is that I have to change the port of the server to 19999, and when I try to run, it does not work. The client is able to send data if its port is also changed to 19999, but it does not work if client is 9999 and server is 19999, which is what I need. New to networking systems so would appreciate any useful links to information or advice.
pic of client (left) and server (right)
Normally, clients will use an ephemeral port for its local port, and connect to the server port. Your client code is attempting to connect to port 9999, which is not the server port, which explains why it is not working. You need to connect to port 19999, since that is the port the server is listening on.
Using an unbound socket causes the client to choose an ephemeral port for its local address when making a connection. If you want the client to bind to a specific port, use bind before you call connect.
# bind locally to 9999
local_addr = (host, 9999)
s.bind(local_addr)
# connect to 19999
remote_addr = (host, 19999)
s.connect(remote_addr)
This question already has answers here:
What is the theoretical maximum number of open TCP connections that a modern Linux box can have
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Does the port change when a server accepts a TCP connection?
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Closed 3 years ago.
I read something I found contradictory with my current understanding of ports. If you google "how many ports does a server have", the first thing to come up states the following:
The server generally only ever uses one port, no matter how many clients are connected. It is the tuple of (client IP, client port,
server IP, server port) that must be unique for each TCP connection -
so the limit of 65535 ports is only relevant for how many connections
a single client can make to a single server.
I thought each time a client establishes a connection to a server, then a socket is creating using a regular port for the connection between the two?
If no, does it mean that a server can have more clients connected to it, than the maximum amount of regular ports?
I thought each time a client establishes a connection to a server, then a socket is creating using a regular port for the connection between the two?
The term "port" in this context is being used to describe, essentially, an address. The port number, along with the IP address, uniquely identifies one endpoint of the network.
Not only does the server endpoint generally only use a single port number, it would be a lot more difficult to make connections to the server if it didn't, because what port number would the client endpoint use to request the connection? DNS allows a client to look up the IP address, if the IP address is not already know, but there's no such facility for port numbers. So the port number has to be known in advance.
So, no…it is not the case that each time a client makes a connection, a socket is created using a "regular port" for the connection between the two. There's no "regular port". There's just "port", all ports are the same, and they are simply a number that identifies the endpoint's address.
If no, does it mean that a server can have more clients connected to it, than the maximum amount of regular ports?
Yes, it can. On the server end, the port number is (generally) always the same. For example, an HTTP server will (generally) use port 80. The listening socket will have as its port number "80", as will the server-side socket for each connection.
The port number can be reused like this, because each socket has other identifying characteristics besides the IP address and port number. In particular, the server's listening socket is unique; there is only one socket on the server end that has that IP address, that port number, and which has no connections (i.e. is listening).
Once a connection is made, a new socket is created to represent that connection. And that socket can be uniquely identified, because unlike the listening socket, it does have a connection (i.e. a remote endpoint) associated with it, along with the IP address and port number. When the client endpoint sends data to the server, the network layer can tell which socket to which that should be delivered, because that data comes from a specific remote endpoint, which also has a unique IP address and port number.
The combination of the server's and client's unique IP addresses and port numbers uniquely identifies that connection, making it distinct from any other socket on the server that may have the same server-side endpoint's IP address and port number.
In the text you quoted, this part is describing exactly this distinct, unique identification of a socket:
It is the tuple of (client IP, client port, server IP, server port) that must be unique for each TCP connection
In this way, the server's IP address and port number can be used an indefinite number of times (not counting other constrained resources on the server, like memory and tables that hold the state of the network connections).
The limitation on port numbers only comes into play when trying to create additional listening sockets (for servers) or additional connections (for clients). Servers typically won't run out of port numbers unless they are implementing a protocol that requires the server to create a connection back to a client's listening socket (this is uncommon), and clients won't run out of port numbers unless they try to make a very large number of connections.
It is this latter limit that this part of the text you quoted is referring to:
the limit of 65535 ports is only relevant for how many connections a single client can make to a single server.
Let's assume I'm in computer A, I have a few servers running on different ports, but all are basically an instance of the same program (just binding to different ports). Now, computer B, a client, does he need to know what port is the software he wishes to connect to on computer A?
The point is, I am implementing some sort of communication similar to sockets. Everything should work fine but I'm not sure how to create the initial-message from a computer to another - I just don't know to what port to send it to. Does the client know the port he's sending to on the server?
Say here (client): clientsocket.connect(('localhost', 8089)), does the client connect a server running on port 8089? If so, what port is his socket on (what port is he using for the client?
Yes. The only way for the network stack on computer A to know which process to deliver an incoming packet is for computer B to set the correct port in the packet. A web server runs on port 80 by default, but a machine running several distinct web servers will run them on distinct ports, and a client must be specific about which server they want to connect to. http://example.com, http://example.com:8080, and http://example.com:12345 would refer to the servers running on example.com on ports 80, 8080, and 12345, respectively.
In order to know which port to use in your client, you need to read the documentation for the server you want to connect to.
Going in the other direction, the port used by the client to receive responses is typically set by the networking stack automatically. The client doesn't need to do anything special to set it, and the server simply sends packets back to the address/port found in the source portion of the incoming packet.
I need to write my first socket program involving TCP connections. In the program I have created there is a client and server, both of which are the machine I am coding on.However,it requires that I pass the port number as a command line argument. How do I accomplish this?
The answer is simple : Make sure your server and your client agree on the port to use. As long as the port is available and can be used, set up the connected so that the client and server use that same port.
Here's a link that explain the different ranges available for TCP and UDP ports.
As an exemple, the port 3074 is used by microsoft for its Xbox live service. Making an application using this port might interfere with the service.
The port used will be defined either in a configuration file or hard-coded in the source code of both the server and the client. You should easily be able to find it with a quick look at the code or the directory which contains the application.
Is it possible to set the fixed port on the client side of the connection?
I connect to the ssh-server using port 22 and the client socket is getting random port to identify the session. An example (output from netstat -atn)
tcp4 0 0 <server>.22 <client>.54117 ESTABLISHED
In this example, client gets port 54117. For the test purposes, I'd like a fixed port to be assigned for the client, let's say 40185.
So I'd love the following output:
tcp4 0 0 <server>.22 <client>.40185 ESTABLISHED
Is it even possible?
You can do it programmaticaly, but the ssh(1) command doesn't allow to do that. The main reason is that you let the kernel select the client port, so you can open more than one ssh(1) session to the same server from different source ports in the same client machine. If you fix the port number in the client and the server, you cannot distinguish the packets belonging to one connection from the ones belonging to the other (same protocol, tcp, same source address, same dest address, same source port and same destination port)
To do it programmaticaly in a client and fix the local port, just call bind(2) system call to fix it, before doing the connect(2) system call (as the server does just before the accept(2) system call)
Be careful in that you cannot have two connections with the same five parameters (source add, source port, tcp protocol, dest port, dest addr)