CODEWITHSUNDEEP
pythonsocketslocalhostportbroadcast
beorn
beorn

Reputation: 45

How to send a message in a specific port with python sockets ? No random ports

I have to test a broadcast with acknowledgement on localhost. So I have some text files that represent the nodes and inside there is a list of neighbors.
I use localhost as the IP and the port is the number of the node.

The problem is when I receive a message (that I sent) from a node like 7000, python replaces it with a random number for example 65724. So now my father is 65724 instead of 7000, so I cannot remove 7000 from the list of neighbors.
I cannot complete my algorithm and that is very frustrating.

I can send a message with the port number that I want, but it's not very elegant.

Could someone tell me how to not allow python to randomize the port?

rmunn saved me, the answer I was looking far is the bind before connect method. Befor sending a message you bind your own port and you connect to the other one.

Upvotes: 0

Views: 3202

Answers (1)

rmunn
rmunn

Reputation: 36708

This is not a Python problem, per se. You are confused about how ports work.

Each TCP communication (sending or receiving) has two IP addresses and two ports: the host IP and host port, and the destination IP and destination port.

If you're communicating with a computer "out there" on the network, your host and destination IPs will be different. In your test case, your host and destination IPs will both be 127.0.0.1 (localhost). But I'm going to go with the "different IPs" case for my example, because it makes it easier to see.

So your IP address is, say 10.1.2.3, and you're talking to a computer at 10.1.2.99. You tell the system that you want to talk to 10.1.2.99 at port 7000, and it opens up a connection. When that happens, it will randomly pick a source port that's not in use. So now there's a two-way communication channel open:

10.1.2.3:65274 <-> 10.1.2.99:7000

Note that you did not pick that host port. EDIT: I originally said "In fact, the system will not allow you to pick the host port; it will be assigned to you" here, but that is wrong. If s is a socket object, you can call s.bind() to set its source port, then call s.connect() to connect to a destination port.

Now, when you're listening for a message, then you pick the port you're listening on, and the computer that's connecting to you will have a random port. So if you were listening for a message on port 8912, the incoming connection (once established) will look like:

10.1.2.3:8912 <-> 10.1.2.99:38290

Note that 38290 was chosen at random by the operating system of the computer at the 10.1.2.99 IP address.

Now for the bit of Python. You mention sockets in your question title, so I'll assume you're using the socket module from Python's standard library. Once you've created a socket object s, use s.getpeername() to find out the address (host and port) that you've connected to, and s.getsockname() to find out the address (host and port) that you've connected from.

Since you talk about expecting the number 7000 and getting a random number, I think you're using the host socket when you should be using the destination socket.

Upvotes: 3

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