How to check if the connection is still alive in Python?

Question

I've been trying to make a sever that multiple clients can connect to but I ran into a problem. I have a list with all the current connected targets and I want to check if they're still alive without having to manually connect to them.

This is the listener method from the Class I've written:

def listenForConnections(self):
        self.s.bind((self.HOST, self.PORT)) 
        while True:
            self.s.listen(128)
            c, addr = self.s.accept()
            self.c = c
            print("\n[+]Got a new connection!")
            self.conns.append(c)

As you can see, it appends a new connection object to a list from which you then can select a target with a getTarget() method from the same class but I won't paste it here since you probably know how it looks like. Keep in mind that the listener runs in a separate thread. Now I would like to write a ping() function of a sort, that checks every couple of seconds, if all the connections in the conns list are still alive but I have no idea how to do it. I thought in the ping method I would have to send some data to the client every couple of seconds but I fear that it would break the whole program because it could be that the client think that it's a valid command sent from the user and not some garbage to check if a connection is alive or not.

Possible duplicate of [How to tell if a connection is dead in python](https://stackoverflow.com/questions/667640/how-to-tell-if-a-connection-is-dead-in-python) — Iain Shelvington, Sep 27 '19 at 05:11

score 0 · Answer 1 · answered Sep 27 '19 at 13:31

If you want to use a PING message to detect if the client is still alive then you have to provide for this in the client/server protocol. As you noted already, the client must be able to see it is a PING message and not a "normal" message. This only works if you can change the client/server protocol. This is more difficult to implement if the client/server protocol is strictly request/response based, as now the server can also initiate messages.

There are other options:

If the client closes the connection correctly, the server can determine this the next time it tries to communicate with the client.
If the client normally sends requests continuously, you can assume the client is dead and close the connection if the client has not send new requests for some time.
You can use TCP/IP keepalive to detect dead clients.

How to check if the connection is still alive in Python?

1 Answers1