Monitoring a tcp port

Question

For fun, I've been toying around with writing a load balancer in python and have been trying to figure the best (correct?) way to test if a port is available and the remote host is still there.

I'm finding that, once connected, it becomes difficult to tell when the remote host goes down. I've turned keep alive on, but can't get it to recognize a downed connection sooner than a minute (I realize polling more often than a minute might be overkill, but lets say I wanted to), even after setting the various TCP_KEEPALIVE options to their lowest.

When I use nonblocking sockets, I've noticed that a recv() will return an error ("resource temporarily unavailable") when it reads from a live socket, but returns "" when reading from a dead one (send and recv of 0 bytes, which might be the cause?). That seems like an odd way to test for it connected, though, and makes it impossible to tell if the connected died but after sending some data.

Aside from connecting/disconnecting for every check, is there something I can do? Can I manually send a tcp keepalive, or can I establish a lower level connection that will let me test the connectivity without sending real data the remote server would potentially process?

Answer 1

I'd recommend not leaving your (single) test socket connected - make a new connection each time you need to poll. Every load balancer / server availability system I've ever seen uses this method instead of a persistent connection.

If the remote server hasn't responded within a reasonable amount of time (e.g. 10s) mark it as "down". Use timers and signals rather than function response codes to handle that timeout.

Answer 2

"it becomes difficult to tell when the remote host goes down"

Correct. This is a feature of TCP. The whole point of TCP is to have an enduring connection between ports. Theoretically an application can drop and reconnect to the port through TCP (the socket libraries don't provide a lot of support for this, but it's part of the TCP protocol).