How to use SO_KEEPALIVE option properly to detect that the client at the other end is down?

Question

I was trying to learn the usage of option SO_KEEPALIVE in socket programming in C language under Linux environment.

I created a server socket and used my browser to connect to it. It was successful and I was able to read the GET request, but I got stuck on the usage of SO_KEEPALIVE.

I checked this link [email protected] but I could not find any example which shows how to use it.

As soon as I detect the client's request on accept() function I set the SO_KEEPALIVE option value 1 on the client socket. Now I don't know, how to check if the client is down, how to change the time interval between the probes sent etc.

I mean, how will I get the signal that the client is down? (Without reading or writing at the client - I thought I will get some signal when probes are not replied back from client), how should I program it after setting the option SO_KEEPALIVE on).

Also if suppose the probes are sent every 3 secs and the client goes down in between I will not get to know that client is down and I may get SIGPIPE.

Anyways importantly I wanna know how to use SO_KEEPALIVE in the code.

Answer 1

To modify the number of probes or the probe intervals, you write values to the /proc filesystem like

 echo 600 > /proc/sys/net/ipv4/tcp_keepalive_time
 echo 60 > /proc/sys/net/ipv4/tcp_keepalive_intvl
 echo 20 > /proc/sys/net/ipv4/tcp_keepalive_probes

Note that these values are global for all keepalive enabled sockets on the system, You can also override these settings on a per socket basis when you set the setsockopt, see section 4.2 of the document you linked.

You can't "check" the status of the socket from userspace with keepalive. Instead, the kernel is simply more aggressive about forcing the remote end to acknowledge packets, and determining if the socket has gone bad. When you attempt to write to the socket, you will get a SIGPIPE if keepalive has determined remote end is down.