学步园

When the network daemon accepts a connection request, the accepted socket will bind
to port 2000 and to the address where the daemon is running (e.g. 192.6.250.100
). If you executed netstat an
, the output would resemble:

Active connections (including servers)

Proto Recv-Q Send-Q  Local Address Foreign Address (state)

tcp 0 0 192.6.250.100.2000 192.6.250.101.4000 ESTABLISHED

tcp 0 0 *.2000 *.* LISTEN

Here the network daemon has established a connection to the client (192.6.250.101.4000
) with a new server socket. The original network daemon server continues to listen for more connection requests.

If the listening network daemon process is killed, attempts to restart the daemon fail if SO_REUSEADDR is not set. The restart fails because the daemon attempts to bind to port 2000 and a wildcard IP address (e.g. *.2000
). The wildcard address matches the address of the established connection (192.6.250.100
), so the bind
aborts to avoid duplicate socket naming.

When SO_REUSEADDR
is set, bind
ignores the wildcard match, so the network daemon can be restarted. An example usage of this option is:

int optval = 1;

setsockopt
 (s, SOL_SOCKET, SO_REUSEADDR, &optval, 

sizeof(optval));

bind (s, &sin, sizeof(sin));