SO_REUSEADDR

SO_REUSEADDR set or not
socketA has already been successfully bound to the address
bind to the address given for socketB
result of the bind operation for socketB

SO_REUSEADDR	socketA	socketB	result
ON/OFF	192.168.0.1:21	192.168.0.1:21	Error (EADDRINUSE)
ON/OFF	192.168.0.1:21	10.0.0.1:21	OK
ON/OFF	10.0.0.1:21	192.168.0.1:21	OK
OFF	0.0.0.0:21	192.168.0.1:21	Error (EADDRINUSE)
OFF	192.168.0.1:21	0.0.0.0:21	Error (EADDRINUSE)
ON	0.0.0.0:21	192.168.0.1:21	OK
ON	192.168.0.1:21	0.0.0.0:21	OK
ON/OFF	0.0.0.0:21	0.0.0.0:21	Error (EADDRINUSE)

set SO_REUSEADDR to be able to bind

	socketA	socketB
TIME_WAIT (after "half dead")		ON
multicast	ON	ON

TIME_WAIT

until all pending data has been successfully sent
until a timeout is hit

socket is closed forcefully

SO_REUSEPORT

SO_REUSEPORT (since Linux 3.9)
        Permits multiple AF_INET or AF_INET6 sockets to be bound to an
        identical socket address. This option must be set on each
        socket (including the first socket) prior to calling bind(2)
        on the socket. To prevent port hijacking, all of the
        processes binding to the same address must have the same
        effective UID. This option can be employed with both TCP and
        UDP sockets.

sample code

socket set SO_REUSEPORT option
Linux kernel 3.9 の新機能 SO_REUSEPORT を試してみる

def listener_work(num):
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.setsockopt(socket.SOL_SOCKET, SO_REUSEPORT, 1)   # set SO_REUSEPORT
    s.bind(("", PORT))
    ...

NGINX

Scaling NGINX Web Server Performance Socket Sharding Using SO REUSEPORT

reuseport

http {
     server {          listen 80 reuseport;
          server_name  localhost;
          ...
     }
}

stream {
     server {          listen 12345 reuseport;
          ...
     }
}

ref.