- 1 year ago
Why isn't SO_REUSEADDR letting my take over a connection in TIME_WAIT?
The SO_REUSEADDR
socket option in both Windows and Linux allows a socket to bind to a local address that is in TIME_WAIT
state. However, it does not "take over" the connection; instead, it allows a new socket to reuse the same local address and port combination that was previously used by a socket that is now in TIME_WAIT
state.
The TIME_WAIT
state occurs after a socket has been closed, and it is a normal part of the TCP protocol. During the TIME_WAIT
state, the operating system keeps the socket in a reserved state for a certain period of time to ensure that any delayed or out-of-order packets related to the closed connection are not misinterpreted by new connections.
When SO_REUSEADDR
is used, a new socket can bind to the same local address and port that is still in TIME_WAIT
state by a previous connection. This is useful in scenarios where you need to quickly restart a server or application on the same address and port without waiting for the TIME_WAIT
period to expire.
However, there are a few things to consider:
Socket Address Reuse:
SO_REUSEADDR
allows multiple sockets to bind to the same address and port, but each socket must have a different remote endpoint (i.e., the remote IP address and port). This means you cannot "take over" an existing connection with the same remote endpoint usingSO_REUSEADDR
.Wait Time:
Even withSO_REUSEADDR
, there may be a brief period where the old connection is still inTIME_WAIT
state, and new connections usingSO_REUSEADDR
might fail during that time. The wait time is typically a few minutes, depending on the TCP/IP implementation.Port Exhaustion:
UsingSO_REUSEADDR
liberally can lead to "port exhaustion," where all available ports are in use, preventing new connections from being established.
If you need to handle existing connections during a restart, you may need to implement application-level logic to gracefully close the existing connections before restarting the server or application. Alternatively, you can consider using a connection pool or load balancer to handle the connections during the restart process.