Table of Contents


A TCP/UDP connection is identified by a tuple of five values:

(<protocol>, <src addr>, <src port>, <dest addr>, <dest port>)
  • protocol is set when a socket is created with the socket() function.
  • source address and port are set with the bind() function.
    • assign 0 for port means any port, the system will choose a specific port from predefined range of source ports
    • assign INADDR_ANY (‘’ for IPv4 or ‘::’ for IPv6) for addr, the socket will bound to all source IP and address of all local interfaces
    • depending on the destination address and the content of the routing table, the system will pick an appropriate source address and replace the “any” binding with a binding to the chosen source IP address.
  • destination address and port are set with the connect() function.

Any unique combination of these values identifies a connection. As a result, no two connections can have the same five values, otherwise the system would not be able to distinguish these connections any longer.

Since UDP is a connectionless protocol, UDP sockets can be used without connecting them. Yet it is allowed to connect them and in some cases very advantageous for your code and general application design. In connectionless mode, UDP sockets that were not explicitly bound when data is sent over them for the first time are usually automatically bound by the system, as an unbound UDP socket cannot receive any (reply) data. Same is true for an unbound TCP socket, it is automatically bound before it will be connected.

Socket Options


The amount of time the kernel will wait before it closes the socket, regardless if it still has pending send data or not, is called the Linger Time. The Linger Time is globally configurable on most systems and by default rather long (two minutes is a common value you will find on many systems). It is also configurable per socket using the socket option SO_LINGER which can be used to make the timeout shorter or longer, and even to disable it completely. Disabling it completely is a very bad idea, though, since closing a TCP socket gracefully is a slightly complex process and involves sending forth and back a couple of packets (as well as resending those packets in case they got lost) and this whole close process is also limited by the Linger Time. If you disable lingering, your socket may not only lose pending data, it is also always closed forcefully instead of gracefully, which is usually not recommended. The details about how a TCP connection is closed gracefully are beyond the scope of this answer, if you want to learn more about, I recommend you have a look at this page. And even if you disabled lingering with SO_LINGER, if your process dies without explicitly closing the socket, BSD (and possibly other systems) will linger nonetheless, ignoring what you have configured. This will happen for example if your code just calls exit() (pretty common for tiny, simple server programs) or the process is killed by a signal (which includes the possibility that it simply crashes because of an illegal memory access). So there is nothing you can do to make sure a socket will never linger under all circumstances.


By default, no two sockets can be bound to the same combination of source address and source port. As long as the source port is different, the source address is actually irrelevant. Binding socketA to A:X and socketB to B:Y, where A and B are addresses and X and Y are ports, is always possible as long as X != Y holds true. However, even if X == Y, the binding is still possible as long as A != B holds true. E.g. socketA belongs to a FTP server program and is bound to and socketB belongs to another FTP server program and is bound to, both bindings will succeed. Keep in mind, though, that a socket may be locally bound to “any address”. If a socket is bound to, it is bound to all existing local addresses at the same time and in that case no other socket can be bound to port 21, regardless which specific IP address it tries to bind to, as conflicts with all existing local IP addresses.

Things start to get OS specific when address reuse comes into play. We start with BSD, since as I said above, it is the mother of all socket implementations.



If SO_REUSEADDR is enabled on a socket prior to binding it, the socket can be successfully bound unless there is a conflict with another socket bound to exactly the same combination of source address and port. Now you may wonder how is that any different than before? The keyword is “exactly”. SO_REUSEADDR mainly changes the way how wildcard addresses (“any IP address”) are treated when searching for conflicts.

Without SO_REUSEADDR, binding socketA to and then binding socketB to will fail (with error EADDRINUSE), since means “any local IP address”, thus all local IP addresses are considered in use by this socket and this includes, too. With SO_REUSEADDR it will succeed, since and are not exactly the same address, one is a wildcard for all local addresses and the other one is a very specific local address. Note that the statement above is true regardless in which order socketA and socketB are bound; without SO_REUSEADDR it will always fail, with SO_REUSEADDR it will always succeed.

Okay, SO_REUSEADDR has an effect on wildcard addresses, good to know. Yet that isn’t it’s only effect it has. There is another well known effect which is also the reason why most people use SO_REUSEADDR in server programs in the first place. For the other important use of this option we have to take a deeper look on how the TCP protocol works.

The question is, how does the system treat a socket in state TIME_WAIT? If SO_REUSEADDR is not set, a socket in state TIME_WAIT is considered to still be bound to the source address and port and any attempt to bind a new socket to the same address and port will fail until the socket has really been closed, which may take as long as the configured Linger Time. So don’t expect that you can rebind the source address of a socket immediately after closing it. In most cases this will fail. However, if SO_REUSEADDR is set for the socket you are trying to bind, another socket bound to the same address and port in state TIME_WAIT is simply ignored, after all its already “half dead”, and your socket can bind to exactly the same address without any problem. In that case it plays no role that the other socket may have exactly the same address and port. Note that binding a socket to exactly the same address and port as a dying socket in TIME_WAIT state can have unexpected, and usually undesired, side effects in case the other socket is still “at work”, but that is beyond the scope of this answer and fortunately those side effects are rather rare in practice.

There is one final thing you should know about SO_REUSEADDR. Everything written above will work as long as the socket you want to bind to has address reuse enabled. It is not necessary that the other socket, the one which is already bound or is in a TIME_WAIT state, also had this flag set when it was bound. The code that decides if the bind will succeed or fail only inspects the SO_REUSEADDR flag of the socket fed into the bind() call, for all other sockets inspected, this flag is not even looked at.


SO_REUSEPORT is what most people would expect SO_REUSEADDR to be. Basically, SO_REUSEPORT allows you to bind an arbitrary number of sockets to exactly the same source address and port as long as all prior bound sockets also had SO_REUSEPORT set before they were bound. If the first socket that is bound to an address and port does not have SO_REUSEPORT set, no other socket can be bound to exactly the same address and port, regardless if this other socket has SO_REUSEPORT set or not, until the first socket releases its binding again. Unlike in case of SO_REUESADDR the code handling SO_REUSEPORT will not only verify that the currently bound socket has SO_REUSEPORT set but it will also verify that the socket with a conflicting address and port had SO_REUSEADDR set when it was bound.

SO_REUSEPORT does not imply SO_REUSEADDR. This means if a socket did not have SO_REUSEPORT set when it was bound and another socket has SO_REUSEPORT set when it is bound to exactly the same address and port, the bind fails, which is expected, but it also fails if the other socket is already dying and is in TIME_WAIT state. To be able bind a socket to the same addresses and port as another socket in TIME_WAIT state requires either SO_REUSEADDR to be set on that socket or SO_REUSEPORT must have been set on both sockets prior to binding them. Of course it is allowed to set both, SO_REUSEPORT and SO_REUSEADDR, on a socket.

There is not much more to say about SO_REUSEPORT other than that it was added later than SO_REUSEADDR, that’s why you will not find it in many socket implementations of other systems, which “forked” the BSD code before this option was added, and that there was no way to bind two sockets to exactly the same socket address in BSD prior to this option.

Connect() Returning EADDRINUSE?

Two sockets bind to exactly the same addr using addr/port reuse. and then both try to connect to the same destination addr and port, thus, the source, dest and protocol are the same, so would fail

Most people know that bind() may fail with the error EADDRINUSE, however, when you start playing around with address reuse, you may run into the strange situation that connect() fails with that error as well. How can this be? How can a remote address, after all that’s what connect adds to a socket, be already in use? Connecting multiple sockets to exactly the same remote address has never been a problem before, so what’s going wrong here?

As I said on the very top of my reply, a connection is defined by a tuple of five values, remember? And I also said, that these five values must be unique otherwise the system cannot distinguish two connections any longer, right? Well, with address reuse, you can bind two sockets of the same protocol to the same source address and port. That means three of those five values are already the same for these two sockets. If you now try to connect both of these sockets also to the same destination address and port, you would create two connected sockets, whose tuples are absolutely identical. This cannot work, at least not for TCP connections (UDP connections are no real connections anyway). If data arrived for either one of the two connections, the system could not tell which connection the data belongs to. At least the destination address or destination port must be different for either connection, so that the system has no problem to identify to which connection incoming data belongs to.

So if you bind two sockets of the same protocol to the same source address and port and try to connect them both to the same destination address and port, connect() will actually fail with the error EADDRINUSE for the second socket you try to connect, which means that a socket with an identical tuple of five values is already connected.

Multicast Addresses

Most people ignore the fact that multicast addresses exist, but they do exist. While unicast addresses are used for one-to-one communication, multicast addresses are used for one-to-many communication. Most people got aware of multicast addresses when they learned about IPv6 but multicast addresses also existed in IPv4, even though this feature was never widely used on the public Internet.

The meaning of SO_REUSEADDR changes for multicast addresses as it allows multiple sockets to be bound to exactly the same combination of source multicast address and port. In other words, for multicast addresses SO_REUSEADDR behaves exactly as SO_REUSEPORT for unicast addresses. Actually the code treats SO_REUSEADDR and SO_REUSEPORT identically for multicast addresses, that means you could say that SO_REUSEADDR implies SO_REUSEPORT for all multicast addresses and the other way round.


offer the same options as BSD and also behave the same way as in BSD.



Prior to Linux 3.9, only the option SO_REUSEADDR existed. This option behaves generally the same as in BSD with two important exceptions. One exception is that if a listening (server) TCP socket is already bound to a wildcard IP address and a specific port, no other TCP socket can be bound to the same port, regardless whether either one or both sockets have this flag set. Not even if it would use a more specific address (as is allowed in case of BSD). This restriction does not apply to non-listening (client) TCP sockets and it is also possible to first bind a listening TCP socket to a specific IP address and port combination and later on bind another one to a wildcard IP address and the same port. The second exception is that for UDP sockets this option behaves exactly like SO_REUSEPORT in BSD, so two UDP sockets can be bound to exactly the same address and port combination as long as both had this flag set before they were bound.

Linux 3.9 added the option SO_REUSEPORT to Linux as well. This option allows two (or more) sockets, TCP or UDP, listening (server) or non-listening (client), to be bound to exactly the same address and port combination as long as all sockets (including the very first one) had this flag set prior to binding them. To prevent “port hijacking”, there is one special limitation, though: All sockets that want to share the same address and port combination must belong to processes that share the same effective user ID! So one user cannot “steal” ports of another user. Additionally the kernel performs some “special magic” for SO_REUSEPORT sockets that isn’t found in any other operating system so far: For UDP sockets, it tries to distribute datagrams evenly, for TCP listening sockets, it tries to distribute incoming connect requests (those accepted by calling accept()) evenly across all the sockets that share the same address and port combination. That means while it is more or less random which socket receives a datagram or connect request in other operating systems that allow full address reuse, Linux tries to optimize distribution so that, for example, multiple instances of a simple server process can easily use SO_REUSEPORT sockets to achieve a kind of simple load balancing and that absolutely for free as the kernel is doing “all the hard work” for them.