Linux下端口复用(SO_REUSEADDR与SO_REUSEPORT)
只考虑AF_INET的情况（同一端口指ip地址与端口号都相同）
- freebsd支持SO_REUSEPORT和SO_REUSEADDR选项,而linux只支持SO_REUSEADDR选项。
- freebsd下,使用SO_REUSEPORT选项，两个tcp的socket可以绑定同一个端口；同样，使用SO_REUSEPORT选项，两个udp的socket可以绑定同一个端口。
- linux下，两个tcp的socket不能绑定同一个端口；而如果使用SO_REUSEADDR选项，两个udp的socket可以绑定同一个端口。
- freebsd下，两个tcp的socket绑定同一端口，只有第一个socket获得数据。
- freebsd下，两个udp的socket绑定同一端口，如果数据包的目的地址是单播地址，则只有第一个socket获得数据，而如果数据包的目的地址是多播地址，则两个socket同时获得相同的数据。
- linux下，两个udp的socket绑定同一端口，如果数据包的目的地址是单播地址，则只有最后一个socket获得数据，而如果数据包的目的地址是多播地址，则两个socket同时获得相同的数据。
Unix 网络 API
SO_REUSEADDR和SO_REUSEPORT
SO_REUSEADDR提供如下四个功能：
- SO_REUSEADDR允许启动一个监听服务器并捆绑其众所周知端口，即使以前建立的将此端口用做他们的本地端口的连接仍存在。这通常是重启监听服务器时出现，若不设置此选项，则bind时将出错。
- SO_REUSEADDR允许在同一端口上启动同一服务器的多个实例（多个进程），只要每个实例捆绑一个不同的本地IP地址即可。在有多块网卡或者用IP Alias技术的机器可以测试这种情况。 对于TCP，我们根本不可能启动捆绑相同IP地址和相同端口号的多个服务器。
- SO_REUSEADDR允许单个进程捆绑同一端口到多个套接口上，只要每个捆绑指定不同的本地IP地址即可。这一般不用于TCP服务器。
- SO_REUSEADDR允许完全重复的捆绑：当一个IP地址和端口绑定到某个套接口上时，还允许此IP地址和端口捆绑到另一个套接口上。一般来说，这个特性仅在支持多播的系统上才有，而且只对UDP套接口而言，不用于TCP（TCP不支持多播）。
SO_REUSEPORT选项有如下语义：
- 此选项允许完全重复捆绑，但仅在想捆绑相同IP地址和端口的套接口都指定了此套接口选项才行。 如果被捆绑的IP地址是一个多播地址，则SO_REUSEADDR和SO_REUSEPORT等效。
使用这两个套接口选项的建议：
- 在所有TCP服务器中，在调用bind之前设置SO_REUSEADDR套接口选项。
- 当编写一个同一时刻在同一主机上可运行多次的多播应用程序时，设置SO_REUSEADDR选项，并将本组的多播地址作为本地IP地址捆绑。
附
Q:编写 TCP/SOCK_STREAM 服务程序时，SO_REUSEADDR到底什么意思？
A:这个套接字选项通知内核，如果端口忙，但TCP状态位于 TIME_WAIT ，可以重用端口。如果端口忙，而TCP状态位于其他状态，重用端口时依旧得到一个错误信息，指明"地址已经使用中"。如果你的服务程序停止后想立即重启，而新套接字依旧使用同一端口，此时SO_REUSEADDR 选项非常有用。必须意识到，此时任何非期望数据到达，都可能导致服务程序反应混乱，不过这只是一种可能，事实上很不可能。
一个套接字由相关五元组构成，协议、本地地址、本地端口、远程地址、远程端口。SO_REUSEADDR 仅仅表示可以重用本地本地地址、本地端口，整个相关五元组还是唯一确定的。所以，重启后的服务程序有可能收到非期望数据。必须慎重使用 SO_REUSEADDR 选项。
if( getsockopt( sockfd , SOL_SOCKET, SO_REUSEPORT,
( char *)&optval, &optlen ) < 0 )
printf( " get socket error /n" );
if( setsockopt( sockfd , SOL_SOCKET, SO_REUSEPORT,
( char *)&optval, sizeof( optval ) ) < 0 )
printf( " set socket error /n" );
编译报错：
libtcp.c:1195: error: `SO_REUSEPORT' undeclared (first use in this function)
libtcp.c:1195: error: (Each undeclared identifier is reported only once
libtcp.c:1195: error: for each function it appears in.)
需要改
/usr/include/asm/socket.h:/* To add :#define SO_REUSEPORT 15 */
from： UNIX Socket FAQ
SO_REUSEADDR allows your server to bind to an address which is in a TIME_WAIT state. It does not allow more than one server to bind to the same address. It was mentioned that use of this flag can create a security risk because another server can bind to a the same port, by binding to a specific address as opposed to INADDR_ANY. The SO_REUSEPORT flag allows multiple processes to bind to the same address provided all of them use the SO_REUSEPORT option.
From Richard Stevens ([email protected]):
This is a newer flag that appeared in the 4.4BSD multicasting code (although that code was from elsewhere, so I am not sure just who invented the new SO_REUSEPORT flag).
What this flag lets you do is rebind a port that is already in use, but only if all users of the port specify the flag. I believe the intent is for multicasting apps, since if you're running the same app on a host, all need to bind the same port. But the flag may have other uses. For example the following is from a post in February:
From Stu Friedberg ([email protected]):
SO_REUSEPORT is also useful for eliminating the try-10-times-to-bind hack in ftpd's data connection setup routine. Without SO_REUSEPORT, only one ftpd thread can bind to TCP (lhost, lport, INADDR_ANY, 0) in preparation for connecting back to the client. Under conditions of heavy load, there are more threads colliding here than the try-10-times hack can accomodate. With SO_REUSEPORT, things work nicely and the hack becomes unnecessary.
I have also heard that DEC OSF supports the flag. Also note that under 4.4BSD, if you are binding a multicast address, then SO_REUSEADDR is condisered the same as SO_REUSEPORT (p. 731 of "TCP/IP Illustrated, Volume 2"). I think under Solaris you just replace SO_REUSEPORT with SO_REUSEADDR.
From a later Stevens posting, with minor editing:
Basically SO_REUSEPORT is a BSD'ism that arose when multicasting was added, even thought it was not used in the original Steve Deering code. I believe some BSD-derived systems may also include it (OSF, now Digital Unix, perhaps?). SO_REUSEPORT lets you bind the same address *and* port, but only if all the binders have specified it. But when binding a multicast address (its main use), SO_REUSEADDR is considered identical to SO_REUSEPORT (p. 731, "TCP/IP Illustrated, Volume 2"). So for portability of multicasting applications I always use SO_REUSEADDR.
from: http://www.cnblogs.com/mydomain/archive/2011/08/23/2150567.html