http://blog.csdn.net/lishenglong666/article/details/45335589
这个问题之前没有怎么留意过，是最近在面试过程中遇到的一个问题，面了两家公司，两家公司竟然都面到到了这个问题，不得不使我开始关注这个问题。说起CLOSE_WAIT状态，如果不知道的话，还是先瞧一下TCP的状态转移图吧。
关闭socket分为主动关闭（Active closure）和被动关闭（Passive closure）两种情况。前者是指有本地主机主动发起的关闭；而后者则是指本地主机检测到远程主机发起关闭之后，作出回应，从而关闭整个连接。将关闭部分的状态转移摘出来，就得到了下图：
产生原因
通过图上，我们来分析，什么情况下，连接处于CLOSE_WAIT状态呢？
在被动关闭连接情况下，在已经接收到FIN，但是还没有发送自己的FIN的时刻，连接处于CLOSE_WAIT状态。
通常来讲，CLOSE_WAIT状态的持续时间应该很短，正如SYN_RCVD状态。但是在一些特殊情况下，就会出现连接长时间处于CLOSE_WAIT状态的情况。
出现大量close_wait的现象，主要原因是某种情况下对方关闭了socket链接，但是我方忙与读或者写，没有关闭连接。代码需要判断socket，一旦读到0，断开连接，read返回负，检查一下errno，如果不是AGAIN，就断开连接。
参考资料4中描述，通过发送SYN-FIN报文来达到产生CLOSE_WAIT状态连接，没有进行具体实验。不过个人认为协议栈会丢弃这种非法报文，感兴趣的同学可以测试一下，然后把结果告诉我;-)
为了更加清楚的说明这个问题，我们写一个测试程序，注意这个测试程序是有缺陷的。
只要我们构造一种情况，使得对方关闭了socket，我们还在read，或者是直接不关闭socket就会构造这样的情况。
server.c:
-
#include <stdio.h> -
#include <string.h> -
#include <netinet/in.h> -
#define MAXLINE 80 -
#define SERV_PORT 8000 -
int main(void) -
{ -
struct sockaddr_in servaddr, cliaddr; -
socklen_t cliaddr_len; -
int listenfd, connfd; -
char buf[MAXLINE]; -
char str[INET_ADDRSTRLEN]; -
int i, n; -
listenfd = socket(AF_INET, SOCK_STREAM, 0); -
int opt = 1; -
setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt)); -
bzero(&servaddr, sizeof(servaddr)); -
servaddr.sin_family = AF_INET; -
servaddr.sin_addr.s_addr = htonl(INADDR_ANY); -
servaddr.sin_port = htons(SERV_PORT); -
bind(listenfd, (struct sockaddr *)&servaddr, sizeof(servaddr)); -
listen(listenfd, 20); -
printf("Accepting connections ...\n"); -
while (1) { -
cliaddr_len = sizeof(cliaddr); -
connfd = accept(listenfd, -
(struct sockaddr *)&cliaddr, &cliaddr_len); -
//while (1) -
{ -
n = read(connfd, buf, MAXLINE); -
if (n == 0) { -
printf("the other side has been closed.\n"); -
break; -
} -
printf("received from %s at PORT %d\n", -
inet_ntop(AF_INET, &cliaddr.sin_addr, str, sizeof(str)), -
ntohs(cliaddr.sin_port)); -
for (i = 0; i < n; i++) -
buf[i] = toupper(buf[i]); -
write(connfd, buf, n); -
} -
//这里故意不关闭socket,或者是在close之前加上一个sleep都可以 -
//sleep(5); -
//close(connfd); -
} -
}
client.c:
-
#include <stdio.h> -
#include <stdlib.h> -
#include <string.h> -
#include <unistd.h> -
#include <sys/socket.h> -
#include <netinet/in.h> -
#define MAXLINE 80 -
#define SERV_PORT 8000 -
int main(int argc, char *argv[]) -
{ -
struct sockaddr_in servaddr; -
char buf[MAXLINE]; -
int sockfd, n; -
char *str; -
if (argc != 2) { -
fputs("usage: ./client message\n", stderr); -
exit(1); -
} -
str = argv[1]; -
sockfd = socket(AF_INET, SOCK_STREAM, 0); -
bzero(&servaddr, sizeof(servaddr)); -
servaddr.sin_family = AF_INET; -
inet_pton(AF_INET, "127.0.0.1", &servaddr.sin_addr); -
servaddr.sin_port = htons(SERV_PORT); -
connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)); -
write(sockfd, str, strlen(str)); -
n = read(sockfd, buf, MAXLINE); -
printf("Response from server:\n"); -
write(STDOUT_FILENO, buf, n); -
write(STDOUT_FILENO, "\n", 1); -
close(sockfd); -
return 0; -
}
结果如下：
-
debian-wangyao:~$ ./client a -
Response from server: -
A -
debian-wangyao:~$ ./client b -
Response from server: -
B -
debian-wangyao:~$ ./client c -
Response from server: -
C -
debian-wangyao:~$ netstat -antp | grep CLOSE_WAIT -
(Not all processes could be identified, non-owned process info -
will not be shown, you would have to be root to see it all.) -
tcp 1 0 127.0.0.1:8000 127.0.0.1:58309 CLOSE_WAIT 6979/server -
tcp 1 0 127.0.0.1:8000 127.0.0.1:58308 CLOSE_WAIT 6979/server -
tcp 1 0 127.0.0.1:8000 127.0.0.1:58307 CLOSE_WAIT 6979/server
解决方法
基本的思想就是要检测出对方已经关闭的socket，然后关闭它。
1.代码需要判断socket，一旦read返回0，断开连接，read返回负，检查一下errno，如果不是AGAIN，也断开连接。(注:在UNP 7.5节的图7.6中，可以看到使用select能够检测出对方发送了FIN，再根据这条规则就可以处理CLOSE_WAIT的连接)
2.给每一个socket设置一个时间戳last_update，每接收或者是发送成功数据，就用当前时间更新这个时间戳。定期检查所有的时间戳，如果时间戳与当前时间差值超过一定的阈值，就关闭这个socket。
3.使用一个Heart-Beat线程，定期向socket发送指定格式的心跳数据包，如果接收到对方的RST报文，说明对方已经关闭了socket，那么我们也关闭这个socket。
4.设置SO_KEEPALIVE选项，并修改内核参数
前提是启用socket的KEEPALIVE机制：
//启用socket连接的KEEPALIVE
int iKeepAlive = 1;
setsockopt(s, SOL_SOCKET, SO_KEEPALIVE, (void *)&iKeepAlive, sizeof(iKeepAlive));
tcp_keepalive_intvl (integer; default: 75; since Linux 2.4)
The number of seconds between TCP keep-alive probes.
tcp_keepalive_probes (integer; default: 9; since Linux 2.2)
The maximum number of TCP keep-alive probes to send before giving up and killing the connection if no response is obtained from the other end.
tcp_keepalive_time (integer; default: 7200; since Linux 2.2)
The number of seconds a connection needs to be idle before TCP begins sending out keep-alive probes. Keep-alives are only sent when the SO_KEEPALIVE socket option is enabled. The default value is 7200 seconds (2 hours). An idle connec‐tion is terminated after approximately an additional 11 minutes (9 probes an interval of 75 seconds apart) when keep-alive is enabled.
echo 120 > /proc/sys/net/ipv4/tcp_keepalive_time
echo 2 > /proc/sys/net/ipv4/tcp_keepalive_intvl
echo 1 > /proc/sys/net/ipv4/tcp_keepalive_probes
除了修改内核参数外，可以使用setsockopt修改socket参数，参考man 7 socket。
-
int KeepAliveProbes=1; -
int KeepAliveIntvl=2; -
int KeepAliveTime=120; -
setsockopt(s, IPPROTO_TCP, TCP_KEEPCNT, (void *)&KeepAliveProbes, sizeof(KeepAliveProbes)); -
setsockopt(s, IPPROTO_TCP, TCP_KEEPIDLE, (void *)&KeepAliveTime, sizeof(KeepAliveTime)); -
setsockopt(s, IPPROTO_TCP, TCP_KEEPINTVL, (void *)&KeepAliveIntvl, sizeof(KeepAliveIntvl));
参考：
http://blog.chinaunix.net/u/20146/showart_1217433.html
http://blog.csdn.net/eroswang/archive/2008/03/10/2162986.aspx
http://haka.sharera.com/blog/BlogTopic/32309.htm
http://learn.akae.cn/media/ch37s02.html
http://faq.csdn.net/read/208036.html
http://www.cndw.com/tech/server/2006040430203.asp
http://davidripple.bokee.com/1741575.html
http://doserver.net/post/keepalive-linux-1.php