套接字选项SO_KEEPALIVE

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套接字选项SO_KEEPALIVE是保活套接字，它会在规定的时间内给对端发送探测分节，用tcpdump抓包可以看到。这个规定的时间就是net.ipv4.tcp_keepalive_time，在系统中可以动态的设置它，它的默认值是7200s。SO_KEEPLIVE选项由setsockopt函数进行设置。

服务端：

#include <unistd.h>#include <sys/types.h>#include <sys/socket.h>#include <netdb.h>#include <stdio.h>#include <stdlib.h>#include <string.h>#include <ctype.h>#include <errno.h>#include <malloc.h>#include <netinet/in.h>#include <arpa/inet.h>#include <sys/ioctl.h>#include <stdarg.h>#include <fcntl.h>#include <time.h>int main(){    int sockSrv = socket(AF_INET, SOCK_STREAM, 0);    struct sockaddr_in addrSrv;    addrSrv.sin_family = AF_INET;    addrSrv.sin_addr.s_addr =htonl(INADDR_ANY);    addrSrv.sin_port = htons(8888);    bind(sockSrv, (const struct sockaddr *)&addrSrv, sizeof(struct sockaddr_in));    listen(sockSrv, 5);    sockaddr_in addrClient;    socklen_t len=sizeof(addrClient);    int sockConn = accept(sockSrv, (struct sockaddr *)&addrClient, &len);    int keepAlive=1;    setsockopt(sockConn,SOL_SOCKET,SO_KEEPALIVE,(void *)&keepAlive,sizeof(keepAlive));    getchar();    close(sockConn);    close(sockSrv);    return 0;}

客户端：

#include <unistd.h>#include <sys/types.h>#include <sys/socket.h>#include <netdb.h>#include <stdio.h>#include <stdlib.h>#include <string.h>#include <ctype.h>#include <errno.h>#include <malloc.h>#include <netinet/in.h>#include <arpa/inet.h>#include <sys/ioctl.h>#include <stdarg.h>#include <fcntl.h>#include <error.h>int main(){     int ret=0;     int sockClient = socket(AF_INET, SOCK_STREAM, 0);     struct sockaddr_in addrSrv;     addrSrv.sin_addr.s_addr=inet_addr("127.0.0.1");     addrSrv.sin_family = AF_INET;     addrSrv.sin_port = htons(8888);     ret=connect(sockClient, ( const struct sockaddr *)&addrSrv, sizeof(struct sockaddr_in));     getchar();     close(sockClient);     return 0;}

tcpdump抓包结果：

[root@localhost mapan]# tcpdump -iany port 8888 -nlps0tcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on any, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes13:57:50.484437 IP 127.0.0.1.40596 > 127.0.0.1.ddi-tcp-1: Flags [S], seq 2495400572, win 65495, options [mss 65495,sackOK,TS val 3129736193 ecr 0,nop,wscale 7], length 013:57:50.484453 IP 127.0.0.1.ddi-tcp-1 > 127.0.0.1.40596: Flags [S.], seq 1098484178, ack 2495400573, win 65483, options [mss 65495,sackOK,TS val 3129736193 ecr 3129736193,nop,wscale 7], length 013:57:50.484471 IP 127.0.0.1.40596 > 127.0.0.1.ddi-tcp-1: Flags [.], ack 1, win 512, options [nop,nop,TS val 3129736193 ecr 3129736193], length 013:59:50.483791 IP 127.0.0.1.ddi-tcp-1 > 127.0.0.1.40596: Flags [.], ack 1, win 512, options [nop,nop,TS val 3129856193 ecr 3129736193], length 013:59:50.483802 IP 127.0.0.1.40596 > 127.0.0.1.ddi-tcp-1: Flags [.], ack 1, win 512, options [nop,nop,TS val 3129856193 ecr 3129736193], length 014:01:50.483787 IP 127.0.0.1.ddi-tcp-1 > 127.0.0.1.40596: Flags [.], ack 1, win 512, options [nop,nop,TS val 3129976193 ecr 3129856193], length 014:01:50.483798 IP 127.0.0.1.40596 > 127.0.0.1.ddi-tcp-1: Flags [.], ack 1, win 512, options [nop,nop,TS val 3129976193 ecr 3129736193], length 0

服务端每2分钟会发送一个探测分节给客户端，客户端以相应的ACK回应，标明客户端一切正常。

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