实战tcpdump看RST

             RST为重置报文段，它会导致TCP连接的快速拆迁，且不需要ack进行确认。

1.针对不存在的端口的连请求

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 <sys/types.h>#include <sys/wait.h>#include <netinet/in.h>#include <arpa/inet.h>#include <signal.h>#include <netinet/tcp.h>#define MAXLINE 4096int main(){   int sockfd,ret;   struct sockaddr_in servaddr;   char sendbuf[32740]={0};   sockfd=socket(AF_INET,SOCK_STREAM,0);   bzero(&servaddr,sizeof(servaddr));   servaddr.sin_family=AF_INET;   servaddr.sin_port=htons(8888);   servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");   ret=connect(sockfd,(struct sockaddr *)&servaddr,sizeof(servaddr));   printf("ret=%d\n",ret);   write(sockfd,sendbuf,sizeof(sendbuf)+1);   getchar();   close(sockfd);   return 0;}

编译并运行，此时没有服务端在8888端口进行监听，tcpdump抓包看。

07:19:32.643476 IP 127.0.0.1.49028 > 127.0.0.1.ddi-tcp-1: Flags [S], seq 1270070893, win 65495, options [mss 65495,sackOK,TS val 3883769366 ecr 0,nop,wscale 7], length 007:19:32.643491 IP 127.0.0.1.ddi-tcp-1 > 127.0.0.1.49028: Flags [R.], seq 0, ack 1270070894, win 0, length 0

客户端发起连接，但受到一个RST包。

2.请求超时

因为客户端可以设置接收数据的超时时间，当客户端调用connect函数发送SYN时，由于客户端收到服务端的SYN/ACK的时间超过了客户端设置的等待时间，造成接收超时。当服务端的SYN/ACK到达时，客户端回应RST。

3.提前关闭

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 <sys/types.h>#include <sys/wait.h>#include <netinet/in.h>#include <arpa/inet.h>#include <signal.h>#define MAXLINE 4096int main(){   int listenfd,acceptfd,n;   socklen_t  clilen;   char recvbuf[100]={0};   struct sockaddr_in cliaddr,servaddr;   listenfd=socket(AF_INET,SOCK_STREAM,0);   servaddr.sin_family=AF_INET;   servaddr.sin_port=htons(8888);   servaddr.sin_addr.s_addr = INADDR_ANY;    bind(listenfd,(struct sockaddr *)&servaddr,sizeof(struct sockaddr_in));   listen(listenfd,5);   clilen=sizeof(cliaddr);   acceptfd=accept(listenfd,(struct sockaddr *)&cliaddr,&clilen);      n=recv(acceptfd,recvbuf,sizeof(recvbuf)-1,0);   printf("n=%d\n",n);    getchar();   close(acceptfd);   close(listenfd);   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 <sys/types.h>#include <sys/wait.h>#include <netinet/in.h>#include <arpa/inet.h>#include <signal.h>#include <netinet/tcp.h>#define MAXLINE 4096int main(){   int sockfd,ret;   struct sockaddr_in servaddr;   char sendbuf[1000]={0};   sockfd=socket(AF_INET,SOCK_STREAM,0);   bzero(&servaddr,sizeof(servaddr));   servaddr.sin_family=AF_INET;   servaddr.sin_port=htons(8888);   servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");   ret=connect(sockfd,(struct sockaddr *)&servaddr,sizeof(servaddr));   printf("ret=%d\n",ret);   write(sockfd,sendbuf,sizeof(sendbuf)+1);   getchar();   close(sockfd);   return 0;}

先启动服务端，再启动客户端。客户端每次发送1001个字节，而服务端只接收了99个字节，还有剩下的字节在接收缓冲区里面。此时先关闭服务端，用tcpdump抓包查看。

16:28:06.149336 IP 127.0.0.1.49192 > 127.0.0.1.ddi-tcp-1: Flags [S], seq 3096824100, win 65495, options [mss 65495,sackOK,TS val 3916682872 ecr 0,nop,wscale 7], length 016:28:06.149354 IP 127.0.0.1.ddi-tcp-1 > 127.0.0.1.49192: Flags [S.], seq 1491431840, ack 3096824101, win 65483, options [mss 65495,sackOK,TS val 3916682872 ecr 3916682872,nop,wscale 7], length 016:28:06.149372 IP 127.0.0.1.49192 > 127.0.0.1.ddi-tcp-1: Flags [.], ack 1, win 512, options [nop,nop,TS val 3916682872 ecr 3916682872], length 016:28:06.149461 IP 127.0.0.1.49192 > 127.0.0.1.ddi-tcp-1: Flags [P.], seq 1:1002, ack 1, win 512, options [nop,nop,TS val 3916682872 ecr 3916682872], length 100116:28:06.149491 IP 127.0.0.1.ddi-tcp-1 > 127.0.0.1.49192: Flags [.], ack 1002, win 528, options [nop,nop,TS val 3916682872 ecr 3916682872], length 016:28:07.699933 IP 127.0.0.1.ddi-tcp-1 > 127.0.0.1.49192: Flags [R.], seq 1, ack 1002, win 528, options [nop,nop,TS val 3916684423 ecr 3916682872], length 0

服务端没有将数据全部接收完成，然后就关闭了，所以服务端产生了一个RST。

4.在一个已关闭的socket上发到数据

#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 <sys/types.h>#include <sys/wait.h>#include <netinet/in.h>#include <arpa/inet.h>#include <signal.h>#define MAXLINE 4096int main(){   int listenfd,acceptfd,n;   socklen_t  clilen;   char recvbuf[100]={0};   struct sockaddr_in cliaddr,servaddr;   listenfd=socket(AF_INET,SOCK_STREAM,0);   servaddr.sin_family=AF_INET;   servaddr.sin_port=htons(8888);   servaddr.sin_addr.s_addr = INADDR_ANY;    bind(listenfd,(struct sockaddr *)&servaddr,sizeof(struct sockaddr_in));   listen(listenfd,5);   clilen=sizeof(cliaddr);   acceptfd=accept(listenfd,(struct sockaddr *)&cliaddr,&clilen);      getchar();   close(acceptfd);   close(listenfd);   return 0;}

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 <sys/types.h>#include <sys/wait.h>#include <netinet/in.h>#include <arpa/inet.h>#include <signal.h>#include <netinet/tcp.h>#define MAXLINE 4096int main(){   int sockfd,ret;   struct sockaddr_in servaddr;   char sendbuf[1000]={0};   sockfd=socket(AF_INET,SOCK_STREAM,0);   bzero(&servaddr,sizeof(servaddr));   servaddr.sin_family=AF_INET;   servaddr.sin_port=htons(8888);   servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");   ret=connect(sockfd,(struct sockaddr *)&servaddr,sizeof(servaddr));   printf("ret=%d\n",ret);   getchar();   write(sockfd,sendbuf,sizeof(sendbuf)+1);   getchar();   close(sockfd);   return 0;}

先打开服务端，在打开客户端。然后关闭服务端，在客户端按下回车键键，用tcpdump抓包查看结果。

16:44:16.226353 IP 127.0.0.1.49194 > 127.0.0.1.ddi-tcp-1: Flags [S], seq 3249455833, win 65495, options [mss 65495,sackOK,TS val 3917652949 ecr 0,nop,wscale 7], length 016:44:16.226370 IP 127.0.0.1.ddi-tcp-1 > 127.0.0.1.49194: Flags [S.], seq 1092997986, ack 3249455834, win 65483, options [mss 65495,sackOK,TS val 3917652949 ecr 3917652949,nop,wscale 7], length 016:44:16.226387 IP 127.0.0.1.49194 > 127.0.0.1.ddi-tcp-1: Flags [.], ack 1, win 512, options [nop,nop,TS val 3917652949 ecr 3917652949], length 016:44:18.402946 IP 127.0.0.1.ddi-tcp-1 > 127.0.0.1.49194: Flags [F.], seq 1, ack 1, win 512, options [nop,nop,TS val 3917655126 ecr 3917652949], length 016:44:18.403887 IP 127.0.0.1.49194 > 127.0.0.1.ddi-tcp-1: Flags [.], ack 2, win 512, options [nop,nop,TS val 3917655127 ecr 3917655126], length 016:44:20.376861 IP 127.0.0.1.49194 > 127.0.0.1.ddi-tcp-1: Flags [P.], seq 1:1002, ack 2, win 512, options [nop,nop,TS val 3917657100 ecr 3917655126], length 100116:44:20.376874 IP 127.0.0.1.ddi-tcp-1 > 127.0.0.1.49194: Flags [R], seq 1092997988, win 0, length 0

客户端和服务端建立连接之后，服务端就关闭了。此时客户端再向服务端发送数据，此时服务端返回RST。

参考地址：http://www.360doc.com/content/13/0702/10/1073512_297069771.shtml