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本文主要讲代理服务器源码，是一位叫Carl Harris大神写的，非常简单易懂，把代理服务器（Proxy Server）本质完全体现出来。相信读懂了这段代码，以后想搞定http代理等其他类型的代理服务器也能行。在附录中会贴出proxy全部源码，仅供学习使用。
一、代理服务器的定义
代理服务器（Proxy Server）是一种重要的服务器安全功能，它的工作主要在开放系统互联(OSI)模型的会话层，从而起到防火墙的作用。代理（英语：Proxy），也称网络代理，是一种特殊的网络服务，允许一个网络终端（一般为客户端）通过这个服务与另一个网络终端（一般为服务器）进行非直接的连接。一些网关、路由器等网络设备具备网络代理功能。一般认为代理服务有利于保障网络终端的隐私或安全，防止攻击。
以上截取了网上Proxy Server的定义。看起来晦涩难懂。简单来说，代理服务器就是起到一个转发功能。比如，你在本机A想要访问国外的服务器C，你本机没权限访问C，需要通过向服务器B发送数据，B再把你的数据发送给C，C返回数据也是先把数据交给了B，然后B再转交给你。这里B服务器别名为代理服务器（Proxy Server）。等会分析到proxy源码，就更加清楚了。
二、proxy源码分析
以下是proxy的主程序。
int main(int argc, char **argv) { int clilen; int childpid; int sockfd, newsockfd; struct sockaddr_in servaddr, cliaddr; parse_args(argc, argv);//prepare an address struct to listen for connect bzero((char*)&servaddr, sizeof(servaddr)); servaddr.sin_family = AF_INET; servaddr.sin_addr.s_addr = htonl(INADDR_ANY); servaddr.sin_port = proxy_port; //get asocket .. if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) { fputs("failed to crate server socket ", stderr); exit(1); } // and bind our address and port to it if(bind(sockfd, (struct sockaddr_in*)&servaddr, sizeof(servaddr)) < 0) { fputs("failed to bind server socket to specified/r/n", stderr); exit(1); } // get ready to accept with at most 5 clients waitting to connect listen(sockfd, 5); // turn ourselves into daemon daemonize(sockfd); //fall into a loop to accept new connections and spawn children while(1) { //accept the next connection clilen = sizeof(cliaddr); newsockfd = accept(sockfd, (struct sockaddr*)&cliaddr, &clilen); if(newsockfd <0 && errno == EINTR) continue; //a signal might interrupt our accept call else if(newsockfd < 0) errorout("failed to accept connection");//sth quiet amiss--kill server //fork a child to handle this connection if((childpid = fork()) == 0) { close(sockfd);// inherit do_proxy(newsockfd); exit(0); } // if fork falied the connection is silently dropped --oops close(newsockfd); } return 0; }
从上面的程序看出，先是用函数parse_args(argc, argv) 解析获得用户输入的信息，包括想要绑定的代理服务器端口号、远程主机的host name 和 远程主机的服务号。获取之后设置到全局变量。接着用socket函数创建tcp 套接字，用bind函数绑定刚才获取到的端口号，用listen监听有没有客户端连接到代理服务器。然后，程序调用daemonize(sockfd)函数，这个函数的作用是创建一个守护进程。程序往下是在while循环里面调用accept函数阻塞，如果发现有客户端连接，就返回一个套接字，并fork出子进程处理这个套接字发过来的请求。do_proxy函数就是专门来干这事的。
do_proxy函数代码如下：
void do_proxy (int usersockfd) { int isosockfd; fd_set rdfdset; int connstat; int iolen; char buf[2048]; /* open a socket to connect to the isolated host */ if ((isosockfd = socket(AF_INET,SOCK_STREAM,0)) < 0) errorout("failed to create socket to host"); /* attempt a connection */ connstat = connect(isosockfd,(struct sockaddr *) &hostaddr, sizeof(hostaddr)); switch (connstat) { case 0: break; case ETIMEDOUT: case ECONNREFUSED: case ENETUNREACH: strcpy(buf,sys_myerrlist[errno]); strcat(buf," "); write(usersockfd, buf, strlen(buf)); close(usersockfd); exit(1); /* die peacefully if we can't establish a connection */ break; default: errorout("failed to connect to host"); } /* now we're connected, serve fall into the data echo loop */ while (1) { /* Select for readability on either of our two sockets */ FD_ZERO(&rdfdset); FD_SET(usersockfd,&rdfdset); FD_SET(isosockfd,&rdfdset); if (select(FD_SETSIZE,&rdfdset,NULL,NULL,NULL) < 0) errorout("select failed"); /* is the client sending data? */ if (FD_ISSET(usersockfd,&rdfdset)) { if ((iolen = read(usersockfd,buf,sizeof(buf))) <= 0) break; /* zero length means the client disconnected */ write(isosockfd,buf,iolen); /* copy to host -- blocking semantics */ } /* is the host sending data? */ if(FD_ISSET(isosockfd,&rdfdset)) { if((iolen = read(isosockfd,buf,sizeof(buf))) <= 0) break; /* zero length means the host disconnected */ write(usersockfd,buf,iolen); /* copy to client -- blocking semantics */ } } /* we're done with the sockets */ close(isosockfd); close(usersockfd); }
do_proxy函数是整个程序的核心，代理服务器的工作原理就是从这里体现出来的。函数有一个参数，是客户端连接好的套接字，也就是accept函数的返回值，可用于和客户端发送或者接收消息。在这个函数中，会再用socket函数创建一个新的套接字，这个套接字是用于和远程服务器连接的。在之前parse_args函数中，我们获取过远程服务器的主机名和服务端口号。接着调用connect函数向远程服务器发起连接请求。从这可以看出，代理服务器既是服务器，同时也是远程服务器的客户端。当两边都建立连接好以后，调用select函数，把两边的套接字描述符都加入监视。select函数可参考TCP socket select用法分析。如果发现是客户端A有发送信息请求访问远程服务器C时，本代理服务器就读取A的套接字描述符，把信息存放到buf中，然后再把buf中的数据写到C。反过来，当C有数据响应要发给A时，同样也是先发送给B，然后B再发送给A。
关于代码，这里再讲一下守护进程，函数如下所示：
void daemonize(int servfd) { int childpid, fd, fdtablesize; // ignore terminal I/O,stop signals signal(SIGTTOU, SIG_IGN); signal(SIGTTIN, SIG_IGN); signal(SIGTSTP, SIG_IGN); /* fork to put us in the background (whether or not the user specified '&' on the command line */ if((childpid=fork()) < 0) { fputs("failed to fork first child/r/n",stderr); exit(1); } else if(childpid >0) exit(0);// terminate parent, continue child //dissociate from process group if (setpgrp(0,getpid()) < 0) { fputs("failed to become process group leader/r/n",stderr); exit(1); } /* lose controlling terminal */ if ((fd = open("/dev/tty", O_RDWR)) >= 0) { ioctl(fd,TIOCNOTTY,NULL); close(fd); } /* close any open file descriptors */ for(fd = 0, fdtablesize = getdtablesize(); fd < fdtablesize; fd++) if (fd != servfd) close(fd); /* set working directory to allow filesystems to be unmounted */ chdir("/"); /* clear the inherited umask */ umask(0); /* setup zombie prevention */ signal(SIGCLD,(sigfunc *)reap_status); }
从上面可以看出，程序调用signal忽略一些信号，然后父进程fork出子进程，父进程退出，子进程调用setpgrp(0,getpid())，与进程组分离。接着使子进程与终端脱离，关闭不相关的文件描述符，调用umask将文件模式创建屏蔽字设置为0，最后处理信号SIGCLD。所有这些过程都是为了创建一个守护进程。让程序能够在后台运行，并且不受终端控制。
三、proxy完整源码
/**************************************************************************** program: proxyd module: proxyd.c summary: provides proxy tcp service for a host on an isolated network. programmer: Carl Harris (ceharris@vt.edu) date: 22 Feb 94 description: This code implements a daemon process which listens for tcp connec- tions on a specified port number. When a connection is established, a child is forked to handle the new client. The child then estab- lishes a tcp connection to a port on the isolated host. The child then falls into a loop in which it writes data to the isolated host for the client and vice-versa. Once a child has been forked, the parent resumes listening for additional connections. The name of the isolated host and the port to serve as proxy for, as well as the port number the server listen on are specified as command line arguments. ****************************************************************************/ #include <stdio.h> #include <stdlib.h> #include <string.h> #include <ctype.h> //#include <unistd.h> #include <errno.h> #include <signal.h> #include <sys/types.h> #include <sys/socket.h> #include <sys/file.h> #include <sys/ioctl.h> #include <sys/wait.h> #include <netinet/in.h> #include <netdb.h> #define TCP_PROTO "tcp" int proxy_port; //port to listen for proxy connections on struct sockaddr_in hostaddr; //host addr assembled form gethostbyname() extern int errno; //define by libc.a char *sys_myerrlist[]; void parse_args(int argc, char **argv); void daemonize(int servfd); void do_proxy(int usersockfd); void reap_status(void); void errout(char *msg); typedef void sigfunc(int );//sigfunc可以声明一个函数类型 /*description: Main level driver. After daemonizing the process, * a socket is opened to listen for connections on the proxy port, * connections are accepted and children are spawned to handle each * new connection. */ int main(int argc, char **argv) { int clilen; int childpid; int sockfd, newsockfd; struct sockaddr_in servaddr, cliaddr; parse_args(argc, argv);//prepare an address struct to listen for connect bzero((char*)&servaddr, sizeof(servaddr)); servaddr.sin_family = AF_INET; servaddr.sin_addr.s_addr = htonl(INADDR_ANY); servaddr.sin_port = proxy_port; //get asocket .. if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) { fputs("failed to crate server socket ", stderr); exit(1); } // and bind our address and port to it if(bind(sockfd, (struct sockaddr_in*)&servaddr, sizeof(servaddr)) < 0) { fputs("failed to bind server socket to specified/r/n", stderr); exit(1); } // get ready to accept with at most 5 clients waitting to connect listen(sockfd, 5); // turn ourselves into daemon daemonize(sockfd); //fall into a loop to accept new connections and spawn children while(1) { //accept the next connection clilen = sizeof(cliaddr); newsockfd = accept(sockfd, (struct sockaddr*)&cliaddr, &clilen); if(newsockfd <0 && errno == EINTR) continue; //a signal might interrupt our accept call else if(newsockfd < 0) errorout("failed to accept connection");//sth quiet amiss--kill server //fork a child to handle this connection if((childpid = fork()) == 0) { close(sockfd);// inherit do_proxy(newsockfd); exit(0); } // if fork falied the connection is silently dropped --oops close(newsockfd); } return 0; } //parse the command line args void parse_args(int argc, char **argv) { int i; struct hostent *hostp;//host entry struct servent *servp; unsigned long inaddr; struct { char proxy_port[16]; char isolate_host[64]; char service_name[32]; }pargs; if(argc < 4) { printf("usage:%s<proxy-port> <host> <service-name|port-number> ", argv[0]); exit(1); } strcpy(pargs.proxy_port, argv[1]); strcpy(pargs.isolate_host, argv[2]); strcpy(pargs.service_name, argv[3]); for(i=0; i<strlen(pargs.proxy_port); i++) if(!isdigit(*(pargs.proxy_port+i))) break; if(i == strlen(pargs.proxy_port)) proxy_port = htons(atoi(pargs.proxy_port));//port short else { printf("invalid proxy port ", pargs.proxy_port); exit(0); } bzero(&hostaddr, sizeof(hostaddr)); hostaddr.sin_family = AF_INET; if((inaddr= inet_addr(pargs.isolate_host))!= INADDR_NONE) bcopy(&inaddr, &hostaddr.sin_addr, sizeof(inaddr)); else if((hostp == gethostbyname(pargs.isolate_host)) != NULL) bcopy(hostp->h_addr,&hostaddr.sin_addr,hostp->h_length); else { printf("%s unknow host ", pargs.isolate_host); exit(1); } if((servp = getservbyname(pargs.service_name, TCP_PROTO)) != NULL) hostaddr.sin_port = servp->s_port; else if(atoi(pargs.service_name) >0) hostaddr.sin_port = htons(atoi(pargs.service_name)); else { printf("%s invalid unknow service or port no. ", pargs.service_name); exit(1); } } /*detach the server process from the current context, creating a pristine, * predictable environment in which it will */ void daemonize(int servfd) { int childpid, fd, fdtablesize; // ignore terminal I/O,stop signals signal(SIGTTOU, SIG_IGN); signal(SIGTTIN, SIG_IGN); signal(SIGTSTP, SIG_IGN); /* fork to put us in the background (whether or not the user specified '&' on the command line */ if((childpid=fork()) < 0) { fputs("failed to fork first child/r/n",stderr); exit(1); } else if(childpid >0) exit(0);// terminate parent, continue child //dissociate from process group if (setpgrp(0,getpid()) < 0) { fputs("failed to become process group leader/r/n",stderr); exit(1); } /* lose controlling terminal */ if ((fd = open("/dev/tty", O_RDWR)) >= 0) { ioctl(fd,TIOCNOTTY,NULL); close(fd); } /* close any open file descriptors */ for(fd = 0, fdtablesize = getdtablesize(); fd < fdtablesize; fd++) if (fd != servfd) close(fd); /* set working directory to allow filesystems to be unmounted */ chdir("/"); /* clear the inherited umask */ umask(0); /* setup zombie prevention */ signal(SIGCLD,(sigfunc *)reap_status); } //handle a SIGCLD signal by reaping the exit status of the perished child, //and discarding it. void reap_status() { int pid; union wait status; while ((pid = wait3(&status,WNOHANG,NULL)) > 0) ; /* loop while there are more dead children */ } //does the actual work of virtually connecting a client to the telnet //service on the isolated host. void do_proxy (int usersockfd) { int isosockfd; fd_set rdfdset; int connstat; int iolen; char buf[2048]; /* open a socket to connect to the isolated host */ if ((isosockfd = socket(AF_INET,SOCK_STREAM,0)) < 0) errorout("failed to create socket to host"); /* attempt a connection */ connstat = connect(isosockfd,(struct sockaddr *) &hostaddr, sizeof(hostaddr)); switch (connstat) { case 0: break; case ETIMEDOUT: case ECONNREFUSED: case ENETUNREACH: strcpy(buf,sys_myerrlist[errno]); strcat(buf," "); write(usersockfd, buf, strlen(buf)); close(usersockfd); exit(1); /* die peacefully if we can't establish a connection */ break; default: errorout("failed to connect to host"); } /* now we're connected, serve fall into the data echo loop */ while (1) { /* Select for readability on either of our two sockets */ FD_ZERO(&rdfdset); FD_SET(usersockfd,&rdfdset); FD_SET(isosockfd,&rdfdset); if (select(FD_SETSIZE,&rdfdset,NULL,NULL,NULL) < 0) errorout("select failed"); /* is the client sending data? */ if (FD_ISSET(usersockfd,&rdfdset)) { if ((iolen = read(usersockfd,buf,sizeof(buf))) <= 0) break; /* zero length means the client disconnected */ write(isosockfd,buf,iolen); /* copy to host -- blocking semantics */ } /* is the host sending data? */ if(FD_ISSET(isosockfd,&rdfdset)) { if((iolen = read(isosockfd,buf,sizeof(buf))) <= 0) break; /* zero length means the host disconnected */ write(usersockfd,buf,iolen); /* copy to client -- blocking semantics */ } } /* we're done with the sockets */ close(isosockfd); close(usersockfd); } //displays an error message on the console and kills the current process. void errorout (char *msg) { FILE *console; console = fopen("/dev/console","a"); fprintf(console,"proxyd: %s ",msg); fclose(console); exit(1); }