OpenSSH系列(扩展一)-ProxyCommand的扩展使用以及实现代码

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ProxyCommand功能的使用 – 透过http代理

1 2	# 注意, 这里的nc需要使用netcat-openbsd(openbsd版本的nc, gnu的nc没有-X功能) ssh -o "ProxyCommand=nc -X connect -x 127.0.0.1:8888 %h %p" corvo@192.168.137.23

源码相关

本来也没必要读源码的, 主要是我想知道ssh是怎样使用代理连接的. 下面是相关代码, 我把重要部分补上了注释. 主要使用了POSIX标准中的pipe功能, 新建一个子进程用户连接代理服务器, 而后父进程再通过stdin, stdout重定向的功能与子进程通信, 也就可以与远端服务器通信了, 此代码不难, 但是建议与博客中的NodeJS实现的代理服务器一起阅读.

代码来源: https://github.com/openssh/libopenssh

1 2	# 直接搜索相关的代码 find . -name "*.c" \| xargs grep 'proxy_command' -n

可以看到, 相关的代码放在ssh/sshconnect.c

// ssh_connect 函数

/* If a proxy command is given, connect using it. */
if (proxy_command != NULL)
    return ssh_proxy_connect(host, port, proxy_command);

/*
 * Connect to the given ssh server using a proxy command.
 */
static struct ssh *
ssh_proxy_connect(const char *host, u_short port, const char *proxy_command)
{
    struct ssh *ssh;
    char *command_string, *tmp;
    int pin[2], pout[2];
    pid_t pid;
    char *shell, strport[NI_MAXSERV];

    if ((shell = getenv("SHELL")) == NULL || *shell == '\0')
        shell = _PATH_BSHELL;

    /* Convert the port number into a string. */
    snprintf(strport, sizeof strport, "%hu", port);

    /*
     * Build the final command string in the buffer by making the
     * appropriate substitutions to the given proxy command.
     *
     * Use "exec" to avoid "sh -c" processes on some platforms
     * (e.g. Solaris)
     */
    xasprintf(&tmp, "exec %s", proxy_command);
    command_string = percent_expand(tmp, "h", host, "p", strport,
        "r", options.user, (char *)NULL);
    xfree(tmp);

    // 初始化两个管道
    /* Create pipes for communicating with the proxy. */
    if (pipe(pin) < 0 || pipe(pout) < 0)
        fatal("Could not create pipes to communicate with the proxy: %.100s",
            strerror(errno));

    debug("Executing proxy command: %.500s", command_string);

    /* Fork and execute the proxy command. */
    if ((pid = fork()) == 0) { // 子进程
        char *argv[10];

        /* Child.  Permanently give up superuser privileges. */
        permanently_drop_suid(original_real_uid);

        /* Redirect stdin and stdout. */
        close(pin[1]);
        if (pin[0] != 0) {
            // 关掉子进程的标准输入, 使用pin[0]作为标准输入
            if (dup2(pin[0], 0) < 0)
                perror("dup2 stdin");
            close(pin[0]);
        }
        // 关掉子进程的标准输出, 使用pout[1]作为标准输出
        close(pout[0]);
        if (dup2(pout[1], 1) < 0)
            perror("dup2 stdout");
        /* Cannot be 1 because pin allocated two descriptors. */
        close(pout[1]);

        /* Stderr is left as it is so that error messages get
           printed on the user's terminal. */
        argv[0] = shell;
        argv[1] = "-c";
        argv[2] = command_string;
        argv[3] = NULL;

        /* Execute the proxy command.  Note that we gave up any
           extra privileges above. */
        signal(SIGPIPE, SIG_DFL);
        // 启动子进程, 子进程将会与远程的服务器相连
        execv(argv[0], argv);
        perror(argv[0]);
        exit(1);
    }
    /* Parent. */
    if (pid < 0)
        fatal("fork failed: %.100s", strerror(errno));
    else
        proxy_command_pid = pid; /* save pid to clean up later */

    /* Close child side of the descriptors. */
    close(pin[0]);
    close(pout[1]);

    /* Free the command name. */
    xfree(command_string);

    /* Set the connection file descriptors. */
    ssh = ssh_packet_set_connection(NULL, pout[0], pin[1]);
    ssh_packet_set_timeout(ssh, options.server_alive_interval,
        options.server_alive_count_max);

    /* Indicate OK return */
    return (ssh);
}

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ProxyCommand功能的使用 – 透过http代理

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