Connect 在客户端返回 -1 和 getaddrinfo

Question

我在 Linux 上运行客户端和服务器代码。

服务器使用 TCP/IP 和 AF_INET 和 SOCK_STREAM 协议。

首先，在客户端，我（错误地？）使用bind 从getaddrinfo 返回的链表中获取正确的addr_info，返回适当的sockaddr_in。我有一个在客户端看起来像这样的方法：

struct sockaddr_in *hostname_to_ip(char *hostname, struct addrinfo *servinfo)
{ 
    int check_sfd;
    struct addrinfo hints, *p;
    //struct sockaddr_in* ret_value;

    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;

    int res = getaddrinfo(hostname, NULL, &hints, &servinfo) 
    if (res != 0) {
        fprintf(stderr, "Error: error in getaddrinfo on hostname: %s\n", gai_strerror(res));
        exit(EXIT_FAILURE);
    }

    // getaddrinfo returned a linked list of relevant addresses
    // loop through the addresses and return the first one available
    for (p = servinfo; p != NULL; p = p->ai_next) {
        check_sfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol);
        if (checksfd < 0)
            continue;

        if (bind(check_sfd, p->ai_addr, p->ai_addrlen) == 0) { // bind successful
            return (struct sockaddr_in*) p->ai_addr;
        }
    }

    return NULL;
}

稍后连接到connect中的返回值。

代码的形式运行良好，客户端和服务器之间的连接已成功建立（并且所有功能都正常工作）。但是，由于没有正确使用freeaddrinfo，我出现了内存泄漏。

所以我决定把代码改成如下：

void hostname_to_ip(char *hostname, int *connection)
{ 
    int check_sfd;
    struct addrinfo hints, *p, *servinfo;
    //struct sockaddr_in* ret_value;

    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;

    int res = getaddrinfo(hostname, NULL, &hints, &servinfo) 
    if (res != 0) {
        fprintf(stderr, "Error: error in getaddrinfo on hostname: %s\n", gai_strerror(s));
        exit(EXIT_FAILURE);
    }

    // getaddrinfo returned a linked list of relevant addresses
    // loop through the addresses and return the first one available
    for (p = servinfo; p != NULL; p = p->ai_next) {
        check_sfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol);
        if (check_sfd == -1)
            continue;

        if (connect(check_sfd, p->ai_addr, p->ai_addrlen) == 0) { // connection successful
        *connection = check_sfd;
        break;
        }

        close(check_sfd);
    }

    if (p == NULL) {
        // print error
        exit(EXIT_FAILURE);
     }

    freeaddrinfo(&servinfo);
}

但是，现在我似乎无法连接到服务器，因为connect 总是返回-1（p 以非NULL 开始，因此与套接字的连接失败）。错误是connection refused。

当我将ai_socktype 更改为SOCK_DGRAM 时，连接成功，但客户端稍后在发送数据时失败（这是有道理的，因为服务器正在使用SOCK_STREAM），这很奇怪。

有什么想法吗？客户端和服务器的代码很长，所以我没有全部附上，但我尝试添加所有相关信息。如有遗漏，请评论，我会补充。

这是客户端上的strace 信息：

execve("./pcc_client", ["./pcc_client", "localhost", "2001", "1000"], [/* 71 vars */]) = 0
brk(NULL)                               = 0x2446000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=88205, ...}) = 0
mmap(NULL, 88205, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f0a0f4fa000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0P\t\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1868984, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f0a0f4f9000
mmap(NULL, 3971488, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f0a0ef21000
mprotect(0x7f0a0f0e1000, 2097152, PROT_NONE) = 0
mmap(0x7f0a0f2e1000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1c0000) = 0x7f0a0f2e1000
mmap(0x7f0a0f2e7000, 14752, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f0a0f2e7000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f0a0f4f8000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f0a0f4f7000
arch_prctl(ARCH_SET_FS, 0x7f0a0f4f8700) = 0
mprotect(0x7f0a0f2e1000, 16384, PROT_READ) = 0
mprotect(0x601000, 4096, PROT_READ)     = 0
mprotect(0x7f0a0f510000, 4096, PROT_READ) = 0
munmap(0x7f0a0f4fa000, 88205)           = 0
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 3
getsockname(3, {sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("0.0.0.0")}, [16]) = 0
socket(PF_LOCAL, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0) = 4
connect(4, {sa_family=AF_LOCAL, sun_path="/var/run/nscd/socket"}, 110) = -1 ENOENT (No such file or directory)
close(4)                                = 0
socket(PF_LOCAL, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0) = 4
connect(4, {sa_family=AF_LOCAL, sun_path="/var/run/nscd/socket"}, 110) = -1 ENOENT (No such file or directory)
close(4)                                = 0
brk(NULL)                               = 0x2446000
brk(0x2467000)                          = 0x2467000
open("/etc/nsswitch.conf", O_RDONLY|O_CLOEXEC) = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=529, ...}) = 0
read(4, "# /etc/nsswitch.conf\n#\n# Example"..., 4096) = 529
read(4, "", 4096)                       = 0
close(4)                                = 0
open("/etc/host.conf", O_RDONLY|O_CLOEXEC) = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=92, ...}) = 0
read(4, "# The \"order\" line is only used "..., 4096) = 92
read(4, "", 4096)                       = 0
close(4)                                = 0
getpid()                                = 12014
open("/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=184, ...}) = 0
read(4, "# Dynamic resolv.conf(5) file fo"..., 4096) = 184
read(4, "", 4096)                       = 0
close(4)                                = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=88205, ...}) = 0
mmap(NULL, 88205, PROT_READ, MAP_PRIVATE, 4, 0) = 0x7f0a0f4fa000
close(4)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libnss_files.so.2", O_RDONLY|O_CLOEXEC) = 4
read(4, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\260!\0\0\0\0\0\0"..., 832) = 832
fstat(4, {st_mode=S_IFREG|0644, st_size=47600, ...}) = 0
mmap(NULL, 2168600, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 4, 0) = 0x7f0a0ed0f000
mprotect(0x7f0a0ed1a000, 2093056, PROT_NONE) = 0
mmap(0x7f0a0ef19000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 4, 0xa000) = 0x7f0a0ef19000
mmap(0x7f0a0ef1b000, 22296, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f0a0ef1b000
close(4)                                = 0
mprotect(0x7f0a0ef19000, 4096, PROT_READ) = 0
munmap(0x7f0a0f4fa000, 88205)           = 0
open("/etc/hosts", O_RDONLY|O_CLOEXEC)  = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=233, ...}) = 0
read(4, "127.0.0.1\tlocalhost\n127.0.1.1\tmi"..., 4096) = 233
read(4, "", 4096)                       = 0
close(4)                                = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 2), ...}) = 0
write(1, "x\n", 2x
)                      = 2
socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 4
write(1, "4\n", 24
)                      = 2
write(1, "\n", 1
)                       = 1
connect(4, {sa_family=AF_INET, sin_port=htons(53511), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 ECONNREFUSED (Connection refused)
close(4)                                = 0
write(1, "Error: couldn't connect to hostn"..., 56Error: couldn't connect to hostname. Connection refused
) = 56
exit_group(1)                           = ?
+++ exited with 1 +++

谢谢！

Answer 1

只要编译器配合并且您不需要支持 IPv6，您对问题的修复就会起作用。但更好的解决方法是首先将端口号传递给getaddrinfo，作为“服务”参数。您必须将其转换为字符串才能执行此操作，但您可以返回使用 AF_UNSPEC 而不必在 getaddrinfo 的返回列表中强制转换或操作 sockaddrs：

void hostname_to_ip(char *hostname, int *connection_socket,
                    unsigned short port_number)
{
    int check_sfd;
    struct addrinfo hints, *p, *servinfo;
    char port_number_s[sizeof("65535")];

    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_NUMERICSERV;
    hints.ai_protocol = 0;

    snprintf(port_number_s, sizeof port_number_s, "%u", port_number);

    int res = getaddrinfo(hostname, port_number_s, &hints, &servinfo);
    if (res == EAI_SYSTEM) {
        fprintf(stderr, "Error looking up %s: %s\n",
                hostname, strerror(errno));
        exit(1);
    } else if (res != 0) {
        fprintf(stderr, "Error looking up %s: %s\n",
                hostname, gai_strerror(res));
        exit(1);
    } else if (servinfo == NULL) {
        fprintf(stderr, "Error looking up %s: No addresses found\n",
                hostname);
        exit(1);
    }

    // getaddrinfo returned a linked list of relevant addresses
    // loop through the addresses and return the first one we can connect to
    for (p = servinfo; p != NULL; p = p->ai_next) {
        check_sfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol);
        if (check_sfd == -1)
            continue;

        if (connect(check_sfd, p->ai_addr, p->ai_addrlen)) {
            *connection_socket = check_sfd;
            freeaddrinfo(servinfo);
            return;
        }
        close(check_sfd);
    }
    // If we get here, we couldn't connect to any of the addresses.
    fprintf(stderr, "Couldn't connect to %s: %s\n", hostname, strerror(errno));
    exit(1);
}

我还修复了一些其他细微的错误：

• port_number 应该是 unsigned short，因为这就是它在 TCP 中的内容。
• 如果getaddrinfo 返回EAI_SYSTEM，则需要打印strerror(errno) 而不是gai_strerror(res)。
• 在某些系统上，getaddrinfo 可以声称已成功但返回零地址。
• 始终在有关连接到网络的错误消息中打印您尝试与之通信的对等方的主机名。与文件操作失败时始终打印文件名的原理相同。
• 如果您在connect 成功时在循环内使用提前返回，则无需在循环后再次检查是否成功。当您要打印错误消息并退出时，不必释放内存。

为您准备的后续练习，按难度升序排列：

• 最好在失败时返回 -1 或在成功时返回打开的套接字，而不是写入输出参数或退出程序。
• 使用getnameinfo 打印“正在连接到128.52.0.2...”消息，就像telnet 一样。
• 使用非阻塞套接字一次连接到所有地址，然后使用 select 或 poll（您的选择）获取第一个可用的套接字并关闭所有其他地址。

Answer 2

感谢@MarkPlotnick，我发现问题与链接列表中返回地址的未设置（或错误设置）端口号有关。以下代码更新修复了它（因为我知道调用函数时的端口号）：

void hostname_to_ip(char *hostname, int *connection_socket, int port_number)
{ 
    int check_sfd;
    struct addrinfo hints, *p, *servinfo;

    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_family = AF_UNSPEC;//AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = 0;
    hints.ai_protocol = 0;

    int res = getaddrinfo(hostname, NULL, &hints, &servinfo);
    if (res != 0) {
        fprintf(stderr, "Error: error in getaddrinfo on hostname: %s\n", gai_strerror(res));
        exit(EXIT_FAILURE);
    }

    // getaddrinfo returned a linked list of relevant addresses
    // loop through the addresses and return the first one available
    for (p = servinfo; p != NULL; p = p->ai_next) {
        check_sfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol);
        if (check_sfd == -1)
            continue;

        struct sockaddr_in *sockstruct = (struct sockaddr_in*) p->ai_addr;
        sockstruct->sin_port = htons(port_number);
        sockstruct->sin_family = AF_INET;
        socklen_t addrsize = sizeof(struct sockaddr_in);

        if (connect(check_sfd, (struct sockaddr*)sockstruct, addrsize) != -1) { // successfuly connected
            *connection_socket = check_sfd;
            break;
        }

        close(check_sfd);
    }

    if (p == NULL) { // couldn't connect to any item in the linked list
        printf("Error: couldn't connect to hostname. %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }

    freeaddrinfo(servinfo);
}