客户端的 getaddrinfo 连接被拒绝

问题描述 投票：0回答：2

我正在 Linux 上运行客户端和服务器代码。

服务器使用带有 

AF_INET
和 
SOCK_STREAM
协议的 TCP/IP。 代码如下：

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相当长，所以我没有附上所有内容，但我尝试添加所有相关信息。如果有什么遗漏请评论，我会补充的。

这是客户端的

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！

c linux sockets tcp client-server
2个回答
2
投票

只要编译器配合，您对问题的修复就会起作用，并且您不需要支持 IPv6。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);
}

我还修复了其他一些细微的错误：

  • port_number
    应该是 
    unsigned short
    因为这就是 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。
  • 在有关连接到网络的错误消息中，始终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，无需释放内存。

为您提供后续练习，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
    那样。
  • 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）获取第一个有效的套接字并关闭所有其他地址。
0
投票

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该问题（因为我知道调用该函数时的端口号）：

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);
}
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