使用C语言获取DNS nameserver并进行域名解析

#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>

int main()
{
  res_init();
  int i = 0;
  for (i = 0;i< _res.nscount;i++) /* _res.nscount为找到的域名服务器的数量 */
    {
      struct sockaddr_in addr = _res.nsaddr_list[i]; /* 域名服务器的地址  */
    }
  int class = ns_c_in;
  int type = QUERY;
  char answer[256]="";
  res_query("www.baidu.com", class, type, answer, sizeof(answer));
  res_close();
  printf("answer=%s\n", answer); /* answer中为域名解析的结果 */
}

这种方法由于使用到了静态全局变量 _res ,因此并不是线程安全的。为此glib提供了线程安全的对应函数 res_ninit , res_nquery , res_nclose :

#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>

int main()
{
  res_state res;
  res_ninit(res);
  int i = 0;
  for (i = 0;i< res->nscount;i++) /* res->nscount存储了域名服务器的个数 */
    {
      struct sockaddr_in addr = res->nsaddr_list[i]; /* 域名服务器的地址 */
    }
  int class = ns_c_in;
  int type = QUERY;
  char answer[256]="";
  res_nquery(res, "www.baidu.com", class, type, answer, sizeof(answer));
  res_nclose(res);
  printf("answer=%s", answer);  /* answer中为域名解析的结果 */
}

其中:

• res_state 的定义在 resolv/bits/types/res_state.h 中

  /* res_state: the global state used by the resolver stub.  */
  #define MAXNS                   3       /* max # name servers we'll track */
  #define MAXDFLSRCH              3       /* # default domain levels to try */
  #define MAXDNSRCH               6       /* max # domains in search path */
  #define MAXRESOLVSORT           10      /* number of net to sort on */
  
  struct __res_state {
    int   retrans;                /* retransmition time interval */
    int   retry;                  /* number of times to retransmit */
    unsigned long options;                /* option flags - see below. */
    int   nscount;                /* number of name servers */
    struct sockaddr_in
      nsaddr_list[MAXNS]; /* address of name server */
    unsigned short id;            /* current message id */
    /* 2 byte hole here.  */
    char  *dnsrch[MAXDNSRCH+1];   /* components of domain to search */
    char  defdname[256];          /* default domain (deprecated) */
    unsigned long pfcode;         /* RES_PRF_ flags - see below. */
    unsigned ndots:4;             /* threshold for initial abs. query */
    unsigned nsort:4;             /* number of elements in sort_list[] */
    unsigned ipv6_unavail:1;      /* connecting to IPv6 server failed */
    unsigned unused:23;
    struct {
      struct in_addr      addr;
      uint32_t    mask;
    } sort_list[MAXRESOLVSORT];
    /* 4 byte hole here on 64-bit architectures.  */
    void * __glibc_unused_qhook;
    void * __glibc_unused_rhook;
    int   res_h_errno;            /* last one set for this context */
    int   _vcsock;                /* PRIVATE: for res_send VC i/o */
    unsigned int _flags;          /* PRIVATE: see below */
    /* 4 byte hole here on 64-bit architectures.  */
    union {
      char        pad[52];        /* On an i386 this means 512b total. */
      struct {
        uint16_t          nscount;
        uint16_t          nsmap[MAXNS];
        int                       nssocks[MAXNS];
        uint16_t          nscount6;
        uint16_t          nsinit;
        struct sockaddr_in6       *nsaddrs[MAXNS];
  #ifdef _LIBC
        unsigned long long int __glibc_extension_index
          __attribute__((packed));
  #else
        unsigned int              __glibc_reserved[2];
  #endif
      } _ext;
    } _u;
  };
  
  typedef struct __res_state *res_state;

• class 和 type 的定义在 resolv/arpa/nameser.h 中

  /*%
   * Currently defined opcodes.
   */
  typedef enum __ns_opcode {
                            ns_o_query = 0,               /*%< Standard query. */
                            ns_o_iquery = 1,      /*%< Inverse query (deprecated/unsupported). */
                            ns_o_status = 2,      /*%< Name server status query (unsupported). */
                            /* Opcode 3 is undefined/reserved. */
                            ns_o_notify = 4,      /*%< Zone change notification. */
                            ns_o_update = 5,      /*%< Zone update message. */
                            ns_o_max = 6
  } ns_opcode;
  
  /*%
   * Values for class field
   */
  typedef enum __ns_class {
                           ns_c_invalid = 0,      /*%< Cookie. */
                           ns_c_in = 1,           /*%< Internet. */
                           ns_c_2 = 2,            /*%< unallocated/unsupported. */
                           ns_c_chaos = 3,                /*%< MIT Chaos-net. */
                           ns_c_hs = 4,           /*%< MIT Hesiod. */
                           /* Query class values which do not appear in resource records */
                           ns_c_none = 254,       /*%< for prereq. sections in update requests */
                           ns_c_any = 255,                /*%< Wildcard match. */
                           ns_c_max = 65536
  } ns_class;

其他函数说明可以参见 man resolver .