DNS资源记录与gethostbyname

1.    DNS资源记录

DNS资源记录类型主要有以下几种：

A ：A记录将主机名映射为IPV4地址，例如：

       ns.abc.com.             IN   A   192.168.100.5
       mail1.abc.com.       IN   A   192.168.100.6

AAAA ：AAAA记录将主机名映射为IPV6地址，格式同上。

CNAME ：CNAME记录别名与正式名称之间的对应关系，格式为：

       Alias   IN   CNAME    Canonical-hostname
MX ：MX记录提供了邮件路由信息，以及邮件交换器的主机名称以及相应的优先值。

PTR : PTR记录将IP地址与主机名进行映射对应，作用正好与A记录相反。

【注意】在Linux或者windows系统上，nslookup命令可以查看这几种类型记录。

 

2.    gethostbyname函数的语法：

函数的语法形式如下：

      struct hostent *gethostbyname(const char *hostname)      

      函数的主要功能是执行依次对CNAME，A记录或者AAAA记录的查询，并返回主机对应的别名、正式名、IPV4或IPV6地址。函数返回类型结构定义如下：

           structhostent {

　　           char *h_name;  //主机的正式名称

　　           char **h_aliases;  //主机的别名列

　　           int h_addrtype;  //主机的地址类型

　　           int h_length; //主机的地址长度（4字节或者16字节）

　　           char **h_addr_list; //主机对应的IPV4或者IPV6地址列

　　           #define h_addr h_addr_list[0] //地址列表中的第一个地址

　　      };

      根据上面的定义，值得注意的是，对于某台主机，可能存在着多个别名以及多个A或AAAA记录，即多个IP地址。

【测试程序】

/**************************************  modify by dragon based on UNP code**************************************/#include <netdb.h>#include <stdio.h>#include <string.h>#define IN6ADDRSZ 16 #defineINT16SZ 2static const char *inet_ntop4(const u_char *src, char *dst, size_t size);static const char *inet_ntop6(const u_char *src, char *dst, size_t size);static const char *inet_ntop(int af, const void *src, char *dst, size_t size);static const char *inet_ntop4(const u_char *src, char *dst, size_t size){static const char fmt[] = "%u.%u.%u.%u";char tmp[sizeof "255.255.255.255"];sprintf(tmp, fmt, src[0], src[1], src[2], src[3]);if (strlen(tmp) > size) {return NULL;}strcpy(dst, tmp);return (dst);}static const char *inet_ntop6(const u_char *src, char *dst, size_t size){/* * Note that int32_t and int16_t need only be "at least" large enough * to contain a value of the specified size.  On some systems, like * Crays, there is no such thing as an integer variable with 16 bits. * Keep this in mind if you think this function should have been coded * to use pointer overlays.  All the world's not a VAX. */char tmp[sizeof "ffff:ffff:ffff:ffff:ffff:ffff:255.255.255.255"], *tp;struct { int base, len; } best, cur;u_int words[IN6ADDRSZ / INT16SZ];int i;/* * Preprocess: *Copy the input (bytewise) array into a wordwise array. *Find the longest run of 0x00's in src[] for :: shorthanding. */memset(words, 0, sizeof words);for (i = 0; i < IN6ADDRSZ; i++)words[i / 2] |= (src[i] << ((1 - (i % 2)) << 3));best.base = -1;cur.base = -1;for (i = 0; i < (IN6ADDRSZ / INT16SZ); i++) {if (words[i] == 0) {if (cur.base == -1)cur.base = i, cur.len = 1;elsecur.len++;} else {if (cur.base != -1) {if (best.base == -1 || cur.len > best.len)best = cur;cur.base = -1;}}}if (cur.base != -1) {if (best.base == -1 || cur.len > best.len)best = cur;}if (best.base != -1 && best.len < 2)best.base = -1;/* * Format the result. */tp = tmp;for (i = 0; i < (IN6ADDRSZ / INT16SZ); i++) {/* Are we inside the best run of 0x00's? */if (best.base != -1 && i >= best.base &&    i < (best.base + best.len)) {if (i == best.base)*tp++ = ':';continue;}/* Are we following an initial run of 0x00s or any real hex? */if (i != 0)*tp++ = ':';/* Is this address an encapsulated IPv4? */if (i == 6 && best.base == 0 &&    (best.len == 6 || (best.len == 5 && words[5] == 0xffff))) {if (!inet_ntop4(src+12, tp, sizeof tmp - (tp - tmp)))return (NULL);tp += strlen(tp);break;}sprintf(tp, "%x", words[i]);tp += strlen(tp);}/* Was it a trailing run of 0x00's? */if (best.base != -1 && (best.base + best.len) == (IN6ADDRSZ / INT16SZ))*tp++ = ':';*tp++ = '\0';/* * Check for overflow, copy, and we're done. */if ((tp - tmp) > size) {return (NULL);}strcpy(dst, tmp);return (dst);}static const char *inet_ntop(int af, const void *src, char *dst, size_t size){switch (af) {case AF_INET:return (inet_ntop4(src, dst, size));case AF_INET6:return (inet_ntop6(src, dst, size));default:return (NULL);}/* NOTREACHED */}intmain(int argc, char **argv){char*ptr, **pptr;charstr[INET6_ADDRSTRLEN];struct hostent*hptr;while (--argc > 0) {ptr = *++argv;if ( (hptr = gethostbyname(ptr)) == NULL) {printf("gethostbyname error for host: %s",ptr);continue;}printf("official hostname: %s\n", hptr->h_name);printf("h_addrtype is %d.\n", hptr->h_addrtype);  //2---AF_INET  10---AF_INET6for (pptr = hptr->h_aliases; *pptr != NULL; pptr++)printf("\talias: %s\n", *pptr);switch (hptr->h_addrtype) {case AF_INET:#ifdefAF_INET6case AF_INET6:#endifpptr = hptr->h_addr_list;for ( ; *pptr != NULL; pptr++)printf("\taddress: %s\n",inet_ntop(hptr->h_addrtype, *pptr, str, sizeof(str)));break;default:printf("unknown address type");break;}}return 0;}