DNS check

最新推荐文章于 2025-08-26 15:33:08 发布

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#header #search #solaris #struct #string #linux

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本文提供了一个C语言程序示例，演示了如何使用系统调用获取主机名及通过DNS查询获得主机的全域名。该程序首先利用uname()函数获取节点名称，然后使用res_init(), res_search()和dn_expand()等函数进行DNS查询，最终输出本地节点名称及其全域名。

1. compile

cc ns.c -lresolv -lsocket -lnsl

2. run

It will print out local node name and its full dsn name. (uname -n)

It DSN work failed, a error string will be printed out, and full dsn name will be same as nodename.

#include <string.h>
#include <sys/types.h>
#include <sys/utsname.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
#include <netdb.h>

/**
* Compile how to:
* cc ns.c -lresolv -lsocket -lnsl
*/

void main(int argc, char * argv[])
{
        static struct utsname n;

        typedef union
        {
                HEADER hdr;
                unsigned char buf[1024];
        } querybuf;

        querybuf answer;
        unsigned char     *cp;
        long l;
        char      name[255];
        char     *ret_ptr;

        if (uname(&n) == -1)
        {
                printf("Cannot run uname(...)/n");
                return;
        }

        ret_ptr = n.nodename;

        /*
         * On certain machines (Solaris, for instance), n.nodename is not in FQDN
         * format. On Linux it is. So, let's ensure that what we have is a
         * FQDN
         */
        l = res_init();
        if (l == -1) {
                printf("Cannot run res_init(...)/n");
                goto done;
        }

        l = res_search(ret_ptr, C_IN, T_ANY, (u_char *)&answer, sizeof(answer));
        if (l == -1 || (unsigned long)l < sizeof(HEADER)) {
                printf("Cannot run res_search(...)/n");
                goto done;
        }

        cp = (unsigned char *)&answer + sizeof(HEADER);

        memset(name, 0, sizeof(name));
        l = dn_expand((unsigned char *)&answer, (unsigned char *)&answer+512, cp, name, 255);
        if (l == -1) {
                printf("Cannot run dn_expand(...)/n");
                goto done;
        }
        ret_ptr = name;

done:
        if (l == -1)
                printf("node name=[%s], fullname=[%s]/n",n.nodename,ret_ptr);
        else
                printf("node name=[%s], fullname=[%s]/n",n.nodename,ret_ptr);

           ret_ptr = 0;
}