How gethostbyname() or getnameinfo() work in backg

问题:

How gethostbyname() or getnameinfo() work in background?

#include <stdlib.h>
#include <stdio.h>
#include <netdb.h>

/* paddr: print the IP address in a standard decimal dotted format */
void
paddr(unsigned char *a)
{
        printf("%d.%d.%d.%d\n", a[0], a[1], a[2], a[3]);
}

main(int argc, char **argv) {
        struct hostent *hp;
        char *host = "google.com";
        int i;

        hp = gethostbyname(host);
        if (!hp) {
                fprintf(stderr, "could not obtain address of %s\n", host);
                return 0;
        }
        for (i=0; hp->h_addr_list[i] != 0; i++)
                paddr((unsigned char*) hp->h_addr_list[i]);
        exit(0);
}

output for google.com:

74.125.236.198
74.125.236.199
74.125.236.206
74.125.236.201
74.125.236.200
74.125.236.196
74.125.236.193
74.125.236.197
74.125.236.194
74.125.236.195
74.125.236.192

output for www.google.com:

74.125.236.210
74.125.236.209
74.125.236.212
74.125.236.208
74.125.236.211

1. Will the above program do a check in the internet to resolve into IP?
2. Why its showing less IP addresses for www.google.com and more for just google.com?

回答1:

On a Linux system the gethostbyname() call implemented in the glibc performs lookups according to the configuration files /etc/host.conf and /etc/nsswitch.conf.

Typically in a default configuration it will first look in the /etc/hosts file if a local entry for the given name exists and if so, returns that. Otherwise it will proceed with the DNS protocol that is in turn configured by /etc/resolv.conf where the nameservers are stated.

Much more complex setups can be configured that lookup LDAP servers, databases etc.

You can also look into some man pages like man 5 nsswitch.conf.