sendmail, 4th Edition

Determine the Local Canonical Name

All versions of sendmail use more or less the same logical process to obtain the canonical name of the local host. As illustrated in the following sample program, sendmail first calls gethostname(3) to obtain the local host’s name within its domain. That name can be either a short name or a fully qualified one depending on how your system is set up. If the call to gethostname(3) fails, the name of the local host is set to localhost:

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/param.h>
#include <netdb.h>
#include <stdio.h>

main(  )
{
        char hostbuf[MAXHOSTNAMELEN];
        struct hostent *hp;

        /* Get the local host name */
        if (gethostname(hostbuf, sizeof(hostbuf)) < 0)
        {
                strcpy(hostbuf, "localhost");
        }
        printf("hostname = "%s"\n", hostbuf);

        /* canonicalize it and get aliases */
        if((hp = gethostbyname(hostbuf)) =  = NULL)
        {
                perror("gethostbyname");
                exit(2);
        }
        printf("canonical = "%s"\n", hp->h_name);
        while (*hp->h_aliases != NULL)
        {
                printf("alias: "%s"\n", *hp->h_aliases);
                ++hp->h_aliases;
        }
}

The local hostname is then given to the gethostbyname routine to obtain the canonical name for the local host. That same routine also returns any aliases (other names for the local host). Note that, if you defined NETINET6 (NET... on page 126) when compiling (for IPv6 support), you must use getipnodebyname(3) in place of gethostbyname(3).

The short (host) name found by gethostbyname(3) or getipnodebyname(3) is assigned as the value of the $w sendmail macro. ...