UNP Host

Unix Network Programming Manual

Getting Host Data

In various situations, you may find that you have one piece of information about a host, but you need others. For example, a host typically has an Internet address, an Internet name and possibly alias names. If you were writing a client process, you might prefer to have the user specify the name of the server machine to use and then find the address. For example, if you use ftp, it is much more convenient to use ftp bozo.clown.edu than ftp 123.32.184.19. This conversion can be accomplished with the gethostbyname function.

     struct hostent *gethostbyname (name)
        char            *name

If you pass the name of a host, it returns a pointer to a structure with the following form:

     hostent 
     {
        char   *h_name;       /* official name of host */
        char   **h_aliases;   /* alias list */
        int    h_addrtype;    /* address type */
        int    h_length;      /* length of address */
        char   **h_addr_list; /* list of  addresses  from name server */
     }
     #define  h_addr h_addr_list[0]

For example, if you execute the following call in a program,

     struct hostent    *host;
     host = gethostbyname ("esus.cs.montana.edu");

and print the host structure, you will get:

     h_name        = esus.cs.montana.edu
     h_aliases     = esus
     h_addrtype    = 2
     h_length      = 4 
     h_addr_list   = 153.90.192.1

Note that there could be multiple aliases and addresses. h_addrtype = 2 indicates the AF_UNIX family.

Similarly, if you have the address of a machine, but not the name, such as after accepting a connection or receiving a datagram, you can find the name by using gethostbyaddr

     struct hostent *gethostbyaddr(addr, len, type)
        char   *addr
        size_t len, 
        int    type;

Where the address is passed as a character string of length len and the type is specifically given. For example the following code finds out what machine has just made a connection with the server.

     sockaddr_in    addr;
     struct hostent *host;

     clientsock = accept (sock, &addr, &addrlen);
     host = gethostbyaddr (&addr.sin_addr, sizeof (struct in_addr), 
                           addr.sin_family);

gethostbyaddr is designed to work for all types of addresses, so each part of the data field has to be treated in a general way, rather than as a known Internet type of address. It should be noted that the addresses are stored in the hostent structure in network byte order format, so they need not be converted before moving them to address structures, such as sockaddr_in. For example,

     struct hostent       *host;
     struct sockaddr_in   addr;

     host = gethostbyname ("esus.cs.montana.edu");
     addr . sin_addr . s_addr = (unsigned int) host -> h_addr_list [0];

will work to move the address into the address structure. However, it is typical to avoid data typing problems by using something like this.

     struct hostent       *host;
     struct sockaddr_in   addr;

     host = gethostbyname ("esus.cs.montana.edu");
     memcpy (&addr . sin_addr . s_addr, host -> h_addr_list [0],
           sizeof (addr . sin_addr . s_addr));