gethostbyaddr()

Get a network host entry, given an Internet address

Synopsis:

#include <netdb.h>

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

Library:

libsocket

Description:

The gethostbyaddr() function searches for information associated with a host, which has the address pointed to by addr within the address family specified by type, opening a connection to the database if necessary. The arguments are:

addr

A pointer to the binary-format (i.e. not NULL-terminated) address in network byte order.

len

The length, in bytes, of addr.

type

The type of address. Currently, this must be AF_INET.

This function returns a pointer to a structure of type hostent that describes an Internet host. This structure contains either the information obtained from a name server, or broken-out fields from a line in /etc/hosts.

You can use sethostent() to request the use of a connected TCP socket for queries. If the stayopen flag is nonzero, all queries to the name server will use TCP and the connection will be retained after each call to gethostbyaddr() or gethostbyname(). If the stayopen flag is zero, queries use UDP datagrams.

Returns:

A pointer to a valid hostent structure, or NULL if an error occurs (h_errno is set).

Errors:

See herror().

Examples:

Use the gethostbyaddr() function to find a host:

struct sockaddr_in  client;
struct hostent* host;

int sock, fd, len;

...

len = sizeof( client );

fd  = accept( sock, (struct sockaddr*)&client, &len );

if( fd == -1 ) {
    perror( "accept" );
    exit( 1 );
}

host = gethostbyaddr( (const void*)&client.sin_addr, 
                      sizeof(struct in_addr), 
                      AF_INET );

printf( "Connection from %s: (%s)\n",
        host ? host->h_name : "<unknown>",
        inet_ntoa( client.sin_addr ) );

...

Files:

/etc/hosts

Host database file.

/etc/resolv.conf

Resolver configuration file.

Classification:

Standard Unix, POSIX 1003.1g (draft)

Caveats:

This function uses static data storage; if the data is needed for future use, it should be copied before any subsequent calls overwrite it. Currently, only the Internet address format is understood.

See also:

endhostent(), gethostbyname(), gethostbyaddr_r(), gethostent(), herror(), hostent, sethostent()

/etc/hosts, /etc/resolv.conf in the Utilities reference