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GETHOSTBYNAME(3) Library Functions Manual GETHOSTBYNAME(3)

gethostbyname, gethostbyname2, gethostbyaddr, gethostent, sethostent, endhostent, herror, hstrerror — get network host entry

library “libc”

#include <netdb.h>

int h_errno;

struct hostent *
gethostbyname(const char *name);

struct hostent *
gethostbyname2(const char *name, int af);

struct hostent *
gethostbyaddr(const void *addr, socklen_t len, int af);

struct hostent *
gethostent(void);

void
sethostent(int stayopen);

void
endhostent(void);

void
herror(const char *string);

const char *
hstrerror(int err);

The getaddrinfo(3) and getnameinfo(3) functions are preferred over the (), gethostbyname2(), and () functions.

The (), gethostbyname2() and () functions each return a pointer to an object with the following structure describing an internet host referenced by name or by address, respectively.

The name argument passed to () or gethostbyname2() should point to a NUL-terminated hostname. The addr argument passed to () should point to an address which is len bytes long, in binary form (i.e., not an IP address in human readable ASCII form). The af argument specifies the address family (e.g. AF_INET, AF_INET6, etc.) of this address.

The structure returned contains either the information obtained from the name server, named(8), broken-out fields from a line in /etc/hosts, or database entries supplied by the yp(8) system. The order of the lookups is controlled by the ‘hosts’ entry in nsswitch.conf(5).

struct	hostent {
	char	*h_name;	/* official name of host */
	char	**h_aliases;	/* alias list */
	int	h_addrtype;	/* host 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]	/* address, for backward compatibility */

The members of this structure are:

h_name
Official name of the host.
h_aliases
A NULL-terminated array of alternate names for the host.
h_addrtype
The type of address being returned; usually AF_INET.
h_length
The length, in bytes, of the address.
h_addr_list
A NULL-terminated array of network addresses for the host. Host addresses are returned in network byte order.
h_addr
The first address in h_addr_list; this is for backward compatibility.

When using the nameserver, () and gethostbyname2() will search for the named host in the current domain and its parents unless the name ends in a dot. If the name contains no dot, and if the environment variable “HOSTALIASES” contains the name of an alias file, the alias file will first be searched for an alias matching the input name. See hostname(7) for the domain search procedure and the alias file format.

The () function is an evolution of gethostbyname() which is intended to allow lookups in address families other than AF_INET, for example AF_INET6.

The () function may be used to request the use of a connected TCP socket for queries. Queries will by default use UDP datagrams. If the stayopen flag is non-zero, a TCP connection to the name server will be used. It will remain open after calls to gethostbyname(), gethostbyname2() or gethostbyaddr() have completed.

The () function closes the TCP connection.

The () function writes a message to the diagnostic output consisting of the string argument string, the constant string ": ", and a message corresponding to the value of h_errno.

The () function returns a string which is the message text corresponding to the value of the err argument.

/etc/hosts
 
/etc/nsswitch.conf
 
/etc/resolv.conf
 

Print out the hostname associated with a specific IP address:

const char *ipstr = "127.0.0.1";
struct in_addr ip;
struct hostent *hp;

if (!inet_aton(ipstr, &ip))
	errx(1, "can't parse IP address %s", ipstr);

if ((hp = gethostbyaddr((const void *)&ip,
    sizeof ip, AF_INET)) == NULL)
	errx(1, "no name associated with %s", ipstr);

printf("name associated with %s is %s\n", ipstr, hp->h_name);

Error return status from gethostbyname(), gethostbyname2() and gethostbyaddr() is indicated by return of a NULL pointer. The integer h_errno may then be checked to see whether this is a temporary failure or an invalid or unknown host. The routine herror() can be used to print an error message describing the failure. If its argument string is non-NULL, it is printed, followed by a colon and a space. The error message is printed with a trailing newline.

The variable h_errno can have the following values:

No such host is known.
This is usually a temporary error and means that the local server did not receive a response from an authoritative server. A retry at some later time may succeed.
Some unexpected server failure was encountered. This is a non-recoverable error.
The requested name is valid but does not have an IP address; this is not a temporary error. This means that the name is known to the name server but there is no address associated with this name. Another type of request to the name server using this domain name will result in an answer; for example, a mail-forwarder may be registered for this domain.

getaddrinfo(3), getnameinfo(3), inet_aton(3), resolver(3), hosts(5), hostname(7), named(8)

The () function is defined, and sethostent() and endhostent() are redefined, when library “libc” is built to use only the routines to lookup in /etc/hosts and not the name server.

The () function reads the next line of /etc/hosts, opening the file if necessary.

The () function opens and/or rewinds the file /etc/hosts. If the stayopen argument is non-zero, the file will not be closed after each call to gethostbyname(), gethostbyname2() or gethostbyaddr().

The () function closes the file.

The herror() function appeared in 4.3BSD. The endhostent(), gethostbyaddr(), gethostbyname(), gethostent(), and sethostent() functions appeared in 4.2BSD. The gethostbyname2() function first appeared in BIND version 4.9.4.

These functions use a thread-specific data storage; if the data is needed for future use, it should be copied before any subsequent calls overwrite it.

Though these functions are thread-safe, still it is recommended to use the getaddrinfo(3) family of functions, instead.

Only the Internet address format is currently understood.

October 4, 2017 FreeBSD-12.0