GETHOSTBYNAME(3)

GETHOSTBYNAME(3) | Library Functions Manual | GETHOSTBYNAME(3)

# NAME

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

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#include <netdb.h>`

`extern 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 type);

struct hostent *
`gethostent`(void);

void
`sethostent`(int stayopen);

void
`endhostent`(void);

void
`herror`(const char *string);

const char *
`hstrerror`(int err);

# DESCRIPTION

The `gethostbyname`(), `gethostbyname2`() and `gethostbyaddr`() functions each return a pointer to an object with the following structure describing an internet host.

    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 alternative names for the host.
h_addrtype
  The type of address being returned; currently always `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.

In the case of `gethostbyname`() and `gethostbyname2`(), the host is specified by name, or using a string representation of a numeric address. In the case of `gethostbyaddr`(), the host is specified using a binary representation of an address.

The returned struct hostent structure may contain the result of a simple string to binary conversion, information obtained from the domain name resolver (see resolver(3)), 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).

When using the domain name resolver, `gethostbyname`() 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 `gethostbyname2`() function is an evolution of `gethostbyname`() which is intended to allow lookups in address families other than `AF_INET`, for example `AF_INET6`. Currently the af argument must be specified as `AF_INET` or `AF_INET6`, else the function will return `NULL` after having set h_errno to `NETDB_INTERNAL`.

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

The `sethostent`() function may be used to request the use of a connected TCP socket for queries. If the stayopen flag is non-zero, this sets the option to send all queries to the name server using TCP and to retain the connection after each call to `gethostbyname`(), `gethostbyname2`(), or `gethostbyaddr`(). Otherwise, queries are performed using UDP datagrams.

The `endhostent`() function closes the TCP connection.

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

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

# FILES

/etc/hosts

# RETURN VALUES

Error return status from `gethostbyent`(), `gethostbyname`(), `gethostbyname2`(), and `gethostbyaddr`() is indicated by return of a null pointer. The external 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:

`HOST_NOT_FOUND`
  No such host is known.
`TRY_AGAIN`
  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.
`NO_RECOVERY`
  Some unexpected server failure was encountered. This is a non-recoverable error.
`NO_DATA`
  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.

# SEE ALSO

resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)

# HISTORY

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-4.9.4. IPv6 support was implemented in WIDE Hydrangea IPv6 protocol stack kit.

# CAVEATS

If the search routines specified in nsswitch.conf(5) decide to read the /etc/hosts file, `gethostbyname`(), `gethostbyname2`(), and `gethostbyaddr`() will read the next line of the file, re-opening the file if necessary.

The `sethostent`() 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`(), `gethostbyaddr`(), or `gethostent`().

The `endhostent`() function closes the file.

# BUGS

These functions use static data storage; if the data is needed for future use, it should be copied before any subsequent calls overwrite it. Only the Internet address format is currently understood.

The `gethostent`() does not currently follow the search order specified in nsswitch.conf(5) and only reads the /etc/hosts file.

August 19, 2013 | NetBSD 10.0