gethostbyname(3) — Linux manual pages

Linux manual pages	Section 3

Name

gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent, h_errno, herror, hstrerror, gethostbyaddr_r, gethostbyname2, gethostbyname2_r, gethostbyname_r, gethostent_r — get network host entry

Synopsis

#include <netdb.h>
extern int h_errno;

struct hostent *gethostbyname( const char *name);

#include <sys/socket.h>        /* for AF_INET */

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

void sethostent( int stayopen);

void endhostent( void);

void herror( const char *s);

const char *hstrerror( int err);

/* System V/POSIX extension */

struct hostent *gethostent( void);

/* GNU extensions */

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

`int gethostent_r(`	struct hostent *`ret`,
	char *`buf`,
	size_t `buflen`,
	struct hostent **`result`,
	int *`h_errnop)`;

`int gethostbyaddr_r(`	const void *`addr`,
	socklen_t `len`,
	int `type`,
	struct hostent *`ret`,
	char *`buf`,
	size_t `buflen`,
	struct hostent **`result`,
	int *`h_errnop)`;

`int gethostbyname_r(`	const char *`name`,
	struct hostent *`ret`,
	char *`buf`,
	size_t `buflen`,
	struct hostent **`result`,
	int *`h_errnop)`;

`int gethostbyname2_r(`	const char *`name`,
	int `af`,
	struct hostent *`ret`,
	char *`buf`,
	size_t `buflen`,
	struct hostent **`result`,
	int *`h_errnop)`;

Note

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
`gethostbyname2`(), `gethostent_r`(), `gethostbyaddr_r`(), `gethostbyname_r`(), `gethostbyname2_r`():
`_BSD_SOURCE` \|\| `_SVID_SOURCE`
`herror`(), `hstrerror`():
Since glibc 2.8:
`_BSD_SOURCE` \|\| `_SVID_SOURCE` \|\| `_GNU_SOURCE`
Before glibc 2.8:
none

DESCRIPTION

The gethostbyname*() and gethostbyaddr*() functions are obsolete. Applications should use getaddrinfo(3) and getnameinfo(3) instead.

The gethostbyname() function returns a structure of type hostent for the given host name. Here name is either a hostname, or an IPv4 address in standard dot notation (as for inet_addr(3)), or an IPv6 address in colon (and possibly dot) notation. (See RFC 1884 for the description of IPv6 addresses.) If name is an IPv4 or IPv6 address, no lookup is performed and gethostbyname() simply copies name into the h_name field and its struct in_addr equivalent into the h_addr_list[0] field of the returned hostent structure. If name doesn't end in a dot and the environment variable HOSTALIASES is set, the alias file pointed to by HOSTALIASES will first be searched for name (see hostname(7) for the file format). The current domain and its parents are searched unless name ends in a dot.

The gethostbyaddr() function returns a structure of type hostent for the given host address addr of length len and address type type. Valid address types are AF_INET and AF_INET6. The host address argument is a pointer to a struct of a type depending on the address type, for example a struct in_addr * (probably obtained via a call to inet_addr(3)) for address type AF_INET.

The sethostent() function specifies, if stayopen is true (1), that a connected TCP socket should be used for the name server queries and that the connection should remain open during successive queries. Otherwise, name server queries will use UDP datagrams.

The endhostent() function ends the use of a TCP connection for name server queries.

The (obsolete) herror() function prints the error message associated with the current value of h_errno on stderr.

The (obsolete) hstrerror() function takes an error number (typically h_errno) and returns the corresponding message string.

The domain name queries carried out by gethostbyname() and gethostbyaddr() use a combination of any or all of the name server named(8), a broken out line from /etc/hosts, and the Network Information Service (NIS or YP), depending upon the contents of the order line in /etc/host.conf. The default action is to query named(8), followed by /etc/hosts.

The hostent structure is defined in <netdb.h> as follows:

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 */
}
#define h_addr h_addr_list[0] /* for backward compatibility */

The members of the hostent structure are:

h_name: The official name of the host.
h_aliases: An array of alternative names for the host, terminated by a NULL pointer.
h_addrtype: The type of address; always AF_INET or AF_INET6 at present.
h_length: The length of the address in bytes.
h_addr_list: An array of pointers to network addresses for the host (in network byte order), terminated by a NULL pointer.
h_addr: The first address in h_addr_list for backward compatibility.

RETURN VALUE

The gethostbyname() and gethostbyaddr() functions return the hostent structure or a NULL pointer if an error occurs. On error, the h_errno variable holds an error number. When non-NULL, the return value may point at static data, see the notes below.

ERRORS

The variable h_errno can have the following values:

HOST_NOT_FOUND: The specified host is unknown.
NO_ADDRESS or NO_DATA: The requested name is valid but does not have an IP address.
NO_RECOVERY: A nonrecoverable name server error occurred.
TRY_AGAIN: A temporary error occurred on an authoritative name server. Try again later.

FILES

/etc/host.conf: resolver configuration file
/etc/hosts: host database file
/etc/nsswitch.conf: name service switch configuration

CONFORMING TO

POSIX.1-2001 specifies gethostbyname(), gethostbyaddr(), sethostent(), endhostent(), gethostent(), and h_errno; gethostbyname(), gethostbyaddr(), and h_errno are marked obsolescent in that standard. POSIX.1-2008 removes the specifications of gethostbyname(), gethostbyaddr(), and h_errno, recommending the use of getaddrinfo(3) and getnameinfo(3) instead.

NOTES

The functions gethostbyname() and gethostbyaddr() may return pointers to static data, which may be overwritten by later calls. Copying the struct hostent does not suffice, since it contains pointers; a deep copy is required.

In the original BSD implementation the len argument of gethostbyname() was an int. The SUSv2 standard is buggy and declares the len argument of gethostbyaddr() to be of type size_t. (That is wrong, because it has to be int, and size_t is not. POSIX.1-2001 makes it socklen_t, which is OK.) See also accept(2).

The BSD prototype for gethostbyaddr() uses const char * for the first argument.

System V/POSIX Extension

POSIX requires the gethostent() call, that should return the next entry in the host data base. When using DNS/BIND this does not make much sense, but it may be reasonable if the host data base is a file that can be read line by line. On many systems a routine of this name reads from the file /etc/hosts. It may be available only when the library was built without DNS support. The glibc version will ignore ipv6 entries. This function is not reentrant, and glibc adds a reentrant version gethostent_r().

GNU Extensions

Glibc2 also has a gethostbyname2() that works like gethostbyname(), but permits to specify the address family to which the address must belong.

Glibc2 also has reentrant versions gethostent_r(), gethostbyaddr_r(), gethostbyname_r() and gethostbyname2_r(). The caller supplies a hostent structure ret which will be filled in on success, and a temporary work buffer buf of size buflen. After the call, result will point to the result on success. In case of an error or if no entry is found result will be NULL. The functions return 0 on success and a nonzero error number on failure. In addition to the errors returned by the nonreentrant versions of these functions, if buf is too small, the functions will return ERANGE, and the call should be retried with a larger buffer. The global variable h_errno is not modified, but the address of a variable in which to store error numbers is passed in h_errnop.

BUGS

gethostbyname() does not recognize components of a dotted IPv4 address string that are expressed in hexadecimal.

COLOPHON

This page is part of release 3.33 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://man7.org/linux/man-pages/.

  Copyright 1993 David Metcalfe (davidprism.demon.co.uk)

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

References consulted:
    Linux libc source code
    Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
    386BSD man pages
Modified 1993-05-22, David Metcalfe
Modified 1993-07-25, Rik Faith (faithcs.unc.edu)
Modified 1997-02-16, Andries Brouwer (aebcwi.nl)
Modified 1998-12-21, Andries Brouwer (aebcwi.nl)
Modified 2000-08-12, Andries Brouwer (aebcwi.nl)
Modified 2001-05-19, Andries Brouwer (aebcwi.nl)
Modified 2002-08-05, Michael Kerrisk
Modified 2004-10-31, Andries Brouwer