NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | FILES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON
GETHOSTBYNAME(3) Linux Programmer's Manual GETHOSTBYNAME(3)
gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent, h_errno,
herror, hstrerror, gethostbyaddr_r, gethostbyname2, gethostbyname2_r, gethost-
byname_r, gethostent_r - get network host entry
#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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
gethostbyname2(), gethostent_r(), gethostbyaddr_r(), gethostbyname_r(), geth-
ostbyname2_r(): _BSD_SOURCE || _SVID_SOURCE
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.
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.
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 non-recoverable name server error occurred.
TRY_AGAIN
A temporary error occurred on an authoritative name server. Try again
later.
/etc/host.conf
resolver configuration file
/etc/hosts
host database file
/etc/nsswitch.conf
name service switch configuration
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.
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.
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().
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 non-zero error number on failure. In
addition to the errors returned by the non-reentrant 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.
gethostbyname() does not recognize components of a dotted IPv4 address string
that are expressed in hexadecimal.
getaddrinfo(3), getnameinfo(3), inet(3), inet_ntop(3), inet_pton(3),
resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
2009-03-15 GETHOSTBYNAME(3)