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]](../stylesheet/note.png) |
Note | |||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
DESCRIPTION
The gethostbyname*(),
gethostbyaddr*(), herror(), and hstrerror() functions are obsolete.
Applications should use getaddrinfo(3), getnameinfo(3), and
gai_strerror(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)). If
name is an IPv4
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() rely on the Name Service
Switch (nsswitch.conf(5)) configured
sources or a local name server (named(8)). The default action
is to query the Name Service Switch (nsswitch.conf(5)) configured
sources, failing that, a local name server (named(8)).
Historical
The nsswitch.conf(5) file is the modern way of controlling the order of host lookups.
In glibc 2.4 and earlier, the order keyword was used to control the
order of host lookups as defined in /etc/host.conf (host.conf(5)).
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_INETorAF_INET6at 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_listfor 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_DATA-
The requested name is valid but does not have an IP address. Another type of request to the name server for this domain may return an answer. The constant
NO_ADDRESSis a synonym forNO_DATA. 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
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
| Interface | Attribute | Value |
gethostbyname() |
Thread safety |
MT-Unsafe race:hostbyname env locale |
gethostbyaddr() |
Thread safety |
MT-Unsafe race:hostbyaddr env locale |
sethostent(),endhostent(),
gethostent_r()
|
Thread safety |
MT-Unsafe race:hostent env locale |
|
|
Thread safety | MT-Safe |
gethostent() |
Thread safety |
MT-Unsafe race:hostent race:hostentbuf env locale |
gethostbyname2() |
Thread safety |
MT-Unsafe race:hostbyname2 env locale |
gethostbyaddr_r(), gethostbyname_r(), gethostbyname2_r() |
Thread safety | MT-Safe env locale |
In the above table, hostent in race:hostent signifies that
if any of the functions sethostent(3), gethostent(3), gethostent_r(3), or
endhostent(3) are used in
parallel in different threads of a program, then data races
could occur.
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, which 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.
SEE ALSO
getaddrinfo(3), getnameinfo(3), inet(3), inet_ntop(3), inet_pton(3), resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)
COLOPHON
This page is part of release 4.07 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
|
Copyright 1993 David Metcalfe (davidprism.demon.co.uk) %%%LICENSE_START(VERBATIM) 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. %%%LICENSE_END 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 |