gethostbyaddr
GETHOSTBYNAME(3) Linux Programmer's Manual GETHOSTBYNAME(3)
NAME
gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent, h_er-
rno, herror, hstrerror, gethostbyaddr_r, gethostbyname2, gethostby-
name2_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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
gethostbyname2(), gethostent_r(), gethostbyaddr_r(), gethostbyname_r(),
gethostbyname2_r():
Since glibc 2.19:
_DEFAULT_SOURCE
Glibc versions up to and including 2.19:
_BSD_SOURCE || _SVID_SOURCE
herror(), hstrerror():
Since glibc 2.19:
_DEFAULT_SOURCE
Glibc 2.8 to 2.19:
_BSD_SOURCE || _SVID_SOURCE
Before glibc 2.8:
none
h_errno:
Since glibc 2.19
_DEFAULT_SOURCE || _POSIX_C_SOURCE < 200809L
Glibc 2.12 to 2.19:
_BSD_SOURCE || _SVID_SOURCE || _POSIX_C_SOURCE < 200809L
Before glibc 2.12:
none
DESCRIPTION
The gethostbyname*(), gethostbyaddr*(), herror(), and hstrerror() func-
tions are obsolete. Applications should use getaddrinfo(3), getname-
info(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 ad-
dress, 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 exam-
ple 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. Oth-
erwise, 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 gethost-
byaddr() 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 or-
der 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_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 net-
work 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_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_ADDRESS is a synonym for
NO_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 at-
tributes(7).
+-------------------+---------------+-------------------------------+
|Interface | Attribute | Value |
+-------------------+---------------+-------------------------------+
|gethostbyname() | Thread safety | MT-Unsafe race:hostbyname env |
| | | locale |
+-------------------+---------------+-------------------------------+
|gethostbyaddr() | Thread safety | MT-Unsafe race:hostbyaddr env |
| | | locale |
+-------------------+---------------+-------------------------------+
|sethostent(), | Thread safety | MT-Unsafe race:hostent env |
|endhostent(), | | locale |
|gethostent_r() | | |
+-------------------+---------------+-------------------------------+
|herror(), | Thread safety | MT-Safe |
|hstrerror() | | |
+-------------------+---------------+-------------------------------+
|gethostent() | Thread safety | MT-Unsafe race:hostent |
| | | race:hostentbuf env locale |
+-------------------+---------------+-------------------------------+
|gethostbyname2() | Thread safety | MT-Unsafe race:hostbyname2 |
| | | env locale |
+-------------------+---------------+-------------------------------+
|gethostbyaddr_r(), | Thread safety | MT-Safe env locale |
|gethostbyname_r(), | | |
|gethostbyname2_r() | | |
+-------------------+---------------+-------------------------------+
In the above table, hostent in race:hostent signifies that if any of
the functions sethostent(), gethostent(), gethostent_r(), or endhos-
tent() 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(), gethost-
byaddr(), and h_errno are marked obsolescent in that standard.
POSIX.1-2008 removes the specifications of gethostbyname(), gethost-
byaddr(), and h_errno, recommending the use of getaddrinfo(3) and get-
nameinfo(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 en-
try 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 li-
brary was built without DNS support. The glibc version will ignore
ipv6 entries. This function is not reentrant, and glibc adds a reen-
trant 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 hos-
tent 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 re-
sult will be NULL. The functions return 0 on success and a nonzero er-
ror number on failure. In addition to the errors returned by the non-
reentrant versions of these functions, if buf is too small, the func-
tions 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 5.05 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/.
2017-09-15 GETHOSTBYNAME(3)
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