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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | FILES | ATTRIBUTES | 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, gethost‐
byname2_r, gethostbyname_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(), 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
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_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_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.
/etc/host.conf
resolver configuration file
/etc/hosts
host database file
/etc/nsswitch.conf
name service switch configuration
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(), │ 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
endhostent() are used in parallel in different threads of a program,
then data races could occur.
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.
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.
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 4.12 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/.
2016-03-15 GETHOSTBYNAME(3)
Pages that refer to this page: getent(1), pmhostname(1), byteorder(3), getaddrinfo(3), gethostid(3), getipnodebyname(3), getnameinfo(3), inet(3), pmdatrace(3), pmgetarchivelabel(3), pmnewcontext(3), pmreconnectcontext(3), rcmd(3), resolver(3), rexec(3), setnetgrent(3), host.conf(5), nsswitch.conf(5), resolv.conf(5), environ(7), hostname(7), ip(7), nscd(8), systemd-resolved.service(8)