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GETHOSTBYNAME(3) Linux Programmer's Manual GETHOSTBYNAME(3)
NAME
gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent, h_errno, herror, hstrerror, gethostbyaddr_r, gethostby-
name2, 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);
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
DESCRIPTION
The gethostbyname*() and gethostbyaddr*() functions are obsolete. Applications should use getaddrinfo(3) and getname-
info(3) instead.
The gethostbyname() function returns a structure of type hostent for the given host name. Here name is either a host-
name, 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 per-
formed 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; gethost-
byname(), 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 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 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), host-
name(7), named(8)
COLOPHON
This page is part of release 3.25 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/.
2009-12-03 GETHOSTBYNAME(3)
