NAME
gethostbyname
,
gethostbyname2
,
gethostbyaddr
, gethostent
,
sethostent
, endhostent
,
hstrerror
, herror
—
get network host entry
SYNOPSIS
#include
<netdb.h>
extern int h_errno;
struct hostent *
gethostbyname
(const
char *name);
struct hostent *
gethostent
(void);
void
sethostent
(int
stayopen);
void
endhostent
(void);
void
herror
(const
char *string);
const char *
hstrerror
(int
err);
#include
<sys/socket.h>
#include <netdb.h>
struct hostent *
gethostbyname2
(const
char *name, int
af);
struct hostent *
gethostbyaddr
(const
void *addr, socklen_t
len, int af);
DESCRIPTION
The
gethostbyname
(),
gethostbyname2
(), and
gethostbyaddr
() functions each return a pointer to
an object with the following structure describing an Internet host
referenced by name or by address, respectively. This structure contains
either information obtained from a name server, broken-out fields from a
line in /etc/hosts, or database entries supplied by
the yp(8) system.
resolv.conf(5) describes how the particular database is
chosen.
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 returned addresses */ }; #define h_addr h_addr_list[0] /* address, for backward compat */
The members of this structure are:
- h_name
- Official name of the host.
- h_aliases
- A null-terminated array of alternate names for the host.
- h_addrtype
- The type of address being returned.
- h_length
- The length, in bytes, of the address.
- h_addr_list
- A null-terminated array of network addresses for the host. Host addresses are returned in network byte order.
- h_addr
- The first address in h_addr_list; this is for backward compatibility.
The function
gethostbyname
()
will search for the named host in the current domain and its parents using
the search lookup semantics detailed in
resolv.conf(5) and
hostname(7).
gethostbyname2
()
is an advanced form of gethostbyname
() which allows
lookups in address families other than AF_INET
.
Currently, the only supported address family besides
AF_INET
is AF_INET6
.
The
gethostbyaddr
()
function will search for the specified address of length
len in the address family af.
The only address family currently supported is
AF_INET
.
The
sethostent
()
function may be used to request the use of a connected TCP socket for
queries. If the stayopen flag is non-zero, this sets
the option to send all queries to the name server using TCP and to retain
the connection after each call to gethostbyname
() or
gethostbyaddr
(). Otherwise, queries are performed
using UDP datagrams.
The
endhostent
()
function closes the TCP connection.
The
herror
()
function prints an error message describing the failure. If its argument
string is non-null, it is prepended to the message
string and separated from it by a colon
(‘:
’) and a space. The error message
is printed with a trailing newline. The contents of the error message is the
same as that returned by
hstrerror
()
with argument h_errno.
ENVIRONMENT
- RES_OPTIONS
- A list of options to override the resolver's internal defaults. See resolver(3) for more information.
FILES
- /etc/hosts
- /etc/resolv.conf
DIAGNOSTICS
Error return status from gethostbyname
(),
gethostbyname2
(), and
gethostbyaddr
() is indicated by return of a null
pointer. The external integer h_errno may then be
checked to see whether this is a temporary failure or an invalid or unknown
host.
The variable h_errno can have the following values:
HOST_NOT_FOUND
- No such host is known.
TRY_AGAIN
- This is usually a temporary error and means that the local server did not receive a response from an authoritative server. A retry at some later time may succeed.
NO_RECOVERY
- Some unexpected server failure was encountered. This is a non-recoverable error.
NO_DATA
- The requested name is valid but does not have an IP address; this is not a temporary error. This means that the name is known to the name server but there is no address associated with this name. Another type of request to the name server using this domain name will result in an answer; for example, a mail-forwarder may be registered for this domain.
NETDB_INTERNAL
- An internal error occurred. This may occur when an address family other
than
AF_INET
orAF_INET6
is specified or when a resource is unable to be allocated. NETDB_SUCCESS
- The function completed successfully.
SEE ALSO
getaddrinfo(3), getnameinfo(3), resolver(3), hosts(5), resolv.conf(5), hostname(7)
HISTORY
The herror
() function appeared in
4.3BSD. The endhostent
(),
gethostbyaddr
(),
gethostbyname
(),
gethostent
(), and
sethostent
() functions appeared in
4.2BSD.
CAVEATS
If the search routines in
resolv.conf(5) decide to read the
/etc/hosts file,
gethostent
() and other functions will read the next
line of the file, re-opening the file if necessary.
The sethostent
() function opens and/or
rewinds the file /etc/hosts. If the
stayopen argument is non-zero, the file will not be
closed after each call to gethostbyname
(),
gethostbyname2
(), or
gethostbyaddr
().
The endhostent
() function closes the
file.
BUGS
These functions use static data storage; if the data is needed for future use, it should be copied before any subsequent calls overwrite it.
Only the Internet address formats are currently understood.
YP does not support any address families other than
AF_INET
and uses the traditional database
format.