NAME
gethostbyname, gethostbyaddr, sethostent, endhostent, herror, hstrerror - 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 char *addr,
int len, int type);
void sethostent(int stayopen);
void endhostent(void);
void herror(const char *s);
const char *hstrerror(int err);
/* GNU extensions */
struct.hostent *gethostbyname2(const char *name, int af);
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);
DESCRIPTION
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, 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 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 sear.hed for name
(see
hostname(7)
for the file format).
The current domain and its parents are sear.hed 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. The only valid address type is currently
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. (See
resolv+(8)).
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
A zero-terminated array of alternative names for the host.
h_addrtype
The type of address; always
AF_INET
at present.
h_length
The length of the address in bytes.
h_addr_list
A zero-terminated array of network addresses for the host in network byte
order.
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.
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 non-recoverable 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
CONFORMING TO
BSD 4.3.
NOTES
The SUS-v2 standard is buggy and declares the
len
parameter of
gethostbyaddr()
to be of type
size_t.
(That is wrong, because it has to be
int,
and
size_t
is not. POSIX 1003.1-2001 makes it
socklen_t,
which is OK.)
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.
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
gethostbyname_r()
and
gethostbyname2_r().
These return 0 on success and nonzero on error. The result of the call
is now stored in the struct with address
ret.
After the call,
*result
will be NULL on error or point to the result on success.
Auxiliary data is stored in the buffer
buf
of length
buflen.
(If the buffer is too small, these functions will return
ERANGE.)
No global variable
h_errno
is modified, but the address of a variable in which to store error numbers
is passed in
h_errnop.
POSIX 1003.1-2001 marks
gethostbyaddr()
and
gethostbyname()
legacy, and introduces
struct.hostent *getipnodebyaddr (const void *restrict addr,
socklen_t len, int type, int *restrict error_num);
struct.hostent *getipnodebyname (const char *name,
int type, int flags, int *error_num);
SEE ALSO
resolver(3),
hosts(5),
hostname(7),
resolv+(8),
named(8)
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static const char *error_msg = "Resolver error";
static const char *const h_errlist[] = {
"Error 0",
"Unknown host", /* 1 HOST_NOT_FOUND */
"Host name lookup failure", /* 2 TRY_AGAIN */
"Unknown server error", /* 3 NO_RECOVERY */
"No address associated with name", /* 4 NO_ADDRESS */
};
static const int h_nerr = { sizeof(h_errlist)/sizeof(h_errlist[0]) };
const char *hstrerror(int err)
{
if (err < 0) {
return(error_msg);
} else if (err < h_nerr) {
return(h_errlist[err]);
}
return(error_msg);
}
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