lwres_gethostbyname2
LWRES_GETHOSTENT(3) LWRES_GETHOSTENT(3)
NAME
lwres_gethostbyname, lwres_gethostbyname2, lwres_gethostbyaddr,
lwres_gethostent, lwres_sethostent, lwres_endhostent, lwres_gethostby-
name_r, lwres_gethostbyaddr_r, lwres_gethostent_r, lwres_sethostent_r,
lwres_endhostent_r - lightweight resolver get network host entry
SYNOPSIS
#include <lwres/netdb.h>
struct hostent * lwres_gethostbyname(const char *name);
struct hostent * lwres_gethostbyname2(const char *name, int af);
struct hostent * lwres_gethostbyaddr(const char *addr, int len, int
type);
struct hostent * lwres_gethostent(void);
void lwres_sethostent(int stayopen);
void lwres_endhostent(void);
struct hostent * lwres_gethostbyname_r(const char *name, struct hos-
tent *resbuf, char *buf, int buflen, int *error);
struct hostent * lwres_gethostbyaddr_r(const char *addr, int len, int
type, struct hostent *resbuf, char *buf, int buflen, int *error);
struct hostent * lwres_gethostent_r(struct hostent *resbuf, char *buf,
int buflen, int *error);
void lwres_sethostent_r(int stayopen);
void lwres_endhostent_r(void);
DESCRIPTION
These functions provide hostname-to-address and address-to-hostname
lookups by means of the lightweight resolver. They are similar to the
standard gethostent(3) functions provided by most operating systems.
They use a struct hostent which is usually defined in <namedb.h>.
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 from name server */
};
#define h_addr h_addr_list[0] /* address, for backward compatibility */
The members of this structure are:
h_name The official (canonical) name of the host.
h_aliases
A NULL-terminated array of alternate names (nicknames) for the
host.
h_addrtype
The type of address being returned — PF_INET or PF_INET6.
h_length
The length of the address in bytes.
h_addr_list
A NULL terminated array of network addresses for the host.
Host addresses are returned in network byte order.
For backward compatibility with very old software, h_addr is the first
address in h_addr_list.
lwres_gethostent(), lwres_sethostent(), lwres_endhostent(),
lwres_gethostent_r(), lwres_sethostent_r() and lwres_endhostent_r()
provide iteration over the known host entries on systems that provide
such functionality through facilities like /etc/hosts or NIS. The
lightweight resolver does not currently implement these functions; it
only provides them as stub functions that always return failure.
lwres_gethostbyname() and lwres_gethostbyname2() look up the hostname
name. lwres_gethostbyname() always looks for an IPv4 address while
lwres_gethostbyname2() looks for an address of protocol family af:
either PF_INET or PF_INET6 — IPv4 or IPV6 addresses respectively. Suc-
cessful calls of the functions return a struct hostentfor the name
that was looked up. NULL is returned if the lookups by lwres_gethost-
byname() or lwres_gethostbyname2() fail.
Reverse lookups of addresses are performed by lwres_gethostbyaddr().
addr is an address of length len bytes and protocol family type —
PF_INET or PF_INET6. lwres_gethostbyname_r() is a thread-safe func-
tion for forward lookups. If an error occurs, an error code is
returned in *error. resbuf is a pointer to a struct hostent which is
initialised by a successful call to lwres_gethostbyname_r() . buf is
a buffer of length len bytes which is used to store the h_name,
h_aliases, and h_addr_list elements of the struct hostent returned in
resbuf. Successful calls to lwres_gethostbyname_r() return resbuf,
which is a pointer to the struct hostent it created.
lwres_gethostbyaddr_r() is a thread-safe function that performs a
reverse lookup of address addr which is len bytes long and is of pro-
tocol family type — PF_INET or PF_INET6. If an error occurs, the error
code is returned in *error. The other function parameters are identi-
cal to those in lwres_gethostbyname_r(). resbuf is a pointer to a
struct hostent which is initialised by a successful call to
lwres_gethostbyaddr_r(). buf is a buffer of length len bytes which is
used to store the h_name, h_aliases, and h_addr_list elements of the
struct hostent returned in resbuf. Successful calls to lwres_gethost-
byaddr_r() return resbuf, which is a pointer to the struct hostent()
it created.
RETURN VALUES
The functions lwres_gethostbyname(), lwres_gethostbyname2(),
lwres_gethostbyaddr(), and lwres_gethostent() return NULL to indicate
an error. In this case the global variable lwres_h_errno will contain
one of the following error codes defined in <lwres/netdb.h>:
HOST_NOT_FOUND
The host or address was not found.
TRY_AGAIN
A recoverable error occurred, e.g., a timeout. Retrying the
lookup may succeed.
NO_RECOVERY
A non-recoverable error occurred.
NO_DATA
The name exists, but has no address information associated with
it (or vice versa in the case of a reverse lookup). The code
NO_ADDRESS is accepted as a synonym for NO_DATA for backwards
compatibility.
lwres_hstrerror(3) translates these error codes to suitable error mes-
sages.
lwres_gethostent() and lwres_gethostent_r() always return NULL.
Successful calls to lwres_gethostbyname_r() and lwres_gethost-
byaddr_r() return resbuf, a pointer to the struct hostent that was
initialised by these functions. They return NULL if the lookups fail
or if buf was too small to hold the list of addresses and names refer-
enced by the h_name, h_aliases, and h_addr_list elements of the struct
hostent. If buf was too small, both lwres_gethostbyname_r() and
lwres_gethostbyaddr_r() set the global variable errno to ERANGE.
SEE ALSO
gethostent(3), lwres_getipnode(3), lwres_hstrerror(3)
BUGS
lwres_gethostbyname(), lwres_gethostbyname2(), lwres_gethostbyaddr()
and lwres_endhostent() are not thread safe; they return pointers to
static data and provide error codes through a global variable.
Thread-safe versions for name and address lookup are provided by
lwres_gethostbyname_r(), and lwres_gethostbyaddr_r() respectively.
The resolver daemon does not currently support any non-DNS name ser-
vices such as /etc/hosts or NIS, consequently the above functions
don’t, either.
BIND9 Jun 30, 2000 LWRES_GETHOSTENT(3)
