NAME
dnsres_init, dnsres_gethostbyname, dnsres_gethostbyname2,
dnsres_gethostbyaddr, dnsres_getaddrinfo - non blocking DNS resolving
library
SYNOPSIS
#include <dnsres.h>
int
dnsres_init(struct dnsres *_resp);
void
dnsres_gethostbyname(struct dnsres* res, const char *name,
void (*cb)(struct hostent *hp, int error, void *arg, void *arg);
void
dnsres_gethostbyname2(const char *name, int af,
void (*cb)(struct hostent *hp, int error, void *arg, void *arg);
void
dnsres_gethostbyaddr(const char *addr, int len, int af,
void (*cb)(struct hostent *hp, int error, void *arg, void *arg);
void
dnsres_getaddrinfo(struct dnsres *, const char *, const char *,
const struct addrinfo *,
void (*)(struct addrinfo *, int, void *), void *);;
DESCRIPTION
The dnsres_init() is used to initialize the dnsres library. If you are
developing a multi-threaded application, you need one struct dnsres per
thread.
The dnsres_gethostbyname(), dnsres_gethostbyname2() and
dnsres_gethostbyaddr() functions each call their provided callback
function with 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
the name server (i.e., resolver(3) and named(8)), 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 dnsres_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 */
};
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 zero-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 dnsres_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).
dnsres_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 dnsres_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.
ENVIRONMENT
HOSTALIASES A file containing local host aliases. See hostname(7) for
more information.
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 dnsres_gethostbyname(), dnsres_gethostbyname2(),
and dnsres_gethostbyaddr() is indicated by a null pointer passed to the
callback function. The integer dr_errno may then be checked in struct
dnsres to see whether this is a temporary failure or an invalid or
unknown host.
The variable h_errno can have the following values:
DNSRES_HOST_NOT_FOUND No such host is known.
DNSRES_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.
DNSRES_NO_RECOVERY Some unexpected server failure was encountered.
This is a non-recoverable error.
DNSRES_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.
DNSRES_NETDB_INTERNAL An internal error occurred. This may occurs when
an address family other than AF_INET or AF_INET6
is specified or when a resource is unable to be
allocated.
DNSRES_NETDB_SUCCESS The function completed successfully.
SEE ALSO
getnameinfo(3), resolver(3), hosts(5), resolv.conf(5), hostname(7),
named(8)
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.
AUTHORS
The dnsres library was hacked together by Niels Provos. Heavy use was
made of the exisiting BSD resolver library.