Reference

dnsres(3)

non blocking DNS resolving library

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.