Quick Navigator

Search Site

Unix VPS
A - Starter
B - Basic
C - Preferred
D - Commercial
MPS - Dedicated
Previous VPSs
* Sign Up! *

Contact Us
Online Help
Domain Status
Man Pages

Virtual Servers

Topology Map

Server Agreement
Year 2038

USA Flag



Man Pages

Manual Reference Pages  -  DNSRES (3)


dnsres_init, dnsres_gethostbyname, dnsres_gethostbyname2, dnsres_gethostbyaddr, dnsres_getaddrinfo - non blocking DNS resolving library


See Also


.Fd #include <sys/types.h>
.Fd #include <sys/socket.h>
.Fd #include <netdb.h>
.Fd #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 struct dnsres *res const char *name int af void (*cb)(struct hostent *hp, int error, void *arg) void *arg void dnsres_gethostbyaddr struct dnsres *res const char *addr int len int af void (*cb)(struct hostent *hp, int error, void *arg) void *arg void dnsres_getaddrinfo struct dnsres *res const char *hostname const char *servname const struct addrinfo *hints void (*cb)(struct addrinfo *ai, int res, void *arg) void *arg


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.
  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.

The dnsres_getaddrinfo function is used to get a list of IP addresses and port numbers for host hostname and service servname.


  A file containing local host aliases. See hostname(7) for more information.
  A list of options to override the resolver’s internal defaults. See resolver(3) for more information.




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:
  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.
  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.


getnameinfo(3), resolver(3), hosts(5), resolv.conf(5), hostname(7), named(8)


The herror function appeared in BSD 4.3 . The endhostent, gethostbyaddr, gethostbyname, gethostent, and sethostent functions appeared in BSD 4.2 .


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.


The dnsres library was hacked together by Niels Provos. Heavy use was made of the exisiting BSD resolver library.
Search for    or go to Top of page |  Section 3 |  Main Index

Powered by GSP Visit the GSP FreeBSD Man Page Interface.
Output converted with manServer 1.07.