dnsres_gethostbyname(3) non blocking DNS resolving library

Other Alias

dnsres_init, dnsres_gethostbyname2, dnsres_gethostbyaddr, dnsres_getaddrinfo


Fd #include <dnsres.h> Ft int Fn dnsres_init struct dnsres *_resp Ft void Fn dnsres_gethostbyname struct dnsres* res const char *name void (*cb)(struct hostent *hp, int error, void *arg void *arg Ft void Fn dnsres_gethostbyname2 const char *name int af void (*cb)(struct hostent *hp, int error, void *arg void *arg Ft void Fn dnsres_gethostbyaddr const char *addr int len int af void (*cb)(struct hostent *hp, int error, void *arg void *arg Ft void Fn dnsres_getaddrinfo struct dnsres * const char * const char * const struct addrinfo * void (*)(struct addrinfo *, int, void *) void *;


The Fn 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 Fn dnsres_gethostbyname , Fn dnsres_gethostbyname2 and Fn 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.conf5 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:

Fa h_name
Official name of the host.
Fa h_aliases
A NULL-terminated array of alternate names for the host.
Fa h_addrtype
The type of address being returned.
Fa h_length
The length, in bytes, of the address.
Fa h_addr_list
A zero-terminated array of network addresses for the host. Host addresses are returned in network byte order.
Fa h_addr
The first address in Fa h_addr_list ; this is for backward compatibility.

The function Fn dnsres_gethostbyname will search for the named host in the current domain and its parents using the search lookup semantics detailed in resolv.conf5 and hostname(7).

Fn dnsres_gethostbyname2 is an advanced form of Fn gethostbyname which allows lookups in address families other than AF_INET Currently, the only supported address family besides AF_INET is AF_INET6

The Fn dnsres_gethostbyaddr function will search for the specified address of length Fa len in the address family Fa af . The only address family currently supported is AF_INET


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 Fn dnsres_gethostbyname , Fn dnsres_gethostbyname2 , and Fn 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.
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.
Some unexpected server failure was encountered. This is a non-recoverable error.
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.
The function completed successfully.


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


If the search routines in resolv.conf5 decide to read the /etc/hosts file, Fn gethostent and other functions will read the next line of the file, re-opening the file if necessary.

The Fn sethostent function opens and/or rewinds the file /etc/hosts If the Fa stayopen argument is non-zero, the file will not be closed after each call to Fn gethostbyname , Fn gethostbyname2 , or Fn gethostbyaddr .

The Fn endhostent function closes the file.


The dnsres library was hacked together by Niels Provos. Heavy use was made of the exisiting BSD resolver library.