lwres_getipnodebyaddr(3) lightweight resolver nodename / address translation API

Other Alias

lwres_getipnodebyname, lwres_freehostent


#include <lwres/netdb.h>

struct hostent * lwres_getipnodebyname(const char *name, int af, int flags, int *error_num);
struct hostent * lwres_getipnodebyaddr(const void *src, size_t len, int af, int *error_num);
void lwres_freehostent(struct hostent *he);


These functions perform thread safe, protocol independent nodename-to-address and address-to-nodename translation as defined in RFC2553.

They use a struct hostent which is 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:


The official (canonical) name of the host.


A NULL-terminated array of alternate names (nicknames) for the host.


The type of address being returned - usually PF_INET or PF_INET6.


The length of the address in bytes.


A NULL terminated array of network addresses for the host. Host addresses are returned in network byte order.

lwres_getipnodebyname() looks up addresses of protocol family af for the hostname name. The flags parameter contains ORed flag bits to specify the types of addresses that are searched for, and the types of addresses that are returned. The flag bits are:


This is used with an af of AF_INET6, and causes IPv4 addresses to be returned as IPv4-mapped IPv6 addresses.


This is used with an af of AF_INET6, and causes all known addresses (IPv6 and IPv4) to be returned. If AI_V4MAPPED is also set, the IPv4 addresses are return as mapped IPv6 addresses.


Only return an IPv6 or IPv4 address if here is an active network interface of that type. This is not currently implemented in the BIND 9 lightweight resolver, and the flag is ignored.


This default sets the AI_V4MAPPED and AI_ADDRCONFIG flag bits.

lwres_getipnodebyaddr() performs a reverse lookup of address src which is len bytes long. af denotes the protocol family, typically PF_INET or PF_INET6.

lwres_freehostent() releases all the memory associated with the struct hostent pointer he. Any memory allocated for the h_name, h_addr_list and h_aliases is freed, as is the memory for the hostent structure itself.


If an error occurs, lwres_getipnodebyname() and lwres_getipnodebyaddr() set *error_num to an appropriate error code and the function returns a NULL pointer. The error codes and their meanings are defined in <lwres/netdb.h>:


No such host is known.


The server recognised the request and the name but no address is available. Another type of request to the name server for the domain might return an answer.


A temporary and possibly transient error occurred, such as a failure of a server to respond. The request may succeed if retried.


An unexpected failure occurred, and retrying the request is pointless.

lwres_hstrerror(3) translates these error codes to suitable error messages.


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