getipnodebyname const char *name int af int flags int *error getipnodebyaddr const void *addr size_t len int af int *error freehostent struct hostent *he

and getipnodebyaddr each return a pointer to a structure (see below) describing an internet host referenced by name or by address, as the function names indicate. This structure contains either the information obtained from the name server, named(8) , or broken-out fields from a line in /etc/hosts . If the local name server is not running, these routines do a lookup in /etc/hosts . 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:

h_name

Official name of the host.

h_aliases

A zero-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.

This structure should be freed after use by calling freehostent .

When using the nameserver, getiphostbyaddr will search for the named host in each parent domain given in the directive of resolv.conf() unless the name contains a dot If the name contains no dot, and if the environment variable HOSTALIASES contains the name of an alias file, the alias file will first be searched for an alias matching the input name. See hostname() for the domain search procedure and the alias file format.

Getiphostbyaddr can be told to look for IPv4 addresses, IPv6 addresses or both IPv4 and IPv6. If IPv4 addresses only are to be looked up then af should be set to AF_INET , otherwise it should be set to AF_INET6 .

There are three flags that can be set

AI_V4MAPPED

Return IPv4 addresses if no IPv6 addresses are found. This flag is ignored unless af is AF_INET6 .

AI_ALL

Return IPv4 addresses as well IPv6 addresses if AI_V4MAPPED is set. This flag is ignored unless af is AF_INET6 .

AI_ADDRCONFIG

Only return addresses of a given type if the system has an active interface with that type.

Also AI_DEFAULT is defined to be (AI_V4MAPPED|AI_ADDRCONFIG) .

Getipnodebyaddr will lookup IPv4 mapped and compatible addresses in the IPv4 name space and IPv6 name space

Freehostent frees the hostent structure allocated be getipnodebyname and getipnodebyaddr . The structures returned by gethostbyname , gethostbyname2 , gethostbyaddr and gethostent should not be passed to freehostent as they are pointers to static areas.

HOSTALIASES

Name of file containing pairs.

/etc/hosts

See hosts(5) .

Error return status from getipnodebyname and getipnodebyaddr is indicated by return of a null pointer. In this case may then be checked to see whether this is a temporary failure or an invalid or unknown host. can have the following values:

NETDB_INTERNAL

This indicates an internal error in the library, unrelated to the network or name service. will be valid in this case; see perror(2) .

HOST_NOT_FOUND

No such host is known.

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.

NO_RECOVERY

Some unexpected server failure was encountered. This is a non-recoverable error, as one might expect.

NO_ADDRESS

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.