gethostbyname char *name gethostbyname2 char *name int af gethostbyaddr char *addr int len, type gethostent sethostent int stayopen endhostent herror char *string

and gethostbyaddr 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; usually AF_INET .

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.

When using the nameserver, gethostbyname 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.

Gethostbyname2 is an evolution of gethostbyname intended to allow lookups in address families other than AF_INET , for example, AF_INET6 . Currently, the af argument must be specified as AF_INET else the function will return NULL after having set to NETDB_INTERNAL .

Sethostent may be used to request the use of a connected TCP socket for queries. If the stayopen flag is non-zero, this sets the option to send all queries to the name server using TCP and to retain the connection after each call to gethostbyname or gethostbyaddr . Otherwise, queries are performed using UDP datagrams.

Endhostent closes the TCP connection.

HOSTALIASES

Name of file containing pairs.

/etc/hosts

See hosts(5) .

Error return status from gethostbyname and gethostbyaddr is indicated by return of a null pointer. The external integer may then be checked to see whether this is a temporary failure or an invalid or unknown host. The routine herror can be used to print an error message describing the failure. If its argument string is non-NULL, it is printed, followed by a colon and a space. The error message is printed with a trailing newline.

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_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.

Gethostent is defined, and sethostent and endhostent are redefined, when libc is built to use only the routines to lookup in /etc/hosts and not the name server:

Gethostent reads the next line of /etc/hosts , opening the file if necessary.

Sethostent is redefined to open and rewind the file. If the stayopen argument is non-zero, the hosts data base will not be closed after each call to gethostbyname or gethostbyaddr .

Endhostent is redefined to close the file.

All information is contained in a static area so it must be copied if it is to be saved. Only the Internet address format is currently understood.