GETHOSTBYNAME (3)
get network
host entry
SYNOPSIS
#include <netdb.h>
extern int h_errno;
struct hostent *gethostbyname(const char * name );
#include <sys/socket.h> " " /* for AF_INET */
struct hostent *gethostbyaddr(const char * addr , int len , int type );
void sethostent(int stayopen );
void endhostent(void);
void herror(const char * s );
const char * hstrerror(int err );
DESCRIPTION
The gethostbyname() function returns a structure of type hostent
for the given host name. Here name is either a host name, or
an IPv4 address in standard dot notation, or an IPv6 address in colon
(and possibly dot) notation. (See RFC 1884 for the description of IPv6
addresses.)
If
name
is an IPv4 or IPv6 address, no lookup is performed and
gethostbyname ()
simply copies
name
into the
h_name
field and its
struct in_addr
equivalent into the
h_addr_list[0]
field of the returned
hostent
structure.
If name doesn't end in a dot
and the environment variable HOSTALIASES is set, the alias file
pointed to by HOSTALIASES will first be searched for name.
(See
hostname (7)
for the file format.)
The current domain and its parents are searched unless name
ends in a dot.
The gethostbyaddr() function returns a structure of type hostent
for the given host address addr of length len and address type
type. The only valid address type is currently AF_INET.
The sethostent() function specifies, if stayopen is true (1),
that a connected TCP socket should be used for the name server queries and
that the connection should remain open during successive queries. Otherwise,
name server queries will use UDP datagrams.
The endhostent() function ends the use of a TCP connection for name
server queries.
The (obsolete) herror() function prints the error message associated
with the current value of h_errno on stderr.
The (obsolete) hstrerror() function takes an error number
(typically h_errno) and returns the corresponding message string.
The domain name queries carried out by gethostbyname() and
gethostbyaddr() use a combination of any or all of the name server
named(8), a broken out line from /etc/hosts, and the Network
Information Service (NIS or YP), depending upon the contents of the
order line in /etc/host.conf. (See
resolv+ (8)).
The default action is to query named(8), followed by
/etc/hosts.
The hostent structure is defined in <netdb.h> as follows:
7
8n 16n 32n
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 */
}
#define h_addr h_addr_list[0] /* for backward compatibility */
The members of the hostent structure are:
h_name
The official name of the host.
h_aliases
A zero-terminated array of alternative names for the host.
h_addrtype
The type of address; always AF_INET at present.
h_length
The length of the address in bytes.
h_addr_list
A zero-terminated array of network addresses for the host in network byte
order.
h_addr
The first address in h_addr_list for backward compatibility.
RETURN VALUE
The gethostbyname() and gethostbyaddr() functions return the
hostent structure or a NULL pointer if an error occurs. On error,
the h_errno variable holds an error number.
ERRORS
The variable h_errno can have the following values:
HOST_NOT_FOUND
The specified host is unknown.
NO_ADDRESS or NO_DATA
The requested name is valid but does not have an IP address.
NO_RECOVERY
A non-recoverable name server error occurred.
TRY_AGAIN
A temporary error occurred on an authoritative name server. Try again
later.
FILES
/etc/host.conf
resolver configuration file
/etc/hosts
CONFORMING TO
BSD 4.3.
The SUS-v2 declares the
len
parameter to be of type
size_t .
RUMOURS
The future glibc2.2 will follow SUS-v2.
SEE ALSO
- resolver (3) -
hosts(5) - hostname (7) -
resolv+(8) - named (8) -
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