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GETHOSTBYNAME(3)
Contents
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gethostbyname, gethostbyname2, gethostbyaddr, gethostent, sethostent,
endhostent, herror, hstrerror -- get network host entry
Standard C Library (libc, -lc)
#include <netdb.h>
extern int h_errno;
struct hostent *
gethostbyname(const char *name);
struct hostent *
gethostbyname2(const char *name, int af);
struct hostent *
gethostbyaddr(const char *addr, int len, int type);
struct hostent *
gethostent(void);
void
sethostent(int stayopen);
void
endhostent(void);
void
herror(const char *string);
const char *
hstrerror(int err);
The getaddrinfo(3) and getnameinfo(3) functions are preferred over the
gethostbyname(), gethostbyname2(), and gethostbyaddr() functions.
The gethostbyname(), gethostbyname2() and gethostbyaddr() functions each
return a pointer to an object with the following structure describing an
internet host referenced by name or by address, respectively.
The name argument passed to gethostbyname() or gethostbyname2() should
point to a NUL-terminated hostname. The addr argument passed to
gethostbyaddr() should point to an address which is len bytes long, in
binary form (i.e. not an IP address in human readable ASCII form). The
type argument specifies the address family (e.g. AF_INET, AF_INET6, etc.)
of this address.
The structure returned contains either the information obtained from the
name server, named(8), broken-out fields from a line in /etc/hosts, or
database entries supplied by the yp(8) system. The order of the lookups
is controlled by the `hosts' entry in nsswitch.conf(5).
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 NULL-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 NULL-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() and gethostbyname2() will
search for the named host in the current domain and its parents unless
the name ends in 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(7) for the domain search procedure and the alias file format.
The gethostbyname2() function is an evolution of gethostbyname() which is
intended to allow lookups in address families other than AF_INET, for
example AF_INET6.
The sethostent() function 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(), gethostbyname2() or
gethostbyaddr(). Otherwise, queries are performed using UDP datagrams.
The endhostent() function closes the TCP connection.
The herror() function writes a message to the diagnostic output consisting
of the string argument string, the constant string ": ", and a message
corresponding to the value of h_errno.
The hstrerror() function returns a string which is the message text corresponding
to the value of the err argument.
/etc/hosts
/etc/nsswitch.conf
/etc/resolv.conf
Print out the hostname associated with a specific IP address:
const char *ipstr = "127.0.0.1";
struct in_addr ip;
struct hostent *hp;
if (!inet_aton(ipstr, &ip))
errx(1, "can't parse IP address %s", ipstr);
if ((hp = gethostbyaddr((const char *)&ip,
sizeof ip, AF_INET)) == NULL)
errx(1, "no name associated with %s", ipstr);
printf("name associated with %s is %s\n", ipstr, hp->h_name);
Error return status from gethostbyname(), gethostbyname2() and
gethostbyaddr() is indicated by return of a NULL pointer. The external
integer h_errno 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.
The variable h_errno can have the following values:
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.
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.
getaddrinfo(3), getnameinfo(3), inet_aton(3), resolver(3), hosts(5),
hostname(7), named(8)
The gethostent() function is defined, and sethostent() and endhostent()
are redefined, when Standard C Library (libc, -lc) is built to use only
the routines to lookup in /etc/hosts and not the name server.
The gethostent() function reads the next line of /etc/hosts, opening the
file if necessary.
The sethostent() function opens and/or rewinds the file /etc/hosts. If
the stayopen argument is non-zero, the file will not be closed after each
call to gethostbyname(), gethostbyname2() or gethostbyaddr().
The endhostent() function closes the file.
The herror() function appeared in 4.3BSD. The endhostent(),
gethostbyaddr(), gethostbyname(), gethostent(), and sethostent() functions
appeared in 4.2BSD. The gethostbyname2() function first appeared
in BIND version 4.9.4.
These functions use static data storage; if the data is needed for future
use, it should be copied before any subsequent calls overwrite it. Only
the Internet address format is currently understood.
The gethostbyname2() function cannot perform AF_INET6 lookups over NIS.
The getaddrinfo(3) function must be used instead.
FreeBSD 5.2.1 May 25, 1995 FreeBSD 5.2.1 [ Back ] |
