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GETHOSTBYNAME(3) FreeBSD Library Functions Manual GETHOSTBYNAME(3)
NAME
gethostbyname, gethostbyname2, gethostbyaddr, gethostent, sethostent,
endhostent, herror, hstrerror, gethostbyname_r, gethostbyname2_r,
gethostbyaddr_r - get network host entry
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <netdb.h>
int h_errno;
struct hostent *
gethostbyname(const char *name);
struct hostent *
gethostbyname2(const char *name, int af);
struct hostent *
gethostbyaddr(const void *addr, socklen_t len, int af);
struct hostent *
gethostent(void);
void
sethostent(int stayopen);
void
endhostent(void);
void
herror(const char *string);
const char *
hstrerror(int err);
int
gethostbyname_r(const char *name, struct hostent *he, char *buffer,
size_t buflen, struct hostent **result, int *h_errnop);
int
gethostbyname2_r(const char *name, int af, struct hostent *he,
char *buffer, size_t buflen, struct hostent **result, int *h_errnop);
int
gethostbyaddr_r(const void *addr, socklen_t len, int af,
struct hostent *hp, char *buf, size_t buflen,
struct hostent **result, int *h_errno, p);
DESCRIPTION
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 structure returned contains either the information obtained from the
name server, 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. Queries will by default use UDP datagrams. If
the stayopen flag is non-zero, a TCP connection to the name server will
be used. It will remain open after calls to gethostbyname(),
gethostbyname2() or gethostbyaddr() have completed.
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.
Functions with the _r suffix provide reentrant versions of their
FILES
/etc/hosts
/etc/nsswitch.conf
/etc/resolv.conf
EXAMPLES
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 void *)&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);
DIAGNOSTICS
Error return status from gethostbyname(), gethostbyname2() and
gethostbyaddr() is indicated by return of a NULL pointer. The 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.
SEE ALSO
getaddrinfo(3), getnameinfo(3), inet_aton(3), resolver(3), hosts(5),
hostname(7)
HISTORY
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. The gethostbyname_r() function first
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.
BUGS
These functions use a thread-specific data storage; if the data is needed
for future use, it should be copied before any subsequent calls overwrite
it.
Though these functions are thread-safe, still it is recommended to use
the getaddrinfo(3) family of functions, instead.
Only the Internet address format is currently understood.
FreeBSD 14.0-RELEASE-p6 June 27, 2022 FreeBSD 14.0-RELEASE-p6