GETHOSTBYNAME(3) Linux Programmer's Manual GETHOSTBYNAME(3)
NAME
gethostbyname, gethostbyaddr, sethostent, endhostent, herror, hstrerror
- get network host entry
SYNOPSIS
#include<netdb.h>
externinth_errno;
structhostent*gethostbyname(constchar*name);
#include<sys/socket.h>/*forAF_INET*/
structhostent*gethostbyaddr(constchar*addr,
int len,int type);
voidsethostent(int stayopen);
voidendhostent(void);
voidherror(constchar*s);
constchar*hstrerror(int err);
/* GNU extensions */
structhostent*gethostbyname2(constchar*name,int af);
intgethostbyname_r(constchar*name,
structhostent*ret,char*buf,size_t buflen,
structhostent**result,int*h_errnop);
intgethostbyname2_r(constchar*name,int af,
structhostent*ret,char*buf,size_t buflen,
structhostent**result,int*h_errnop);
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 pos-
sibly 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. Oth-
erwise, 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 gethost-
byaddr() 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:
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.
RETURNVALUE
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
host database file
CONFORMINGTO
BSD 4.3.
NOTES
The SUS-v2 standard is buggy and declares the len parameter of gethost-
byaddr() to be of type size_t. (That is wrong, because it has to be
int, and size_t is not. POSIX 1003.1-2001 makes it socklen_t, which is
OK.)
The functions gethostbyname() and gethostbyaddr() may return pointers
to static data, which may be overwritten by later calls. Copying the
struct hostent does not suffice, since it contains pointers - a deep
copy is required.
Glibc2 also has a gethostbyname2() that works like gethostbyname(), but
permits to specify the address family to which the address must belong.
Glibc2 also has reentrant versions gethostbyname_r() and gethostby-
name2_r(). These return 0 on success and nonzero on error. The result
of the call is now stored in the struct with address ret. After the
call, *result will be NULL on error or point to the result on success.
Auxiliary data is stored in the buffer buf of length buflen. (If the
buffer is too small, these functions will return ERANGE.) No global
variable h_errno is modified, but the address of a variable in which to
store error numbers is passed in h_errnop.
POSIX 1003.1-2001 marks gethostbyaddr() and gethostbyname() legacy, and
introduces
structhostent*getipnodebyaddr(constvoid*restrict addr,
socklen_t len,int type,int*restrict error_num);
structhostent*getipnodebyname(constchar*name,
int type,int flags,int*error_num);
SEEALSO
resolver(3), hosts(5), hostname(7), resolv+(8), named(8)
BSD 2000-08-12 GETHOSTBYNAME(3)
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