gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent, herror, hstrerror — get network host entry
#include <netdb.h> extern int h_errno;
struct hostent *gethostbyname( |
const char * | name); |
#include <sys/socket.h> /* for AF_INET */
struct hostent *gethostbyaddr( |
const void * | addr, |
| int | len, | |
| int | type); |
void
sethostent( |
int | stayopen); |
void
endhostent( |
void); |
void
herror( |
const char * | s); |
const
char *hstrerror( |
int | err); |
/* System V/POSIX extension */
struct hostent *gethostent( |
void); |
/* GNU extensions */
struct hostent *gethostbyname2( |
const char * | name, |
| int | af); |
int
gethostent_r( |
struct hostent * | ret, |
| char * | buf, | |
| size_t | buflen, | |
| struct hostent ** | result, | |
| int * | h_errnop); |
int
gethostbyname_r( |
const char * | name, |
| struct hostent * | ret, | |
| char * | buf, | |
| size_t | buflen, | |
| struct hostent ** | result, | |
| int * | h_errnop); |
int
gethostbyname2_r( |
const char * | name, |
| int | af, | |
| struct hostent * | ret, | |
| char * | buf, | |
| size_t | buflen, | |
| struct hostent ** | result, | |
| int * | h_errnop); |
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.
Valid address types are AF_INET
and AF_INET6. The host address
argument is a pointer to a struct of a type depending on the
address type, for example a struct
in_addr * (probably obtained via a call to
inet_addr(3)) for address
type 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. 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_nameThe official name of the host.
h_aliasesAn array of alternative names for the host, terminated by a NULL pointer.
h_addrtypeThe type of address; always AF_INET or AF_INET6 at present.
h_lengthThe length of the address in bytes.
h_addr_listAn array of pointers to network addresses for the host (in network byte order), terminated by a NULL pointer.
h_addrThe first address in h_addr_list for
backward compatibility.
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. When non-NULL, the return value may
point at static data, see the notes below.
The variable h_errno can have the
following values:
HOST_NOT_FOUNDThe specified host is unknown.
NO_ADDRESS or NO_DATAThe requested name is valid but does not have an IP address.
NO_RECOVERYA non-recoverable name server error occurred.
TRY_AGAINA temporary error occurred on an authoritative name server. Try again later.
/etc/host.confresolver configuration file
/etc/hostshost database file
/etc/nsswitch.confname service switch configuration
POSIX requires the gethostent() call, that should return the
next entry in the host data base. When using DNS/BIND this
does not make much sense, but it may be reasonable if the
host data base is a file that can be read line by line. On
many systems a routine of this name reads from the file
/etc/hosts. It may be available
only when the library was built without DNS support. The
glibc version will ignore ipv6 entries. This function is not
reentrant, and glibc adds a reentrant version gethostent_r().
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 gethostbyname2_r(). These return 0 on
success and non-zero 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.
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.
The SUS-v2 standard is buggy and declares the len parameter of gethostbyaddr() to be of type size_t. (That is wrong,
because it has to be int, and size_t is not. POSIX.1-2001
makes it socklen_t,
which is OK.)
The BSD prototype for gethostbyaddr() uses const char * for the first
argument.
POSIX.1-2001 marks gethostbyaddr() and gethostbyname() obsolescent. See getaddrinfo(3), getnameinfo(3), gai_strerror(3).
getaddrinfo(3), getipnodebyaddr(3), getipnodebyname(3), getnameinfo(3), inet_ntop(3), inet_pton(3), resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)
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