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NAME="NET-COMMON-TCPIP-MANPAGES-GETHOSTBYNAME">gethostbyname</H1
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>GETHOSTBYNAME(3) System Library Functions Manual GETHOSTBYNAME(3)
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NAME
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gethostbyname, gethostbyname2, gethostbyaddr, gethostent, sethostent,
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endhostent, hstrerror, herror - get network host entry
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SYNOPSIS
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#include <netdb.h>
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extern int h_errno;
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struct hostent *
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gethostbyname(const char *name);
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struct hostent *
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gethostbyname2(const char *name, int af);
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struct hostent *
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gethostbyaddr(const char *addr, int len, int af);
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struct hostent *
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gethostent(void);
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void
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sethostent(int stayopen);
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void
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endhostent(void);
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void
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herror(const char *string);
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const char *
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hstrerror(int err);
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DESCRIPTION
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The gethostbyname() and gethostbyaddr() functions each return a pointer
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to an object with the following structure describing an internet host
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referenced by name or by address, respectively. This structure contains
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either information obtained from the name server (i.e., resolver(3) and
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named(8)), broken-out fields from a line in /etc/hosts, or database
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entries supplied by the yp(8) system. resolv.conf(5) describes how the
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particular database is chosen.
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struct hostent {
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char *h_name; /* official name of host */
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char **h_aliases; /* alias list */
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int h_addrtype; /* host address type */
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int h_length; /* length of address */
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char **h_addr_list; /* list of addresses from name server */
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};
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#define h_addr h_addr_list[0] /* address, for backward compatibility */
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The members of this structure are:
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h_name Official name of the host.
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h_aliases A zero-terminated array of alternate names for the host.
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h_addrtype The type of address being returned.
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h_length The length, in bytes, of the address.
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h_addr_list A zero-terminated array of network addresses for the host.
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Host addresses are returned in network byte order.
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h_addr The first address in h_addr_list; this is for backward com-
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patibility.
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The function gethostbyname() will search for the named host in the cur-
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rent domain and its parents using the search lookup semantics detailed in
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resolv.conf(5) and hostname(7).
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gethostbyname2() is an advanced form of gethostbyname() which allows
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lookups in address families other than AF_INET, for example AF_INET6.
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The gethostbyaddr() function will search for the specified address of
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length len in the address family af. The only address family currently
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supported is AF_INET.
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The sethostent() function may be used to request the use of a connected
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TCP socket for queries. If the stayopen flag is non-zero, this sets the
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option to send all queries to the name server using TCP and to retain the
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connection after each call to gethostbyname() or gethostbyaddr(). Other-
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wise, queries are performed using UDP datagrams.
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The endhostent() function closes the TCP connection.
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The herror() function prints an error message describing the failure. If
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its argument string is non-null, it is prepended to the message string
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and separated from it by a colon (`:') and a space. The error message is
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printed with a trailing newline. The contents of the error message is
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the same as that returned by hstrerror() with argument h_errno.
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FILES
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/etc/hosts
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/etc/resolv.conf
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DIAGNOSTICS
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Error return status from gethostbyname(), gethostbyname2(), and
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gethostbyaddr() is indicated by return of a null pointer. The external
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integer h_errno may then be checked to see whether this is a temporary
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failure or an invalid or unknown host.
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The variable h_errno can have the following values:
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HOST_NOT_FOUND No such host is known.
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TRY_AGAIN This is usually a temporary error and means that the
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local server did not receive a response from an authori-
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tative server. A retry at some later time may succeed.
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NO_RECOVERY Some unexpected server failure was encountered. This is
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a non-recoverable error.
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NO_DATA The requested name is valid but does not have an IP
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address; this is not a temporary error. This means that
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the name is known to the name server but there is no
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address associated with this name. Another type of
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request to the name server using this domain name will
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result in an answer; for example, a mail-forwarder may be
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registered for this domain.
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SEE ALSO
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resolver(3), getaddrinfo(3), getnameinfo(3), hosts(5), resolv.conf(5),
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hostname(7), named(8)
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CAVEAT
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If the search routines in resolv.conf(5) decide to read the /etc/hosts
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file, gethostent() and other functions will read the next line of the
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file, re-opening the file if necessary.
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The sethostent() function opens and/or rewinds the file /etc/hosts. If
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the stayopen argument is non-zero, the file will not be closed after each
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call to gethostbyname(), gethostbyname2(), or gethostbyaddr().
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The endhostent() function closes the file.
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HISTORY
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The herror() function appeared in 4.3BSD. The endhostent(),
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gethostbyaddr(), gethostbyname(), gethostent(), and sethostent() func-
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tions appeared in 4.2BSD.
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BUGS
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These functions use static data storage; if the data is needed for future
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use, it should be copied before any subsequent calls overwrite it. Only
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the Internet address formats are currently understood.
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YP does not support any address families other than AF_INET and uses the
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traditional database format.
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BSD March 13, 1997 BSD
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