.\" $OpenBSD: gethostbyname.3,v 1.25 2007/05/31 19:19:30 jmc Exp $ .\" .\" Copyright (c) 1983, 1987, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .Dd $Mdocdate: May 31 2007 $ .Dt GETHOSTBYNAME 3 .Os .Sh NAME .Nm gethostbyname , .Nm gethostbyname2 , .Nm gethostbyaddr , .Nm gethostent , .Nm sethostent , .Nm endhostent , .Nm hstrerror , .Nm herror .Nd get network host entry .Sh SYNOPSIS .Fd #include .Vt extern int h_errno ; .Ft struct hostent * .Fn gethostbyname "const char *name" .Ft struct hostent * .Fn gethostbyname2 "const char *name" "int af" .Ft struct hostent * .Fn gethostbyaddr "const void *addr" "socklen_t len" "int af" .Ft struct hostent * .Fn gethostent void .Ft void .Fn sethostent "int stayopen" .Ft void .Fn endhostent void .Ft void .Fn herror "const char *string" .Ft const char * .Fn hstrerror "int err" .Sh DESCRIPTION The .Fn gethostbyname , .Fn gethostbyname2 , and .Fn gethostbyaddr functions each return a pointer to an object with the following structure describing an Internet host referenced by name or by address, respectively. This structure contains either information obtained from the name server (i.e., .Xr resolver 3 and .Xr named 8 ) , broken-out fields from a line in .Pa /etc/hosts , or database entries supplied by the .Xr yp 8 system. .Xr resolv.conf 5 describes how the particular database is chosen. .Bd -literal -offset indent 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 returned addresses */ }; #define h_addr h_addr_list[0] /* address, for backward compat */ .Ed .Pp The members of this structure are: .Bl -tag -width h_addr_list .It Fa h_name Official name of the host. .It Fa h_aliases A null-terminated array of alternate names for the host. .It Fa h_addrtype The type of address being returned. .It Fa h_length The length, in bytes, of the address. .It Fa h_addr_list A null-terminated array of network addresses for the host. Host addresses are returned in network byte order. .It Fa h_addr The first address in .Fa h_addr_list ; this is for backward compatibility. .El .Pp The function .Fn gethostbyname will search for the named host in the current domain and its parents using the search lookup semantics detailed in .Xr resolv.conf 5 and .Xr hostname 7 . .Pp .Fn gethostbyname2 is an advanced form of .Fn gethostbyname which allows lookups in address families other than .Dv AF_INET . Currently, the only supported address family besides .Dv AF_INET is .Dv AF_INET6 . .Pp The .Fn gethostbyaddr function will search for the specified address of length .Fa len in the address family .Fa af . The only address family currently supported is .Dv AF_INET . .Pp The .Fn sethostent function may be used to request the use of a connected .Tn TCP socket for queries. If the .Fa stayopen flag is non-zero, this sets the option to send all queries to the name server using .Tn TCP and to retain the connection after each call to .Fn gethostbyname or .Fn gethostbyaddr . Otherwise, queries are performed using .Tn UDP datagrams. .Pp The .Fn endhostent function closes the .Tn TCP connection. .Pp The .Fn herror function prints an error message describing the failure. If its argument .Fa string is non-null, it is prepended to the message string and separated from it by a colon .Pq Ql \&: and a space. The error message is printed with a trailing newline. The contents of the error message is the same as that returned by .Fn hstrerror with argument .Fa h_errno . .Sh ENVIRONMENT .Bl -tag -width HOSTALIASES .It HOSTALIASES A file containing local host aliases. See .Xr hostname 7 for more information. .It RES_OPTIONS A list of options to override the resolver's internal defaults. See .Xr resolver 3 for more information. .El .Sh FILES .Bl -tag -width /etc/resolv.conf -compact .It Pa /etc/hosts .It Pa /etc/resolv.conf .El .Sh DIAGNOSTICS Error return status from .Fn gethostbyname , .Fn gethostbyname2 , and .Fn gethostbyaddr is indicated by return of a null pointer. The external integer .Va h_errno may then be checked to see whether this is a temporary failure or an invalid or unknown host. .Pp The variable .Va h_errno can have the following values: .Bl -tag -width HOST_NOT_FOUND .It Dv HOST_NOT_FOUND No such host is known. .It Dv 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. .It Dv NO_RECOVERY Some unexpected server failure was encountered. This is a non-recoverable error. .It Dv 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. .It Dv NETDB_INTERNAL An internal error occurred. This may occur when an address family other than .Dv AF_INET or .Dv AF_INET6 is specified or when a resource is unable to be allocated. .It Dv NETDB_SUCCESS The function completed successfully. .El .Sh SEE ALSO .Xr getaddrinfo 3 , .Xr getnameinfo 3 , .Xr resolver 3 , .Xr hosts 5 , .Xr resolv.conf 5 , .Xr hostname 7 , .Xr named 8 .Sh HISTORY The .Fn herror function appeared in .Bx 4.3 . The .Fn endhostent , .Fn gethostbyaddr , .Fn gethostbyname , .Fn gethostent , and .Fn sethostent functions appeared in .Bx 4.2 . .Sh CAVEATS If the search routines in .Xr resolv.conf 5 decide to read the .Pa /etc/hosts file, .Fn gethostent and other functions will read the next line of the file, re-opening the file if necessary. .Pp The .Fn sethostent function opens and/or rewinds the file .Pa /etc/hosts . If the .Fa stayopen argument is non-zero, the file will not be closed after each call to .Fn gethostbyname , .Fn gethostbyname2 , or .Fn gethostbyaddr . .Pp The .Fn endhostent function closes the file. .Sh BUGS These functions use static data storage; if the data is needed for future use, it should be copied before any subsequent calls overwrite it. .Pp Only the Internet address formats are currently understood. .Pp YP does not support any address families other than .Dv AF_INET and uses the traditional database format.