462 lines
11 KiB
Groff
462 lines
11 KiB
Groff
.\" Copyright (c) 1985, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)resolver.3 8.1 (Berkeley) 6/4/93
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.\" $FreeBSD$
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.\"
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.Dd May 29, 2009
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.Dt RESOLVER 3
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.Os
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.Sh NAME
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.Nm res_query ,
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.Nm res_search ,
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.Nm res_mkquery ,
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.Nm res_send ,
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.Nm res_init ,
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.Nm dn_comp ,
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.Nm dn_expand ,
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.Nm dn_skipname ,
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.Nm ns_get16 ,
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.Nm ns_get32 ,
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.Nm ns_put16 ,
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.Nm ns_put32
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.Nd resolver routines
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/types.h
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.In netinet/in.h
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.In arpa/nameser.h
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.In resolv.h
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.Ft int
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.Fo res_query
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.Fa "const char *dname"
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.Fa "int class"
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.Fa "int type"
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.Fa "u_char *answer"
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.Fa "int anslen"
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.Fc
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.Ft int
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.Fo res_search
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.Fa "const char *dname"
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.Fa "int class"
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.Fa "int type"
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.Fa "u_char *answer"
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.Fa "int anslen"
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.Fc
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.Ft int
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.Fo res_mkquery
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.Fa "int op"
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.Fa "const char *dname"
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.Fa "int class"
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.Fa "int type"
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.Fa "const u_char *data"
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.Fa "int datalen"
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.Fa "const u_char *newrr_in"
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.Fa "u_char *buf"
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.Fa "int buflen"
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.Fc
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.Ft int
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.Fo res_send
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.Fa "const u_char *msg"
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.Fa "int msglen"
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.Fa "u_char *answer"
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.Fa "int anslen"
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.Fc
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.Ft int
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.Fn res_init void
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.Ft int
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.Fo dn_comp
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.Fa "const char *exp_dn"
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.Fa "u_char *comp_dn"
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.Fa "int length"
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.Fa "u_char **dnptrs"
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.Fa "u_char **lastdnptr"
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.Fc
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.Ft int
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.Fo dn_expand
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.Fa "const u_char *msg"
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.Fa "const u_char *eomorig"
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.Fa "const u_char *comp_dn"
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.Fa "char *exp_dn"
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.Fa "int length"
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.Fc
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.Ft int
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.Fn dn_skipname "const u_char *comp_dn" "const u_char *eom"
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.Ft u_int
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.Fn ns_get16 "const u_char *src"
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.Ft u_long
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.Fn ns_get32 "const u_char *src"
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.Ft void
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.Fn ns_put16 "u_int src" "u_char *dst"
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.Ft void
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.Fn ns_put32 "u_long src" "u_char *dst"
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.Sh DESCRIPTION
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These routines are used for making, sending and interpreting
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query and reply messages with Internet domain name servers.
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.Pp
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Global configuration and state information that is used by the
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resolver routines is kept in the structure
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.Va _res .
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Most of the values have reasonable defaults and can be ignored.
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Options
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stored in
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.Va _res.options
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are defined in
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.In resolv.h
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and are as follows.
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Options are stored as a simple bit mask containing the bitwise ``or''
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of the options enabled.
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.Bl -tag -width RES_USE_INET6
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.It Dv RES_INIT
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True if the initial name server address and default domain name are
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initialized (i.e.,
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.Fn res_init
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has been called).
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.It Dv RES_DEBUG
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Print debugging messages.
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.It Dv RES_AAONLY
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Accept authoritative answers only.
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With this option,
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.Fn res_send
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should continue until it finds an authoritative answer or finds an error.
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Currently this is not implemented.
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.It Dv RES_USEVC
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Use
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.Tn TCP
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connections for queries instead of
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.Tn UDP
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datagrams.
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.It Dv RES_STAYOPEN
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Used with
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.Dv RES_USEVC
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to keep the
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.Tn TCP
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connection open between
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queries.
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This is useful only in programs that regularly do many queries.
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.Tn UDP
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should be the normal mode used.
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.It Dv RES_IGNTC
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Unused currently (ignore truncation errors, i.e., do not retry with
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.Tn TCP ) .
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.It Dv RES_RECURSE
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Set the recursion-desired bit in queries.
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This is the default.
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.Pf ( Fn res_send
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does not do iterative queries and expects the name server
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to handle recursion.)
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.It Dv RES_DEFNAMES
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If set,
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.Fn res_search
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will append the default domain name to single-component names
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(those that do not contain a dot).
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This option is enabled by default.
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.It Dv RES_DNSRCH
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If this option is set,
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.Fn res_search
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will search for host names in the current domain and in parent domains; see
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.Xr hostname 7 .
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This is used by the standard host lookup routine
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.Xr gethostbyname 3 .
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This option is enabled by default.
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.It Dv RES_NOALIASES
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This option turns off the user level aliasing feature controlled by the
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.Dq Ev HOSTALIASES
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environment variable.
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Network daemons should set this option.
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.It Dv RES_USE_INET6
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Enables support for IPv6-only applications.
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This causes IPv4 addresses to be returned as an IPv4 mapped address.
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For example,
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.Li 10.1.1.1
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will be returned as
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.Li ::ffff:10.1.1.1 .
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The option is meaningful with certain kernel configuration only.
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.It Dv RES_USE_EDNS0
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Enables support for OPT pseudo-RR for EDNS0 extension.
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With the option, resolver code will attach OPT pseudo-RR into DNS queries,
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to inform of our receive buffer size.
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The option will allow DNS servers to take advantage of non-default receive
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buffer size, and to send larger replies.
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DNS query packets with EDNS0 extension is not compatible with
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non-EDNS0 DNS servers.
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.El
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.Pp
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The
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.Fn res_init
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routine
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reads the configuration file (if any; see
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.Xr resolver 5 )
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to get the default domain name,
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search list and
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the Internet address of the local name server(s).
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If no server is configured, the host running
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the resolver is tried.
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The current domain name is defined by the hostname
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if not specified in the configuration file;
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it can be overridden by the environment variable
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.Ev LOCALDOMAIN .
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This environment variable may contain several blank-separated
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tokens if you wish to override the
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.Em "search list"
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on a per-process basis.
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This is similar to the
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.Ic search
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command in the configuration file.
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Another environment variable
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.Dq Ev RES_OPTIONS
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can be set to
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override certain internal resolver options which are otherwise
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set by changing fields in the
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.Va _res
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structure or are inherited from the configuration file's
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.Ic options
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command.
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The syntax of the
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.Dq Ev RES_OPTIONS
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environment variable is explained in
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.Xr resolver 5 .
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Initialization normally occurs on the first call
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to one of the following routines.
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.Pp
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The
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.Fn res_query
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function provides an interface to the server query mechanism.
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It constructs a query, sends it to the local server,
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awaits a response, and makes preliminary checks on the reply.
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The query requests information of the specified
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.Fa type
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and
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.Fa class
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for the specified fully-qualified domain name
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.Fa dname .
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The reply message is left in the
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.Fa answer
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buffer with length
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.Fa anslen
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supplied by the caller.
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.Pp
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The
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.Fn res_search
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routine makes a query and awaits a response like
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.Fn res_query ,
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but in addition, it implements the default and search rules
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controlled by the
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.Dv RES_DEFNAMES
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and
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.Dv RES_DNSRCH
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options.
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It returns the first successful reply.
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.Pp
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The remaining routines are lower-level routines used by
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.Fn res_query .
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The
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.Fn res_mkquery
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function
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constructs a standard query message and places it in
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.Fa buf .
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It returns the size of the query, or \-1 if the query is
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larger than
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.Fa buflen .
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The query type
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.Fa op
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is usually
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.Dv QUERY ,
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but can be any of the query types defined in
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.In arpa/nameser.h .
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The domain name for the query is given by
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.Fa dname .
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The
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.Fa newrr_in
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argument
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is currently unused but is intended for making update messages.
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.Pp
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The
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.Fn res_send
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routine
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sends a pre-formatted query and returns an answer.
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It will call
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.Fn res_init
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if
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.Dv RES_INIT
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is not set, send the query to the local name server, and
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handle timeouts and retries.
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The length of the reply message is returned, or
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\-1 if there were errors.
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.Pp
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The
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.Fn dn_comp
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function
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compresses the domain name
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.Fa exp_dn
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and stores it in
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.Fa comp_dn .
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The size of the compressed name is returned or \-1 if there were errors.
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The size of the array pointed to by
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.Fa comp_dn
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is given by
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.Fa length .
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The compression uses
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an array of pointers
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.Fa dnptrs
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to previously-compressed names in the current message.
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The first pointer points to
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the beginning of the message and the list ends with
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.Dv NULL .
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The limit to the array is specified by
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.Fa lastdnptr .
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A side effect of
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.Fn dn_comp
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is to update the list of pointers for
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labels inserted into the message
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as the name is compressed.
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If
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.Fa dnptr
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is
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.Dv NULL ,
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names are not compressed.
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If
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.Fa lastdnptr
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is
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.Dv NULL ,
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the list of labels is not updated.
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.Pp
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The
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.Fn dn_expand
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entry
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expands the compressed domain name
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.Fa comp_dn
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to a full domain name
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The compressed name is contained in a query or reply message;
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.Fa msg
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is a pointer to the beginning of the message.
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The uncompressed name is placed in the buffer indicated by
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.Fa exp_dn
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which is of size
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.Fa length .
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The size of compressed name is returned or \-1 if there was an error.
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.Pp
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The
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.Fn dn_skipname
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function skips over a compressed domain name, which starts at a location
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pointed to by
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.Fa comp_dn .
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The compressed name is contained in a query or reply message;
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.Fa eom
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is a pointer to the end of the message.
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The size of compressed name is returned or \-1 if there was
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an error.
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.Pp
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The
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.Fn ns_get16
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function gets a 16-bit quantity from a buffer pointed to by
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.Fa src .
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.Pp
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The
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.Fn ns_get32
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function gets a 32-bit quantity from a buffer pointed to by
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.Fa src .
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.Pp
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The
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.Fn ns_put16
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function puts a 16-bit quantity
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.Fa src
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to a buffer pointed to by
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.Fa dst .
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.Pp
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The
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.Fn ns_put32
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function puts a 32-bit quantity
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.Fa src
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to a buffer pointed to by
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.Fa dst .
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.Sh IMPLEMENTATION NOTES
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This implementation of the resolver is thread-safe, but it will not
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function properly if the programmer attempts to declare his or her own
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.Va _res
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structure in an attempt to replace the per-thread version referred to
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by that macro.
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.Pp
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The following compile-time option can be specified to change the default
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behavior of resolver routines when necessary.
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.Bl -tag -width RES_ENFORCE_RFC1034
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.It Dv RES_ENFORCE_RFC1034
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If this symbol is defined during compile-time,
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.Fn res_search
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will enforce RFC 1034 check, namely, disallow using of underscore character
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within host names.
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This is used by the standard host lookup routines like
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.Xr gethostbyname 3 .
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For compatibility reasons this option is not enabled by default.
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.El
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.Sh RETURN VALUES
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The
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.Fn res_init
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function will return 0 on success, or \-1 in a threaded program if
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per-thread storage could not be allocated.
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.Pp
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The
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.Fn res_mkquery ,
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.Fn res_search ,
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and
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.Fn res_query
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functions return the size of the response on success, or \-1 if an
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error occurs.
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The integer
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.Vt h_errno
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may be checked to determine the reason for error.
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See
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.Xr gethostbyname 3
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for more information.
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.Sh FILES
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.Bl -tag -width /etc/resolv.conf
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.It Pa /etc/resolv.conf
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The configuration file,
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see
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.Xr resolver 5 .
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.El
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.Sh SEE ALSO
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.Xr gethostbyname 3 ,
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.Xr resolver 5 ,
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.Xr hostname 7 ,
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.Xr named 8
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.Pp
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.%T RFC1032 ,
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.%T RFC1033 ,
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.%T RFC1034 ,
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.%T RFC1035 ,
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.%T RFC974
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.Rs
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.%T "Name Server Operations Guide for BIND"
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.Re
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.Sh HISTORY
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The
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.Nm
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function appeared in
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.Bx 4.3 .
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