33dee81933
The getaddrinfo(3), getipnodebyname(3) and resolver(3) can coincide now with what should be totally reentrant, and h_errno values will now be preserved correctly, but this does not affect interfaces such as gethostbyname(3) which are still mostly non-reentrant. In all of these relevant functions, the thread-safety has been pushed down as far as it seems possible right now. This means that operations that are selected via nsdispatch(3) (i.e. files, yp, dns) are protected still under global locks that getaddrinfo(3) defines, but where possible the locking is greatly reduced. The most noticeable improvement is that multiple DNS lookups can now be run at the same time, and this shows major improvement in performance of DNS-lookup threaded programs, and solves the "Mozilla tab serialization" problem. No single-threaded applications need to be recompiled. Multi-threaded applications that reference "_res" to change resolver(3) options will need to be recompiled, and ones which reference "h_errno" will also if they desire the correct h_errno values. If the applications already understood that _res and h_errno were not thread-safe and had their own locking, they will see no performance improvement but will not actually break in any way. Please note that when NSS modules are used, or when nsdispatch(3) defaults to adding any lookups of its own to the individual libc _nsdispatch() calls, those MUST be reentrant as well.
436 lines
11 KiB
Groff
436 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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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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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 June 4, 1993
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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
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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., don't 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. 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. 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. 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 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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.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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.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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