.\"	$OpenBSD: res_init.3,v 1.6 2021/11/24 20:06:32 jca Exp $
.\"
.\" Copyright (c) 1985, 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: November 24 2021 $
.Dt RES_INIT 3
.Os
.Sh NAME
.Nm res_query ,
.Nm res_search ,
.Nm res_mkquery ,
.Nm res_send ,
.Nm res_init ,
.Nm dn_comp ,
.Nm dn_expand
.Nd resolver routines
.Sh SYNOPSIS
.In sys/types.h
.In netinet/in.h
.In arpa/nameser.h
.In resolv.h
.Ft int
.Fo res_query
.Fa "const char *dname"
.Fa "int class"
.Fa "int type"
.Fa "unsigned char *answer"
.Fa "int anslen"
.Fc
.Ft int
.Fo res_search
.Fa "const char *dname"
.Fa "int class"
.Fa "int type"
.Fa "unsigned char *answer"
.Fa "int anslen"
.Fc
.Ft int
.Fo res_mkquery
.Fa "int op"
.Fa "const char *dname"
.Fa "int class"
.Fa "int type"
.Fa "const unsigned char *data"
.Fa "int datalen"
.Fa "const unsigned char *newrr"
.Fa "unsigned char *buf"
.Fa "int buflen"
.Fc
.Ft int
.Fo res_send
.Fa "const unsigned char *msg"
.Fa "int msglen"
.Fa "unsigned char *answer"
.Fa "int anslen"
.Fc
.Ft int
.Fn res_init "void"
.Ft int
.Fo dn_comp
.Fa "const char *exp_dn"
.Fa "unsigned char *comp_dn"
.Fa "int length"
.Fa "unsigned char **dnptrs"
.Fa "unsigned char **lastdnptr"
.Fc
.Ft int
.Fo dn_expand
.Fa "const unsigned char *msg"
.Fa "const unsigned char *eomorig"
.Fa "const unsigned char *comp_dn"
.Fa "char *exp_dn"
.Fa "int length"
.Fc
.Sh DESCRIPTION
These routines are used for making, sending, and interpreting
query and reply messages with Internet domain name servers.
.Pp
Global configuration and state information that is used by the
resolver routines is kept in the structure
.Va _res .
Most of the values have reasonable defaults and can be ignored.
Options stored in
.Va _res.options
are defined in
.In resolv.h
and are as follows.
Options are stored as a simple bit mask containing the bitwise OR
of the options enabled.
.Bl -tag -width RES_USE_DNSSEC
.It Dv RES_INIT
True if the initial name server address and default domain name are
initialized (i.e.\&
.Fn res_init
has been called).
.It Dv RES_DEBUG
Print debugging messages,
if libc is compiled with
.Dv DEBUG .
By default on
.Ox
this option does nothing.
.It Dv RES_AAONLY
Accept authoritative answers only.
With this option,
.Fn res_send
should continue until it finds an authoritative answer or finds an error.
On
.Ox
this option does nothing.
.It Dv RES_USEVC
Use TCP connections for queries instead of UDP datagrams.
.It Dv RES_PRIMARY
Query the primary name server only.
On
.Ox
this option does nothing.
.It Dv RES_IGNTC
Ignore truncation errors, i.e. don't retry with TCP.
.It Dv RES_RECURSE
Set the recursion-desired bit in queries.
.Pf ( Fn res_send
does not do iterative queries and expects the name server
to handle recursion.)
This option is enabled by default.
.It Dv RES_DEFNAMES
If set,
.Fn res_search
will append the default domain name to single-component names
(those that do not contain a dot).
This option is enabled by default.
.It Dv RES_STAYOPEN
Used with
.Dv RES_USEVC
to keep the TCP connection open between queries.
This is useful only in programs that regularly do many queries.
UDP should be the normal mode used.
.It Dv RES_DNSRCH
If this option is set,
.Fn res_search
will search for host names in the current domain and in parent domains; see
.Xr hostname 7 .
This is used by the standard host lookup routine
.Xr gethostbyname 3 .
This option is enabled by default.
.It Dv RES_INSECURE_1
Do not require the IP source address on the reply packet
to be equal to the server's address.
.It Dv RES_INSECURE_2
Do not check if the query section of the reply packet
is equal to that of the query packet.
.It Dv RES_NOALIASES
This option has no effect.
In the past, it turned off the legacy
.Ev HOSTALIASES
feature.
.It Dv RES_TRUSTAD
If set, the resolver routines will set the AD flag in DNS queries and
preserve the value of the AD flag in DNS replies.
If not set, the resolver routines will clear the AD flag in responses.
Direct use of this option to enable AD bit processing is discouraged.
Instead the use of trusted name servers should be annotated with
.Dq options trust-ad
in
.Xr resolv.conf 5 .
This option is automatically enabled if
.Xr resolv.conf 5
only lists name servers on localhost.
.It Dv RES_USE_INET6
With this option
.Xr gethostbyname 3
will return IPv6 addresses if available.
This option is deprecated; software should use the
.Xr getaddrinfo 3
interface instead of modifying the behavior of
.Xr gethostbyname 3 .
On some operating systems this option also causes IPv4 addresses to be
returned as IPv4-mapped IPv6 addresses.
For example, 10.1.1.1 will be returned as ::ffff:10.1.1.1.
IPv4-mapped IPv6 addresses are not supported on
.Ox .
.It Dv RES_USE_EDNS0
Attach an OPT pseudo-RR for the EDNS0 extension,
as specified in RFC 2671.
This informs DNS servers of a client's receive buffer size,
allowing them to take advantage of a non-default receive buffer size,
and thus to send larger replies.
DNS query packets with the EDNS0 extension are not compatible with
non-EDNS0 DNS servers.
.Ox
uses 4096 bytes as input buffer size.
.It Dv RES_USE_DNSSEC
Request that the resolver uses
Domain Name System Security Extensions (DNSSEC),
as defined in RFCs 4033, 4034, and 4035.
The resolver routines will use the EDNS0 extension and set the DNSSEC DO
flag in queries, asking the name server to signal validated records by
setting the AD flag in the reply and to attach additional DNSSEC
records.
The resolver routines will clear the AD flag in replies unless the name
servers are considered trusted.
Also, client applications are often only interested in the value of the
AD flag, making the additional DNSSEC records a waste of network
bandwidth.
See the description for
.Dq options trust-ad
in
.Xr resolv.conf 5 .
.It Dv RES_USE_CD
Set the Checking Disabled flag on queries.
.El
.Pp
The
.Fn res_init
routine reads the configuration file (if any; see
.Xr resolv.conf 5 )
to get the default domain name, search list, and the Internet address
of the local name server(s).
If no server is configured, the host running
the resolver is tried.
The current domain name is defined by the hostname
if not specified in the configuration file;
it can be overridden by the environment variable
.Ev LOCALDOMAIN .
This environment variable may contain several blank-separated
tokens if you wish to override the search list on a per-process basis.
This is similar to the
.Ic search
command in the configuration file.
Another environment variable
.Ev RES_OPTIONS
can be set to override certain internal resolver options which
are otherwise set by changing fields in the
.Va _res
structure or are inherited from the configuration file's
.Ic options
command.
The syntax of the
.Ev RES_OPTIONS
environment variable is explained in
.Xr resolv.conf 5 .
Initialization normally occurs on the first call
to one of the following routines.
.Pp
The
.Fn res_query
function provides an interface to the server query mechanism.
It constructs a query, sends it to the local server,
awaits a response, and makes preliminary checks on the reply.
The query requests information of the specified
.Fa type
and
.Fa class
for the specified fully qualified domain name
.Fa dname .
The reply message is left in the
.Fa answer
buffer with length
.Fa anslen
supplied by the caller.
Values for the
.Fa class
and
.Fa type
fields
are defined in
.In arpa/nameser.h .
.Pp
The
.Fn res_search
routine makes a query and awaits a response like
.Fn res_query ,
but in addition, it implements the default and search rules controlled by the
.Dv RES_DEFNAMES
and
.Dv RES_DNSRCH
options.
It returns the first successful reply.
.Pp
The remaining routines are lower-level routines used by
.Fn res_query .
The
.Fn res_mkquery
function constructs a standard query message and places it in
.Fa buf .
It returns the size of the query, or \-1 if the query is larger than
.Fa buflen .
The query type
.Fa op
is usually
.Dv QUERY ,
but can be any of the query types defined in
.In arpa/nameser.h .
The domain name for the query is given by
.Fa dname .
.Fa newrr
is currently unused but is intended for making update messages.
.Pp
The
.Fn res_send
routine sends a pre-formatted query and returns an answer.
It will call
.Fn res_init
if
.Dv RES_INIT
is not set, send the query to the local name server, and
handle timeouts and retries.
The length of the reply message is returned, or \-1 if there were errors.
.Pp
The
.Fn dn_comp
function compresses the domain name
.Fa exp_dn
and stores it in
.Fa comp_dn .
The size of the compressed name is returned or \-1 if there were errors.
The size of the array pointed to by
.Fa comp_dn
is given by
.Fa length .
The compression uses an array of pointers
.Fa dnptrs
to previously compressed names in the current message.
The first pointer points
to the beginning of the message and the list ends with
.Dv NULL .
The limit to the array is specified by
.Fa lastdnptr .
A side effect of
.Fn dn_comp
is to update the list of pointers for labels inserted into the message
as the name is compressed.
If
.Fa dnptrs
is
.Dv NULL ,
names are not compressed.
If
.Fa lastdnptr
is
.Dv NULL ,
the list of labels is not updated.
.Pp
The
.Fn dn_expand
entry expands the compressed domain name
.Fa comp_dn
to a full domain name.
The compressed name is contained in a query or reply message;
.Fa msg
is a pointer to the beginning of the message.
The uncompressed name is placed in the buffer indicated by
.Fa exp_dn
which is of size
.Fa length .
The size of compressed name is returned or \-1 if there was an error.
.Sh FILES
.Bl -tag -width "/etc/resolv.confXX"
.It Pa /etc/resolv.conf
The configuration file.
.El
.Sh SEE ALSO
.Xr gethostbyname 3 ,
.Xr resolv.conf 5 ,
.Xr hostname 7
.Sh STANDARDS
.Rs
.%A M. Stahl
.%D November 1987
.%R RFC 1032
.%T Domain Administrators Guide
.Re
.Pp
.Rs
.%A M. Lottor
.%D November 1987
.%R RFC 1033
.%T Domain Administrators Operations Guide
.Re
.Pp
.Rs
.%A P. Mockapetris
.%D November 1987
.%R RFC 1034
.%T Domain Names \(en Concepts and Facilities
.Re
.Pp
.Rs
.%A P. Mockapetris
.%D November 1987
.%R RFC 1035
.%T Domain Names \(en Implementation and Specification
.Re
.Pp
.Rs
.%A J. Klensin
.%D October 2008
.%R RFC 5321
.%T Simple Mail Transfer Protocol
.Re
.Sh HISTORY
The functions
.Fn res_mkquery ,
.Fn res_send ,
.Fn res_init ,
.Fn dn_comp ,
and
.Fn dn_expand
appeared in
.Bx 4.3 .
The functions
.Fn res_query
and
.Fn res_search
appeared in
.Bx 4.3 Tahoe .