libc/net/resolver.3

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.Dd January 8, 2024
.Dt RESOLVER 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 ,
.Nm dn_skipname ,
.Nm ns_get16 ,
.Nm ns_get32 ,
.Nm ns_put16 ,
.Nm ns_put32
.Nd resolver routines
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In resolv.h
.Ft int
.Fo res_query
.Fa "const char *dname"
.Fa "int class"
.Fa "int type"
.Fa "u_char *answer"
.Fa "int anslen"
.Fc
.Ft int
.Fo res_search
.Fa "const char *dname"
.Fa "int class"
.Fa "int type"
.Fa "u_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 u_char *data"
.Fa "int datalen"
.Fa "const u_char *newrr_in"
.Fa "u_char *buf"
.Fa "int buflen"
.Fc
.Ft int
.Fo res_send
.Fa "const u_char *msg"
.Fa "int msglen"
.Fa "u_char *answer"
.Fa "int anslen"
.Fc
.Ft int
.Fn res_init void
.Ft int
.Fo dn_comp
.Fa "const char *exp_dn"
.Fa "u_char *comp_dn"
.Fa "int length"
.Fa "u_char **dnptrs"
.Fa "u_char **lastdnptr"
.Fc
.Ft int
.Fo dn_expand
.Fa "const u_char *msg"
.Fa "const u_char *eomorig"
.Fa "const u_char *comp_dn"
.Fa "char *exp_dn"
.Fa "int length"
.Fc
.Ft int
.Fn dn_skipname "const u_char *comp_dn" "const u_char *eom"
.Ft u_int
.Fn ns_get16 "const u_char *src"
.Ft u_long
.Fn ns_get32 "const u_char *src"
.Ft void
.Fn ns_put16 "u_int src" "u_char *dst"
.Ft void
.Fn ns_put32 "u_long src" "u_char *dst"
.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_INET6
.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.
.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.
Currently this is not implemented.
.It Dv RES_USEVC
Use
.Tn TCP
connections for queries instead of
.Tn UDP
datagrams.
.It Dv RES_STAYOPEN
Used with
.Dv RES_USEVC
to keep the
.Tn TCP
connection open between
queries.
This is useful only in programs that regularly do many queries.
.Tn UDP
should be the normal mode used.
.It Dv RES_IGNTC
Unused currently (ignore truncation errors, i.e., do not retry with
.Tn TCP ) .
.It Dv RES_RECURSE
Set the recursion-desired bit in queries.
This is the default.
.Pf ( Fn res_send
does not do iterative queries and expects the name server
to handle recursion.)
.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_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_NOALIASES
This option turns off the user level aliasing feature controlled by the
.Dq Ev HOSTALIASES
environment variable.
Network daemons should set this option.
.It Dv RES_USE_INET6
Enables support for IPv6-only applications.
This causes IPv4 addresses to be returned as an IPv4 mapped address.
For example,
.Li 10.1.1.1
will be returned as
.Li ::ffff:10.1.1.1 .
The option is meaningful with certain kernel configuration only.
.It Dv RES_USE_EDNS0
Enables support for OPT pseudo-RR for EDNS0 extension.
With the option, resolver code will attach OPT pseudo-RR into DNS queries,
to inform of our receive buffer size.
The option will allow DNS servers to take advantage of non-default receive
buffer size, and to send larger replies.
DNS query packets with EDNS0 extension is not compatible with
non-EDNS0 DNS servers.
.El
.Pp
The
.Fn res_init
routine
reads the configuration file (if any; see
.Xr resolver 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
.Em "search list"
on a per-process basis.
This is similar to the
.Ic search
command in the configuration file.
Another environment variable
.Dq 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
.Dq Ev RES_OPTIONS
environment variable is explained in
.Xr resolver 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.
.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 .
The
.Fa newrr_in
argument
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 dnptr
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.
.Pp
The
.Fn dn_skipname
function skips over a compressed domain name, which starts at a location
pointed to by
.Fa comp_dn .
The compressed name is contained in a query or reply message;
.Fa eom
is a pointer to the end of the message.
The size of compressed name is returned or \-1 if there was
an error.
.Pp
The
.Fn ns_get16
function gets a 16-bit quantity from a buffer pointed to by
.Fa src .
.Pp
The
.Fn ns_get32
function gets a 32-bit quantity from a buffer pointed to by
.Fa src .
.Pp
The
.Fn ns_put16
function puts a 16-bit quantity
.Fa src
to a buffer pointed to by
.Fa dst .
.Pp
The
.Fn ns_put32
function puts a 32-bit quantity
.Fa src
to a buffer pointed to by
.Fa dst .
.Sh IMPLEMENTATION NOTES
This implementation of the resolver is thread-safe, but it will not
function properly if the programmer attempts to declare his or her own
.Va _res
structure in an attempt to replace the per-thread version referred to
by that macro.
.Pp
The following compile-time option can be specified to change the default
behavior of resolver routines when necessary.
.Bl -tag -width RES_ENFORCE_RFC1034
.It Dv RES_ENFORCE_RFC1034
If this symbol is defined during compile-time,
.Fn res_search
will enforce RFC 1034 check, namely, disallow using of underscore character
within host names.
This is used by the standard host lookup routines like
.Xr gethostbyname 3 .
For compatibility reasons this option is not enabled by default.
.El
.Sh RETURN VALUES
The
.Fn res_init
function will return 0 on success, or \-1 in a threaded program if
per-thread storage could not be allocated.
.Pp
The
.Fn res_mkquery ,
.Fn res_search ,
and
.Fn res_query
functions return the size of the response on success, or \-1 if an
error occurs.
The integer
.Vt h_errno
may be checked to determine the reason for error.
See
.Xr gethostbyname 3
for more information.
.Sh FILES
.Bl -tag -width /etc/resolv.conf
.It Pa /etc/resolv.conf
The configuration file,
see
.Xr resolver 5 .
.El
.Sh SEE ALSO
.Xr gethostbyname 3 ,
.Xr resolver 5 ,
.Xr hostname 7
.Pp
.%T RFC1032 ,
.%T RFC1033 ,
.%T RFC1034 ,
.%T RFC1035 ,
.%T RFC974
.Sh HISTORY
The
.Nm
function appeared in
.Bx 4.3 .