man-pages/man3/resolver.3

.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\" 
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\" 
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified 1993-07-25 by Rik Faith (faith@cs.unc.edu)
.\" Modified 2004-10-31 by aeb
.\"
.TH RESOLVER 3 2004-10-31 "BSD" "Linux Programmer's Manual"
.SH NAME
res_init, res_query, res_search, res_querydomain, res_mkquery, res_send, 
dn_comp, dn_expand \- resolver routines
.SH SYNOPSIS
.nf
.B #include <netinet/in.h>
.B #include <arpa/nameser.h>
.B #include <resolv.h>
.B extern struct state _res;
.sp
.B int res_init(void);
.sp
.BI "int res_query(const char *" dname ", int " class ", int " type ,
.RS
.BI "unsigned char *" answer ", int " anslen );
.RE
.sp
.BI "int res_search(const char *" dname ", int " class ", int " type ,
.RS
.BI "unsigned char *" answer ", int " anslen );
.RE
.sp
.BI "int res_querydomain(const char *" name ", const char *" domain ,
.RS
.BI "int " class ", int " type ", unsigned char *" answer ,
.BI "int " anslen );
.RE
.sp
.BI "int res_mkquery(int " op ", const char *" dname ", int " class ,
.RS
.BI "int " type ", char *" data ", int " datalen ", struct rrec *" newrr ,
.BI "char *" buf ", int " buflen );
.RE
.sp
.BI "int res_send(const char *" msg ", int " msglen ", char *" answer , 
.RS
.BI "int " anslen );
.RE
.sp
.BI "int dn_comp(unsigned char *" exp_dn ", unsigned char *" comp_dn ,
.RS
.BI "int " length ", unsigned char **" dnptrs ", unsigned char *" exp_dn ,
.BI "unsigned char **" lastdnptr );
.RE
.sp
.BI "int dn_expand(unsigned char *" msg ", unsigned char *" eomorig ,
.RS
.BI "unsigned char *" comp_dn ", unsigned char *" exp_dn ,
.BI "int " length );
.RE
.fi
.sp
Link with \-lresolv.
.SH DESCRIPTION
These functions make queries to and interpret the responses from Internet
domain name servers.
.PP
The \fBres_init\fP() function reads the configuration files (see
resolv.conf(5)) to get the default domain name, search order and name
server address(es).  If no server is given, the local host is tried.
If no domain is given, that associated with the local host is used.
It can be overridden with the environment variable LOCALDOMAIN.
\fBres_init\fP() is normally executed by the first call to one of the
other functions.
.PP
The \fBres_query\fP() function queries the name server for the
fully-qualified domain name \fIname\fP of specified \fItype\fP and
\fIclass\fP.  The reply is left in the buffer \fIanswer\fP of length
\fIanslen\fP supplied by the caller.
.PP
The \fBres_search\fP() function makes a query and waits for the response
like \fBres_query\fP(), but in addition implements the default and search
rules controlled by RES_DEFNAMES and RES_DNSRCH (see description of
\fI_res\fP options below).
.PP
The \fBres_querydomain\fP() function makes a query using \fBres_query\fP()
on the concatenation of \fIname\fP and \fIdomain\fP.
.PP
The following functions are lower-level routines used by \fBres_query\fP().
.PP
The \fBres_mkquery\fP() function constructs a query message in \fIbuf\fP
of length \fIbuflen\fP for the domain name \fIdname\fP.  The query type
\fIop\fP is usually QUERY, but can be any of the types defined in
\fI<arpa/nameser.h>\fP.  \fInewrr\fP is currently unused.
.PP
The \fBres_send\fP() function sends a pre-formatted query given in
\fImsg\fP of length \fImsglen\fP and returns the answer in \fIanswer\fP
which is of length \fIanslen\fP.  It will call \fBres_init\fP(), if it 
has not already been called.
.PP
The \fBdn_comp\fP() function compresses the domain name \fIexp_dn\fP
and stores it in the buffer \fIcomp_dn\fP of length \fIlength\fP.
The compression uses an array of pointers \fIdnptrs\fP to previously
compressed names in the current message.  The first pointer points
to the beginning of the message and the list ends with NULL.  The limit
of the array is specified by \fIlastdnptr\fP.  if \fIdnptr\fP is NULL,
domain names are not compressed.  If \fIlastdnptr\fP is NULL, the list
of labels is not updated.
.PP
The \fPdn_expand\fP() function expands the compressed domain name
\fIcomp_dn\fP to a full domain name, which is placed in the buffer
\fIexp_dn\fP of size \fIlength\fP.  The compressed name is contained
in a query or reply message, and \fImsg\fP points to the beginning of
the message.
.PP
The resolver routines use global configuration and state information
contained in the structure \fI_res\fP, which is defined in 
\fI<resolv.h>\fP.  The only field that is normally manipulated by the 
user is \fI_res.options\fP.  This field can contain the bitwise ``or''
of the following options:
.TP
.B RES_INIT
True if \fBres_init\fP() has been called.
.TP
.B RES_DEBUG
Print debugging messages.
.TP
.B RES_AAONLY
Accept authoritative answers only.  \fBres_send\fP() continues until
it fins an authoritative answer or returns an error.  [Not currently
implemented].
.TP
.B RES_USEVC
Use TCP connections for queries rather than UDP datagrams.
.TP
.B RES_PRIMARY
Query primary domain name server only.
.TP
.B RES_IGNTC
Ignore truncation errors.  Don't retry with TCP.  [Not currently
implemented].
.TP
.B RES_RECURSE
Set the recursion desired bit in queries.  Recursion is carried out
by the domain name server, not by \fBres_send\fP().  [Enabled by 
default].
.TP
.B RES_DEFNAMES
If set, \fBres_search\fP() will append the default domain name to
single component names, ie. those that do not contain a dot.
[Enabled by default].
.TP
.B RES_STAYOPEN
Used with RES_USEVC to keep the TCP connection open between queries.
.TP
.B RES_DNSRCH
If set, \fBres_search\fP() will search for host names in the current
domain and in parent domains.  This option is used by
.BR gethostbyname (3).
[Enabled by default].
.SH "RETURN VALUE"
The \fBres_init\fP() function returns 0 on success, or \-1 if an error
occurs.
.PP
The \fBres_query\fP(), \fBres_search\fP(), \fBres_querydomain\fP(),
\fBres_mkquery\fP() and \fBres_send\fP() functions return the length
of the response, or \-1 if an error occurs.
.PP
The \fBdn_comp\fP() and \fBdn_expand\fP() functions return the length
of the compressed name, or \-1 if an error occurs.
.SH FILES
.nf
/etc/resolv.conf          resolver configuration file
/etc/host.conf            resolver configuration file
.fi
.SH "CONFORMING TO"
4.3BSD
.SH "SEE ALSO"
.BR gethostbyname (3),
.BR resolver (5),
.BR hostname (7),
.BR resolv.conf (5),
.BR named (8)