.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk) .\" .\" %%%LICENSE_START(VERBATIM) .\" 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 .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date.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. .\" %%%LICENSE_END .\" .\" 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 2014-05-28 "GNU" "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 .B #include .B #include .B extern struct __res_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 ", const unsigned char *" data ", int " datalen , .BI "const unsigned char *" newrr ", unsigned char *" buf ", int " buflen ); .RE .sp .BI "int res_send(const unsigned char *" msg ", int " msglen , .RS .BI "unsigned char *" answer ", int " anslen ); .RE .sp .BI "int dn_comp(const char *" exp_dn ", unsigned char *" comp_dn , .RS .BI "int " length ", unsigned char **" dnptrs ", unsigned char **" lastdnptr ); .RE .sp .BI "int dn_expand(const unsigned char *" msg ", const unsigned char *" eomorig , .RS .BI "const unsigned char *" comp_dn ", char *" exp_dn , .BI "int " length ); .RE .fi .sp Link with \fI\-lresolv\fP. .SH DESCRIPTION These functions make queries to and interpret the responses from Internet domain name servers. .PP The .BR res_init () function reads the configuration files (see .BR 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 .BR LOCALDOMAIN . .BR res_init () is normally executed by the first call to one of the other functions. .PP The .BR res_query () 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 .BR res_search () function makes a query and waits for the response like .BR res_query (), but in addition implements the default and search rules controlled by .B RES_DEFNAMES and .B RES_DNSRCH (see description of \fI_res\fP options below). .PP The .BR res_querydomain () function makes a query using .BR res_query () on the concatenation of \fIname\fP and \fIdomain\fP. .PP The following functions are lower-level routines used by .BR res_query (). .PP The .BR res_mkquery () 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 .BR QUERY , but can be any of the types defined in \fI\fP. \fInewrr\fP is currently unused. .PP The .BR res_send () function sends a preformatted 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 .BR res_init (), if it has not already been called. .PP The .BR dn_comp () 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 .BR dn_expand () 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\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 .BR res_init () has been called. .TP .B RES_DEBUG Print debugging messages. This option is available only if glibc was built with debugging enabled, .\" See resolv/README. .\" Support for RES_DEBUG was made conditional in glibc 2.2. which is not the default. .TP .B RES_AAONLY Accept authoritative answers only. .BR res_send () continues until it finds 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 .BR res_send (). [Enabled by default]. .TP .B RES_DEFNAMES If set, .BR res_search () will append the default domain name to single component names\(emthat is, those that do not contain a dot. [Enabled by default]. .TP .B RES_STAYOPEN Used with .B RES_USEVC to keep the TCP connection open between queries. .TP .B RES_DNSRCH If set, .BR res_search () will search for hostnames in the current domain and in parent domains. This option is used by .BR gethostbyname (3). [Enabled by default]. .PP This list is not complete. You can find some other flags described in .BR resolv.conf (5). .SH RETURN VALUE The .BR res_init () function returns 0 on success, or \-1 if an error occurs. .PP The .BR res_query (), .BR res_search (), .BR res_querydomain (), .BR res_mkquery () and .BR res_send () functions return the length of the response, or \-1 if an error occurs. .PP The .BR dn_comp () and .BR dn_expand () 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 resolv.conf (5), .BR resolver (5), .BR hostname (7), .BR named (8) .SH COLOPHON This page is part of release 3.74 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%http://www.kernel.org/doc/man\-pages/.