.\" $NetBSD: resolver.3,v 1.31 2016/12/18 17:34:36 christos Exp $ .\" Copyright (c) 2004 by Internet Systems Consortium, Inc. ("ISC") .\" .\" Permission to use, copy, modify, and distribute this software for any .\" purpose with or without fee is hereby granted, provided that the above .\" copyright notice and this permission notice appear in all copies. .\" .\" THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES .\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR .\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES .\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT .\" OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. .\" .\" Copyright (c) 1985, 1995 The Regents of the University of California. .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms are permitted provided .\" that: (1) source distributions retain this entire copyright notice and .\" comment, and (2) distributions including binaries display the following .\" acknowledgement: ``This product includes software developed by the .\" University of California, Berkeley and its contributors'' in the .\" documentation or other materials provided with the distribution and in .\" all advertising materials mentioning features or use of this software. .\" 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 ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. .\" .\" @(#)resolver.3 6.5 (Berkeley) 6/23/90 .\" Id: resolver.man3,v 1.2 2009/01/21 00:12:34 each Exp .\" .Dd December 18, 2016 .Dt RESOLVER 3 .Os .Sh NAME .Nm res_ninit , .Nm res_ourserver_p , .Nm fp_resstat , .Nm res_hostalias , .Nm res_pquery , .Nm res_nquery , .Nm res_nsearch , .Nm res_nquerydomain , .Nm res_nmkquery , .Nm res_nsend , .Nm res_nupdate , .Nm res_nmkupdate , .Nm res_nclose , .Nm res_nsendsigned , .Nm res_findzonecut , .Nm res_getservers , .Nm res_setservers , .Nm res_ndestroy , .Nm dn_comp , .Nm dn_expand , .\" .Nm hstrerror , .Nm res_init , .Nm res_isourserver , .Nm fp_nquery , .Nm p_query , .Nm hostalias , .Nm res_query , .Nm res_search , .Nm res_querydomain , .Nm res_mkquery , .Nm res_send , .Nm res_update , .Nm res_close , .\" .Nm herror .Nd resolver routines .Sh LIBRARY .Lb libc .Lb libresolv .Sh SYNOPSIS .In resolv.h .In res_update.h .Vt typedef struct __res_state *res_state ; .Pp .Ft int .Fn res_ninit "res_state statp" .Ft int .Fn res_ourserver_p "const res_state statp" "const struct sockaddr_in *addr" .Ft void .Fn fp_resstat "const res_state statp" "FILE *fp" .Ft "const char *" .Fn res_hostalias "const res_state statp" "const char *name" "char *buf" "size_t buflen" .Ft int .Fn res_pquery "const res_state statp" "const u_char *msg" "int msglen" "FILE *fp" .Ft int .Fn res_nquery "res_state statp" "const char *dname" "int class" "int type" "u_char *answer" "int anslen" .Ft int .Fn res_nsearch "res_state statp" "const char *dname" "int class" "int type" "u_char * answer" "int anslen" .Ft int .Fn res_nquerydomain "res_state statp" "const char *name" "const char *domain" "int class" "int type" "u_char *answer" "int anslen" .Ft int .Fo res_nmkquery .Fa "res_state statp" .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" .Fa "u_char *buf" .Fa "int buflen" .Fc .Ft int .Fn res_nsend "res_state statp" "const u_char *msg" "int msglen" "u_char *answer" "int anslen" .Ft int .Fn res_nupdate "res_state statp" "ns_updrec *rrecp_in" .Ft int .Fn res_nmkupdate "res_state statp" "ns_updrec *rrecp_in" "u_char *buf" "int buflen" .Ft void .Fn res_nclose "res_state statp" .Ft int .Fn res_nsendsigned "res_state statp" "const u_char *msg" "int msglen" "ns_tsig_key *key" "u_char *answer" "int anslen" .Ft int .Fn res_findzonecut "res_state statp" "const char *dname" "ns_class class" "int options" "char *zname" "size_t zsize" "struct in_addr *addrs" "int naddrs" .Ft int .Fn res_getservers "res_state statp" "union res_sockaddr_union *set" "int cnt" .Ft void .Fn res_setservers "res_state statp" "const union res_sockaddr_union *set" "int cnt" .Ft void .Fn res_ndestroy "res_state statp" .Ft int .Fn dn_comp "const char *exp_dn" "u_char *comp_dn" "int length" "u_char **dnptrs" "u_char **lastdnptr" .Ft int .Fn dn_expand "const u_char *msg" "const u_char *eomorig" "const u_char *comp_dn" "char *exp_dn" "int length" .\" .Ft "const char *" .\" .Fn hstrerror "int err" .Ss DEPRECATED .In sys/types.h .In netinet/in.h .In arpa/nameser.h .In resolv.h .In res_update.h .Ft int .Fn res_init "void" .Ft int .Fn res_isourserver "const struct sockaddr_in *addr" .Ft int .Fn fp_nquery "const u_char *msg" "int msglen" "FILE *fp" .Ft void .Fn p_query "const u_char *msg" "FILE *fp" .Ft "const char *" .Fn hostalias "const char *name" .Ft int .Fn res_query "const char *dname" "int class" "int type" "u_char *answer" "int anslen" .Ft int .Fn res_search "const char *dname" "int class" "int type" "u_char *answer" "int anslen" .Ft int .Fn res_querydomain "const char *name" "const char *domain" "int class" "int type" "u_char *answer" "int anslen" .Ft int .Fo res_mkquery .Fa "int op" .Fa "const char *dname" .Fa "int class" .Fa "int type" .Fa "const char *data" .Fa "int datalen" .Fa "struct rrec *newrr" .Fa "u_char *buf" .Fa "int buflen" .Fc .Ft int .Fn res_send "const u_char *msg" "int msglen" "u_char *answer" "int anslen" .Ft int .Fn res_update "ns_updrec *rrecp_in" .Ft void .Fn res_close "void" .\" .Ft void .\" .Fn herror "const char *s" .Sh DESCRIPTION These routines are used for making, sending and interpreting query and reply messages with Internet domain name servers. .Pp State information is kept in .Fa statp and is used to control the behavior of these functions. .Fa statp should be set to all zeros prior to the first call to any of these functions. .Pp The functions .Fn res_init , .Fn res_isourserver , .Fn fp_nquery , .Fn p_query , .Fn hostalias , .Fn res_query , .Fn res_search , .Fn res_querydomain , .Fn res_mkquery , .Fn res_send , .Fn res_update , .Fn res_close .\" and .\" .Fn herror are deprecated and are supplied for compatability with old source code. They use global configuration and state information that is kept in the structure .Ft _res rather than that referenced through .Ft statp . .Pp Most of the values in .Ft statp and .Ft _res are initialized on the first call to .Fn res_ninit / .Fn res_init to reasonable defaults and can be ignored. Options stored in .Ft statp->options / .Ft _res.options are defined in .Pa resolv.h and are as follows. Options are stored as a simple bit mask containing the bitwise .Dq 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_ninit / .Fn res_init has been called). .It Dv RES_DEBUG Print debugging messages. .It Dv RES_AAONLY Accept authoritative answers only. Should continue until it finds an authoritative answer or finds an error. Currently this is not implemented. .It Dv RES_USEVC Use TCP connections for queries instead of UDP datagrams. .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_IGNTC Ignore truncation errors, i.e., don't retry with TCP. .It Dv RES_RECURSE Set the recursion-desired bit in queries. This is the default. (\c .Fn res_nsend / .Fn res_send does not do iterative queries and expects the name server to handle recursion.) .It Dv RES_DEFNAMES If set, .Fn res_nsearch / .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_nsearch / .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_USE_INET6 Enables support for IPv6-only applications. This causes IPv4 addresses to be returned as an IPv4 mapped address. For example, 10.1.1.1 will be returned as ::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. .It Dv RES_NOALIASES This option turns off the user level aliasing feature controlled by the .Ev HOSTALIASES environment variable. Network daemons should set this option. .It Dv RES_ROTATE This options causes .Fn res_nsend / .Fn res_send to rotate the list of nameservers in .Fa statp->nsaddr_list / .Fa _res.nsaddr_list . .It Dv RES_KEEPTSIG This option causes .Fn res_nsendsigned to leave the message unchanged after TSIG verification; otherwise the TSIG record would be removed and the header updated. .It Dv RES_NOTLDQUERY This option causes .Fn res_nsearch to not attempt to resolve an unqualified name as if it were a top level domain (TLD). This option can cause problems if the site has "localhost" as a TLD rather than having localhost on one or more elements of the search list. This option has no effect if neither .Dv RES_DEFNAMES or .Dv RES_DNSRCH are set. .El .Pp The .Fn res_ninit / .Fn res_init routines read 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 .Fa search list on a per-process basis. This is similar to the .Fa 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 .Ft statp / .Ft _res structure or are inherited from the configuration file's .Fa 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 other resolver routines. .Pp In .Nx the initialization code also sets up a .Xr kqueue 2 and creates a .Xr kevent 2 watching a file descriptor that points to the resolver file. Every resolver function calls the internal function .Fn __res_check which checks for a new .Xr kevent 2 related to the .Xr resolv.conf 5 file, and reloads the file if necessary. This does not work if the file is accessed through a symlink and the symlink changes to point to a different file. To fix the symlink issue one could add a system call per resolver call to get the current time, and reload every so often. This is not done currently, but it is under consideration. .Pp The memory referred to by .Ft statp must be set to all zeros prior to the first call to .Fn res_ninit . .Fn res_ndestroy should be called to free memory allocated by .Fn res_ninit after last use. .Pp The .Fn res_nquery / .Fn res_query functions provide interfaces to the server query mechanism. They construct a query, send it to the local server, await a response, and make 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. .Fn res_nquery / .Fn res_query return \-1 on error or the length of the answer. .Pp The .Fn res_nsearch / .Fn res_search routines make a query and awaits a response like .Fn res_nquery / .Fn res_query , but in addition, they implement the default and search rules controlled by the .Dv RES_DEFNAMES and .Dv RES_DNSRCH options. They return the length of the first successful reply which is stored in .Ft answer or \-1 on error. .Pp The remaining routines are lower-level routines used by .Fn res_nquery / .Fn res_query . The .Fn res_nmkquery / .Fn res_mkquery functions construct a standard query message and place it in .Fa buf . They return 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 .Aq Pa 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_nsend / .Fn res_send / .Fn res_nsendsigned routines send a pre-formatted query and return an answer. They will call .Fn res_ninit / .Fn res_init if .Dv RES_INIT is not set, send the query to the local name server, and handle timeouts and retries. Additionally, .Fn res_nsendsigned will use TSIG signatures to add authentication to the query and verify the response. In this case, only one nameserver will be contacted. The length of the reply message is returned, or \-1 if there were errors. .Pp .Fn res_nquery / .Fn res_query , .Fn res_nsearch / .Fn res_search and .Fn res_nsend / .Fn res_send return a length that may be bigger than .Fa anslen . In that case the query should be retried with a bigger buffer. .Em NOTE : The answer to the second query may be larger still so supplying a buffer that bigger that the answer returned by the previous query is recommended. .Pp .Fa answer .Em MUST be big enough to receive a maximum UDP response from the server or parts of the answer will be silently discarded. The default maximum UDP response size is 512 bytes. .Pp The function .Fn res_ourserver_p returns true when .Fa inp is one of the servers in .Fa statp->nsaddr_list / .Fa _res.nsaddr_list . .Pp The functions .Fn fp_nquery / .Fn p_query print out the query and any answer in .Fa msg on .Fa fp . .Fn p_query is equivalent to .Fn fp_nquery with .Fa msglen set to 512. .Pp The function .Fn fp_resstat prints out the active flag bits in .Fa statp->options preceeded by the text ";; res options:" on .Fa file . .Pp The functions .Fn res_hostalias / .Fn hostalias look up name in the file referred to by the .Ev HOSTALIASES files and return a fully qualified hostname if found or .Dv NULL if not found or an error occurred. .Fn res_hostalias uses .Fa buf to store the result in, .Fn hostalias uses a static buffer. .Pp The functions .Fn res_getservers and .Fn res_setservers are used to get and set the list of server to be queried. .Pp The functions .Fn res_nupdate / .Fn res_update take a list of ns_updrec .Fa rrecp_in . They identify the containing zone for each record and group the records according to containing zone maintaining in zone order then send an update request to the servers for these zones. The number of zones updated is returned or \-1 on error. Note that .Fn res_nupdate will perform TSIG authenticated dynamic update operations if the key is not .Dv NULL . .Pp The function .Fn res_findzonecut discovers the closest enclosing zone cut for a specified domain name, and finds the IP addresses of the zone's master servers. .Pp The functions .Fn res_nmkupdate / .Fn res_mkupdate take a linked list of ns_updrec .Fa rrecp_in and construct an UPDATE message in .Fa buf . .Fn res_nmkupdate / .Fn res_mkupdate return the length of the constructed message on no error or one of the following error values. .Bl -inset -width "-5" .It \-1 An error occurred parsing .Fa rrecp_in . .It \-2 The buffer .Fa buf was too small. .It \-3 The first record was not a zone section or there was a section order problem. The section order is S_ZONE, S_PREREQ and S_UPDATE. .It \-4 A number overflow occurred. .It \-5 Unknown operation or no records. .El .Pp The functions .Fn res_nclose / .Fn res_close close any open socket file descriptors referenced through .Fa statp / .Fa _res . These functions were designed to be used to emulate .Xr endhostent 3 , and don't release other resources held in .Ft res_state ; to free all_resources, call .Fn res_ndestroy . .Pp The function .Fn res_ndestroy calls .Fn res_nclose then frees any memory allocated by .Fn res_ninit . .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. .Fa eomorig is a pointer to the first location after 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 variables .Ft statp->res_h_errno / .Ft _res.res_h_errno and external variable .Ft h_errno are set whenever an error occurs during resolver operation. The following definitions are given in .Aq Pa netdb.h : .Bd -literal #define NETDB_INTERNAL -1 /* see errno */ #define NETDB_SUCCESS 0 /* no problem */ #define HOST_NOT_FOUND 1 /* Authoritative Answer Host not found */ #define TRY_AGAIN 2 /* Non-Authoritative not found, or SERVFAIL */ #define NO_RECOVERY 3 /* Non-Recoverable: FORMERR, REFUSED, NOTIMP */ #define NO_DATA 4 /* Valid name, no data for requested type */ .Ed .\" .Pp .\" The .\" .Fn herror .\" function writes a message to the diagnostic output consisting of the string .\" parameter .\" .Fa s , .\" the constant string ": ", and a message corresponding to the value of .\" .Ft h_errno . .\" .Pp .\" The .\" .Fn hstrerror .\" function returns a string which is the message text corresponding to the .\" value of the .\" .Fa err .\" parameter. .Pp The following functions are only in .Dv libresolv : .Fn res_findzonecut , .Fn res_nmkupdate , .Fn res_nsendsigned , and .Fn res_nupdate . All the rest are in both .Dv libc and .Dv libresolv . .Sh FILES .Bl -tag -width "/etc/resolv.conf " .It Pa /etc/resolv.conf The configuration file, see .Xr resolv.conf 5 . .El .Sh SEE ALSO .Xr getaddrinfo 3 , .Xr gethostbyaddr 3 , .Xr gethostbyname 3 , .Xr getnameinfo 3 , .Xr resolv.conf 5 , .Xr hostname 7 , .Xr named 8 .Pp .%T RFC 974 , .%T RFC 1032 , .%T RFC 1033 , .%T RFC 1034 , .%T RFC 1035 , .%T RFC 1535 .Rs .%T "Name Server Operations Guide for BIND" .Re .Sh HISTORY The .Nm function appeared in .Bx 4.3 .