RESOLVER(3) Library Functions Manual RESOLVER(3)

res_ninit, res_ourserver_p, fp_resstat, res_hostalias, res_pquery, res_nquery, res_nsearch, res_nquerydomain, res_nmkquery, res_nsend, res_nupdate, res_nmkupdate, res_nclose, res_nsendsigned, res_findzonecut, res_getservers, res_setservers, res_ndestroy, dn_comp, dn_expand, res_init, res_isourserver, fp_nquery, p_query, hostalias, res_query, res_search, res_querydomain, res_mkquery, res_send, res_update, res_close — resolver routines

Standard C Library (libc, -lc)
DNS Resolver Library (libresolv, -lresolv)

#include <resolv.h>
#include <res_update.h>

typedef struct __res_state *res_state;


int
res_ninit(res_state statp);

int
res_ourserver_p(const res_state statp, const struct sockaddr_in *addr);

void
fp_resstat(const res_state statp, FILE *fp);

const char *
res_hostalias(const res_state statp, const char *name, char *buf, size_t buflen);

int
res_pquery(const res_state statp, const u_char *msg, int msglen, FILE *fp);

int
res_nquery(res_state statp, const char *dname, int class, int type, u_char *answer, int anslen);

int
res_nsearch(res_state statp, const char *dname, int class, int type, u_char * answer, int anslen);

int
res_nquerydomain(res_state statp, const char *name, const char *domain, int class, int type, u_char *answer, int anslen);

int
res_nmkquery(res_state statp, int op, const char *dname, int class, int type, const u_char *data, int datalen, const u_char *newrr, u_char *buf, int buflen);

int
res_nsend(res_state statp, const u_char *msg, int msglen, u_char *answer, int anslen);

int
res_nupdate(res_state statp, ns_updrec *rrecp_in);

int
res_nmkupdate(res_state statp, ns_updrec *rrecp_in, u_char *buf, int buflen);

void
res_nclose(res_state statp);

int
res_nsendsigned(res_state statp, const u_char *msg, int msglen, ns_tsig_key *key, u_char *answer, int anslen);

int
res_findzonecut(res_state statp, const char *dname, ns_class class, int options, char *zname, size_t zsize, struct in_addr *addrs, int naddrs);

int
res_getservers(res_state statp, union res_sockaddr_union *set, int cnt);

void
res_setservers(res_state statp, const union res_sockaddr_union *set, int cnt);

void
res_ndestroy(res_state statp);

int
dn_comp(const char *exp_dn, u_char *comp_dn, int length, u_char **dnptrs, u_char **lastdnptr);

int
dn_expand(const u_char *msg, const u_char *eomorig, const u_char *comp_dn, char *exp_dn, int length);

#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
#include <res_update.h>

int
res_init(void);

int
res_isourserver(const struct sockaddr_in *addr);

int
fp_nquery(const u_char *msg, int msglen, FILE *fp);

void
p_query(const u_char *msg, FILE *fp);

const char *
hostalias(const char *name);

int
res_query(const char *dname, int class, int type, u_char *answer, int anslen);

int
res_search(const char *dname, int class, int type, u_char *answer, int anslen);

int
res_querydomain(const char *name, const char *domain, int class, int type, u_char *answer, int anslen);

int
res_mkquery(int op, const char *dname, int class, int type, const char *data, int datalen, struct rrec *newrr, u_char *buf, int buflen);

int
res_send(const u_char *msg, int msglen, u_char *answer, int anslen);

int
res_update(ns_updrec *rrecp_in);

void
res_close(void);

These routines are used for making, sending and interpreting query and reply messages with Internet domain name servers.

State information is kept in statp and is used to control the behavior of these functions. statp should be set to all zeros prior to the first call to any of these functions.

The functions (), (), fp_nquery(), p_query(), hostalias(), res_query(), res_search(), (), res_mkquery(), res_send(), res_update(), res_close() are deprecated and are supplied for compatibility with old source code. They use global configuration and state information that is kept in the structure _res rather than that referenced through statp.

Most of the values in statp and _res are initialized on the first call to () / res_init() to reasonable defaults and can be ignored. Options stored in statp->options / _res.options are defined in resolv.h and are as follows. Options are stored as a simple bit mask containing the bitwise “OR” of the options enabled.

True if the initial name server address and default domain name are initialized (i.e., res_ninit() / res_init() has been called).
Print debugging messages.
Accept authoritative answers only. Should continue until it finds an authoritative answer or finds an error. Currently this is not implemented.
Use TCP connections for queries instead of UDP datagrams.
Used with 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.
Ignore truncation errors, i.e., don't retry with TCP.
Set the recursion-desired bit in queries. This is the default. (res_nsend() / res_send() does not do iterative queries and expects the name server to handle recursion.)
If set, res_nsearch() / 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.
If this option is set, res_nsearch() / res_search() will search for host names in the current domain and in parent domains; see hostname(7). This is used by the standard host lookup routine gethostbyname(3). This option is enabled by default.
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.
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.
This option turns off the user level aliasing feature controlled by the HOSTALIASES environment variable. Network daemons should set this option.
This options causes res_nsend() / res_send() to rotate the list of nameservers in statp->nsaddr_list / _res.nsaddr_list.
This option causes res_nsendsigned() to leave the message unchanged after TSIG verification; otherwise the TSIG record would be removed and the header updated.
This option causes 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 RES_DEFNAMES or RES_DNSRCH are set.

The () / res_init() routines read the configuration file (if any; see 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 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 search command in the configuration file. Another environment variable RES_OPTIONS can be set to override certain internal resolver options which are otherwise set by changing fields in the statp / _res structure or are inherited from the configuration file's options command. The syntax of the RES_OPTIONS environment variable is explained in resolv.conf(5). Initialization normally occurs on the first call to one of the other resolver routines.

In NetBSD the initialization code also sets up a kqueue(2) and creates a kevent(2) watching a file descriptor that points to the resolver file. Every resolver function calls the internal function () which checks for a new kevent(2) related to the 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.

The memory referred to by statp must be set to all zeros prior to the first call to (). res_ndestroy() should be called to free memory allocated by res_ninit() after last use.

The () / () 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 type and class for the specified fully-qualified domain name dname. The reply message is left in the answer buffer with length anslen supplied by the caller. res_nquery() / res_query() return -1 on error or the length of the answer.

The () / () routines make a query and awaits a response like res_nquery() / res_query(), but in addition, they implement the default and search rules controlled by the RES_DEFNAMES and RES_DNSRCH options. They return the length of the first successful reply which is stored in answer or -1 on error.

The remaining routines are lower-level routines used by () / (). The () / () functions construct a standard query message and place it in buf. They return the size of the query, or -1 if the query is larger than buflen. The query type op is usually QUERY, but can be any of the query types defined in ⟨arpa/nameser.h⟩. The domain name for the query is given by dname. newrr is currently unused but is intended for making update messages.

The () / () / () routines send a pre-formatted query and return an answer. They will call res_ninit() / res_init() if RES_INIT is not set, send the query to the local name server, and handle timeouts and retries. Additionally, 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.

() / (), res_nsearch() / res_search() and res_nsend() / res_send() return a length that may be bigger than anslen. In that case the query should be retried with a bigger buffer. : 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.

answer 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.

The function () returns true when inp is one of the servers in statp->nsaddr_list / _res.nsaddr_list.

The functions () / () print out the query and any answer in msg on fp. p_query() is equivalent to fp_nquery() with msglen set to 512.

The function () prints out the active flag bits in statp->options preceded by the text ";; res options:" on file.

The functions () / () look up name in the file referred to by the HOSTALIASES files and return a fully qualified hostname if found or NULL if not found or an error occurred. res_hostalias() uses buf to store the result in, hostalias() uses a static buffer.

The functions () and () are used to get and set the list of server to be queried.

The functions () / () take a list of ns_updrec 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 res_nupdate() will perform TSIG authenticated dynamic update operations if the key is not NULL.

The function () discovers the closest enclosing zone cut for a specified domain name, and finds the IP addresses of the zone's master servers.

The functions () / () take a linked list of ns_updrec rrecp_in and construct an UPDATE message in buf. res_nmkupdate() / res_mkupdate() return the length of the constructed message on no error or one of the following error values.

-1
An error occurred parsing rrecp_in.
-2
The buffer buf was too small.
-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.
-4
A number overflow occurred.
-5
Unknown operation or no records.

The functions () / () close any open socket file descriptors referenced through statp / _res. These functions were designed to be used to emulate endhostent(3), and don't release other resources held in res_state; to free all_resources, call res_ndestroy().

The function () calls res_nclose() then frees any memory allocated by res_ninit().

The () function compresses the domain name exp_dn and stores it in comp_dn. The size of the compressed name is returned or -1 if there were errors. The size of the array pointed to by comp_dn is given by length. The compression uses an array of pointers dnptrs 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 to the array is specified by lastdnptr. A side effect of dn_comp() is to update the list of pointers for labels inserted into the message as the name is compressed. If dnptr is NULL, names are not compressed. If lastdnptr is NULL, the list of labels is not updated.

The () entry expands the compressed domain name comp_dn to a full domain name. The compressed name is contained in a query or reply message; msg is a pointer to the beginning of the message. eomorig is a pointer to the first location after the message. The uncompressed name is placed in the buffer indicated by exp_dn which is of size length. The size of compressed name is returned or -1 if there was an error.

The variables statp->res_h_errno / _res.res_h_errno and external variable h_errno are set whenever an error occurs during resolver operation. The following definitions are given in ⟨netdb.h⟩:

#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 */

The following functions are only in libresolv: (), res_nmkupdate(), (), and res_nupdate(). All the rest are in both libc and libresolv.

/etc/resolv.conf
The configuration file, see resolv.conf(5).

getaddrinfo(3), gethostbyaddr(3), gethostbyname(3), getnameinfo(3), resolv.conf(5), hostname(7), named(8)

RFC 974, RFC 1032, RFC 1033, RFC 1034, RFC 1035, RFC 1535

Name Server Operations Guide for BIND.

The res_ninit function appeared in 4.3BSD.

February 7, 2018 NetBSD 11.0