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RESOLVER(3) Library Functions Manual RESOLVER(3)

res_query, res_search, res_mkquery, res_send, res_init, dn_comp, dn_expand
resolver routines

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

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

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

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

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

int
res_init(void);

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

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

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

Global configuration and state information that is used by the resolver routines is kept in the structure _res. Most of the values have reasonable defaults and can be ignored. Options stored in _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_init() has been called).
Print debugging messages.
Accept authoritative answers only. With this option, res_send() 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.
Unused currently (ignore truncation errors, i.e., don't retry with TCP).
Set the recursion-desired bit in queries. This is the default. (res_send() does not do iterative queries and expects the name server to handle recursion.)
If set, 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_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 not meaningful on OpenBSD.
Enables support for OPT pseudo-RR for EDNS0 extension. With this 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.

The res_init() routine reads 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 _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 following routines.

The 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 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. Values for the class and type fields are defined in ⟨arpa/nameser.h⟩.

The res_search() routine makes a query and awaits a response like res_query(), but in addition, it implements the default and search rules controlled by the RES_DEFNAMES and RES_DNSRCH options. It returns the first successful reply.

The remaining routines are lower-level routines used by res_query(). The res_mkquery() function constructs a standard query message and places it in buf. It returns 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 res_send() routine sends a pre-formatted query and returns an answer. It will call res_init() if 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.

The dn_comp() 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 dnptrs is NULL, names are not compressed. If lastdnptr is NULL, the list of labels is not updated.

The dn_expand() 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. 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.

/etc/resolv.conf
The configuration file.

gethostbyname(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_query function appeared in 4.3BSD.
May 31, 2007 OpenBSD-5.1