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Manual Pages  — RESOLVER


res_query, res_search, res_mkquery, res_send, res_init, dn_comp, dn_expand, dn_skipname, ns_get16, ns_get32, ns_put16, ns_put32 – resolver routines



Standard C Library (libc, -lc)


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

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

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

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

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


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

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

dn_skipname(const u_char *comp_dn, const u_char *eom);

ns_get16(const u_char *src);

ns_get32(const u_char *src);

ns_put16(u_int src, u_char *dst);

ns_put32(u_long src, u_char *dst);


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.
RES_INIT True if the initial name server address and default domain name are initialized (i.e., res_init() has been called).
RES_DEBUG Print debugging messages.
RES_AAONLY 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.
RES_USEVC Use TCP connections for queries instead of UDP datagrams.
RES_STAYOPEN 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.
RES_IGNTC Unused currently (ignore truncation errors, i.e., do not retry with TCP).
RES_RECURSE 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.)
RES_DEFNAMES 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.
RES_DNSRCH 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.
  This option turns off the user level aliasing feature controlled by the " HOSTALIASES" environment variable. Network daemons should set this option.
  Enables support for IPv6-only applications. This causes IPv4 addresses to be returned as an IPv4 mapped address. For example, will be returned as ::ffff: 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.

The res_init() routine reads the configuration file (if any; see resolver(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 resolver(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.

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. The newrr_in argument 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 dnptr 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.

The dn_skipname() function skips over a compressed domain name, which starts at a location pointed to by comp_dn. The compressed name is contained in a query or reply message; eom is a pointer to the end of the message. The size of compressed name is returned or -1 if there was an error.

The ns_get16() function gets a 16-bit quantity from a buffer pointed to by src.

The ns_get32() function gets a 32-bit quantity from a buffer pointed to by src.

The ns_put16() function puts a 16-bit quantity src to a buffer pointed to by dst.

The ns_put32() function puts a 32-bit quantity src to a buffer pointed to by dst.


This implementation of the resolver is thread-safe, but it will not function properly if the programmer attempts to declare his or her own _res structure in an attempt to replace the per-thread version referred to by that macro.

The following compile-time option can be specified to change the default behavior of resolver routines when necessary.
  If this symbol is defined during compile-time, res_search() will enforce RFC 1034 check, namely, disallow using of underscore character within host names. This is used by the standard host lookup routines like gethostbyname(3). For compatibility reasons this option is not enabled by default.


The res_init() function will return 0 on success, or -1 in a threaded program if per-thread storage could not be allocated.

The res_mkquery(), res_search(), and res_query() functions return the size of the response on success, or -1 if an error occurs. The integer h_errno may be checked to determine the reason for error. See gethostbyname(3) for more information.


  The configuration file, see resolver(5).


gethostbyname(3), resolver(5), hostname(7)

RFC1032, RFC1033, RFC1034, RFC1035, RFC974


The res_query function appeared in BSD 4.3 .

RESOLVER (3) September 15, 2022

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