Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
RES_INIT(3)	       FreeBSD Library Functions Manual		   RES_INIT(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>

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

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

     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);

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


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

     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 reason-
     able defaults and can be ignored.	Options	stored in _res.options are de-
     fined 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 do-
		     main name are initialized (i.e. res_init()	has been

     RES_DEBUG	     Print debugging messages, if libc is compiled with	DEBUG.
		     By	default	on OpenBSD this	option does nothing.

     RES_AAONLY	     Accept authoritative answers only.	 With this option,
		     res_send()	should continue	until it finds an authorita-
		     tive answer or finds an error.  On	OpenBSD	this option
		     does nothing.

     RES_USEVC	     Use TCP connections for queries instead of	UDP datagrams.

     RES_PRIMARY     Query the primary name server only.  On OpenBSD this op-
		     tion does nothing.

     RES_IGNTC	     Ignore truncation errors, i.e. don't retry	with TCP.

     RES_RECURSE     Set the recursion-desired bit in queries.	(res_send()
		     does not do iterative queries and expects the name	server
		     to	handle recursion.)  This option	is enabled by default.

     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_STAYOPEN    Used with RES_USEVC to keep the TCP connection open be-
		     tween queries.  This is useful only in programs that reg-
		     ularly do many queries.  UDP should be the	normal mode

     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 de-

     RES_INSECURE_1  Do	not require the	IP source address on the reply packet
		     to	be equal to the	server's address.

     RES_INSECURE_2  Do	not check if the query section of the reply packet is
		     equal to that of the query	packet.

     RES_NOALIASES   This option has no	effect.	 In the	past, it turned	off
		     the legacy	HOSTALIASES feature.

     RES_USE_INET6   With this option gethostbyname(3) will return IPv6	ad-
		     dresses if	available.  This option	is deprecated; soft-
		     ware should use the getaddrinfo(3)	interface instead of
		     modifying the behavior of gethostbyname(3).  On some op-
		     erating systems this option also causes IPv4 addresses to
		     be	returned as IPv4-mapped	IPv6 addresses.	 For example, will be returned as ::ffff:
		     IPv4-mapped IPv6 addresses	are not	supported on OpenBSD.

     RES_USE_EDNS0   Attach an OPT pseudo-RR for the EDNS0 extension, as spec-
		     ified in RFC 2671.	 This informs DNS servers of a
		     client's receive buffer size, allowing them to take ad-
		     vantage of	a non-default receive buffer size, and thus to
		     send larger replies.  DNS query packets with the EDNS0
		     extension are not compatible with non-EDNS0 DNS servers.
		     OpenBSD uses 4096 bytes as	input buffer size.

     RES_USE_DNSSEC  Request that the resolver uses Domain Name	System Secu-
		     rity Extensions (DNSSEC), as defined in RFCs 4033,	4034,
		     and 4035.

     RES_USE_CD	     Set the Checking Disabled flag on queries.

     The res_init() routine reads the configuration file (if any; see
     resolv.conf(5)) to	get the	default	domain name, search list, and the In-
     ternet address of the local name server(s).  If no	server is configured,
     the host running the resolver is tried.  The current domain name is de-
     fined 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	over-
     ride 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

     The res_query() function provides an interface to the server query	mecha-
     nism.  It constructs a query, sends it to the local server, awaits	a re-
     sponse, and makes preliminary checks on the reply.	 The query requests
     information of the	specified type and class for the specified fully qual-
     ified 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 lo-
     cal name server, and handle timeouts and retries.	The length of the re-
     ply 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 com-
     pressed.  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 uncom-
     pressed 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

     /etc/resolv.conf	 The configuration file.

     gethostbyname(3), resolv.conf(5), hostname(7)

     M.	Stahl, Domain Administrators Guide, RFC	1032, November 1987.

     M.	Lottor,	Domain Administrators Operations Guide,	RFC 1033, November

     P.	Mockapetris, Domain Names - Concepts and Facilities, RFC 1034,
     November 1987.

     P.	Mockapetris, Domain Names - Implementation and Specification, RFC
     1035, November 1987.

     J.	Klensin, Simple	Mail Transfer Protocol,	RFC 5321, October 2008.

     The functions res_mkquery(), res_send(), res_init(), dn_comp(), and
     dn_expand() appeared in 4.3BSD.  The functions res_query()	and
     res_search() appeared in 4.3BSD-Tahoe.

FreeBSD	13.0			April 25, 2020			  FreeBSD 13.0


Want to link to this manual page? Use this URL:

home | help