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CAP_DNS(3)	       FreeBSD Library Functions Manual		    CAP_DNS(3)

     cap_gethostbyname,	cap_gethostbyname2, cap_gethostbyaddr,
     cap_getnameinfo, cap_dns_type_limit, cap_dns_family_limit -- library for
     getting network host entry	in capability mode

     library "libcap_dns"

     #include <sys/nv.h>
     #include <libcasper.h>
     #include <casper/cap_dns.h>

     struct hostent *
     cap_gethostbyname(const cap_channel_t *chan, const	char *name);

     struct hostent *
     cap_gethostbyname2(const cap_channel_t *chan, const char *name, int af);

     struct hostent *
     cap_gethostbyaddr(const cap_channel_t *chan, const	void *addr,
	 socklen_t len,	int af);

     cap_getnameinfo(const cap_channel_t *chan,	const void *name,
	 int namelen);

     cap_dns_type_limit(cap_channel_t *chan, const char	* const	*types,
	 size_t	ntypes);

     cap_dns_family_limit(const	cap_channel_t *chan, const int *families,
	 size_t	nfamilies);

     The functions cap_gethostbyname(),	cap_gethostbyname2(),
     cep_gethostbyaddr() and cap_getnameinfo() are respectively	equivalent to
     gethostbyname(3), gethostbyname2(3), gethostbyaddr(3) and getnameinfo(3)
     except that the connection	to the system.dns service needs	to be pro-

     The cap_dns_type_limit() function limits the functions allowed in the
     service.  The types variable can be set to	ADDR2NAME or NAME2ADDR.	 See
     the LIMITS	section	for more details.  The ntpyes variable contains	the
     number of types provided.

     The cap_dns_family_limit()	functions allows to limit address families.
     For details see LIMITS.  The nfamilies variable contains the number of
     families provided.

     The preferred way of setting limits is to use the cap_dns_type_limit()
     and cap_dns_family_limit()	functions, but the limits of service can be
     set also using cap_limit_set(3).  The nvlist(9) for that function can
     contain the following values and types:

	   type	(NV_TYPE_STRING)
	   The type can	have two values: ADDR2NAME or NAME2ADDR.  The ADDR
	   means that reverse DNS lookups are allowed with cap_getnameinfo()
	   and cap_gethostbyaddr() functions.  In case when type is set	to
	   NAME	the name resolution is allowed with cap_getaddrinfo(),
	   cap_gethostbyname(),	and cap_gethostbyname2() functions.

	   family (NV_TYPE_NUMBER)
	   The family limits service to	one of the address families (e.g.
	   AF_INET, AF_INET6, etc.).

     The following example first opens a capability to casper and then uses
     this capability to	create the system.dns casper service and uses it to
     resolve an	IP address.

     cap_channel_t *capcas, *capdns;
     const char	*typelimit = "ADDR";
     int familylimit;
     const char	*ipstr = "";
     struct in_addr ip;
     struct hostent *hp;

     /*	Open capability	to Casper. */
     capcas = cap_init();
     if	(capcas	== NULL)
	     err(1, "Unable to contact Casper");

     /*	Enter capability mode sandbox. */
     if	(cap_enter() < 0 && errno != ENOSYS)
	     err(1, "Unable to enter capability	mode");

     /*	Use Casper capability to create	capability to the system.dns service. */
     capdns = cap_service_open(capcas, "system.dns");
     if	(capdns	== NULL)
	     err(1, "Unable to open system.dns service");

     /*	Close Casper capability, we don't need it anymore. */

     /*	Limit system.dns to reverse DNS	lookups. */
     if	(cap_dns_type_limit(capdns, &typelimit,	1) < 0)
	     err(1, "Unable to limit access to the system.dns service");

     /*	Limit system.dns to reserve IPv4 addresses */
     familylimit = AF_INET;
     if	(cap_dns_family_limit(capdns, &familylimit, 1) < 0)
	     err(1, "Unable to limit access to the system.dns service");

     /*	Convert	IP address in C-string to in_addr. */
     if	(!inet_aton(ipstr, &ip))
	     errx(1, "Unable to	parse IP address %s.", ipstr);

     /*	Find hostname for the given IP address.	*/
     hp	= cap_gethostbyaddr(capdns, (const void	*)&ip, sizeof(ip), AF_INET);
     if	(hp == NULL)
	     errx(1, "No name associated with %s.", ipstr);

     printf("Name associated with %s is	%s.\n",	ipstr, hp->h_name);

     cap_enter(2), err(3), gethostbyaddr(3), gethostbyname(3),
     gethostbyname2(3),	getnameinfo(3),	capsicum(4), nv(9)

     The cap_dns service first appeared	in FreeBSD 10.3.

     The cap_dns service was implemented by Pawel Jakub	Dawidek
     <> under sponsorship from	the FreeBSD Foundation.

     This manual page was written by
     Mariusz Zaborski <>.

FreeBSD	13.0			  May 5, 2020			  FreeBSD 13.0


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