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CAP_DNS(3) 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);

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

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

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

The functions (), (), () and () are respectively equivalent to gethostbyname(2), gethostbyname2(2), gethostbyaddr(2) and getnameinfo(2) except that the connection to the system.dns service needs to be provided.

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

The () 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: ADDR or NAME. The ADDR means that functions cap_gethostbyname(), cap_gethostbyname2() and () are allowed. In case when type is set to NAME the cap_getnameinfo() function is allowed.
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 = "127.0.0.1";
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. */
cap_close(capcas);

/* 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 was implemented by Pawel Jakub Dawidek <pawel@dawidek.net> under sponsorship from the FreeBSD Foundation.

This manual page was written by
Mariusz Zaborski <oshogbo@FreeBSD.org>.

March 18, 2018 FreeBSD-12.0