CAP_DNS(3)

💖 CAP_DNS(3) — Library Functions Manual 💖

okay so bestie, we’re in cap_dns territory, and the manual name is Library Functions Manual — cute, official, and here for the capability-mode DNS lore. Let’s go 💅

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🌸 NAME 🌸

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

📚 LIBRARY 📚

library “libcap_dns” — that’s the one, bestie ✨

🧚 SYNOPSIS 🧚

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

int
cap_getaddrinfo(cap_channel_t *chan, const char *hostname,
    const char *servname, const struct addrinfo *hints,
    struct addrinfo **res);

int
cap_getnameinfo(cap_channel_t *chan, const struct sockaddr *sa,
    socklen_t salen, char *host, size_t hostlen, char *serv,
    size_t servlen, int flags);

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_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);

✨ DESCRIPTION ✨

🚨 Obsolete alert, bestie: This service is obsolete and cap_net(3) should be used instead. No cap. The cap_getaddrinfo(), and cap_getnameinfo(), functions are preferred over the cap_gethostbyname(), cap_gethostbyname2(), and cap_gethostbyaddr() functions.

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 provided. (yep, the page literally says cep_gethostbyaddr() there — we’re keeping it 100, iykyk 👀)

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. (yes, ntpyes — that’s the spelling on the page, bestie 🎀)

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

🎀 LIMITS 🎀

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.
  • ADDR2NAME means that reverse DNS lookups are allowed with cap_getnameinfo() and cap_gethostbyaddr() functions.
  • In case when type is set to NAME2ADDR 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.). 💅

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🦋 EXAMPLES 🦋

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. It’s giving main-character capability energy, fr fr ✨

cap_channel_t *capcas, *capdns;
int familylimit, error;
const char *ipstr = "127.0.0.1";
const char *typelimit = "ADDR2NAME";
char hname[NI_MAXHOST];
struct addrinfo hints, *res;

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

/* Cache NLA for gai_strerror. */
caph_cache_catpages();

/* Enter capability mode sandbox. */
if (caph_enter() < 0)
        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 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 struct sockaddr. */
memset(&hints, 0, sizeof(hints));
hints.ai_family = familylimit;
hints.ai_flags = AI_NUMERICHOST;
error = cap_getaddrinfo(capdns, ipstr, NULL, &hints, &res);
if (error != 0)
       errx(1, "cap_getaddrinfo(): %s: %s", ipstr, gai_strerror(error));

/* 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");

/* Find hostname for the given IP address. */
error = cap_getnameinfo(capdns, res->ai_addr, res->ai_addrlen, hname, sizeof(hname),
    NULL, 0, 0);
if (error != 0)
        errx(1, "cap_getnameinfo(): %s: %s", ipstr, gai_strerror(error));

printf("Name associated with %s is %s.\n", ipstr, hname);

👀 SEE ALSO 👀

All the lil friends: cap_enter(2), caph_enter(3), err(3), gethostbyaddr(3), gethostbyname(3), gethostbyname2(3), getnameinfo(3), capsicum(4), nv(9) 💜

🌟 HISTORY 🌟

The cap_dns service first appeared in FreeBSD 10.3. Iconic timeline moment ⭐

🫶 AUTHORS 🫶

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>. Slay 💅

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🎀 footer moment, bestie: this one’s the Debian page, dated August 15, 2020, for CAP_DNS(3). That’s the whole vibe — bye and go resolve responsibly 💖✨🦋