CAP_NET(3) FreeBSD Library Functions Manual CAP_NET(3)
NAME
cap_bind, cap_connect, cap_getaddrinfo, cap_gethostbyaddr,
cap_gethostbyname, cap_gethostbyname2, cap_getnameinfo, cap_net_free,
cap_net_limit, cap_net_limit_addr2name, cap_net_limit_addr2name_family,
cap_net_limit_bind, cap_net_limit_connect, cap_net_limit_init,
cap_net_limit_name2addr, cap_net_limit_name2addr_family, - library for
networking in capability mode
LIBRARY
library "libcap_net"
SYNOPSIS
#include <sys/nv.h>
#include <libcasper.h>
#include <casper/cap_net.h>
int
cap_bind(cap_channel_t *chan, int s, const struct sockaddr *addr,
socklen_t addrlen);
int
cap_connect(cap_channel_t *chan, int s, const struct sockaddr *name,
socklen_t namelen);
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);
cap_net_limit_t *
cap_net_limit_init(cap_channel_t *chan, uint64_t mode);
int
cap_net_limit(cap_net_limit_t *limit);
void
cap_net_free(cap_net_limit_t *limit);
cap_net_limit_t *
cap_net_limit_addr2name_family(cap_net_limit_t *limit, int *family,
size_t size);
cap_net_limit_t *
cap_net_limit_addr2name(cap_net_limit_t *limit,
const struct sockaddr *sa, socklen_t salen);
cap_net_limit_t *
cap_net_limit_name2addr_family(cap_net_limit_t *limit, int *family,
size_t size);
cap_net_limit_t *
cap_net_limit_name2addr(cap_net_limit_t *limit, const char *name,
const char *serv);
cap_net_limit_t *
cap_net_limit_connect(cap_net_limit_t *limit, const struct sockaddr *sa,
socklen_t salen);
cap_net_limit_t *
cap_net_limit_bind(cap_net_limit_t *limit, const struct sockaddr *sa,
socklen_t salen);
DESCRIPTION
The functions cap_bind(), cap_connect(), cap_gethostbyname(),
cap_gethostbyname2(), cap_gethostbyaddr() and cap_getnameinfo() are
respectively equivalent to bind(2), connect(2), gethostbyname(3),
gethostbyname2(3), gethostbyaddr(3) and getnameinfo(3) except that the
connection to the system.net service needs to be provided.
LIMITS
By default, the cap_net capability provides unrestricted access to the
network namespace. Applications typically only require access to a small
portion of the network namespace: cap_net_limit() interface can be used
to restrict access to the network. cap_net_limit_init() returns an
opaque limit handle used to store a list of capabilities. The restricts
the functionality of the service. Modes are encoded using the following
flags:
CAPNET_ADDR2NAME reverse DNS lookups are allowed with
cap_getnameinfo
CAPNET_NAME2ADDR name resolution is allowed with
cap_getaddrinfo
CAPNET_DEPRECATED_ADDR2NAME reverse DNS lookups are allowed with
cap_gethostbyaddr
CAPNET_DEPRECATED_NAME2ADDR name resolution is allowed with
cap_gethostbyname and cap_gethostbyname2
CAPNET_BIND bind syscall is allowed
CAPNET_CONNECT connect syscall is allowed
CAPNET_CONNECTDNS connect syscall is allowed to the values
returned from privies call to
the cap_getaddrinfo or cap_gethostbyname
cap_net_limit_addr2name_family() limits the cap_getnameinfo() and
cap_gethostbyaddr() to do reverse DNS lookups to specific family
(AF_INET, AF_INET6, etc.)
cap_net_limit_addr2name() limits the cap_getnameinfo() and
cap_gethostbyaddr() to do reverse DNS lookups only on those specific
structures.
cap_net_limit_name2addr_family() limits the cap_getaddrinfo(),
cap_gethostbyname() and cap_gethostbyname2() to do the name resolution on
specific family (AF_INET, AF_INET6, etc.)
cap_net_limit_addr2name() restricts cap_getaddrinfo(),
cap_gethostbyname() and cap_gethostbyname2() to a set of domains.
cap_net_limit_bind() limits cap_bind() to bind only on those specific
structures.
cap_net_limit_connect() limits cap_connect() to connect only on those
specific structures. If the CAPNET_CONNECTDNS is set the limits are
extended to the values returned by cap_getaddrinfo(), cap_gethostbyname()
and cap_gethostbyname2() In case of the cap_getaddrinfo() the restriction
is strict. In case of the cap_gethostbyname() and cap_gethostbyname2()
any port will be accepted in the cap_connect() function.
cap_net_limit() applies a set of sysctl limits to the capability, denying
access to sysctl variables not belonging to the set.
Once a set of limits is applied, subsequent calls to cap_net_limit() will
fail unless the new set is a subset of the current set.
The cap_net_limit() will consume the limits. If the cap_net_limit() was
not called the rights may be freed using cap_net_free(). Multiple calls
to cap_net_limit_addr2name_family(), cap_net_limit_addr2name(),
cap_net_limit_name2addr_family(), cap_net_limit_name2addr(),
cap_net_limit_connect(), and cap_net_limit_bind() is supported, each call
is extending preview capabilities.
EXAMPLES
The following example first opens a capability to casper and then uses
this capability to create the system.net casper service and uses it to
resolve a host and connect to it.
cap_channel_t *capcas, *capnet;
cap_net_limit_t *limit;
int familylimit, error, s;
const char *host = "example.com";
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_casper() < 0)
err(1, "Unable to enter capability mode");
/* Use Casper capability to create capability to the system.net service. */
capnet = cap_service_open(capcas, "system.net");
if (capnet == NULL)
err(1, "Unable to open system.net service");
/* Close Casper capability. */
cap_close(capcas);
/* Limit system.net to reserve IPv4 addresses, to host example.com . */
limit = cap_net_limit_init(capnet, CAPNET_NAME2ADDR | CAPNET_CONNECTDNS);
if (limit == NULL)
err(1, "Unable to create limits.");
cap_net_limit_name2addr(limit, host, "80");
familylimit = AF_INET;
cap_net_limit_name2addr_family(limit, &familylimit, 1);
if (cap_net_limit(limit) < 0)
err(1, "Unable to apply limits.");
/* Find IP addresses for the given host. */
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
error = cap_getaddrinfo(capnet, host, "80", &hints, &res);
if (error != 0)
errx(1, "cap_getaddrinfo(): %s: %s", host, gai_strerror(error));
s = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
if (s < 0)
err(1, "Unable to create socket");
if (cap_connect(capnet, s, res->ai_addr, res->ai_addrlen) < 0)
err(1, "Unable to connect to host");
SEE ALSO
bind(2), cap_enter(2), connect(2), caph_enter(3), err(3),
gethostbyaddr(3), gethostbyname(3), gethostbyname2(3), getnameinfo(3),
capsicum(4), nv(9)
AUTHORS
Mariusz Zaborski <oshogbo@FreeBSD.org>
FreeBSD 13.1-RELEASE-p6 August 15, 2020 FreeBSD 13.1-RELEASE-p6
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