.\" Copyright (c) 2020 Mariusz Zaborski <oshogbo@FreeBSD.org>
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.Dd November 15, 2021
.Dt CAP_NET 3
.Os
.Sh NAME
.Nm cap_bind ,
.Nm cap_connect ,
.Nm cap_getaddrinfo ,
.Nm cap_gethostbyaddr ,
.Nm cap_gethostbyname ,
.Nm cap_gethostbyname2 ,
.Nm cap_getnameinfo ,
.Nm cap_net_free ,
.Nm cap_net_limit ,
.Nm cap_net_limit_addr2name ,
.Nm cap_net_limit_addr2name_family ,
.Nm cap_net_limit_bind ,
.Nm cap_net_limit_connect ,
.Nm cap_net_limit_init ,
.Nm cap_net_limit_name2addr ,
.Nm cap_net_limit_name2addr_family ,
.Nd "library for networking in capability mode"
.Sh LIBRARY
.Lb libcap_net
.Sh SYNOPSIS
.In sys/nv.h
.In libcasper.h
.In casper/cap_net.h
.Ft int
.Fn cap_bind "cap_channel_t *chan" "int s" "const struct sockaddr *addr" "socklen_t addrlen"
.Ft int
.Fn cap_connect "cap_channel_t *chan" "int s" "const struct sockaddr *name" "socklen_t namelen"
.Ft int
.Fn cap_getaddrinfo "cap_channel_t *chan" "const char *hostname" "const char *servname" "const struct addrinfo *hints" "struct addrinfo **res"
.Ft int
.Fn 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"
.Ft "struct hostent *"
.Fn cap_gethostbyname "const cap_channel_t *chan" "const char *name"
.Ft "struct hostent *"
.Fn cap_gethostbyname2 "const cap_channel_t *chan" "const char *name" "int af"
.Ft "struct hostent *"
.Fn cap_gethostbyaddr "const cap_channel_t *chan" "const void *addr" "socklen_t len" "int af"
.Ft "cap_net_limit_t *"
.Fn cap_net_limit_init "cap_channel_t *chan" "uint64_t mode"
.Ft int
.Fn cap_net_limit "cap_net_limit_t *limit"
.Ft void
.Fn cap_net_free "cap_net_limit_t *limit"
.Ft "cap_net_limit_t *"
.Fn cap_net_limit_addr2name_family "cap_net_limit_t *limit" "int *family" "size_t size"
.Ft "cap_net_limit_t *"
.Fn cap_net_limit_addr2name "cap_net_limit_t *limit" "const struct sockaddr *sa" "socklen_t salen"
.Ft "cap_net_limit_t *"
.Fn cap_net_limit_name2addr_family "cap_net_limit_t *limit" "int *family" "size_t size"
.Ft "cap_net_limit_t *"
.Fn cap_net_limit_name2addr "cap_net_limit_t *limit" "const char *name" "const char *serv"
.Ft "cap_net_limit_t *"
.Fn cap_net_limit_connect "cap_net_limit_t *limit" "const struct sockaddr *sa" "socklen_t salen"
.Ft "cap_net_limit_t *"
.Fn cap_net_limit_bind "cap_net_limit_t *limit" "const struct sockaddr *sa" "socklen_t salen"
.Sh DESCRIPTION
The functions
.Fn cap_bind ,
.Fn cap_connect ,
.Fn cap_gethostbyname ,
.Fn cap_gethostbyname2 ,
.Fn cap_gethostbyaddr
and
.Fn cap_getnameinfo
provide a set of APIs equivalent to
.Xr bind 2 ,
.Xr connect 2 ,
.Xr gethostbyname 3 ,
.Xr gethostbyname2 3 ,
.Xr gethostbyaddr 3
and
.Xr getnameinfo 3
except that a connection to the
.Nm system.net
service needs to be provided.
.Sh 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:
The
.Fn cap_net_limit
function can be used to restrict access to the network.
The
.Fn cap_net_limit_init
returns an opaque limit handle used to store a list of capabilities.
The
.Fv mode
restricts the functionality of the service.
Modes are encoded using the following flags:
.Pp
.Bd -literal -offset indent -compact
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 previous call to
				the cap_getaddrinfo or cap_gethostbyname
.Ed
.Pp
.Fn cap_net_limit_addr2name_family
limits the
.Fn cap_getnameinfo
and
.Fn cap_gethostbyaddr
to do reverse DNS lookups to specific family (AF_INET, AF_INET6, etc.)
.Pp
.Fn cap_net_limit_addr2name
limits the
.Fn cap_getnameinfo
and
.Fn cap_gethostbyaddr
to do reverse DNS lookups only on those specific structures.
.Pp
.Fn cap_net_limit_name2addr_family
limits the
.Fn cap_getaddrinfo ,
.Fn cap_gethostbyname
and
.Fn cap_gethostbyname2
to do the name resolution on specific family (AF_INET, AF_INET6, etc.)
.Pp
.Fn cap_net_limit_addr2name
restricts
.Fn cap_getaddrinfo ,
.Fn cap_gethostbyname
and
.Fn cap_gethostbyname2
to a set of domains.
.Pp
.Fn cap_net_limit_bind
limits
.Fn cap_bind
to bind only on those specific structures.
.Pp
.Fn cap_net_limit_connect
limits
.Fn cap_connect
to connect only on those specific structures.
If the CAPNET_CONNECTDNS is set the limits are extended to the values returned
by
.Fn cap_getaddrinfo ,
.Fn cap_gethostbyname
and
.Fn cap_gethostbyname2
In case of the
.Fn cap_getaddrinfo
the restriction is strict.
In case of the
.Fn cap_gethostbyname
and
.Fn cap_gethostbyname2
any port will be accepted in the
.Fn cap_connect
function.
.Pp
.Fn cap_net_limit
applies a set of sysctl limits to the capability, denying access to sysctl
variables not belonging to the set.
.Pp
Once a set of limits is applied, subsequent calls to
.Fn cap_net_limit
will fail unless the new set is a subset of the current set.
.Pp
The
.Fn cap_net_limit
will consume the limits.
If the
.Fn cap_net_limit
was not called the rights may be freed using
.Fn cap_net_free .
Multiple calls to
.Fn cap_net_limit_addr2name_family ,
.Fn cap_net_limit_addr2name ,
.Fn cap_net_limit_name2addr_family ,
.Fn cap_net_limit_name2addr ,
.Fn cap_net_limit_connect ,
and
.Fn cap_net_limit_bind
is supported, each call is extending preview capabilities.
.Sh EXAMPLES
The following example first opens a capability to casper and then uses this
capability to create the
.Nm system.net
casper service and uses it to resolve a host and connect to it.
.Bd -literal
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");
.Ed
.Sh SEE ALSO
.Xr bind 2 ,
.Xr cap_enter 2 ,
.Xr connect 2 ,
.Xr caph_enter 3 ,
.Xr err 3 ,
.Xr gethostbyaddr 3 ,
.Xr gethostbyname 3 ,
.Xr gethostbyname2 3 ,
.Xr getnameinfo 3 ,
.Xr capsicum 4 ,
.Xr nv 9
.Sh AUTHORS
.An Mariusz Zaborski Aq Mt oshogbo@FreeBSD.org