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.Dd May 2, 2015
.Dt LIBCAPSICUM 3
.Os
.Sh NAME
.Nm cap_init ,
.Nm cap_wrap ,
.Nm cap_unwrap ,
.Nm cap_sock ,
.Nm cap_clone ,
.Nm cap_close ,
.Nm cap_limit_get ,
.Nm cap_limit_set ,
.Nm cap_send_nvlist ,
.Nm cap_recv_nvlist ,
.Nm cap_xfer_nvlist ,
.Nm cap_service_open
.Nd "library for handling application capabilities"
.Sh LIBRARY
.Lb libcapsicum
.Sh SYNOPSIS
.In libcapsicum.h
.In nv.h
.Ft "cap_channel_t *"
.Fn cap_init "void"
.Ft "cap_channel_t *"
.Fn cap_wrap "int sock"
.Ft "int"
.Fn cap_unwrap "cap_channel_t *chan"
.Ft "int"
.Fn cap_sock "const cap_channel_t *chan"
.Ft "cap_channel_t *"
.Fn cap_clone "const cap_channel_t *chan"
.Ft "void"
.Fn cap_close "cap_channel_t *chan"
.Ft "int"
.Fn cap_limit_get "const cap_channel_t *chan" "nvlist_t **limitsp"
.Ft "int"
.Fn cap_limit_set "const cap_channel_t *chan" "nvlist_t *limits"
.Ft "int"
.Fn cap_send_nvlist "const cap_channel_t *chan" "const nvlist_t *nvl"
.Ft "nvlist_t *"
.Fn cap_recv_nvlist "const cap_channel_t *chan" "int flags"
.Ft "nvlist_t *"
.Fn cap_xfer_nvlist "const cap_channel_t *chan" "nvlist_t *nvl" "int flags"
.In libcapsicum_service.h
.Ft "cap_channel_t *"
.Fn cap_service_open "const cap_channel_t *chan" "const char *name"
.Sh DESCRIPTION
The
.Nm libcapsicum
library allows to manage application capabilities through the
.Xr casperd 8
daemon.
.Pp
The application capability (represented by the
.Vt cap_channel_t
type) is a communication channel between the caller and the
.Xr casperd 8
daemon or an instance of one of its services.
A capability to the
.Xr casperd 8
daemon obtained with the
.Fn cap_init
function allows to create capabilities to casper's services via the
.Fn cap_service_open
function.
.Pp
The
.Fn cap_init
function opens capability to the
.Xr casperd 8
daemon.
.Pp
The
.Fn cap_wrap
function creates
.Vt cap_channel_t
based on the given socket.
The function is used when capability is inherited through
.Xr execve 2
or send over
.Xr unix 4
domain socket as a regular file descriptor and has to be represented as
.Vt cap_channel_t
again.
.Pp
The
.Fn cap_unwrap
function is the opposite of the
.Fn cap_wrap
function.
It frees the
.Vt cap_channel_t
structure and returns
.Xr unix 4
domain socket associated with it.
.Pp
The
.Fn cap_clone
function clones the given capability.
.Pp
The
.Fn cap_close
function closes the given capability.
.Pp
The
.Fn cap_sock
function returns
.Xr unix 4
domain socket descriptor associated with the given capability for use with
system calls like
.Xr kevent 2 ,
.Xr poll 2
and
.Xr select 2 .
.Pp
The
.Fn cap_limit_get
function stores current limits of the given capability in the
.Fa limitsp
argument.
If the function return
.Va 0
and
.Dv NULL
is stored in
.Fa limitsp
it means there are no limits set.
.Pp
The
.Fn cap_limit_set
function sets limits for the given capability.
The limits are provided as nvlist.
The exact format depends on the service the capability represents.
.Pp
The
.Fn cap_send_nvlist
function sends the given nvlist over the given capability.
This is low level interface to communicate with casper services.
Most services should provide higher level API.
.Pp
The
.Fn cap_recv_nvlist
function receives the given nvlist over the given capability.
The
.Fa flags
argument defines what type the top nvlist is expected to be.
If the nvlist flags do not match the flags passed to
.Fn cap_recv_nvlist ,
the nvlist will not be returned.
.Pp
The
.Fn cap_xfer_nvlist
function sends the given nvlist, destroys it and receives new nvlist in
response over the given capability.
The
.Fa flags
argument defines what type the top nvlist is expected to be.
If the nvlist flags do not match the flags passed to
.Fn cap_xfer_nvlist ,
the nvlist will not be returned.
It does not matter if the function succeeds or fails, the nvlist given
for sending will always be destroyed once the function returns.
.Pp
The
.Fn cap_service_open
function opens casper service of the given name through casper capability
obtained via the
.Fn cap_init
function.
The function returns capability that provides access to opened service.
.Sh RETURN VALUES
The
.Fn cap_clone ,
.Fn cap_init ,
.Fn cap_recv_nvlist ,
.Fn cap_service_open ,
.Fn cap_wrap
and
.Fn cap_xfer_nvlist
functions return
.Dv NULL
and set the
.Va errno
variable on failure.
.Pp
The
.Fn cap_limit_get ,
.Fn cap_limit_set
and
.Fn cap_send_nvlist
functions return
.Dv -1
and set the
.Va errno
variable on failure.
.Pp
The
.Fn cap_close ,
.Fn cap_sock
and
.Fn cap_unwrap
functions always succeed.
.Sh EXAMPLES
The following example first opens capability to the
.Xr casperd 8
daemon, then using this capability creates new capability to the
.Nm system.dns
casper service and uses the latter capability to resolve IP address.
.Bd -literal
cap_channel_t *capcas, *capdns;
nvlist_t *limits;
const char *ipstr = "127.0.0.1";
struct in_addr ip;
struct hostent *hp;

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

/* 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 and IPv4 addresses. */
limits = nvlist_create(0);
nvlist_add_string(limits, "type", "ADDR");
nvlist_add_number(limits, "family", (uint64_t)AF_INET);
if (cap_limit_set(capdns, limits) < 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);
.Ed
.Sh SEE ALSO
.Xr cap_enter 2 ,
.Xr execve 2 ,
.Xr kevent 2 ,
.Xr poll 2 ,
.Xr select 2 ,
.Xr cap_gethostbyaddr 3 ,
.Xr err 3 ,
.Xr gethostbyaddr 3 ,
.Xr inet_aton 3 ,
.Xr nv 3 ,
.Xr capsicum 4 ,
.Xr unix 4 ,
.Xr casperd 8
.Sh AUTHORS
The
.Nm libcapsicum
library was implemented by
.An Pawel Jakub Dawidek Aq Mt pawel@dawidek.net
under sponsorship from the FreeBSD Foundation.