.\" Copyright (c) 2018 Mariusz Zaborski <oshogbo@FreeBSD.org>
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.Dd March 18, 2018
.Dt CAP_DNS 3
.Os
.Sh NAME
.Nm cap_gethostbyname ,
.Nm cap_gethostbyname2 ,
.Nm cap_gethostbyaddr ,
.Nm cap_getnameinfo ,
.Nm cap_dns_type_limit ,
.Nm cap_dns_family_limit
.Nd "library for getting network host entry in capability mode"
.Sh LIBRARY
.Lb libcap_dns
.Sh SYNOPSIS
.In sys/nv.h
.In libcasper.h
.In casper/cap_dns.h
.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 "int"
.Fn cap_getnameinfo "const cap_channel_t *chan" "const void *name" "int namelen"
.Ft "int"
.Fn cap_dns_type_limit "cap_channel_t *chan" "const char * const *types" "size_t ntypes"
.Ft "int"
.Fn cap_dns_family_limit "const cap_channel_t *chan" "const int *families" "size_t nfamilies"
.Sh DESCRIPTION
The functions
.Fn cap_gethostbyname ,
.Fn cap_gethostbyname2 ,
.Fn cep_gethostbyaddr
and
.Fn cap_getnameinfo
are respectively equivalent to
.Xr gethostbyname 2 ,
.Xr gethostbyname2 2 ,
.Xr gethostbyaddr 2
and
.Xr getnameinfo 2
except that the connection to the
.Nm system.dns
service needs to be provided.
.Pp
The
.Fn cap_dns_type_limit
function limits the functions allowed in the service.
The
.Fa types
variable can be set to
.Dv ADDR
or
.Dv NAME .
See the
.Sx LIMITS
section for more details.
The
.Fa ntpyes
variable contains the number of
.Fa types
provided.
.Pp
The
.Fn cap_dns_family_limit
functions allows to limit address families.
For details see
.Sx LIMITS .
The
.Fa nfamilies
variable contains the number of
.Fa families
provided.
.Sh LIMITS
The preferred way of setting limits is to use the
.Fn cap_dns_type_limit
and
.Fn cap_dns_family_limit
functions, but the limits of service can be set also using
.Xr cap_limit_set 3 .
The
.Xr nvlist 9
for that function can contain the following values and types:
.Bl -ohang -offset indent
.It type ( NV_TYPE_STRING )
The
.Va type
can have two values:
.Dv ADDR
or
.Dv NAME .
The
.Dv ADDR
means that functions
.Fn cap_gethostbyname ,
.Fn cap_gethostbyname2
and
.Fn cap_gethostbyaddr
are allowed.
In case when
.Va type
is set to
.Dv NAME
the
.Fn cap_getnameinfo
function is allowed.
.It family ( NV_TYPE_NUMBER )
The
.Va family
limits service to one of the address families (e.g.
.Dv AF_INET , AF_INET6 ,
etc.).
.Sh EXAMPLES
The following example first opens a capability to casper and then uses this
capability to create the
.Nm system.dns
casper service and uses it to resolve an IP address.
.Bd -literal
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);
.Ed
.Sh SEE ALSO
.Xr cap_enter 2 ,
.Xr err 3 ,
.Xr gethostbyaddr 3 ,
.Xr gethostbyname 3 ,
.Xr gethostbyname2 3 ,
.Xr getnameinfo 3 ,
.Xr capsicum 4 ,
.Xr nv 9
.Sh AUTHORS
The
.Nm cap_dns
service was implemented by
.An Pawel Jakub Dawidek Aq Mt pawel@dawidek.net
under sponsorship from the FreeBSD Foundation.
.Pp
This manual page was written by
.An Mariusz Zaborski Aq Mt oshogbo@FreeBSD.org .