.\" Copyright (c) 2018 Mariusz Zaborski <oshogbo@FreeBSD.org>
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.Dd November 18, 2018
.Dt CAP_DNS 3
.Os
.Sh NAME
.Nm cap_getaddrinfo ,
.Nm cap_getnameinfo ,
.Nm cap_gethostbyname ,
.Nm cap_gethostbyname2 ,
.Nm cap_gethostbyaddr ,
.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 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 "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
.Bf -symbolic
The
.Fn cap_getaddrinfo ,
and
.Fn cap_getnameinfo ,
functions are preferred over the
.Fn cap_gethostbyname ,
.Fn cap_gethostbyname2 ,
and
.Fn cap_gethostbyaddr
functions.
.Ef
.Pp
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 ADDR2NAME
or
.Dv NAME2ADDR .
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 ADDR2NAME
or
.Dv NAME2ADDR .
The
.Dv ADDR2NAME
means that reverse DNS lookups are allowed with
.Fn cap_getnameinfo
and
.Fn cap_gethostbyaddr
functions.
In case when
.Va type
is set to
.Dv NAME2ADDR
the name resolution is allowed with
.Fn cap_getaddrinfo ,
.Fn cap_gethostbyname ,
and
.Fn cap_gethostbyname2
functions.
.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;
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);
.Ed
.Sh SEE ALSO
.Xr cap_enter 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
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 .