[GitHub/mt8127/android_kernel_alcatel_ttab.git] / Documentation / networking / dns_resolver.txt

			     ===================
			     DNS Resolver Module
			     ===================

Contents:

 - Overview.
 - Compilation.
 - Setting up.
 - Usage.
 - Mechanism.
 - Debugging.


========
OVERVIEW
========

The DNS resolver module provides a way for kernel services to make DNS queries
by way of requesting a key of key type dns_resolver.  These queries are
upcalled to userspace through /sbin/request-key.

These routines must be supported by userspace tools dns.upcall, cifs.upcall and
request-key.  It is under development and does not yet provide the full feature
set.  The features it does support include:

 (*) Implements the dns_resolver key_type to contact userspace.

It does not yet support the following AFS features:

 (*) Dns query support for AFSDB resource record.

This code is extracted from the CIFS filesystem.


===========
COMPILATION
===========

The module should be enabled by turning on the kernel configuration options:

	CONFIG_DNS_RESOLVER	- tristate "DNS Resolver support"


==========
SETTING UP
==========

To set up this facility, the /etc/request-key.conf file must be altered so that
/sbin/request-key can appropriately direct the upcalls.  For example, to handle
basic dname to IPv4/IPv6 address resolution, the following line should be
added:

	#OP	TYPE		DESC	CO-INFO	PROGRAM ARG1 ARG2 ARG3 ...
	#======	============	=======	=======	==========================
	create	dns_resolver  	*	*	/usr/sbin/cifs.upcall %k

To direct a query for query type 'foo', a line of the following should be added
before the more general line given above as the first match is the one taken.

	create	dns_resolver  	foo:*	*	/usr/sbin/dns.foo %k


=====
USAGE
=====

To make use of this facility, one of the following functions that are
implemented in the module can be called after doing:

	#include <linux/dns_resolver.h>

 (1) int dns_query(const char *type, const char *name, size_t namelen,
		   const char *options, char **_result, time_t *_expiry);

     This is the basic access function.  It looks for a cached DNS query and if
     it doesn't find it, it upcalls to userspace to make a new DNS query, which
     may then be cached.  The key description is constructed as a string of the
     form:

		[<type>:]<name>

     where <type> optionally specifies the particular upcall program to invoke,
     and thus the type of query to do, and <name> specifies the string to be
     looked up.  The default query type is a straight hostname to IP address
     set lookup.

     The name parameter is not required to be a NUL-terminated string, and its
     length should be given by the namelen argument.

     The options parameter may be NULL or it may be a set of options
     appropriate to the query type.

     The return value is a string appropriate to the query type.  For instance,
     for the default query type it is just a list of comma-separated IPv4 and
     IPv6 addresses.  The caller must free the result.

     The length of the result string is returned on success, and a negative
     error code is returned otherwise.  -EKEYREJECTED will be returned if the
     DNS lookup failed.

     If _expiry is non-NULL, the expiry time (TTL) of the result will be
     returned also.

The kernel maintains an internal keyring in which it caches looked up keys.
This can be cleared by any process that has the CAP_SYS_ADMIN capability by
the use of KEYCTL_KEYRING_CLEAR on the keyring ID.


===============================
READING DNS KEYS FROM USERSPACE
===============================

Keys of dns_resolver type can be read from userspace using keyctl_read() or
"keyctl read/print/pipe".


=========
MECHANISM
=========

The dnsresolver module registers a key type called "dns_resolver".  Keys of
this type are used to transport and cache DNS lookup results from userspace.

When dns_query() is invoked, it calls request_key() to search the local
keyrings for a cached DNS result.  If that fails to find one, it upcalls to
userspace to get a new result.

Upcalls to userspace are made through the request_key() upcall vector, and are
directed by means of configuration lines in /etc/request-key.conf that tell
/sbin/request-key what program to run to instantiate the key.

The upcall handler program is responsible for querying the DNS, processing the
result into a form suitable for passing to the keyctl_instantiate_key()
routine.  This then passes the data to dns_resolver_instantiate() which strips
off and processes any options included in the data, and then attaches the
remainder of the string to the key as its payload.

The upcall handler program should set the expiry time on the key to that of the
lowest TTL of all the records it has extracted a result from.  This means that
the key will be discarded and recreated when the data it holds has expired.

dns_query() returns a copy of the value attached to the key, or an error if
that is indicated instead.

See <file:Documentation/security/keys-request-key.txt> for further
information about request-key function.


=========
DEBUGGING
=========

Debugging messages can be turned on dynamically by writing a 1 into the
following file:

        /sys/module/dnsresolver/parameters/debug
Commit	Line	Data
1a4240f4 WL	1	===================
	2	DNS Resolver Module
	3	===================
	4
	5	Contents:
	6
	7	- Overview.
	8	- Compilation.
	9	- Setting up.
	10	- Usage.
	11	- Mechanism.
	12	- Debugging.
	13
	14
	15	========
	16	OVERVIEW
	17	========
	18
	19	The DNS resolver module provides a way for kernel services to make DNS queries
	20	by way of requesting a key of key type dns_resolver. These queries are
	21	upcalled to userspace through /sbin/request-key.
	22
	23	These routines must be supported by userspace tools dns.upcall, cifs.upcall and
	24	request-key. It is under development and does not yet provide the full feature
	25	set. The features it does support include:
	26
	27	(*) Implements the dns_resolver key_type to contact userspace.
	28
	29	It does not yet support the following AFS features:
	30
	31	(*) Dns query support for AFSDB resource record.
	32
	33	This code is extracted from the CIFS filesystem.
	34
	35
	36	===========
	37	COMPILATION
	38	===========
	39
	40	The module should be enabled by turning on the kernel configuration options:
	41
	42	CONFIG_DNS_RESOLVER - tristate "DNS Resolver support"
	43
	44
	45	==========
	46	SETTING UP
	47	==========
	48
	49	To set up this facility, the /etc/request-key.conf file must be altered so that
	50	/sbin/request-key can appropriately direct the upcalls. For example, to handle
	51	basic dname to IPv4/IPv6 address resolution, the following line should be
	52	added:
	53
	54	#OP TYPE DESC CO-INFO PROGRAM ARG1 ARG2 ARG3 ...
	55	#====== ============ ======= ======= ==========================
	56	create dns_resolver * * /usr/sbin/cifs.upcall %k
	57
	58	To direct a query for query type 'foo', a line of the following should be added
	59	before the more general line given above as the first match is the one taken.
	60
	61	create dns_resolver foo:* * /usr/sbin/dns.foo %k
	62
	63
1a4240f4 WL	64	=====
	65	USAGE
	66	=====
	67
	68	To make use of this facility, one of the following functions that are
	69	implemented in the module can be called after doing:
	70
	71	#include <linux/dns_resolver.h>
	72
	73	(1) int dns_query(const char type, const char name, size_t namelen,
	74	const char options, char _result, time_t _expiry);
	75
	76	This is the basic access function. It looks for a cached DNS query and if
	77	it doesn't find it, it upcalls to userspace to make a new DNS query, which
	78	may then be cached. The key description is constructed as a string of the
	79	form:
	80
	81	[<type>:]<name>
	82
	83	where <type> optionally specifies the particular upcall program to invoke,
	84	and thus the type of query to do, and <name> specifies the string to be
	85	looked up. The default query type is a straight hostname to IP address
	86	set lookup.
	87
	88	The name parameter is not required to be a NUL-terminated string, and its
	89	length should be given by the namelen argument.
	90
	91	The options parameter may be NULL or it may be a set of options
	92	appropriate to the query type.
	93
	94	The return value is a string appropriate to the query type. For instance,
	95	for the default query type it is just a list of comma-separated IPv4 and
	96	IPv6 addresses. The caller must free the result.
	97
ff9517a6 DH	98	The length of the result string is returned on success, and a negative
	99	error code is returned otherwise. -EKEYREJECTED will be returned if the
	100	DNS lookup failed.
1a4240f4 WL	101
	102	If _expiry is non-NULL, the expiry time (TTL) of the result will be
	103	returned also.
	104
700920eb DH	105	The kernel maintains an internal keyring in which it caches looked up keys.
	106	This can be cleared by any process that has the CAP_SYS_ADMIN capability by
	107	the use of KEYCTL_KEYRING_CLEAR on the keyring ID.
	108
1a4240f4	109
1362fa07 DH	110	===============================
	111	READING DNS KEYS FROM USERSPACE
	112	===============================
	113
	114	Keys of dns_resolver type can be read from userspace using keyctl_read() or
	115	"keyctl read/print/pipe".
	116
	117
1a4240f4 WL	118	=========
	119	MECHANISM
	120	=========
	121
	122	The dnsresolver module registers a key type called "dns_resolver". Keys of
	123	this type are used to transport and cache DNS lookup results from userspace.
	124
	125	When dns_query() is invoked, it calls request_key() to search the local
	126	keyrings for a cached DNS result. If that fails to find one, it upcalls to
	127	userspace to get a new result.
	128
	129	Upcalls to userspace are made through the request_key() upcall vector, and are
	130	directed by means of configuration lines in /etc/request-key.conf that tell
	131	/sbin/request-key what program to run to instantiate the key.
	132
	133	The upcall handler program is responsible for querying the DNS, processing the
	134	result into a form suitable for passing to the keyctl_instantiate_key()
	135	routine. This then passes the data to dns_resolver_instantiate() which strips
	136	off and processes any options included in the data, and then attaches the
	137	remainder of the string to the key as its payload.
	138
	139	The upcall handler program should set the expiry time on the key to that of the
	140	lowest TTL of all the records it has extracted a result from. This means that
	141	the key will be discarded and recreated when the data it holds has expired.
	142
	143	dns_query() returns a copy of the value attached to the key, or an error if
	144	that is indicated instead.
	145
d410fa4e RD	146	See <file:Documentation/security/keys-request-key.txt> for further
d410fa4e RD	147	information about request-key function.
1a4240f4 WL	148
	149
	150	=========
	151	DEBUGGING
	152	=========
	153
	154	Debugging messages can be turned on dynamically by writing a 1 into the
	155	following file:
	156
	157	/sys/module/dnsresolver/parameters/debug