158 lines
5.1 KiB
Plaintext
158 lines
5.1 KiB
Plaintext
===================
|
|
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
|