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- ===================
- 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