156 lines
5.0 KiB
ReStructuredText
156 lines
5.0 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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===================
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DNS Resolver Module
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===================
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.. Contents:
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- Overview.
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- Compilation.
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- Setting up.
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- Usage.
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- Mechanism.
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- Debugging.
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Overview
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========
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The DNS resolver module provides a way for kernel services to make DNS queries
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by way of requesting a key of key type dns_resolver. These queries are
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upcalled to userspace through /sbin/request-key.
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These routines must be supported by userspace tools dns.upcall, cifs.upcall and
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request-key. It is under development and does not yet provide the full feature
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set. The features it does support include:
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(*) Implements the dns_resolver key_type to contact userspace.
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It does not yet support the following AFS features:
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(*) Dns query support for AFSDB resource record.
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This code is extracted from the CIFS filesystem.
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Compilation
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===========
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The module should be enabled by turning on the kernel configuration options::
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CONFIG_DNS_RESOLVER - tristate "DNS Resolver support"
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Setting up
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==========
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To set up this facility, the /etc/request-key.conf file must be altered so that
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/sbin/request-key can appropriately direct the upcalls. For example, to handle
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basic dname to IPv4/IPv6 address resolution, the following line should be
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added::
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#OP TYPE DESC CO-INFO PROGRAM ARG1 ARG2 ARG3 ...
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#====== ============ ======= ======= ==========================
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create dns_resolver * * /usr/sbin/cifs.upcall %k
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To direct a query for query type 'foo', a line of the following should be added
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before the more general line given above as the first match is the one taken::
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create dns_resolver foo:* * /usr/sbin/dns.foo %k
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Usage
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=====
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To make use of this facility, one of the following functions that are
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implemented in the module can be called after doing::
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#include <linux/dns_resolver.h>
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::
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int dns_query(const char *type, const char *name, size_t namelen,
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const char *options, char **_result, time_t *_expiry);
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This is the basic access function. It looks for a cached DNS query and if
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it doesn't find it, it upcalls to userspace to make a new DNS query, which
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may then be cached. The key description is constructed as a string of the
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form::
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[<type>:]<name>
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where <type> optionally specifies the particular upcall program to invoke,
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and thus the type of query to do, and <name> specifies the string to be
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looked up. The default query type is a straight hostname to IP address
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set lookup.
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The name parameter is not required to be a NUL-terminated string, and its
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length should be given by the namelen argument.
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The options parameter may be NULL or it may be a set of options
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appropriate to the query type.
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The return value is a string appropriate to the query type. For instance,
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for the default query type it is just a list of comma-separated IPv4 and
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IPv6 addresses. The caller must free the result.
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The length of the result string is returned on success, and a negative
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error code is returned otherwise. -EKEYREJECTED will be returned if the
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DNS lookup failed.
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If _expiry is non-NULL, the expiry time (TTL) of the result will be
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returned also.
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The kernel maintains an internal keyring in which it caches looked up keys.
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This can be cleared by any process that has the CAP_SYS_ADMIN capability by
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the use of KEYCTL_KEYRING_CLEAR on the keyring ID.
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Reading DNS Keys from Userspace
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===============================
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Keys of dns_resolver type can be read from userspace using keyctl_read() or
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"keyctl read/print/pipe".
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Mechanism
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=========
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The dnsresolver module registers a key type called "dns_resolver". Keys of
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this type are used to transport and cache DNS lookup results from userspace.
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When dns_query() is invoked, it calls request_key() to search the local
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keyrings for a cached DNS result. If that fails to find one, it upcalls to
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userspace to get a new result.
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Upcalls to userspace are made through the request_key() upcall vector, and are
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directed by means of configuration lines in /etc/request-key.conf that tell
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/sbin/request-key what program to run to instantiate the key.
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The upcall handler program is responsible for querying the DNS, processing the
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result into a form suitable for passing to the keyctl_instantiate_key()
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routine. This then passes the data to dns_resolver_instantiate() which strips
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off and processes any options included in the data, and then attaches the
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remainder of the string to the key as its payload.
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The upcall handler program should set the expiry time on the key to that of the
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lowest TTL of all the records it has extracted a result from. This means that
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the key will be discarded and recreated when the data it holds has expired.
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dns_query() returns a copy of the value attached to the key, or an error if
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that is indicated instead.
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See <file:Documentation/security/keys/request-key.rst> for further
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information about request-key function.
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Debugging
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=========
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Debugging messages can be turned on dynamically by writing a 1 into the
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following file::
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/sys/module/dnsresolver/parameters/debug
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