.. SPDX-License-Identifier: GPL-3.0-or-later .. _runtime-cfg: Run-time reconfiguration ======================== Knot Resolver offers several ways to modify its configuration at run-time: - Using control socket driven by an external system - Using Lua program embeded in Resolver's configuration file Both ways can also be combined: For example the configuration file can contain a little Lua function which gathers statistics and returns them in JSON string. This can be used by an external system which uses control socket to call this user-defined function and to retrieve its results. .. _control-sockets: Control sockets --------------- Control socket acts like "an interactive configuration file" so all actions available in configuration file can be executed interactively using the control socket. One possible use-case is reconfiguring the resolver instances from another program, e.g. a maintenance script. .. note:: Each instance of Knot Resolver exposes its own control socket. Take that into account when scripting deployments with :ref:`systemd-multiple-instances`. When Knot Resolver is started using Systemd (see section :ref:`quickstart-startup`) it creates a control socket in path ``/run/knot-resolver/control/$ID``. Connection to the socket can be made from command line using e.g. ``socat``: .. code-block:: bash $ socat - UNIX-CONNECT:/run/knot-resolver/control/1 When successfully connected to a socket, the command line should change to something like ``>``. Then you can interact with kresd to see configuration or set a new one. There are some basic commands to start with. .. code-block:: lua > help() -- shows help > net.interfaces() -- lists available interfaces > net.list() -- lists running network services The *direct output* of commands sent over socket is captured and sent back, while also printed to the daemon standard outputs (in :func:`verbose` mode). This gives you an immediate response on the outcome of your command. Error or debug logs aren't captured, but you can find them in the daemon standard outputs. Control sockets are also a way to enumerate and test running instances, the list of sockets corresponds to the list of processes, and you can test the process for liveliness by connecting to the UNIX socket. .. function:: map(lua_snippet) Executes the provided string as lua code on every running resolver instance and returns the results as a table. Key ``n`` is always present in the returned table and specifies the total number of instances the command was executed on. The table also contains results from each instance accessible through keys ``1`` to ``n`` (inclusive). If any instance returns ``nil``, it is not explicitly part of the table, but you can detect it by iterating through ``1`` to ``n``. .. code-block:: lua > map('worker.id') -- return an ID of every active instance { '2', '1', ['n'] = 2, } > map('worker.id == "1" or nil') -- example of `nil` return value { [2] = true, ['n'] = 2, } The order of instances isn't guaranteed or stable. When you need to identify the instances, you may use ``kluautil.kr_table_pack()`` function to return multiple values as a table. It uses similar semantics with ``n`` as described above to allow ``nil`` values. .. code-block:: lua > map('require("kluautil").kr_table_pack(worker.id, stats.get("answer.total"))') { { '2', 42, ['n'] = 2, }, { '1', 69, ['n'] = 2, }, ['n'] = 2, } If the command fails on any instance, an error is returned and the execution is in an undefined state (the command might not have been executed on all instances). When using the ``map()`` function to execute any code that might fail, your code should be wrapped in `pcall() `_ to avoid this issue. .. code-block:: lua > map('require("kluautil").kr_table_pack(pcall(net.tls, "cert.pem", "key.pem"))') { { true, -- function suceeded true, -- function return value(s) ['n'] = 2, }, { false, -- function failed 'error occurred...', -- the returned error message ['n'] = 2, }, ['n'] = 2, } Lua scripts ----------- As it was mentioned in section :ref:`config-syntax`, Resolver's configuration file contains program in Lua programming language. This allows you to write dynamic rules and helps you to avoid repetitive templating that is unavoidable with static configuration. For example parts of configuration can depend on :func:`hostname` of the machine: .. code-block:: lua if hostname() == 'hidden' then net.listen(net.eth0, 5353) else net.listen('127.0.0.1') net.listen(net.eth1.addr[1]) end Another example would show how it is possible to bind to all interfaces, using iteration. .. code-block:: lua for name, addr_list in pairs(net.interfaces()) do net.listen(addr_list) end .. tip:: Some users observed a considerable, close to 100%, performance gain in Docker containers when they bound the daemon to a single interface:ip address pair. One may expand the aforementioned example with browsing available addresses as: .. code-block:: lua addrpref = env.EXPECTED_ADDR_PREFIX for k, v in pairs(addr_list["addr"]) do if string.sub(v,1,string.len(addrpref)) == addrpref then net.listen(v) ... You can also use third-party Lua libraries (available for example through LuaRocks_) as on this example to download cache from parent, to avoid cold-cache start. .. code-block:: lua local http = require('socket.http') local ltn12 = require('ltn12') local cache_size = 100*MB local cache_path = '/var/cache/knot-resolver' cache.open(cache_size, 'lmdb://' .. cache_path) if cache.count() == 0 then cache.close() -- download cache from parent http.request { url = 'http://parent/data.mdb', sink = ltn12.sink.file(io.open(cache_path .. '/data.mdb', 'w')) } -- reopen cache with 100M limit cache.open(cache_size, 'lmdb://' .. cache_path) end Helper functions ^^^^^^^^^^^^^^^^ Following built-in functions are useful for scripting: .. envvar:: env (table) Retrieve environment variables. Example: .. code-block:: lua env.USER -- equivalent to $USER in shell .. function:: fromjson(JSONstring) :return: Lua representation of data in JSON string. Example: .. code-block:: lua > fromjson('{"key1": "value1", "key2": {"subkey1": 1, "subkey2": 2}}') [key1] => value1 [key2] => { [subkey1] => 1 [subkey2] => 2 } .. function:: hostname([fqdn]) :return: Machine hostname. If called with a parameter, it will set kresd's internal hostname. If called without a parameter, it will return kresd's internal hostname, or the system's POSIX hostname (see gethostname(2)) if kresd's internal hostname is unset. This also affects ephemeral (self-signed) certificates generated by kresd for DNS over TLS. .. function:: package_version() :return: Current package version as string. Example: .. code-block:: lua > package_version() 2.1.1 .. function:: resolve(name, type[, class = kres.class.IN, options = {}, finish = nil, init = nil]) :param string name: Query name (e.g. 'com.') :param number type: Query type (e.g. ``kres.type.NS``) :param number class: Query class *(optional)* (e.g. ``kres.class.IN``) :param strings options: Resolution options (see :c:type:`kr_qflags`) :param function finish: Callback to be executed when resolution completes (e.g. `function cb (pkt, req) end`). The callback gets a packet containing the final answer and doesn't have to return anything. :param function init: Callback to be executed with the :c:type:`kr_request` before resolution starts. :return: boolean, ``true`` if resolution was started The function can also be executed with a table of arguments instead. This is useful if you'd like to skip some arguments, for example: .. code-block:: lua resolve { name = 'example.com', type = kres.type.AAAA, init = function (req) end, } Example: .. code-block:: lua -- Send query for root DNSKEY, ignore cache resolve('.', kres.type.DNSKEY, kres.class.IN, 'NO_CACHE') -- Query for AAAA record resolve('example.com', kres.type.AAAA, kres.class.IN, 0, function (pkt, req) -- Check answer RCODE if pkt:rcode() == kres.rcode.NOERROR then -- Print matching records local records = pkt:section(kres.section.ANSWER) for i = 1, #records do local rr = records[i] if rr.type == kres.type.AAAA then print ('record:', kres.rr2str(rr)) end end else print ('rcode: ', pkt:rcode()) end end) .. function:: tojson(object) :return: JSON text representation of `object`. Example: .. code-block:: lua > testtable = { key1 = "value1", "key2" = { subkey1 = 1, subkey2 = 2 } } > tojson(testtable) {"key1":"value1","key2":{"subkey1":1,"subkey2":2}} .. _async-events: Asynchronous events ------------------- Lua language used in configuration file allows you to script actions upon various events, for example publish statistics each minute. Following example uses built-in function :func:`event.recurrent()` which calls user-supplied anonymous function: .. code-block:: lua modules.load('stats') -- log statistics every second local stat_id = event.recurrent(1 * second, function(evid) log(table_print(stats.list())) end) -- stop printing statistics after first minute event.after(1 * minute, function(evid) event.cancel(stat_id) end) Note that each scheduled event is identified by a number valid for the duration of the event, you may use it to cancel the event at any time. To persist state between two invocations of a fuction Lua uses concept called closures_. In the following example function ``speed_monitor()`` is a closure function, which provides persistent variable called ``previous``. .. code-block:: lua modules.load('stats') -- make a closure, encapsulating counter function speed_monitor() local previous = stats.list() -- monitoring function return function(evid) local now = stats.list() local total_increment = now['answer.total'] - previous['answer.total'] local slow_increment = now['answer.slow'] - previous['answer.slow'] if slow_increment / total_increment > 0.05 then log('WARNING! More than 5 %% of queries was slow!') end previous = now -- store current value in closure end end -- monitor every minute local monitor_id = event.recurrent(1 * minute, speed_monitor()) Another type of actionable event is activity on a file descriptor. This allows you to embed other event loops or monitor open files and then fire a callback when an activity is detected. This allows you to build persistent services like monitoring probes that cooperate well with the daemon internal operations. See :func:`event.socket()`. Filesystem watchers are possible with :func:`worker.coroutine()` and cqueues_, see the cqueues documentation for more information. Here is an simple example: .. code-block:: lua local notify = require('cqueues.notify') local watcher = notify.opendir('/etc') watcher:add('hosts') -- Watch changes to /etc/hosts worker.coroutine(function () for flags, name in watcher:changes() do for flag in notify.flags(flags) do -- print information about the modified file print(name, notify[flag]) end end end) .. include:: ../daemon/bindings/event.rst .. include:: ../modules/etcd/README.rst .. _closures: https://www.lua.org/pil/6.1.html .. _cqueues: https://25thandclement.com/~william/projects/cqueues.html .. _LuaRocks: https://luarocks.org/