Knot DNS Resolver daemon

The server is in the daemon directory, it works out of the box without any configuration.

$ kresd -h # Get help
$ kresd -a ::1

Enabling DNSSEC

The resolver supports DNSSEC including RFC 5011 automated DNSSEC TA updates and RFC 7646 negative trust anchors. To enable it, you need to provide trusted root keys. Bootstrapping of the keys is automated, and kresd fetches root trust anchors set over a secure channel from IANA. From there, it can perform RFC 5011 automatic updates for you.

$ kresd -k root.keys # File for root keys
[ ta ] bootstrapped root anchor "19036 8 2 49AAC11D7B6F6446702E54A1607371607A1A41855200FD2CE1CDDE32F24E8FB5"
[ ta ] warning: you SHOULD check the key manually, see: https://data.iana.org/root-anchors/draft-icann-dnssec-trust-anchor.html#sigs
[ ta ] key: 19036 state: Valid
[ ta ] next refresh: 86400000

Alternatively, you can set it in configuration file with trust_anchors.file = 'root.keys'. If the file doesn't exist, it will be automatically populated with root keys validated using root anchors retrieved over HTTPS.

This is equivalent to using unbound-anchor:

$ unbound-anchor -a "root.keys" || echo "warning: check the key at this point"
$ echo "auto-trust-anchor-file: \"root.keys\"" >> unbound.conf
$ unbound -c unbound.conf

Manually providing root anchors

The root anchors bootstrap may fail for various reasons, in this case you need to provide IANA or alternative root anchors. The format of the keyfile is the same as for Unbound or BIND and contains DS/DNSKEY records.

1. Check the current TA published on IANA website
2. Fetch current keys (DNSKEY), verify digests
3. Deploy them

$ kdig DNSKEY . @k.root-servers.net +noall +answer | grep "DNSKEY[[:space:]]257" > root.keys
$ ldns-key2ds -n root.keys # Only print to stdout
... verify that digest matches TA published by IANA ...
$ kresd -k root.keys

You've just enabled DNSSEC!

CLI interface

The daemon features a CLI interface, type help() to see the list of available commands.

$ kresd /var/run/knot-resolver
[system] started in interactive mode, type 'help()'
> cache.count()
53

Verbose output

If the verbose logging is compiled in, i.e. not turned off by -DNOVERBOSELOG, you can turn on verbose tracing of server operation with the -v option. You can also toggle it on runtime with verbose(true|false) command.

$ kresd -v

Scaling out

The server can clone itself into multiple processes upon startup, this enables you to scale it on multiple cores. Multiple processes can serve different addresses, but still share the same working directory and cache. You can add, start and stop processes during runtime based on the load.

$ kresd -f 4 rundir > kresd.log &
$ kresd -f 2 rundir > kresd_2.log & # Extra instances
$ pstree $$ -g
bash(3533)─┬─kresd(19212)─┬─kresd(19212)
           │              ├─kresd(19212)
           │              └─kresd(19212)
           ├─kresd(19399)───kresd(19399)
           └─pstree(19411)
$ kill 19399 # Kill group 2, former will continue to run
bash(3533)─┬─kresd(19212)─┬─kresd(19212)
           │              ├─kresd(19212)
           │              └─kresd(19212)
           └─pstree(19460)

Notice the absence of an interactive CLI. You can attach to the the consoles for each process, they are in rundir/tty/PID.

$ nc -U rundir/tty/3008 # or socat - UNIX-CONNECT:rundir/tty/3008
> cache.count()
53

The direct output of the CLI command is captured and sent over the socket, while also printed to the daemon standard outputs (for accountability). 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.

This is also a way to enumerate and test running instances, the list of files in tty corresponds to the list of running processes, and you can test the process for liveliness by connecting to the UNIX socket.

Running supervised

Knot Resolver can run under a supervisor to allow for graceful restarts, watchdog process and socket activation. This way the supervisor binds to sockets and lends them to the resolver daemon. If the resolver terminates or is killed, the sockets remain open and no queries are dropped.

The watchdog process must notify kresd about active file descriptors, and kresd will automatically determine the socket type and bound address, thus it will appear as any other address. There's a tiny supervisor script for convenience, but you should have a look at real process managers.

$ python scripts/supervisor.py ./daemon/kresd -a 127.0.0.1
$ [system] interactive mode
> quit()
> [2016-03-28 16:06:36.795879] process finished, pid = 99342, status = 0, uptime = 0:00:01.720612
[system] interactive mode
>

The daemon also supports systemd socket activation, it is automatically detected and requires no configuration on users's side.

Configuration

In it's simplest form it requires just a working directory in which it can set up persistent files like cache and the process state. If you don't provide the working directory by parameter, it is going to make itself comfortable in the current working directory.

$ kresd /var/run/kresd

And you're good to go for most use cases! If you want to use modules or configure daemon behavior, read on.

There are several choices on how you can configure the daemon, a RPC interface, a CLI, and a configuration file. Fortunately all share common syntax and are transparent to each other.

Configuration example

-- interfaces
net = { '127.0.0.1', '::1' }
-- load some modules
modules = { 'policy' }
-- 10MB cache
cache.size = 10*MB

Configuration syntax

The configuration is kept in the config file in the daemon working directory, and it's going to get loaded automatically. If there isn't one, the daemon is going to start with sane defaults, listening on localhost. The syntax for options is like follows: group.option = value or group.action(parameters). You can also comment using a -- prefix.

A simple example would be to load static hints.

modules = {
        'hints' -- no configuration
}

If the module accepts configuration, you can call the module.config({...}) or provide options table. The syntax for table is { key1 = value, key2 = value }, and it represents the unpacked JSON-encoded string, that the modules use as the :ref:`input configuration <mod-properties>`.

modules = {
        hints = '/etc/hosts'
}

Modules are inherently ordered by their declaration. Some modules are built-in, so it would be normally impossible to place for example hints before rrcache. You can enforce specific order by precedence operators > and <.

modules = {
   'hints  > iterate', -- Hints AFTER iterate
   'policy > hints',   -- Policy AFTER hints
   'view   < rrcache'  -- View BEFORE rrcache
}
modules.list() -- Check module call order

This is useful if you're writing a module with a layer, that evaluates an answer before writing it into cache for example.

Dynamic configuration

Knowing that the the configuration is a Lua in disguise enables you to write dynamic rules. It also helps you to avoid repetitive templating that is unavoidable with static configuration.

if hostname() == 'hidden' then
        net.listen(net.eth0, 5353)
else
        net = { '127.0.0.1', net.eth1.addr[1] }
end

Another example would show how it is possible to bind to all interfaces, using iteration.

for name, addr_list in pairs(net.interfaces()) do
        net.listen(addr_list)
end

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 packages (available for example through LuaRocks) as on this example to download cache from parent, to avoid cold-cache start.

local http = require('socket.http')
local ltn12 = require('ltn12')

if cache.count() == 0 then
        -- download cache from parent
        http.request {
                url = 'http://parent/cache.mdb',
                sink = ltn12.sink.file(io.open('cache.mdb', 'w'))
        }
        -- reopen cache with 100M limit
        cache.size = 100*MB
end

Events and services

The Lua supports a concept called closures, this is extremely useful for scripting actions upon various events, say for example - prune the cache within minute after loading, publish statistics each 5 minutes and so on. Here's an example of an anonymous function with :func:`event.recurrent()`:

-- every 5 minutes
event.recurrent(5 * minute, function()
        cache.prune()
end)

Note that each scheduled event is identified by a number valid for the duration of the event, you may cancel it at any time. You can do this with anonymous functions, if you accept the event as a parameter, but it's not very useful as you don't have any non-global way to keep persistent variables.

-- make a closure, encapsulating counter
function pruner()
        local i = 0
        -- pruning function
        return function(e)
                cache.prune()
                -- cancel event on 5th attempt
                i = i + 1
                if i == 5 then
                        event.cancel(e)
                fi
        end
end

-- make recurrent event that will cancel after 5 times
event.recurrent(5 * minute, pruner())

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 HTTP servers or monitoring probes that cooperate well with the daemon internal operations.

For example a simple web server that doesn't block:

local server, headers = require 'http.server', require 'http.headers'
local cqueues = require 'cqueues'
-- Start socket server
local s = server.listen { host = 'localhost', port = 8080 }
assert(s:listen())
-- Compose per-request coroutine
local cq = cqueues.new()
cq:wrap(function()
   s:run(function(stream)
      -- Create response headers
      local headers = headers.new()
      headers:append(':status', '200')
      headers:append('connection', 'close')
      -- Send response and close connection
      assert(stream:write_headers(headers, false))
      assert(stream:write_chunk('OK', true))
      stream:shutdown()
      stream.connection:shutdown()
   end)
   s:close()
end)
-- Hook to socket watcher
event.socket(cq:pollfd(), function (ev, status, events)
   cq:step(0)
end)

• File watchers

Configuration reference

This is a reference for variables and functions available to both configuration file and CLI.

Environment

Network configuration

For when listening on localhost just doesn't cut it.

net = { '127.0.0.1', net.eth0, net.eth1.addr[1] }
net.ipv4 = false

Trust anchors and DNSSEC

Modules configuration

The daemon provides an interface for dynamic loading of :ref:`daemon modules <modules-implemented>`.

modules = {
        hints = {file = '/etc/hosts'}
}

modules.load('hints')
hints.config({file = '/etc/hosts'})

Cache configuration

The cache in Knot DNS Resolver is persistent with LMDB backend, this means that the daemon doesn't lose the cached data on restart or crash to avoid cold-starts. The cache may be reused between cache daemons or manipulated from other processes, making for example synchronised load-balanced recursors possible.

Timers and events

The timer represents exactly the thing described in the examples - it allows you to execute closures after specified time, or event recurrent events. Time is always described in milliseconds, but there are convenient variables that you can use - sec, minute, hour. For example, 5 * hour represents five hours, or 5*60*60*100 milliseconds.

Watch for file descriptor activity. This allows embedding other event loops or simply firing events when a pipe endpoint becomes active. In another words, asynchronous notifications for daemon.

Map over multiple forks

When daemon is running in forked mode, each process acts independently. This is good because it reduces software complexity and allows for runtime scaling, but not ideal because of additional operational burden. For example, when you want to add a new policy, you'd need to add it to either put it in the configuration, or execute command on each process independently. The daemon simplifies this by promoting process group leader which is able to execute commands synchronously over forks.

Scripting worker

Worker is a service over event loop that tracks and schedules outstanding queries, you can see the statistics or schedule new queries. It also contains information about specified worker count and process rank.

Using CLI tools

• kresd-host.lua - a drop-in replacement for host(1) utility

Queries the DNS for information. The hostname is looked up for IP4, IP6 and mail.

Example:

$ kresd-host.lua -f root.key -v nic.cz
nic.cz. has address 217.31.205.50 (secure)
nic.cz. has IPv6 address 2001:1488:0:3::2 (secure)
nic.cz. mail is handled by 10 mail.nic.cz. (secure)
nic.cz. mail is handled by 20 mx.nic.cz. (secure)
nic.cz. mail is handled by 30 bh.nic.cz. (secure)

• kresd-query.lua - run the daemon in zero-configuration mode, perform a query and execute given callback.

This is useful for executing one-shot queries and hooking into the processing of the result, for example to check if a domain is managed by a certain registrar or if it's signed.

Example:

$ kresd-query.lua www.sub.nic.cz 'assert(kres.dname2str(req:resolved().zone_cut.name) == "nic.cz.")' && echo "yes"
yes
$ kresd-query.lua -C 'trust_anchors.config("root.keys")' nic.cz 'assert(req:resolved():hasflag(kres.query.DNSSEC_WANT))'
$ echo $?
0