Custom HTTP services

This chapter describes how to create custom HTTP services inside Knot Resolver. Please read HTTP module basics in chapter Other HTTP services before continuing.

Each network address+protocol+port combination configured using net.listen() is associated with kind of endpoint, e.g. doh_legacy or webmgmt.

Each of these kind names is associated with table of HTTP endpoints, and the default table can be replaced using http.config() configuration call which allows your to provide your own HTTP endpoints.

Items in the table of HTTP endpoints are small tables describing a triplet - {mime, on_serve, on_websocket}. In order to register a new service in webmgmt kind of HTTP endpoint add the new endpoint description to respective table:

-- custom function to handle HTTP /health requests
local on_health = {'application/json',
function (h, stream)
        -- API call, return a JSON table
        return {state = 'up', uptime = 0}
end,
function (h, ws)
        -- Stream current status every second
        local ok = true
        while ok do
                local push = tojson('up')
                ok = ws:send(tojson({'up'}))
                require('cqueues').sleep(1)
        end
        -- Finalize the WebSocket
        ws:close()
end}

modules.load('http')
-- copy all existing webmgmt endpoints
my_mgmt_endpoints = http.configs._builtin.webmgmt.endpoints
-- add custom endpoint to the copy
my_mgmt_endpoints['/health'] = on_health
-- use custom HTTP configuration for webmgmt
http.config({
        endpoints = my_mgmt_endpoints
}, 'webmgmt')

Then you can query the API endpoint, or tail the WebSocket using curl.

$ curl -k https://localhost:8453/health
{"state":"up","uptime":0}
$ curl -k -i -N -H "Connection: Upgrade" -H "Upgrade: websocket" -H "Host: localhost:8453/health"  -H "Sec-Websocket-Key: nope" -H "Sec-Websocket-Version: 13" https://localhost:8453/health
HTTP/1.1 101 Switching Protocols
upgrade: websocket
sec-websocket-accept: eg18mwU7CDRGUF1Q+EJwPM335eM=
connection: upgrade

?["up"]?["up"]?["up"]

Since the stream handlers are effectively coroutines, you are free to keep state and yield using cqueues library.

This is especially useful for WebSockets, as you can stream content in a simple loop instead of chains of callbacks.

Last thing you can publish from modules are “snippets”. Snippets are plain pieces of HTML code that are rendered at the end of the built-in webpage. The snippets can be extended with JS code to talk to already exported restful APIs and subscribe to WebSockets.

http.snippets['/health'] = {'Health service', '<p>UP!</p>'}

Custom RESTful services

A RESTful service is likely to respond differently to different type of methods and requests, there are three things that you can do in a service handler to send back results. First is to just send whatever you want to send back, it has to respect MIME type that the service declared in the endpoint definition. The response code would then be 200 OK, any non-string responses will be packed to JSON. Alternatively, you can respond with a number corresponding to the HTTP response code or send headers and body yourself.

-- Our upvalue
local value = 42

-- Expose the service
local service = {'application/json',
function (h, stream)
        -- Get request method and deal with it properly
        local m = h:get(':method')
        local path = h:get(':path')
        log('method %s path %s', m, path)
        -- Return table, response code will be '200 OK'
        if m == 'GET' then
                return {key = path, value = value}
        -- Save body, perform check and either respond with 505 or 200 OK
        elseif m == 'POST' then
                local data = stream:get_body_as_string()
                if not tonumber(data) then
                        return 500, 'Not a good request'
                end
                value = tonumber(data)
        -- Unsupported method, return 405 Method not allowed
        else
                return 405, 'Cannot do that'
        end
end}
modules.load('http')
http.config({
        endpoints = { ['/service'] = service }
}, 'myservice')
-- do not forget to create socket of new kind using
-- net.listen(..., { kind = 'myservice' })
-- or configure systemd socket kresd-myservice.socket

In some cases you might need to send back your own headers instead of default provided by HTTP handler, you can do this, but then you have to return false to notify handler that it shouldn’t try to generate a response.

local headers = require('http.headers')
function (h, stream)
        -- Send back headers
        local hsend = headers.new()
        hsend:append(':status', '200')
        hsend:append('content-type', 'binary/octet-stream')
        assert(stream:write_headers(hsend, false))
        -- Send back data
        local data = 'binary-data'
        assert(stream:write_chunk(data, true))
        -- Disable default handler action
        return false
end