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Lua filter scripts

LUA scripts can be used as filters in skipper. The current implementation supports Lua 5.1.

Route filters

The lua scripts can be added to a route description with the lua() filter, the first parameter for the filter is the script. This can be either a file name (ending with .lua) or inline code, e.g. as

  • file lua("/path/to/file.lua") - if a file path is not absolute, the path is relative to skipper’s working directory.
  • inline lua("function request(c, p); print(c.request.url); end")

Any other additional parameters for the filter will be passed as a second table parameter to the called functions.

Any parameter starting with “lua-” should not be used to pass values for the script - those will be used for configuring the filter.

Script requirements

A filter script needs at least one global function: request or response. If present, they are called with a skipper filter context and the params passed in the route as table like

-- route looks like
-- any: * -> lua("./test.lua", "myparam=foo", "other=bar", "justkey") -> <shunt>
function request(ctx, params)
    print(params[1])      -- myparam=foo
    print(params[2])      -- other=bar
    print(params[3])      -- justkey
    print(params[4])      -- nil
    print(params.myparam) -- foo
    print(params.other)   -- bar
    print(params.justkey) -- (empty string)
    print(params.x)       -- nil

Parameter table allows index access as well as key-value access

Lua print builtin function writes skipper info log messages.


sleep(number) function pauses execution for at least number milliseconds. A negative or zero duration causes sleep to return immediately.

Enable and Disable lua sources

The flag -lua-sources allows to set 5 different values:

  • “file”: Allows to use reference to file for scripts
  • “inline”: Allows to use inline scripts
  • “inline”, “file”: Allows to use reference to file and inline scripts
  • “none”: Disable Lua filters
  • ”“: the same as “inline”, “file”, the default value for binary and library users

Available lua modules

Besides the standard modules - except for debug - the following additional modules have been preloaded and can be used with e.g. local http = require("http"), see also the examples below

For differences between the standard modules and the gopher-lua implementation check the gopher-lua documentation.

Any other module can be loaded in non-byte code form from the lua path (by default for require("mod") this is ./mod.lua, /usr/local/share/lua/5.1/mod.lua and /usr/local/share/lua/5.1/mod/init.lua).

You may selectively enable standard and additional Lua modules using -lua-modules flag:

Note that preloaded additional modules require package module.

For standard modules you may enable only a subset of module symbols:


Use none to disable all modules:


See also

Lua states

There is no guarantee that the request() and response() functions of a lua script run in the same lua state during one request. Setting a variable in the request and accessing it in the response will most likely fail and lead to hard debuggable errors. Use the ctx.state_bag to propagate values from request to response - and any other filter in the chain.

Request and response

The request() function is run for an incoming request and response() for backend response.


Request headers can be accessed via ctx.request.header table like

ua = ctx.request.header["user-agent"]
and iterated like
for k, v in ctx.request.header() do
    print(k, "=", v);

Header table is a functable that returns iterator

Header names are normalized by the net/http go module like usual. Setting a header is done by assigning to the header table. Setting a header to nil or an empty string deletes the header - setting to nil is preferred.

ctx.request.header["user-agent"] = "skipper.lua/0.0.1"
ctx.request.header["Authorization"] = nil -- delete authorization header

header table returns empty string for missing keys

Response headers ctx.response.header work the same way - this is of course only valid in the response() phase.

Multiple header values

Request and response header tables provide access to a first value of a header.

To access multiple values use add and values methods:

function request(ctx, params)
    ctx.request.header.add("X-Foo", "Bar")
    ctx.request.header.add("X-Foo", "Baz")

    -- all X-Foo values
    for _, v in pairs(ctx.request.header.values("X-Foo")) do

    -- all values
    for k, _ in ctx.request.header() do
        for _, v in pairs(ctx.request.header.values(k)) do
            print(k, "=", v)

Other request fields

  • backend_url - (read only) returns the backend url specified in the route or an empty value in case it’s a shunt or loopback
  • host - (read only) the ‘Host’ header that was in the incoming request to the proxy
  • outgoing_host - (read/write) the host that will be set for the outgoing proxy request as the ‘Host’ header.
  • remote_addr - (read only) the remote host, usually IP:port
  • content_length - (read only) content length
  • proto - (read only) something like “HTTP/1.1”
  • method - (read only) request method, e.g. “GET” or “POST”
  • url - (read/write) request URL as string
  • url_path - (read/write) request URL path as string
  • url_query - (read/write) request URL query parameter table, similar to header table but returns nil for missing keys
  • url_raw_query - (read/write) encoded request URL query values, without ‘?’ as string
  • cookie - (read only) request cookie table, similar to header table but returns nil for missing keys

Other response fields

  • status_code - (read/write) response status code as number, e.g. 200

Serving requests from lua

Requests can be served with ctx.serve(table), you must return after this call. Possible keys for the table:

  • status_code (number) - required (but currently not enforced)
  • header (table)
  • body (string)

See also redirect and internal server error examples below

Path parameters

Path parameters (if any) can be read via ctx.path_param table

Path("/api/:id") -> lua("function request(ctx, params); print(; end") -> <shunt>

path_param table returns nil for missing keys


The state bag can be used to pass string, number and table values from one filter to another in the same chain. It is shared by all filters in one request (lua table values are only available to lua filters).

function request(ctx, params)
    -- the value of "mykey" will be available to all filters in the chain now:
    ctx.state_bag["mykey"] = "foo"

function response(ctx, params)

state_bag table returns nil for missing keys


The examples serve as examples. If there is a go based plugin available, use that instead. For overhead estimate see benchmark.

OAuth2 token as basic auth password

local base64 = require("base64")

function request(ctx, params)
    token = string.gsub(ctx.request.header["Authorization"], "^%s*[Bb]earer%s+", "", 1)
    user = ctx.request.header["x-username"]
    if user == "" then
        user = params.username
    ctx.request.header["Authorization"] = "Basic " .. base64.encode(user .. ":"  .. token)
    -- print(ctx.request.header["Authorization"])

validate token

local http = require("http")
function request(ctx, params)
    token = string.gsub(ctx.request.header["Authorization"], "^%s*[Bb]earer%s+", "", 1)
    if token == "" then
        ctx.serve({status_code=401, body="Missing Token"})

    res, err = http.get(""..token)
    if err ~= nil then
        print("Failed to get tokeninfo: " .. err)
        ctx.serve({status_code=401, body="Failed to validate token: "..err})
    if res.status_code ~= 200 then
        ctx.serve({status_code=401, body="Invalid token"})

strip query

function request(ctx, params)
    ctx.request.url = string.gsub(ctx.request.url, "%?.*$", "")
    -- print("URL="..ctx.request.url)


function request(ctx, params)

internal server error

function request(ctx, params)
    -- let 10% of all requests fail with 500
    if math.random() < 0.1 then
            body="Internal Server Error.\n",

set request header from params

function request(ctx, params)
    ctx.request.header[params[1]] = params[2]
    if params[1]:lower() == "host" then
        ctx.request.outgoing_host = params[2]


redirectTo vs lua redirect

See skptesting/

Route for “skipper” is * -> redirectTo(302, "http://localhost:9980") -> <shunt>, route for “lua” is * -> lua("function request(c,p); c.serve({status_code=302, header={location='http://localhost:9980'}});end") -> <shunt>

Benchmark results

[benchmarking skipper-redirectTo]
Running 12s test @
  2 threads and 128 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     4.19ms    5.38ms  69.50ms   85.10%
    Req/Sec    26.16k     2.63k   33.22k    64.58%
  Latency Distribution
     50%    1.85ms
     75%    6.38ms
     90%   11.66ms
     99%   23.34ms
  626122 requests in 12.04s, 91.36MB read
Requests/sec:  51996.22
Transfer/sec:      7.59MB
[benchmarking skipper-redirectTo done]

[benchmarking redirect-lua]
Running 12s test @
  2 threads and 128 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     6.81ms    9.69ms 122.19ms   85.95%
    Req/Sec    21.17k     2.83k   30.63k    73.75%
  Latency Distribution
     50%    2.21ms
     75%   10.22ms
     90%   19.88ms
     99%   42.54ms
  507434 requests in 12.06s, 68.72MB read
Requests/sec:  42064.69
Transfer/sec:      5.70MB
[benchmarking redirect-lua done]
show lua performance is ~80% of native.

The benchmark was run with the default pool size of script.InitialPoolSize = 3; script.MaxPoolSize = 10. With script.InitialPoolSize = 128; script.MaxPoolSize = 128 (tweaked for this benchmark) you get >95% of native performance in lua:

[benchmarking skipper-redirectTo]
Running 12s test @
  2 threads and 128 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     4.15ms    5.24ms  62.27ms   84.88%
    Req/Sec    25.81k     2.64k   32.74k    70.00%
  Latency Distribution
     50%    1.88ms
     75%    6.49ms
     90%   11.43ms
     99%   22.49ms
  617499 requests in 12.03s, 90.10MB read
Requests/sec:  51336.87
Transfer/sec:      7.49MB
[benchmarking skipper-redirectTo done]

[benchmarking redirect-lua]
Running 12s test @
  2 threads and 128 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     3.79ms    4.98ms  91.19ms   87.15%
    Req/Sec    25.14k     4.71k   51.45k    72.38%
  Latency Distribution
     50%    1.61ms
     75%    5.17ms
     90%   10.05ms
     99%   21.83ms
  602630 requests in 12.10s, 81.61MB read
Requests/sec:  49811.24
Transfer/sec:      6.75MB
[benchmarking redirect-lua done]

Similar results are achieved when testing stripQuery() vs the lua version from above.