Filtering reverse HTTP proxy


Reverse HTTP proxy to filter requests by different rules. Can be used between production webserver and the application server to prevent abuse of the application backend.

The original purpose of this program was to defend SearXNG, but it can be used to guard any web application.

Installation and setup

$ go get
$ "$GOPATH/bin/filtron" --help


A rule has two required attributes: name and actions

A rule can contain all of the following attributes:

  • limit integer - Defines how many matching requests allowed to access the application within interval seconds. (Can be omitted if 0)
  • interval integer - Time range in seconds to reset rule numbers (Can be omitted if limit is 0)
  • filters list of selectors
  • aggregations list of selectors (if filters specified it activates only in case of the filter matches)
  • subrules list of rules (if filters specified it activates only in case of the filter matches)
  • disabled bool - Disable a rule (default is false)
  • stop bool - Finish request validation immediately and skip remaining rules (default is false)

JSON representation of a rule:

    "name": "example rule",
    "interval": 60,
    "limit": 10,
    "filters": ["GET:q", "Header:User-Agent=^curl"],
    "actions": [
        {"name": "log",
         "params": {"destination": "stderr"}},
        {"name": "block",
         "params": {"message": "Not allowed"}}

Explanation: Allow only 10 requests a minute where q represented as GET parameter and the user agent header starts with curl. Request is logged to STDERR and blocked with a custom error message if limit is exceeded. See more examples here.


Rule's actions are sequentially activated if a request exceeds rule's limit

Note: Only the rule's first action will be executed that serves custom response

Currently implemented actions


Log the request


Serve HTTP 429 response instead of passing the request to the application


Execute a shell command. cmd (string) and args (list of selectors) are required params (Example: {"name": "shell", "params": {"cmd": "echo %v is the IP", "args": ["IP"]}})


If all the selectors found, it increments a counter. Rule blocks the request if counter reaches limit


Counts the values returned by selectors. Rule blocks the request if any value's number reaches limit


Each rule can contain any number of subrules. Activates on parent rule's filter match.


Request's different parts can be extracted using selector expressions.

Selectors are strings that can match any attribute of a HTTP request with the following syntax:

  • ! can negate the selector
  • RequestAttribute (required) selects specific part of a request - possible values:
    • Single value
      • IP
      • Host
      • Path
      • Method
    • Multiple values
      • GET
      • POST
      • Param - it is an alias for both GET and POST
      • Cookie
      • Header
  • SubAttribute if RequestAttribute is not a single value, this can specify the inner attribute
  • Expression possible value:
    • a regular expression to filter the selected attribute values.
    • nslookup(Hostname) to filter the selected attribute values with the IP addresses of Hostname. Filtron resolves Hostname to its IP addresses when the rule is loaded (IPv4 and IPv6).


IP returns the client's IP address

GET:x returns the x GET parameter if exists

!Header:Accept-Language returns true if there is no Accept-Language HTTP header

Path=^/(x|y)$ matches if the path is /x or /y

IP=nslookup( matches if the client's IP address is one of the IP addresses of


Filtron can be configured through its REST API which listens on by default.

API endpoints


Loaded rules in JSON format


Reload the rule file specified at startup


UI built on the API



Bugs or suggestions? Visit the issue tracker.