Heimdal

Heimdal is a Layer 7 web application firewall that selectively proxies traffic onto a target host based on CIDR matches for IP addresses and the HTTP methods allowed for that match.

It uses the X-Forwarded-For HTTP header to determine the remote IP address of a connection.

For example, in AWS an Elastic Load Balancer might add two entries to an X-Forwarded-For header:

X-Forwarded-For: 37.59.40.223, 172.140.17.1

Where the first dotted IP address is the actual remote IP of the request and the second is the load balancer appliance itself.

Rule configuration

The rules are encoded in a basic JSON format:

{
  "rules": [
    {
      "effect": "ALLOW",
      "methods": [
        "GET", "HEAD", "OPTIONS"
      ],
      "source": [
        "0.0.0.0/0"
      ]
    },
    {
      "effect": "ALLOW",
      "methods": [
        "PUT", "POST", "PATCH"
      ],
      "source": [
        "46.102.195.175/32",
        "62.254.125.126/32"
      ]
    }
  ]
}

This allows GET, HEAD, OPTIONS requests from anywhere, but only allows PUT, POST, PATCH operations from two distinct IP addresses.

All non-matching traffic will receive a 403 Forbidden response immediately, without consulting the target.

Installation and Running

Heimdal needs to be installed behind a load balancer appliance or other proxy that adds an X-Forwarded-For header.

You can pass a rules file for Heimdal to use with a Docker volume command, mapping a file into the container as /etc/heimdal/rules.json.

Environment Variable	Description	Mandatory
LISTENER_PORT	Heimdal's listening port for requests	Yes
TARGET_HOST	Proxy target hostname	Yes
TARGET_PORT	Proxy target port	Yes
RULES_FILE	Location of the rules file to use - defaults to /etc/heimdal/rules.json	No

sudo docker run -d --rm \
    --env LISTENER_PORT='<listener port>' \
    --env TARGET_HOST='<target host>' \
    --env TARGET_PORT='<target port>' \
    -v <absolute path to rules file>:/etc/heimdal/rules.json \
    heimdal

Note

Input for -v needs to be an absolute path.

Etymology

Heimdal uses the quickproxy library which in turn uses the Tornado web server.