haproxy-lua-dnsbl

DNSBL (DNS blacklisting) module for HAProxy Lua. Dynamically block requests based on the response of the DNS query or use any of the added extra request headers to make decisions down the line.

Features

• Query multiple DNSBL providers (Tor exit lists, Spamhaus, etc.)
• Cache results using HAProxy stick-tables for optimal performance
• Configurable track-sc index (sc0, sc1, sc2) for flexible stick-table usage
• Detailed HTTP headers for logging and downstream processing
• Fail-open design - errors don't block legitimate traffic

Supported DNSBL Providers

Provider	Domain	Description
Dan.me.uk Tor List	.torexit.dan.me.uk	Tor exit node list
Tor Project	.exitlist.torproject.org	Official Tor exit list
Spamhaus SBL	sbl.spamhaus.org	Spam sources
Spamhaus XBL	xbl.spamhaus.org	Exploits/botnets
Spamhaus PBL	pbl.spamhaus.org	Policy block list
Spamhaus ZEN	zen.spamhaus.org	Combined list

Quick Start

global
    lua-load /usr/share/lua/5.3/dnsbl.lua

backend st_dnsbl_cache
    stick-table type ipv6 size 1m expire 30m store gpc0,gpc1

frontend http-in
    bind *:80
    http-request track-sc0 src table st_dnsbl_cache
    http-request lua.dnsbl_query st_dnsbl_cache .torexit.dan.me.uk "" ""
    http-request lua.dnsbl_block st_dnsbl_cache
    default_backend servers

Installation

haproxy-lua-dnsbl depends on utils and socket library. Download copies to your Lua path:

# Install to Lua path (e.g., /usr/share/lua/5.3/)
cp src/dnsbl.lua /usr/share/lua/5.3/

# Install dependencies
wget -O /usr/share/lua/5.3/utils.lua \
  https://raw.githubusercontent.com/dobrevit/haproxy-lua-utils/main/src/utils.lua

Documentation

Full documentation is available in the docs folder:

• Documentation Index
• Architecture Overview - How the module works
• API Reference - Complete function documentation
• Configuration Guide - HAProxy setup instructions
• Understanding DNSBL - How DNS blacklists work
• HTTP Headers Reference - Headers set by the module
• Troubleshooting - Common issues and solutions

Examples

The examples folder contains ready-to-use configurations:

Example	Description
Basic	Minimal working configuration
Behind Proxy	Using X-Forwarded-For header
Multiple DNSBLs	Querying multiple providers
Docker	Complete Docker test environment
Logging	Custom log formats with DNSBL headers

Usage

Basic Usage

http-request lua.dnsbl_query <backend> <domain> <src_var> <src_header> [sc_index]
http-request lua.dnsbl_block <backend>

Parameters

Parameter	Description
backend	Backend name containing the stick-table
domain	DNSBL domain (e.g., .torexit.dan.me.uk)
src_var	HAProxy variable with client IP (optional)
src_header	HTTP header with client IP (optional)
sc_index	Track-sc index: 0, 1, or 2 (optional, default: 0)

Examples

# Basic - direct client IP
http-request lua.dnsbl_query st_cache .torexit.dan.me.uk "" ""

# Behind proxy - use X-Forwarded-For
http-request lua.dnsbl_query st_cache .torexit.dan.me.uk "" X-Forwarded-For

# Use sc1 instead of sc0
http-request lua.dnsbl_query st_cache .torexit.dan.me.uk "" "" 1

# Multiple providers with different track-sc indices
http-request track-sc0 src table st_tor
http-request track-sc1 src table st_spam
http-request lua.dnsbl_query st_tor .torexit.dan.me.uk "" "" 0
http-request lua.dnsbl_query st_spam xbl.spamhaus.org "" "" 1

How it Works

The library uses the socket.dns class to make DNS queries. The DNSBL domain is expected to return:

• NXDOMAIN - IP is not blacklisted (request allowed)
• A record response - IP is blacklisted (request blocked)

Results are cached using HAProxy's stick-table with gpc0 and gpc1 counters:

• gpc0 = 1 → IP was checked and allowed
• gpc1 = 1 → IP was checked and blocked

HTTP Headers

The module sets these headers on each request:

Header	Description
X-DNSBL-Action	Result: CACHE-ALLOW, LOOKUP-DENY, etc.
X-DNSBL-Is-Allowed	1 if allowed, 0 if blocked
X-DNSBL-Client-IP	IP address that was checked
X-DNSBL-Query	DNS query that was made
X-DNSBL-Zone	Spamhaus zone (if applicable)
X-DNSBL-Description	Block reason (if applicable)

Changelog

v0.4.0 (Current)

• Added support for .exitlist.torproject.org domain
• Added support for Spamhaus domains (sbl, xbl, pbl, zen)
• Added configurable track-sc index (sc0, sc1, sc2)
• Added X-DNSBL-Zone and X-DNSBL-Description headers
• Added comprehensive documentation and examples

v0.3.0

• Initial public release
• Support for .torexit.dan.me.uk domain

Known Limitations

• Only A record DNS queries are supported (not AAAA)
• Spamhaus may rate-limit queries from public DNS resolvers

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/dobrevit/haproxy-lua-dnsbl/issues.

License

MIT License

Copyright (c) 2023 Dobrev IT Ltd., Martin Dobrev

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.