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OpenSMTPD filter integration for the SenderScore reputation
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README.md

filter-senderscore

Description

This filter performs a SenderScore lookup and allows OpenSMTPD to either block or slow down a session based on the reputation of the source IP address.

Features

The filter currently supports:

  • blocking hosts with reputation below a certain value
  • adding X-Spam header to hosts with reputation below a certain value
  • apply to a session a time penalty proportional to the IP reputation

Dependencies

The filter is written in Golang and doesn't have any dependencies beyond standard library.

It requires OpenSMTPD 6.6.0 or higher.

How to install

Install from your operating system's preferred package manager if available. On OpenBSD:

$ doas pkg_add filter-senderscore
quirks-3.167 signed on 2019-08-11T14:18:58Z
filter-senderscore-v0.1.0: ok
$

Alternatively, clone the repository, build and install the filter:

$ cd filter-senderscore/
$ go build
$ doas install -m 0555 filter-senderscore /usr/local/bin/filter-senderscore

How to configure

The filter itself requires no configuration.

It must be declared in smtpd.conf and attached to a listener:

filter "senderscore" proc-exec "/usr/local/bin/filter-senderscore -blockBelow 50 -junkBelow 80 -slowFactor 1000"

listen on all filter "senderscore"

-blockBelow will display an error banner for sessions with reputation score below value then disconnect.

-junkBelow will prepend the 'X-Spam: yes' header to messages

-slowFactor will delay all answers to a reputation-related percentage of its value in milliseconds.

-scoreHeader will add an X-SenderScore header with reputation value if known.

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