libopendmarc

libopendmarc is an open-source C library that implements the core DMARC evaluation logic used by the OpenDMARC project. The library provides an API that allows applications to validate DMARC for a message by supplying the necessary domain information, DKIM and SPF results, and the DMARC DNS policy. It focuses on the policy evaluation step and is designed to be embedded in mail servers or filtering tools.

DMARC evaluation performed by libopendmarc includes domain alignment checks between the header From domain and the domains used for SPF and DKIM authentication, as well as DNS-based policy retrieval. The library applies the domain’s DMARC policy (p=none, quarantine, or reject), and can generate results and reporting data suitable for logging, delivery decisions, or further processing. It supports producing data suitable for aggregate and forensic DMARC reports, aligning with the DMARC specification.

Architecture and usage: libopendmarc is a standalone library intended to be linked into larger applications. It is used by the opendmarc Milter component to integrate DMARC validation into mail-transfer workflows, and it can also be integrated into other software that requires DMARC evaluation. While it handles the DMARC policy decision, DKIM and SPF verification are typically performed by other components in the OpenDMARC suite.

Licensing: libopendmarc is distributed under a BSD-style license.

See also: OpenDMARC, DMARC, DKIM, SPF.