68 lines
2.8 KiB
Markdown
68 lines
2.8 KiB
Markdown
# Filter DKIMout
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[](https://github.com/breard-r/opensmtpd-filter-dkimout/actions/workflows/ci.yml)
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DKIM filter for [OpenSMTPD](https://www.opensmtpd.org/).
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## Project status
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This is a work in progress, it is not supposed to work yet.
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## Building and packaging
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```
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cargo build --release
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```
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Packagers may want to set the `VARLIBDIR` to a custom value (default is `/var/lib`):
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```
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VARLIBDIR="/usr/local/var/lib" cargo build --release
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```
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## Frequently Asked Questions
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### Does this filter signs outgoing emails using DKIM or check the DKIM signature of incoming emails?
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It only signs outgoing emails.
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### Why create another filter for that?
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Currently, the options to sign outgoing emails with DKIM are the following:
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- [DKIMproxy](https://dkimproxy.sourceforge.net/usage.html)
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- [filter-dkimsign](https://imperialat.at/dev/filter-dkimsign/)
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- [filter-rspamd](https://github.com/poolpOrg/filter-rspamd)
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DKIMproxy is not an OpenSMTPD filter and is therefore more inconvenient to use. Moreover, its development stopped in 2013 and it is therefore dangerous to use.
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The two other are fine, however I think they lack a few features, like automatic key rotation and publication of obsolete private keys.
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### Why would anyone publish private keys, even obsolete ones? Are you crazy?
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DKIM's goal is to fight spam, that's all, and for that it only need the keys to be safe when the recipients receives the email. But because it includes a cryptographic proof over the content it is being used for other usages, mostly as a legal proof long after the email has been sent and received. Publishing the obsolete/revoked private keys allows the sender to regain deniability.
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Matthew Green wrote an excellent article on this subject: [Ok Google: please publish your DKIM secret keys](https://blog.cryptographyengineering.com/2020/11/16/ok-google-please-publish-your-dkim-secret-keys/).
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### Where is the documentation?
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The complete documentation can be found in the `filter-dkimout (8)` man page.
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### One of my keys has been compromised, how do I revoke it?
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Keys are stored in an SQLite format 3 database. You may access it using the `sqlite3` CLI tool or any other compatible tool.
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The simplest way to revoke a key is to set its `not_after` field at the current timestamp. A new key will automatically be generated. You may also set the `revocation` field to a different timestamp in order to publish the key when desired.
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```
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UPDATE key_db \
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SET \
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not_after = unixepoch(), \
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revocation = unixepoch('now', '+20 days') \
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WHERE \
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selector = 'dkim-755512d8f51b4da6936d565a1ddbaf17';
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```
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