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121 lines
4.3 KiB
Markdown
121 lines
4.3 KiB
Markdown
DKIM is a security measure targeting email spoofing. It is greatly recommended one activates it. See [the Wikipedia page](https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail) for more details on DKIM.
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### Enabling DKIM signature
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To enable DKIM signature, **you must have created at least one email account**.
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Once its done, just run the following command to generate the signature:
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```BASH
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./setup.sh config dkim
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```
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The script assumes you're being in the directory where the `config/` directory is located. The default keysize when generating the signature is 4096 bits for now. If you need to change it (e.g. your DNS-Provider limits the size), then provide the size as the first parameter of the command:
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```BASH
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./setup.sh config dkim <keysize>
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```
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For LDAP systems that do not have any directly created user account you can run the following command (since `8.0.0`) to generate the signature by additionally providing the desired domain name (if you have multiple domains use the command multiple times or provide a comma-separated list of domains):
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```BASH
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./setup.sh config dkim <key-size> <domain.tld>[,<domain2.tld>]
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```
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Now the keys are generated, you can configure your DNS server with DKIM signature, simply by adding a TXT record.
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If you have direct access to your DNS zone file, then it's only a matter of pasting the content of `config/opendkim/keys/domain.tld/mail.txt` in your `domain.tld.hosts` zone.
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``` TXT
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$ dig mail._domainkey.domain.tld TXT
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---
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;; ANSWER SECTION
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mail._domainkey.<DOMAIN> 300 IN TXT "v=DKIM1; k=rsa; p=AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN"
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```
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<details>
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<summary>Configuration using a web interface</summary>
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1. Generate a new record of the type `TXT`.
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2. Paste `mail._domainkey` the `Name` txt field.
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3. In the `Target` or `Value` field fill in `v=DKIM1; k=rsa; p=AZERTYUGHJKLMWX...`.
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4. In `TTL` (time to live): Time span in seconds. How long the DNS server should cache the `TXT` record.
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5. Save.
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</details>
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After generating DKIM keys, you should restart the mail server. DNS edits may take a few minutes to hours to propagate.
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Note: Sometimes the key in `config/opendkim/keys/domain.tld/mail.txt` can be on multiple lines. If so then you need to concatenate the values in the TXT record:
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```
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$ dig mail._domainkey.domain.tld TXT
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---
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;; ANSWER SECTION
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mail._domainkey IN TXT ( "v=DKIM1; k=rsa; "
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"p=AZERTYUIOPQSDF..."
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"asdfQWERTYUIOPQSDF..." )
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```
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the target (or value) field must then have all the parts together: `v=DKIM1; k=rsa; p=AZERTYUIOPQSDF...asdfQWERTYUIOPQSDF...`
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## Verify-only
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If you want DKIM to only _verify_ incoming emails, the following version of /etc/opendkim.conf may be useful (right now there is no easy mechanism for installing it other than forking the repo):
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``` TXT
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# This is a simple config file verifying messages only
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#LogWhy yes
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Syslog yes
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SyslogSuccess yes
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Socket inet:12301@localhost
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PidFile /var/run/opendkim/opendkim.pid
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ReportAddress postmaster@my-domain.com
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SendReports yes
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Mode v
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```
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## Debugging
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### Tools
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* [DKIM-verifer](https://addons.mozilla.org/en-US/thunderbird/addon/dkim-verifier): A add-on for the mail client Thunderbird.
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### DKIM TXT Record
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You can debug your TXT records with the `dig` tool.
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```
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dig TXT mail._domainkey.domain.tld
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```
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Output:
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```
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; <<>> DiG 9.10.3-P4-Debian <<>> TXT mail._domainkey.domain.tld
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;; global options: +cmd
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;; Got answer:
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;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 39669
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;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
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;; OPT PSEUDOSECTION:
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; EDNS: version: 0, flags:; udp: 512
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;; QUESTION SECTION:
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;mail._domainkey.domain.tld. IN TXT
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;; ANSWER SECTION:
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mail._domainkey.domain.tld. 3600 IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCxBSjG6RnWAdU3oOlqsdf2WC0FOUmU8uHVrzxPLW2R3yRBPGLrGO1++yy3tv6kMieWZwEBHVOdefM6uQOQsZ4brahu9lhG8sFLPX4MaKYN/NR6RK4gdjrZu+MYSdfk3THgSbNwIDAQAB"
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;; Query time: 50 msec
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;; SERVER: 127.0.1.1#53(127.0.1.1)
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;; WHEN: Wed Sep 07 18:22:57 CEST 2016
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;; MSG SIZE rcvd: 310
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```
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## Switch off DKIM
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Simply remove the DKIM key by recreating (not just relaunching) the mailserver container. |