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This guide gives a complete workflow for Virtualmin-managed mail, locally managed BIND zones, and domains whose DNS is hosted elsewhere.
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DomainKeys Identified Mail (DKIM) adds a cryptographic signature to outgoing mail. OpenDKIM uses a private key held on the mail server to create that signature. A receiving server takes the selector and domain from the message’s DKIM-Signature header, looks up the matching public key in DNS, and verifies the signature.
The DNS name has this form:
selector._domainkey.example.comThe selector is the value after s= in the message header. The signing domain is the value after d=. Verification fails when the private key and DNS public key are different, the selector is wrong, the TXT record is missing, or the receiving server cannot reach the authoritative DNS server.
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- No signature at all: signing may be disabled, OpenDKIM may be absent or unconfigured, or mail may be bypassing the Postfix path that Virtualmin configured.
- Record not found: the selector is mistyped, the record is at the wrong provider, or nameserver delegation is incorrect.
- Public key does not match: the DNS key is from an older selector or key rotation, or the TXT value was copied incorrectly.
- Works on the server but not on the internet: the local BIND zone is not authoritative for the domain, or another DNS provider is serving the public zone.
Confirm which system controls mail and DNS
Before editing a record, map the domain’s architecture. Virtualmin can manage a virtual server’s mail, DNS, both, or neither. DKIM settings in Virtualmin are available only when that virtual server has both the DNS and email features enabled.
When Virtualmin hosts mail and authoritative DNS
Virtualmin can generate the key pair and show the TXT record to publish. If the domain’s delegated nameservers point to the same server or cluster running BIND, you can add the record through Webmin’s BIND DNS Server module or Virtualmin’s DNS tools.
When Virtualmin hosts mail but DNS is external
Enable signing in Virtualmin, but create the TXT record at the external DNS provider. Editing a local BIND zone does not change public answers when the registrar delegates the domain to another provider.
When DNS is local but mail is elsewhere
The server that actually sends the message must hold the private key and produce the signature. A DKIM record in your Webmin zone cannot make a different mail service sign messages. Obtain that service’s selector and public key, then publish the supplied record in the authoritative zone.
Use delegation checks when the answer is unclear
Public resolvers follow the domain’s delegated authoritative nameservers, not the server you happen to administer. A wrong NS delegation or missing glue record can make an otherwise correct BIND zone invisible. Check the registrar delegation with whois and compare it with the nameservers you believe are authoritative.
Choose the right DNS editing path
| Situation | Where to create the DKIM TXT record | What to verify |
|---|---|---|
| Virtualmin and BIND are authoritative | Webmin’s BIND DNS Server module or Virtualmin DNS tools | The domain’s public NS records point to those BIND servers |
| Mail is on Virtualmin, DNS is hosted elsewhere | The external DNS provider’s zone editor | The provider is listed as authoritative by the domain delegation |
| Mail is hosted by another service | The authoritative DNS provider, using the mail service’s supplied selector and key | The outgoing message’s d= and s= values match the published record |
Managed DNS can simplify nameserver, TTL, and propagation administration, but it does not remove the need to publish the exact selector and key generated for the active mail system.
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Repair or install OpenDKIM
If Virtualmin reports that an OpenDKIM configuration file is missing, install the package before changing DNS. In the Virtualmin interface, open Email Messages → DomainKeys Identified Mail (the label may appear under the current interface’s Email Settings page). Use Install Now when it is offered.
The documented package commands are:
# Enterprise Linux systems
dnf install opendkim
# Debian-derived systems
apt-get install opendkimAfter installation, return to the DKIM page and let Virtualmin detect the package and its configuration. Do not copy a key from another server unless you deliberately intend to keep the same key pair and selector; Virtualmin must be signing with the private key that corresponds to the public key in DNS.
If installation fails
Inspect /root/virtualmin-install.log for the package error. Check repository configuration, network access, package metadata, and any third-party repositories that may have replaced or conflicted with the operating system’s packages. Virtualmin notes that nonstandard packages can require the Webmin module to be told where configuration files are located. A package that installed successfully can still be unusable if the module is looking in a different path.
- Resolve repository or dependency errors first; repeated attempts to create DNS records will not fix a missing signer.
- Confirm that the Webmin module recognizes the installed OpenDKIM configuration rather than assuming the package name alone is sufficient.
- After correcting paths or packages, reopen the DKIM settings page and regenerate or display the record for the active selector.
Enable DKIM signing in Virtualmin
For a virtual server with both email and DNS features active, use the domain’s DKIM settings page:
- Open Email Settings → DomainKeys Identified Mail for the virtual server.
- Set Signing of outgoing mail enabled? to Yes.
- Enter a selector, such as
2026-10, or retain the selector already used by the domain. - Save the settings.
- Copy the public-key publication instructions Virtualmin displays. Keep the selector and the entire TXT value together while you publish them.
Once enabled, mail relayed through Virtualmin for that domain is signed regardless of which mail client submitted it, provided it travels through the configured mail path.
Use the command line when you need repeatability
The equivalent Virtualmin command is:
virtualmin set-dkim --enableThe documented options let you set the selector, key size, and incoming verification behavior:
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virtualmin set-dkim --enable --select 2026-10 --size 2048
virtualmin set-dkim --verify
virtualmin set-dkim --no-verifyUse the selector and size options on the command that enables or updates the domain according to your Virtualmin version’s accepted syntax. The key-size value is the number of bits. By default, incoming mail is not checked for a valid DKIM signature unless --verify is given.
Publish the exact public key in DNS
Create a TXT record at the selector that Virtualmin actually configured. For a selector named 2026-10 and domain example.com, the owner name is:
2026-10._domainkey.example.comPaste the complete value supplied by Virtualmin, including the v=DKIM1 and p= fields. Do not replace punctuation, remove characters, insert quotation marks into the value unnecessarily, or use a key from a previous selector.
Adding the record in Webmin or Virtualmin BIND
- Open the domain’s zone in Webmin’s BIND DNS Server module or the Virtualmin DNS editor.
- Create a TXT record with the full selector owner name, the displayed value, and a suitable TTL.
- Save or apply the zone change.
- Verify the result with a public lookup rather than relying only on the local zone-file view.
Webmin’s BIND documentation describes a TXT record as associating an arbitrary text message with a name. The record still matters only if that BIND server is authoritative for the domain.
Adding the record at an external DNS provider
Open the provider’s DNS zone for the domain and add a TXT record. Some interfaces expect only the relative host name, such as 2026-10._domainkey; others accept the fully qualified name. Follow that interface’s convention, but ensure the resulting public name is exactly 2026-10._domainkey.example.com.
Virtualmin’s modify-dns API can represent the record with name, type, TTL, and value fields. This is useful for scripted changes, but the same authority rule applies: the API must modify the zone that public nameservers serve.
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Handle long DKIM keys correctly
DNS providers may display a long public key as several quoted TXT strings. Preserve every segment and the provider’s required formatting. DNS resolvers concatenate the strings for the TXT response; omitting one segment produces a different key. Confirm the concatenated answer with dig after saving.
Verify the public DNS path
Run the lookup from a system that can query public DNS:
dig TXT 2026-10._domainkey.example.com
host -t TXT 2026-10._domainkey.example.comThe answer should contain the complete DKIM value, including v=DKIM1 and p=. An empty answer, an NXDOMAIN response, or a key belonging to another selector indicates a DNS publication problem, not an OpenDKIM signature calculation problem.
Query the authoritative servers directly
If ordinary resolver results are stale or inconsistent, identify the authoritative nameservers from the domain delegation and query each one directly with dig. Differences between authoritative answers point to an unsynchronized zone or a record added at the wrong provider. A recursive resolver may continue returning an older answer until the record’s TTL expires.
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Use whois for delegation and glue problems
whois example.comUse the result to confirm which nameservers the registrar delegates to the domain and whether the host names require glue records. If those nameservers do not include the BIND server you edited, the local record cannot be retrieved publicly.
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Send a test message through the same account and submission path that is failing. Open the message’s full headers and locate DKIM-Signature. Record:
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s=, the selector;d=, the signing domain; and- the receiving system’s DKIM result.
Use those values to form the DNS query. If the header says s=oldkey while you published 2026-10._domainkey, the server is still using the old selector. If no DKIM-Signature header exists, revisit signing enablement, OpenDKIM installation, and the mail path rather than editing the TXT record.
A message can be signed correctly while DNS is still wrong, and DNS can be correct while the message uses a different selector. Both sides must agree.
Diagnose the common failure patterns
| Symptom | Likely cause | Corrective action |
|---|---|---|
| No DKIM signature | Signing disabled; OpenDKIM missing or not recognized; message bypasses the configured Postfix path | Enable signing for the virtual server, repair the package/configuration, and test through the configured relay |
| DKIM record not found | Selector typo, missing TXT record, wrong DNS provider, or broken NS/glue delegation | Read s= from the message, query that exact name with dig and host, then check delegation with whois |
| Public key does not match | Old key remains after rotation or TXT value was copied incompletely | Publish the complete key generated for the active selector and retest |
| Record visible locally but not online | Edited BIND is not authoritative, or an external provider serves the zone | Follow the delegated nameservers and edit the authoritative zone |
| Virtualmin cannot create DKIM configuration | Package/repository failure, missing configuration path, or unsupported nonstandard package | Review /root/virtualmin-install.log, fix repositories or paths, and reopen the DKIM module |
| Long TXT value verifies incorrectly | One quoted TXT segment was omitted or altered | Restore every segment exactly and inspect the concatenated public response with dig |
Rotate selectors and keys without breaking mail
A selector is a version label for a key pair. Rotating it is safer than overwriting a key that existing messages or senders still use.
- Choose a new selector, such as a new year-and-month value.
- Configure Virtualmin to sign with the new selector and let it generate or select the corresponding private key.
- Publish the new selector’s public TXT record in the authoritative DNS zone.
- Use
digto confirm that the new record is publicly visible. - Send a test message and verify that its
s=value is the new selector. - Keep the old TXT record while any sender still uses the old selector. Remove it only after the old signing configuration is no longer active.
If Virtualmin continues signing with an old selector after a change, publish that old selector again or correct the active configuration before removing anything. Deleting a still-used key causes immediate verification failures.
Decide whether to verify incoming mail
| Mode | Effect | When it fits |
|---|---|---|
| Signing only | Outgoing mail receives DKIM signatures; incoming signatures are not checked by default | You need outbound authentication without changing incoming-mail handling |
| Signing plus verification | Outgoing mail is signed and incoming DKIM signatures are checked | You have a policy and monitoring plan for handling verification results |
Enable incoming verification with virtualmin set-dkim --verify or the corresponding interface setting. Disable it with virtualmin set-dkim --no-verify when verification is causing unwanted bounces, but understand that this changes how suspicious or malformed incoming mail is handled. Verification alone does not authorize rejection; your mail-filtering policy determines what happens to a failed check.
A repeatable recovery checklist
- Confirm Virtualmin owns the mail path and that the virtual server has both email and DNS features enabled.
- Determine the domain’s authoritative nameservers; do not assume the local BIND zone is public.
- Install or repair OpenDKIM using Virtualmin’s Install Now,
dnf install opendkim, orapt-get install opendkim. - Enable outgoing signing and write down the configured selector.
- Publish the complete public key at
selector._domainkey.example.comin the authoritative DNS zone. - Query that exact name with
digandhost; query authoritative servers directly if necessary. - Use
whoiswhen nameserver delegation or glue records are suspect. - Inspect a test message’s
s=andd=values and compare them with the DNS name you queried. - During rotation, retain old selector records until all signing traffic has moved to the new key.
Following the checklist isolates the failure domain: package and signing issues stay on the server, while selector, TXT, delegation, and authority issues stay in DNS. DKIM succeeds only when the private key used to sign, the selector in the header, and the public key served by authoritative DNS form one matching set.
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