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To test NGINX mutual TLS (mTLS), exercise the endpoint with a valid client certificate, no certificate, an untrusted certificate, an invalid certificate, and a trusted certificate with the wrong application identity. In strict mode, ssl_verify_client on; must reject every case except the valid certificate. Log $ssl_client_verify for every request: SUCCESS proves NGINX verified a certificate, NONE means no certificate arrived, and FAILED:reason means a certificate arrived but verification failed.
What NGINX mTLS actually checks
Mutual TLS adds client authentication to ordinary server-side TLS. The server still presents its own certificate, while NGINX requests a certificate from the client and validates that certificate against a configured trust store.
ssl_client_certificatepoints to the CA certificates NGINX trusts for client authentication.ssl_verify_clientcontrols enforcement and defaults tooff. A server certificate alone does not enable mTLS.ssl_verify_depthlimits the permitted certificate-chain depth.$ssl_client_verifyrecords the result asSUCCESS,FAILED:reason, orNONE.
With ssl_verify_client on;, every client must present a valid certificate during the TLS handshake. A client with no certificate, an untrusted issuer, an expired certificate, or another verification failure is rejected before protected HTTP content should be served.
Use a known-good strict configuration first
Start with the smallest configuration that makes certificate verification mandatory. This separates TLS authentication problems from application authorization problems.
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server {
listen 443 ssl;
server_name mtls.example.test;
ssl_certificate /etc/nginx/tls/server.crt;
ssl_certificate_key /etc/nginx/tls/server.key;
ssl_client_certificate /etc/nginx/tls/client-ca.pem;
ssl_verify_client on;
ssl_verify_depth 2;
location / {
proxy_set_header X-Client-Verify $ssl_client_verify;
proxy_set_header X-Client-Subject $ssl_client_s_dn;
proxy_pass http://app;
}
}The decisive lines are ssl_client_certificate and ssl_verify_client on;. The CA file must contain the CA certificate, or certificates, that issued the client certificate. The server certificate and the client CA serve different purposes and are not interchangeable.
Use a test hostname that resolves to this virtual server and ensure the certificate’s names and the request’s SNI match. Record the NGINX and OpenSSL versions before testing; directive behavior and variable availability can be version-qualified.
Prepare the endpoint and capture evidence
Validate and reload safely
- Run
nginx -twith the same configuration that the service uses. - Fix every syntax, file-permission, certificate, and private-key error reported by the test.
- Reload NGINX only after the test succeeds, then confirm the running process accepted the new configuration.
- Repeat the test after any client certificate, CA bundle, NGINX, or OpenSSL change.
Add temporary mTLS fields to the access log
A handshake result is not visible from an ordinary status code alone. Add a temporary log format that records the verification result and the certificate identity used for the request.
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log_format mtls '$remote_addr $request_id "$request" '
'status=$status verify=$ssl_client_verify '
'subject="$ssl_client_s_dn" issuer="$ssl_client_i_dn" '
'fingerprint=$ssl_client_fingerprint serial=$ssl_client_serial';
access_log /var/log/nginx/mtls-access.log mtls;Keep the log line for each test request and correlate it with the client command, timestamp, and hostname. Treat certificate subjects, serial numbers, and fingerprints as sensitive operational data; retain this expanded format only as long as needed.
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Run the complete client-certificate test matrix
Use the same URL, SNI name, and trusted server CA for every case. Change only the client-certificate condition being tested.
| Case | Client command or condition | Strict mode expectation | What the log should show |
|---|---|---|---|
| Valid certificate | curl --cacert /etc/nginx/tls/server-ca.pem --cert /etc/nginx/tls/client.crt --key /etc/nginx/tls/client.key https://mtls.example.test/ |
TLS handshake and request succeed | SUCCESS |
| No certificate | curl --cacert /etc/nginx/tls/server-ca.pem https://mtls.example.test/ |
Handshake is rejected; protected content is not returned | No normal request log, or a rejection entry without a successful client verification |
| Untrusted issuer | Send a certificate signed by a CA absent from client-ca.pem |
Handshake is rejected | If the request reaches HTTP policy, FAILED:reason |
| Expired or not-yet-valid certificate | Send a certificate outside its validity period | Verification fails and access is denied | FAILED:reason when a request is logged |
| Malformed certificate | Send a damaged, improperly encoded, or otherwise invalid certificate | Handshake or verification failure | FAILED:reason, where available |
| Wrong application identity | Send a validly chained certificate whose subject is not authorized by the application | TLS may succeed; application authorization must deny it | SUCCESS, followed by an application-level denial |
The positive curl test needs a client certificate and its matching private key. The --cacert file is the CA bundle used by curl to verify the NGINX server; it is separate from NGINX’s client CA file. Adjust the sample paths to the files in your test environment.
Inspect the TLS handshake with OpenSSL
Use openssl s_client when you need to determine whether NGINX actually requests a certificate, which virtual server answered, or which server certificate was presented.
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openssl s_client
-connect mtls.example.test:443
-servername mtls.example.test
-showcerts </dev/nullIn the output, inspect the selected server certificate, handshake messages, and any certificate-request information. To test the positive path, provide the client credentials and the server CA:
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openssl s_client
-connect mtls.example.test:443
-servername mtls.example.test
-cert /etc/nginx/tls/client.crt
-key /etc/nginx/tls/client.key
-CAfile /etc/nginx/tls/server-ca.pem </dev/nullThe exact output varies with the OpenSSL release, TLS version, certificate encoding, and SNI selection. A certificate-request message proves that the server asked for a certificate; it does not by itself prove that the certificate was trusted or that the HTTP application authorized its identity. Confirm the result in the NGINX log.
How to tell whether a client check was skipped
Interpret the verification variable correctly
SUCCESS: NGINX received a certificate and verified its chain according to the configured client CA and verification settings.NONE: The client sent no certificate. This is not an authenticated client.FAILED:reason: A certificate was presented, but NGINX could not verify it. The reason identifies the class of failure.
Only SUCCESS is evidence that NGINX performed successful client-certificate verification. The subject, issuer, serial, and fingerprint fields help identify what was presented, but they do not turn NONE or FAILED:* into an authenticated result.
Compare the log with the response
- Run the no-certificate request and save the exact URL, response, and timestamp.
- Find the matching access-log line.
- If the line says
NONEand the client received protected content, the authorization layer bypassed mTLS. - Run the untrusted-certificate case. A
FAILED:reasonvalue followed by protected content is the same authorization defect for a presented-but-unverified certificate. - Run the wrong-identity case. A
SUCCESSvalue with access granted to an unauthorized subject means chain verification works but identity authorization is missing or incorrect.
Strict, optional, and optional_no_ca are different security designs
| Setting | Handshake behavior | No certificate | Untrusted certificate | Where authorization happens | Revocation use |
|---|---|---|---|---|---|
on |
NGINX requires a valid certificate | Handshake rejected | Handshake rejected | NGINX verifies the chain; the application must still authorize identity | Compatible with OCSP |
optional |
NGINX requests a certificate but permits the handshake without one | $ssl_client_verify=NONE |
Verification result is FAILED:reason |
HTTP policy must reject every value except SUCCESS |
Compatible with OCSP |
optional_no_ca |
NGINX requests a certificate without requiring its issuer to be in the configured CA list | Permitted | Certificate can arrive without NGINX proving its chain | A designated external verifier must authenticate it before identity is trusted | External verifier owns revocation and trust decisions |
optional is not “strict, but friendlier.” It deliberately allows a connection without a certificate. If the endpoint is meant to be mTLS-only, use on. If optional client authentication is intentional, enforce the access decision explicitly.
Enforce optional mode with an allow-only-success map
Place the map in the http context, then deny requests whose result is anything other than the literal success value.
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map $ssl_client_verify $mtls_allowed {
default 0;
SUCCESS 1;
}
server {
listen 443 ssl;
server_name mtls.example.test;
ssl_certificate /etc/nginx/tls/server.crt;
ssl_certificate_key /etc/nginx/tls/server.key;
ssl_client_certificate /etc/nginx/tls/client-ca.pem;
ssl_verify_client optional;
location / {
if ($mtls_allowed = 0) { return 403; }
proxy_set_header X-Client-Verify $ssl_client_verify;
proxy_set_header X-Client-Subject $ssl_client_s_dn;
proxy_pass http://app;
}
}The default map value covers both NONE and every FAILED:reason. Do not create an allow rule that merely checks whether a subject header is non-empty.
Handle optional_no_ca as an external-authentication boundary
With optional_no_ca, the presence of a certificate proves only that the client sent certificate data. It does not prove the issuer, validity, or identity. The service that performs verification must authenticate the certificate before NGINX-forwarded identity headers are trusted. Treat those headers as untrusted input until that external decision is complete.
Add revocation checking with OCSP when required
Certificate-chain validation does not automatically tell you whether a certificate was revoked. NGINX’s OCSP client validation is used with ssl_verify_client on or optional and requires DNS resolution for the configured responder.
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- Good: continue according to the normal certificate and identity policy.
- Revoked: deny the client.
- Unknown: apply the site’s documented fail-open or fail-closed rule; do not silently treat it as good.
Test OCSP separately from the basic mTLS matrix. A client can have a valid chain and still be revoked, while a responder outage can produce an unknown result. Record the resolver and NGINX/OpenSSL versions used so an operational failure is distinguishable from a certificate-policy failure.
Troubleshoot results that look like skipped verification
The endpoint accepts a client with no certificate
- Check the effective configuration for
ssl_verify_client off;. Because the directive defaults tooff, assuming that a client CA file enables enforcement is a common mistake. - Confirm that the request reached the intended
serverblock. A different SNI name, address, or default virtual server can have different TLS settings. - Check that the reload succeeded and that the running worker processes use the file you edited.
- Inspect the log value.
NONEplus protected content means an authorization bypass, not a successful certificate check. - If another proxy terminates TLS before NGINX, test the TLS-terminating proxy. The downstream NGINX process cannot verify a client certificate that never reaches it.
The valid client is rejected
- Verify that the client certificate chains to a CA in
ssl_client_certificate. - Send the matching private key and ensure the key is readable by the NGINX worker.
- Check the certificate validity period and intermediate certificates.
- Review
ssl_verify_depth; a chain deeper than the configured limit will fail. - Confirm that the client is connecting with the expected SNI name and that the server CA used by curl or OpenSSL validates the server certificate.
The certificate verifies, but the wrong user is admitted
NGINX’s SUCCESS result establishes certificate verification, not business authorization. Build an explicit authorization rule around the approved subject, issuer, serial, fingerprint, or application identity mapping. Then test a second certificate from the same trusted CA whose identity is not approved. That case should produce SUCCESS in the log and a denial from the authorization layer.
Headers make an unauthenticated request look authenticated
Never trust an identity header supplied by an arbitrary client. Strip or overwrite incoming versions at the TLS terminator, and set forwarded identity headers only from NGINX variables after the verification decision. This is especially important with optional_no_ca, where NGINX itself has not established trust.
A repeatable change-control checklist
- Record NGINX and OpenSSL versions, the active virtual-server name, and the certificate and CA file paths.
- Run
nginx -tand reload only after it passes. - Capture a log line for a valid certificate, no certificate, an untrusted issuer, an invalid certificate, and a wrong-but-trusted identity.
- Verify that only the valid certificate produces
SUCCESSand that every protected response follows the intended authorization result. - If using
optional, confirm that the map denies bothNONEand everyFAILED:reason. - If using
optional_no_ca, document the external verifier and prove that forwarded identity is ignored until that verifier succeeds. - If revocation matters, test OCSP Good, Revoked, and Unknown outcomes under the documented fail-open or fail-closed policy.
- Remove or restrict the expanded diagnostic log after testing, while retaining the minimum fields needed for incident investigation.
The decisive verdict is simple: in a strict NGINX mTLS endpoint, a no-certificate request must not obtain protected content, and the only successful verification value is SUCCESS. Anything else is either a rejected handshake or an authorization condition that your configuration must explicitly deny.
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