The Tool Desk
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Start by identifying the furthest stage reached
Use client and broker logs to establish what happened immediately before the failure. If there is no TCP connection, investigate the endpoint and network path. If TCP connects but TLS fails, examine negotiation and certificates. If the client receives CONNACK, TLS completed and the broker is responding at the MQTT layer.
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| Last confirmed event | Likely failure area | Evidence to collect |
|---|---|---|
| No socket connection | Endpoint, DNS, route, firewall, listener or port | Configured hostname and port, DNS result, socket error, network and broker-listener logs |
| TCP connects, TLS does not complete | TLS version or configuration, SNI, ALPN, certificate chain or server-name verification | TLS alert or verification error, client and broker TLS logs, presented certificate chain |
| TLS completes, but no successful MQTT session | Client certificate, MQTT CONNECT fields or broker policy | Whether a client certificate was presented, CONNECT details, CONNACK code and broker logs |
A username or password cannot explain a failure that occurs before MQTT CONNECT reaches the broker. Conversely, a CONNACK refusal is evidence of an MQTT-level response, not a failed TLS handshake.
1. Confirm the endpoint and resolve DNS
Check that the client is connecting to the intended broker address for the right deployment, account or region. Record the exact hostname or IP, the resolved address, the port, and whether the client uses a DNS name or connects directly by IP.
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- Make sure the client API expects a hostname rather than a URL. AWS IoT Core documents that its SDKs require the endpoint hostname; passing a full URL can result in an invalid-hostname error. See AWS IoT Core device connection guidance.
- For a provider-managed endpoint, obtain the endpoint assigned to the intended account and region, or the configured custom domain.
- If the name does not resolve, or the client fails before opening a socket, investigate DNS, routing and firewall rules. Verify that the broker is listening on the address and port the client is using.
Do not assume an address shown in a dashboard is interchangeable with an application URL or with an IP address. The name used at connection time can also matter later, during certificate hostname verification.
2. Verify TCP connectivity and the listener
A refused or timed-out TCP socket occurs before TLS certificate checks and MQTT CONNECT. Confirm that the client and broker agree on the intended listener, port and transport security. A deployment may expose separate listeners for plaintext MQTT, MQTT over TLS or MQTT over WebSockets; the correct choice depends on the broker configuration.
For AWS IoT Core, the documented default-endpoint mapping includes X.509-authenticated MQTT on port 8883 without ALPN, and X.509-authenticated MQTT on port 443 with ALPN x-amzn-mqtt-ca. Other authentication and WebSocket paths have different requirements. These are AWS endpoint rules, not universal MQTT conventions. Consult the AWS IoT Core protocols and ports reference before changing a client configuration.
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3. Check TLS negotiation requirements
If TCP succeeds but the secure session does not, capture the precise TLS error and compare client and broker logs. Check that the TLS versions and cipher configuration supported by the client are compatible with the broker’s current policy. AWS IoT Core documents support for TLS 1.2 and TLS 1.3; that is an AWS service detail, not a minimum that applies to every MQTT broker.
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Check SNI and ALPN when the endpoint requires them
Server Name Indication (SNI) lets the client identify the server name during the TLS handshake. AWS IoT Core states that non-SDK clients must send SNI and that connections without it are refused. Its requirements apply to features including multi-account registration, configurable endpoints, custom domains and VPC endpoints. See AWS IoT Core connection guidance and the protocol reference.
Application libraries do not always set SNI or ALPN the way a provider expects. For AWS X.509 MQTT on port 443, verify that the client sends the applicable ALPN value, x-amzn-mqtt-ca, for the documented default-endpoint path. A mismatch among endpoint, port, authentication method and ALPN can stop the connection before MQTT CONNECT is processed.
4. Separate certificate trust from hostname verification
Certificate checks answer two independent questions: does the client trust the certificate issuer, and does the certificate identify the server name the client actually used? Passing one check does not guarantee passing the other.
Confirm the client trusts the issuer
The client needs the appropriate CA certificate or chain in its trust store. In its Greengrass troubleshooting guide, AWS says that a missing CA certificate is the most common cause of the documented “Unable to verify server certificate” issue. That guidance concerns the Greengrass scenario; it does not establish that missing CA material is the most common cause of MQTT TLS failures generally. The guide demonstrates retrieving a presented chain with OpenSSL: AWS Greengrass certificate troubleshooting.
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Check the certificate name against the connection address
Compare the DNS name or IP passed to the client with the certificate’s Subject Alternative Name (SAN). If the client connects by IP, that IP must be covered by the certificate for IP-based identity verification to succeed. AWS describes a Greengrass case where the device IP was absent from SAN and server-name verification failed.
The AWS guide’s example command is openssl s_client -showcerts -connect <device IP>:8883. It targets the documented Greengrass listener; substitute the hostname or IP and port used by your broker. To inspect a saved certificate, use openssl x509 -in <certificate-file> -text and examine its issuer and SAN entries.
In the specific Greengrass scenario, AWS notes that Mbed TLS supports DNS-name verification only in SAN. Do not assume that this statement describes every embedded TLS library or configuration. Verify the behavior of the library and certificate used in your own client.
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5. If mutual TLS is enabled, verify the client certificate
When the broker requires mutual TLS, check that the client presents the intended certificate and its matching private key, and that the broker trusts the issuing CA. The broker must also accept the certificate’s identity and status under its own configuration. Enrollment, revocation, activation and identity-mapping rules vary by broker, so use that broker’s documentation and logs for those details.
Keep certificate roles distinct. Mosquitto warns that CA, server and client certificates should use different subject parameters; otherwise, the broker or client may have difficulty distinguishing them. See the Mosquitto TLS manual.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.6. Read CONNACK as an MQTT-layer response
Once TLS is established and the client sends MQTT CONNECT, check whether the broker returns CONNACK and record its code or reason. A CONNACK means the failure has reached the MQTT layer. It is then appropriate to investigate protocol version, client identifier, credentials and authorization rather than treating the incident as an initial TLS failure.
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| MQTT 3.1.1 return code | Meaning | What to check |
|---|---|---|
| 0 | Connection accepted | Look for a later disconnect or application-level problem. |
| 1 | Unsupported protocol version | Compare the client’s MQTT version with the broker’s supported versions. |
| 2 | Client identifier rejected | Check the client ID’s format and any broker requirements. |
| 3 | Server unavailable | Check broker health and service-side availability. |
| 4 | Malformed username or password | Check the credential format and how the client supplies it. |
| 5 | Not authorized | Check authentication, authorization rules and any certificate-to-client identity mapping. |
MQTT 5 uses more detailed reason codes, including malformed packet, protocol error, unsupported protocol version, invalid client identifier and bad username or password. The code set and the information a broker returns depend on protocol version and implementation; consult the broker’s documentation and logs for the response you received.
A repeated connect-and-disconnect pattern can also involve client ID policy rather than TLS. AWS IoT Core does not allow two connections with the same client ID to remain active concurrently: a newly accepted connection can displace the existing one. AWS also says a client must wait for CONNACK before sending additional control packets. Check the AWS IoT Core MQTT documentation when diagnosing those behaviors.
Use a known-good client to isolate application settings
If you have Mosquitto CLI available, test the same broker from the same network with the correct CA, hostname and port. AWS documentation demonstrates mosquitto_sub as a way to verify a broker connection, with a CA file and optional username and password: AWS Greengrass troubleshooting examples.
If the CLI connects but your application does not, compare the exact settings rather than assuming the network is the cause: endpoint, CA trust store, server name, SNI and ALPN behavior, client certificate and private key, MQTT version, client ID, credentials and CONNECT fields. A successful test with one client narrows the problem to differences in configuration or implementation; it does not prove that every application setting is correct.
Keep the diagnosis tied to the failed stage
Do not switch brokers or rewrite credentials until logs show which stage is failing. A new endpoint may help test a client stack, but it cannot repair a missing trusted CA, a certificate SAN mismatch or an invalid MQTT CONNECT. First capture the last confirmed event, then change the setting that belongs to that layer.
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