Mister PKI

SSL Certificates * SSL Tools * Certificate Decoder

TLS errors can break websites, APIs, and internal services instantly. When TLS handshakes fail, clients cannot establish secure connections and applications may stop functioning entirely.

This guide explains the most common TLS handshake errors, what causes them, and how DevOps engineers troubleshoot them in production environments.

This article is part of the PKI for DevOps Engineers training series. In the previous lesson we discussed automating certificate renewal. In this lesson we focus on diagnosing TLS errors.

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Table of Contents

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What is a TLS handshake?

The TLS handshake is the process used to establish a secure encrypted connection between a client and server.

During the handshake:

  1. the client connects to the server
  2. the server sends its certificate
  3. the client verifies the certificate
  4. the client and server negotiate encryption keys
  5. the encrypted session begins

If any step fails, the handshake will terminate and the connection will be rejected.

If you want a deeper explanation of this process, see our lesson on how TLS and PKI work.

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Common TLS errors

Several common errors occur when certificate validation or TLS negotiation fails.

tls handshake failed

This error usually indicates that the TLS negotiation process could not complete successfully.

Common causes include:

  • expired certificates
  • unsupported TLS versions
  • invalid certificate chains
  • cipher suite mismatches
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certificate verify failed

This error occurs when the client cannot verify the certificate presented by the server.

This often indicates:

  • missing intermediate certificates
  • unknown certificate authorities
  • incorrect trust store configuration
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hostname mismatch

This error occurs when the hostname used by the client does not match the certificate.

Modern TLS validation checks the Subject Alternative Name (SAN) field inside the certificate.

If you want to understand how SAN works, see our lesson on X.509 certificates.

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Certificate validation errors

Several TLS errors occur specifically because of certificate problems.

Common examples include:

  • certificate expired
  • certificate not yet valid
  • self signed certificate

These errors are often discovered during certificate monitoring or expiration checks.

See our article on certificate expiration monitoring for more information.

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Certificate chain problems

Certificate chain errors occur when the server does not provide the full chain of trust.

Examples include:

  • unable to get local issuer certificate
  • self signed certificate in certificate chain

These errors usually occur when intermediate certificates are missing or misconfigured.

If you want to understand how certificate chains work, see our lesson on certificate chains.

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Debug TLS errors with OpenSSL

One of the most useful tools for diagnosing TLS problems is OpenSSL.

You can inspect TLS connections using the following command:

openssl s_client -connect example.com:443

This command displays:

  • the certificate chain
  • TLS protocol version
  • cipher suite
  • certificate verification results

For more details, see our article on openssl s_client.

You can also verify certificates locally using openssl verify.

openssl verify certificate.crt

If you want to inspect certificate fields, you can also view the certificate contents.

See our guide on viewing certificates with OpenSSL.

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Preventing TLS errors

Many TLS errors can be prevented with proper certificate management practices.

Recommended practices include:

  • monitor certificate expiration
  • automate certificate renewal
  • validate certificate chains during deployment
  • verify TLS configuration after installation

These practices significantly reduce the risk of TLS outages in production systems.

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Frequently Asked Questions

What causes TLS handshake failures?

Handshake failures are usually caused by certificate validation problems, unsupported TLS versions, or misconfigured certificate chains.

How do I debug TLS errors?

OpenSSL tools such as openssl s_client and openssl verify are commonly used to diagnose TLS problems.

What is the most common TLS error?

Certificate expiration and missing intermediate certificates are among the most common TLS issues.

How can TLS errors be prevented?

Certificate monitoring and automated renewal systems help prevent most TLS outages.

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Next Lesson

In the next lesson we will examine how DevOps teams design scalable PKI architectures for modern infrastructure.

Lesson 11 – PKI Architecture for DevOps →

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