TLS: SSL_ERROR_SYSCALL / connection reset during handshake

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

# Pin down which side and which stage
openssl s_client -connect host:443 -servername host -tls1_2 </dev/null
openssl s_client -connect host:443 -servername host -tls1_3 </dev/null

# Watch the wire. A RST right after ClientHello means a middlebox
sudo tcpdump -ni any host <ip> and port 443

# Test without SNI to detect SNI-based filtering
openssl s_client -connect host:443 -noservername </dev/null

How to diagnose TLS errors

TLS errors have a small number of root causes: the certificate is expired, the hostname does not match any SAN entry, the chain is incomplete (the server did not send its intermediates), the issuing CA is not trusted by this client, or the two sides share no cipher or protocol version. The incomplete-chain case is the most deceptive, because browsers often paper over it with cached intermediates while curl, Java and Go fail.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

1. Inspect the live handshake: openssl s_client -connect host:443 -servername host -showcerts. It shows the full chain the server actually sends.
2. Check dates and SANs: openssl x509 -noout -dates -subject -ext subjectAltName. The Common Name is ignored by modern clients: only SANs matter.
3. If browsers work but curl or your language runtime does not, suspect a missing intermediate. Browsers cache intermediates; other clients do not.
4. Verify the trust store the failing client uses. Containers frequently ship without ca-certificates installed, and Java, Node and Python each have their own store.
5. For protocol or cipher errors, check the negotiated version and the server's supported list. TLS 1.0 and 1.1 are disabled by default in current clients.

Tools worth reaching for

• openssl s_client
• openssl x509 -noout -text
• curl -vI
• testssl.sh
• sslyze

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 openssl.org

Related TLS errors

• Apache mod_ssl errorApache's SSL module encountered an error, often due to misconfigured SSL directives.
• Certificate chain incompleteSSL certificate chain is missing intermediate certificates, causing trust validation to fail.
• Certificate has expiredThe SSL certificate is past its expiration date and is no longer valid for secure connections.
• Client certificate requiredServer requires client certificate authentication but none was provided.
• curl: (56) OpenSSL SSL_read: Connection reset by peerThe SSL connection was abruptly closed by the remote server, often due to protocol mismatch…
• ERR_SSL_PROTOCOL_ERRORBrowser detected an SSL protocol violation or misconfiguration. Often caused by mixed…
• Hostname verification failedThe SSL certificate's hostname doesn't match the requested domain name.
• Invalid SSL certificateThe SSL certificate is malformed, self-signed, or doesn't meet security requirements.

See all 24 TLS errors →

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Something missing or wrong?

This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.