ALPN Checker

Check supported ALPN protocols (h2, http/1.1, etc.)

About ALPN Checker

Our free ALPN (Application-Layer Protocol Negotiation) Checker tests which application protocols a server supports during the TLS handshake. Enter any domain and instantly discover supported protocols like HTTP/2 (h2), HTTP/1.1, and more. Essential for troubleshooting HTTP/2 and HTTP/3 compatibility.

How to Use

1Enter the domain name you want to test in the input field.
2Click the Check button to initiate the ALPN protocol scan.
3Review the results showing supported protocols and TLS handshake details.

Use Cases

•Verify HTTP/2 (h2) support before enabling it on your server
•Troubleshoot ALPN negotiation issues with load balancers and CDNs
•Audit third-party services for modern protocol support

Frequently Asked Questions

ALPN (Application-Layer Protocol Negotiation) allows a client and server to agree on which protocol to use during the TLS handshake. It is essential for HTTP/2 and HTTP/3 connections and improves performance by eliminating extra round trips.

The checker tests for common ALPN protocols including h2 (HTTP/2), http/1.1, and h2c. It reports all protocols the server advertises during the TLS handshake negotiation phase.

Our checker tests port 443 by default, which is the standard HTTPS port. ALPN negotiation occurs during the TLS handshake and is supported on any port that uses TLS encryption.

Tips & Best Practices

•Enable ALPN for HTTP/2 and HTTP/3 support
•Configure preferred ALPN protocols in order of preference
•Test ALPN negotiation with different client configurations
•Monitor ALPN usage to ensure proper protocol selection

Technical Details

Extension: Application-Layer Protocol Negotiation (ALPN) detection
Protocols: h2 (HTTP/2), http/1.1, h3 (HTTP/3), grpc, custom protocols
Support: TLS extension negotiation during ClientHello/ServerHello
Output: ALPN status with supported protocols and recommendations