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Alt-Svc Protocol Errors — HTTP Alternative Service Connection Failures

Informational3xx redirection

Overview

Fix Alt-Svc header errors causing failed protocol upgrades to HTTP/3 QUIC, incorrect alternative service routing, and fallback failures.

Key Details

  • Alt-Svc (Alternative Services) allows servers to advertise that a resource is available via a different protocol or host
  • Commonly used to upgrade connections from HTTP/2 to HTTP/3 (QUIC)
  • The header format is: Alt-Svc: h3=":443"; ma=86400 (HTTP/3 on port 443, max-age 24 hours)
  • Browsers cache Alt-Svc entries and attempt the alternative service on subsequent requests
  • Failed alternative service connections should fall back to the original protocol transparently

Common Causes

  • Firewall blocking UDP traffic on port 443 required for HTTP/3 QUIC connections
  • Alt-Svc advertising a service that is not actually available or configured
  • QUIC server certificate not matching the original hostname
  • Alt-Svc max-age too long, causing clients to attempt a service that has been decommissioned

Steps

  1. 1Verify HTTP/3 QUIC is properly configured and accessible: test with 'curl --http3 https://yourdomain.com'
  2. 2Ensure firewall allows UDP traffic on the port specified in Alt-Svc (typically 443)
  3. 3Set a reasonable max-age (ma): start with 3600 (1 hour) and increase after confirming stability
  4. 4Use 'Alt-Svc: clear' to remove cached alternative services when decommissioning HTTP/3
  5. 5Monitor fallback rates — high fallback indicates the alternative service is not reliably reachable

Tags

alt-svchttp3quicprotocol-upgradealternative-service

More in 3xx Redirection

http-300-multiple-choicesHTTP 300 Multiple Choices — What It Means & How to Fix It
Warning
http-301-moved-permanentlyHTTP 301 Moved Permanently — What It Means & How to Fix It
Warning
http-302-foundHTTP 302 Found — What It Means & How to Fix It
Warning
http-303-see-otherHTTP 303 See Other — What It Means & How to Fix It
Warning
http-304-not-modifiedHTTP 304 Not Modified — What It Means & How to Fix It
Warning
http-305-use-proxyHTTP 305 Use Proxy — What It Means & How to Fix It
Warning

Frequently Asked Questions

Alt-Svc primarily enables protocol upgrades from HTTP/2 to HTTP/3 (QUIC). It tells browsers that the same content is available via a faster protocol, allowing them to switch transparently.