HTTP transport tooling

Alt-Svc Header Builder & HTTP/3 Upgrade Planner

Compose valid Alt-Svc headers for HTTP/2 and HTTP/3, compare alternative origins, model client cache lifetime, and copy deployment snippets for Node, Nginx, Apache, and Cloudflare. Useful for gradual HTTP/3 rollout, CDN migrations, and debugging transport hints.

BuildsAlt-Svc

Protocolsh2, h3, h3-29

OutputsHeader + snippets

PlanningCache + rollout notes

Header setup

Alt-Svc lets an origin advertise an alternative host, port, or protocol. Browsers may keep using the current connection for the first response, cache the hint, then try the advertised service on later requests.

Primary origin host

Primary port

Cache max-age (seconds)

persist flag

Quick mode

Planning toggles

Same authority as origin Include h2 fallback Cross-host/CDN migration Prefer QUIC-only rollout

Alternative services

Each alternative is rendered as protocol=\"host:port\". Use :443 to keep the same host, or set an explicit alternative authority for migrations.

Generated Alt-Svc header

Rollout summary

Behavior preview

Validation notes

Server snippets

When to use this

HTTP/3 rolloutAdvertise h3=":443" while keeping HTTPS stable

CDN migrationShift clients toward an alternate edge hostname

RollbackSend Alt-Svc: clear to flush cached hints

TuningUse shorter ma during testing, longer in steady state

Alt-Svc is only a hint, not a redirect. Clients may ignore it, wait until a later request, or refuse a broken protocol/authority if certificates, ALPN, or reachability do not line up.

Good rollout pattern: start with a short cache like 300–3600 seconds, verify QUIC and certificate coverage, then increase max-age once telemetry looks healthy.

Common risks

Advertising a host that is not covered by your TLS certificate
Using a long ma before validating real client behavior
Forgetting to clear stale hints during rollback
Assuming Alt-Svc changes browser address bars or SEO signals

Deployment checklist

Confirm ALPN support for each advertised protocol
Confirm alt host and port are reachable from public networks
Match origin authority, SNI, and cert chain expectations
Monitor handshake failures before raising max-age