Configure Caddy on-demand TLS so certificates for customer/unknown domains are obtained dynamically at first TLS handshake, protected by a required 'ask' permission-check endpoint.
domain: caddyserver.com · 8 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Add the global 'on_demand_tls' option to the Caddyfile with an 'ask' endpoint that Caddy will query before issuing any certificate:
{
on_demand_tls {
ask http://localhost:9123/ask
}
}
Enable on-demand TLS in the site block using a dynamic address and the tls directive's 'on_demand' subdirective:
https:// {
tls {
on_demand
}
reverse_proxy localhost:8080
}
Implement the ask endpoint (your own backend) to receive a GET request with a query string '?domain=<name>' and respond with a 2xx status if that domain is permitted (e.g. it exists in a customer accounts table), or any other status to reject it.
Make the ask endpoint respond in a few milliseconds — the docs recommend a constant-time database lookup by domain name and explicitly warn against loops, DNS queries, or other network requests inside it, since it blocks the first handshake.
Ensure each customer domain's DNS is pointed at your Caddy instance before a client connects, since the certificate is only requested at handshake time for the SNI presented.
Do not omit the on_demand_tls/ask restriction — the docs explicitly warn that using 'on_demand' without it is insecure for a publicly accessible instance, since anyone can point DNS at your server and exhaust resources.
Deploy and test by pointing a new domain's DNS at the server and making an HTTPS request; the first handshake will be held for a few seconds while Caddy calls the ask endpoint and completes ACME issuance, then subsequent handshakes are fast and cached.
Official docs: https://caddyserver.com/docs/automatic-https#on-demand-tls , https://caddyserver.com/docs/caddyfile/directives/tls , https://caddyserver.com/docs/caddyfile/options
Known gotchas
On-demand TLS must be both enabled (tls directive's on_demand) and restricted (global on_demand_tls with an ask endpoint) — the docs state the ask/permission restriction is 'required for use in production environments, to prevent abuse'.
The ask endpoint must return fast; if the endpoint returns anything other than a 2xx status, Caddy cancels issuance and errors the TLS handshake for that domain.
on_demand_tls restrictions are global and are not configurable per-site or per-domain.
The deprecated 'interval' and 'burst' rate-limiting suboptions of on_demand_tls are explicitly flagged as NOT recommended in current docs — remove them if present in an older config.
Only the first handshake for a new domain is slow (waiting on the ask call plus ACME issuance); if your ACME CA takes more than a few seconds to issue, that delay directly hurts that first client's experience.
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