Caddy Custom-Domain Proxy

Custom church domains (mychurch.org → the church's B1 website) terminate at a single Windows EC2 box running Caddy. The box owns the TLS certificates, resolves each domain to its {sub}.b1.church site, and reverse-proxies with a rewritten Host header. Its entire configuration is two files — a static Caddyfile and a hosts.map refreshed from the Membership API — so it survives restarts with zero runtime state. This page covers how the box is built from scratch, how it operates, and the field-tested gotchas that will bite anyone rebuilding it.

For how a request resolves to a church/site once it reaches B1App, see Website Routing & Multi-Site.

Components

Piece	What it is
EC2 instance	Windows Server; Elastic IP 3.23.251.61 (baked into church DNS worldwide — the IP is permanent, instances are disposable)
C:\caddy\caddy.exe	Custom Caddy build with the techknowlogick/certmagic-s3 storage module — stock Caddy cannot read the cert store
C:\caddy\Caddyfile	The entire proxy config: on-demand TLS, host→upstream map, www→apex redirects, :80→https
C:\caddy\hosts.map	One {domain} {sub}.b1.church line per routable domain, imported into the Caddyfile's map block
sync-hostmap.ps1 + CaddyHostmapSync task	Scheduled task (every 5 min + at boot, as SYSTEM) refreshes hosts.map from the API and gracefully reloads Caddy only on change
Windows service caddy (WinSW wrapper)	Runs caddy.exe run --config C:\caddy\Caddyfile --adapter caddyfile; auto-restart on failure. Caddy is not SCM-aware, so a wrapper is required
S3 bucket churchapps-caddy-certs	Shared certificate storage (region us-east-2, prefix certs) — certs survive instance rebuilds
IAM role CaddyRole	Grants the instance S3 access; Caddy uses the AWS default credential chain (no keys in config)

The two API endpoints the box depends on

Both are anonymous, on the Membership API:

Endpoint	Role
GET /membership/domains/authorize?domain={host}	Caddy's on-demand TLS ask gate: 200 {"authorized":true} when the host (or, for a www. host, its apex) is a row in domains; 404 otherwise. This is the abuse control — Caddy will not issue a certificate for a host this endpoint rejects
GET /membership/domains/hostmap	text/plain, sorted, deduplicated {domain} {sub}.b1.church lines (site-aware: a domain assigned to a secondary site dials that site's subdomain). Source of the map

Request flow

1. Browser resolves mychurch.org → 3.23.251.61 (apex A record, or CNAME proxy.b1.church).
2. Caddy terminates TLS. Certificate on hand in S3 → serve; unknown SNI → authorize is asked; 200 → issue on demand via Let's Encrypt; 404 → the handshake is refused (no certificate, no response — an unknown host gets TLS-refused, not an HTTP error).
3. The map resolves the Host to {sub}.b1.church; www.{apex} gets a 302 to the apex; an authorized-but-unmapped host (a brand-new domain inside the ≤5-minute sync window) gets a clean 404.
4. reverse_proxy dials {sub}.b1.church:443 with SNI and Host rewritten to the upstream, so Vercel's edge serves the B1App site.
5. Port 80 passes ACME HTTP-01 challenges and 308-redirects everything else to https.

New-domain propagation: a domain saved in B1Admin becomes routable within ~5 minutes (the sync task); its certificate is minted on the first HTTPS hit.

Building the box from scratch

Condensed from the field-tested procedure (full step-by-step with copy-paste commands lives in the ops workspace, not this repo). Prerequisites first — the build is dead without them:

1. IAM: attach CaddyRole (S3 access to the cert bucket) to the instance. Verify via IMDSv2 from the box — note a bare IMDS GET returning 401 just means IMDSv2 is enforced, not "no role".
2. API health: authorize must return 404 for a bogus domain and hostmap must return 200 before anything else.

Then:

3. Binary: download a custom build from Caddy's build service — https://caddyserver.com/api/download?os=windows&arch=amd64&p=github.com/techknowlogick/certmagic-s3 (~57 MB vs ~45 MB stock; v2.11.4 at time of writing). The module choice matters: techknowlogick/certmagic-s3 uses bucket/region/prefix keys matching the existing cert layout; the ss098 fork uses host/endpoint and will not find the existing certificates.
4. Files: Caddyfile + sync-hostmap.ps1 into C:\caddy\; seed the map once with sync-hostmap.ps1 -NoReload.
5. Gates before first start: caddy list-modules must show the s3 storage module; caddy adapt must emit "module":"s3","bucket":"churchapps-caddy-certs","region":"us-east-2","prefix":"certs" in its storage block; caddy validate must pass.
6. Service: install via WinSW (service id caddy, auto-restart on failure, rolling logs). Runs as LocalSystem, which reaches IMDS for the role credentials.
7. Sync task: register CaddyHostmapSync (SYSTEM, every 5 min + at startup, 4-minute execution limit).
8. Verify pre-cutover by force-resolving domains to 127.0.0.1 with curl --resolve (the box has no real traffic until the EIP moves): an existing domain must serve with a valid carried-over cert; www. must 302; an unknown host must be TLS-refused; and Restart-Service caddy must come back serving with no manual re-priming — that restart test is the entire point of the static design.
9. Go-live: reassociate the Elastic IP 3.23.251.61 to the new instance. Church DNS never changes.

Field-tested gotchas (learned the hard way — do not regress)

Gotcha	Symptom	Fix
tls_server_name {vars.upstream} in the reverse_proxy transport	Every proxied domain 502s: map placeholders resolve empty at TLS-dial time ("either ServerName or InsecureSkipVerify must be specified")	Use the transport-native placeholder: tls_server_name {http.reverse_proxy.upstream.host}
Duplicate keys or junk lines in hosts.map	Caddy's map handler hard-errors on a duplicate input key — one bad line can take the whole config down	The sync script normalizes whitespace, drops malformed lines (rejecting wholesale only if >20% are bad), dedupes first-wins, and writes BOM-free UTF-8 (a BOM corrupts the first map key). The API also filters empty/space-containing domain rows at the source
Register-ScheduledTask -RepetitionDuration ([TimeSpan]::MaxValue)	Task registration silently fails (out-of-range XML, non-terminating error)	Build the repetition as a MSFT_TaskRepetitionPattern CIM instance with Interval = "PT5M" and no duration; add a 4-minute ExecutionTimeLimit (the first SYSTEM run can hang on a cold TLS/CRL lookup)

Aviso

The admin API binds to localhost:2019 only. The legacy runtime mode exposed it remotely so the Membership API could push route configs; the static design needs no remote pushes, and the smaller surface is deliberate. caddy reload (run locally by the sync script) is the only admin-API consumer.

Legacy runtime push

CaddyHelper in the API (and the /membership/domains/caddy + /caddy/init endpoints) still exist as the rollback path to the old runtime-configured mode. They are scheduled for deletion once the static box has been stable for a couple of weeks — after that, authorize + hostmap are the only integration points.

Operations

• Logs: WinSW rolling logs in C:\caddy\ (service stdout/err — reverse-proxy errors land in caddy-service.err.log); sync history in C:\caddy\sync-hostmap.log.
• Force a map refresh: Start-ScheduledTask -TaskName CaddyHostmapSync.
• Config change: edit C:\caddy\Caddyfile, then caddy validate + caddy reload (or Restart-Service caddy — restarts are safe by design).
• Mass domain deletion trips the sync script's shrink guard by design; move the old hosts.map aside and re-run the task to accept an intentional large shrink.
• DNS instructions for churches are unchanged forever: apex A 3.23.251.61 or CNAME proxy.b1.church.

Related Pages

• Website Routing & Multi-Site — how the proxied request resolves to a church/site in B1App
• API Deployment — deploying the Membership API that serves authorize/hostmap