Wayne Hayes a9e2a736fc caddy: build via caddyserver.com download URL, not local xcaddy

The xcaddy/Go compile burns ~1GB RAM this VPS can't spare (per ~/docs/caddy.md
"Custom Binary"). Pull the prebuilt L4-enabled binary from the Caddy build
server instead and swap it over the stock binary in the official image. Built
and verified: caddy v2.11.3 with layer4.handlers.proxy + proxy_protocol.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

2026-06-03 22:39:33 -04:00

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tailwart edge — layer-4 mail proxy

A custom Caddy (with the caddy-l4 app) that pipes the public mail ports to the Stalwart sidecar over the tailnet. Pure TCP pass-through with PROXY protocol — Stalwart still terminates all the TLS. Runs anywhere with a public IP that's on the tailnet and tagged tag:reverse-proxy; doesn't need to share a host with the mailbox.

Why layer 4 and not a normal Caddy vhost

Web apps reverse-proxy at layer 7 (route by Host/SNI, Caddy terminates TLS). Mail can't: port 25 has no SNI (STARTTLS comes after connect), and you want one global :25 listener, not per-domain routing. So the edge is a dumb L4 pipe and Stalwart owns the TLS. The novelty you spotted: this is the same stream-style proxying nginx/Caddy can do for any TCP — it just usually isn't used for it.

Build & run

docker compose up -d --build      # builds the image, runs it

The Dockerfile doesn't compile Caddy — it pulls the prebuilt L4-enabled binary from caddyserver.com/api/download (the house method, see ~/docs/caddy.md "Custom Binary"), dodging the ~1GB-RAM local xcaddy build this VPS can't afford. The build still fails loudly if caddy-l4 isn't in the downloaded binary. To add plugins, append &p=<url-encoded module path> to CADDY_DOWNLOAD in the Dockerfile.

Edit the upstream

caddy.json dials stalwart.tail7b1641.ts.net:<port>. If your STALWART_MAGIC_NAME / TS_TAILNET differ, update the five dial lines. (JSON can't read .env; this is the one spot the MagicDNS name is hardcoded — same trade-off as pgAdmin's servers.json.)

The HTTP side (JMAP / autoconfig / admin) is separate

That part is ordinary layer 7. Don't put it here if this box already runs the main Caddy on :443 — you'll collide. Instead add a vhost to the existing Caddy:

mail.infinidim.net {
    reverse_proxy stalwart.tail7b1641.ts.net:8080
}

Prerequisites on the host running this

Joined to the tailnet, tagged tag:reverse-proxy (so the ACL lets it reach tag:stalwart).
Public firewall opens for whichever mail ports you expose (25 minimum).
Nothing else bound to those ports.

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