tailwart/caddy

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 Dockerfile (xcaddy + caddy-l4), runs it
caddy list-modules | grep layer4  # (inside the image) proof the module loaded

The build fails loudly if caddy-l4 isn't in the resulting binary.

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.