Tor SMTP Gateway (svc-net-tor-smtp)

Description

A small SMTP relay that carries .onion recipients over Tor. The active mail provider (Stalwart) cannot open a SOCKS connection of its own, and a .onion address cannot be reached any other way, so the provider hands every .onion recipient to this gateway as an ordinary SMTP relay target and the gateway dials <recipient-domain>:25 through Tor’s SOCKS5 listener.

Overview

Stalwart  ──(relay route: rcpt ends_with .onion)──▶  tor-smtp:1525
tor-smtp  ──(SOCKS5, remote DNS)──▶  Tor  ──▶  <recipient>.onion:25

Clearnet mail is untouched — it stays on the provider’s normal MX delivery. The gateway only ever accepts .onion recipients; it is not an open relay.

Cosmos

The diagram places Tor SMTP Gateway (svc-net-tor-smtp) in the Infinito.Nexus cosmos: the components it deploys (capabilities), the central services it consumes (dependencies), and its outward reach (federation and bridged external networks).

flowchart LR
    subgraph role [svc-net-tor-smtp 🐳🐝]
        svc_tor_smtp["tor-smtp"]
    end
    subgraph dependents [Dependents]
        dpt_web_app_stalwart["web-app-stalwart 🐳🐝"]
    end
    svc_tor_smtp -. "0..1" .-> dpt_web_app_stalwart

Solid 1:1 edges are fixed relationships; dashed 0..1 edges are conditional (enabled only in matching deployments); red 0..0 edges are turned off in this role. Node markers show the role’s deploy modes (💻 host, 🐳 compose, 🐝 swarm); ❌ marks a service that is explicitly turned off, and ⚙️ an Ansible role dependency declared in meta/main.yml.

Features

  • Onion-only relay: accepts a recipient only when its domain ends with .onion; every clearnet recipient is refused with 550.

  • Remote resolution: dials through Tor’s SOCKS5 with proxy_rdns, so the .onion name is resolved by Tor and never looked up locally.

  • One hop per domain: recipients sharing a .onion domain are delivered in a single SMTP transaction.

  • Clearnet untouched: normal SMTP reputation, DKIM and SPF delivery never pass through Tor.

Quick Setup

Development

Clone, set up the workstation, and deploy Tor SMTP Gateway (svc-net-tor-smtp) onto the local stack:

git clone https://github.com/infinito-nexus/core.git
cd core
make onboard
make compose-deploy mode=reinstall apps=svc-net-tor-smtp full_cycle=false

Production

Run the published image to provision the inventory and deploy Tor SMTP Gateway (svc-net-tor-smtp) to a managed server (the mounted volume persists the inventory):

APP=svc-net-tor-smtp
HOST=<your-server>
DOMAIN=<your-domain>
TLS_MODE=self_signed
SSH_PUBLIC_KEY="<your-ssh-public-key>"

docker run --rm -it \
  -v "$PWD/inventories:/etc/infinito.nexus/inventories" \
  -e APP="$APP" -e HOST="$HOST" -e DOMAIN="$DOMAIN" -e TLS_MODE="$TLS_MODE" -e SSH_PUBLIC_KEY="$SSH_PUBLIC_KEY" \
  ghcr.io/infinito-nexus/core/debian bash -c '
    INVENTORY=/etc/infinito.nexus/inventories/production
    infinito administration inventory provision "$INVENTORY" \
      --inventory-file "$INVENTORY/devices.yml" \
      --host "$HOST" \
      --include "$APP" \
      --vars "{\"TLS_MODE\": \"$TLS_MODE\", \"DOMAIN_PRIMARY\": \"$DOMAIN\", \"users\": {\"administrator\": {\"authorized_keys\": [\"$SSH_PUBLIC_KEY\"]}}}" &&
    infinito administration deploy dedicated "$INVENTORY/devices.yml" \
      --password-file "$INVENTORY/.password" \
      --diff -vv'

Design Decisions

  • A dedicated role, not Stalwart config. Stalwart has no outbound SOCKS support (stalwartlabs/stalwart#644), so the Tor hop lives in a separate service that only the .onion route uses.

  • proxy_rdns is on. A .onion has no DNS entry, so Tor must resolve it — the name is never resolved locally.

  • Co-located with svc-net-tor. It dials that role’s SOCKS listener, so it is manager-pinned like the Tor daemon.

Credits

Implemented by Alejandro Roman Ibanez. Part of the Infinito.Nexus Project and maintained by Kevin Veen-Birkenbach. Licensed under the Infinito.Nexus Community License (Non-Commercial).