feat: turnkey Postfix relay setup script for Debian/Ubuntu

.github/workflows/lint.yml

@@ -183,7 +183,7 @@ jobs:
       - name: Syntax-check bash scripts
         run: |
           set -e
-          for f in bash-curl-zerosmtp.sh bash-swaks-zerosmtp.sh check-connection.sh; do
+          for f in bash-curl-zerosmtp.sh bash-swaks-zerosmtp.sh check-connection.sh setup-postfix-relay.sh; do
             echo "Checking $f"
             bash -n "$f"
           done

docs/SYSTEM-MTA.md

@@ -25,6 +25,21 @@ Postfix is Debian's and most Ubuntu servers' default MTA, so this is almost
 always the right starting point if Postfix is already installed
 (`dpkg -l postfix`).
 
+### Fastest path: one script, username and password only
+
+[`setup-postfix-relay.sh`](https://github.com/msgwing/ZeroSMTP/blob/main/setup-postfix-relay.sh)
+installs Postfix, sets satellite mode, configures the relay, and applies the
+sender rewrite from the section below — all of it, in one run:
+
+```bash
+sudo ./setup-postfix-relay.sh
+```
+
+It asks for your ZeroSMTP username and password (or read them from
+`ZEROSMTP_USERNAME`/`ZEROSMTP_PASSWORD` for an unattended run) and does the
+rest. Safe to re-run. The steps below are what it automates, for anyone who
+wants to see or adjust them by hand instead.
+
 ```bash
 sudo apt install -y postfix mailutils libsasl2-modules
 ```

setup-postfix-relay.sh

@@ -0,0 +1,127 @@
+#!/bin/bash
+# setup-postfix-relay.sh
+# Turnkey Postfix satellite/smarthost setup for mx.msgwing.com, Debian/Ubuntu.
+# Installs Postfix, points it at ZeroSMTP, and rewrites the envelope/From
+# sender so mail doesn't bounce with "not owned by user" - the two steps
+# docs/SYSTEM-MTA.md documents as separate manual sections, done together
+# here. Needs only a ZeroSMTP username and password; everything else is
+# derived or has a documented default. Run with sudo. Safe to re-run.
+#
+# Usage:
+#   sudo ./setup-postfix-relay.sh
+#   sudo ZEROSMTP_USERNAME=you@msgwing.com ZEROSMTP_PASSWORD=secret ./setup-postfix-relay.sh
+#
+# The password is never taken as a command-line argument (it would sit in
+# shell history and `ps`) - pass it via the environment variable above for
+# unattended runs, or let the script prompt for it.
+
+set -euo pipefail
+
+error_exit() {
+  printf 'ERROR: %s\n' "$1" >&2
+  exit 1
+}
+
+trap 'error_exit "Script interrupted"' INT TERM
+
+[[ $EUID -eq 0 ]] || error_exit "Run this with sudo - it installs packages and writes to /etc/postfix."
+
+[[ -r /etc/os-release ]] || error_exit "Cannot read /etc/os-release - this script targets Debian/Ubuntu only."
+# shellcheck disable=SC1091
+. /etc/os-release
+case "${ID:-}:${ID_LIKE:-}" in
+  debian:*|ubuntu:*|*:*debian*) ;;
+  *) error_exit "Detected ID='${ID:-unknown}', not Debian or Ubuntu. See docs/SYSTEM-MTA.md for other systems." ;;
+esac
+
+ZEROSMTP_PORT="${ZEROSMTP_PORT:-587}"
+case "$ZEROSMTP_PORT" in
+  587|465) ;;
+  *) error_exit "ZEROSMTP_PORT must be 587 (STARTTLS) or 465 (implicit TLS), got '$ZEROSMTP_PORT'." ;;
+esac
+
+USERNAME="${ZEROSMTP_USERNAME:-}"
+if [[ -z "$USERNAME" ]]; then
+  read -r -p "ZeroSMTP username (your-name@msgwing.com): " USERNAME
+fi
+[[ -z "$USERNAME" ]] && error_exit "No username given."
+[[ "$USERNAME" == *@msgwing.com ]] || error_exit "Username should be the full address issued at msgwing.com (e.g. you@msgwing.com), got '$USERNAME'."
+
+PASSWORD="${ZEROSMTP_PASSWORD:-}"
+if [[ -z "$PASSWORD" ]]; then
+  read -r -s -p "ZeroSMTP password: " PASSWORD
+  echo
+fi
+[[ -z "$PASSWORD" ]] && error_exit "No password given."
+
+echo "== Installing Postfix =="
+# DEBIAN_FRONTEND=noninteractive alone still leaves postfix's own package
+# with no chosen "general type" the first time it installs, which can stall
+# the install waiting on a debconf prompt. Preseeding both questions before
+# apt runs is what actually makes this non-interactive.
+debconf-set-selections <<EOF
+postfix postfix/main_mailer_type select Satellite system
+postfix postfix/mailname string $(hostname -f 2>/dev/null || hostname)
+postfix postfix/relayhost string [mx.msgwing.com]:$ZEROSMTP_PORT
+EOF
+export DEBIAN_FRONTEND=noninteractive
+apt-get update -qq
+apt-get install -y -qq postfix mailutils libsasl2-modules
+
+echo "== Configuring relay ($USERNAME via mx.msgwing.com:$ZEROSMTP_PORT) =="
+DB_TYPE="$(postconf -h default_database_type)"
+
+printf '[mx.msgwing.com]:%s %s:%s\n' "$ZEROSMTP_PORT" "$USERNAME" "$PASSWORD" \
+  > /etc/postfix/sasl_passwd
+chmod 600 /etc/postfix/sasl_passwd
+postmap /etc/postfix/sasl_passwd
+
+# Envelope sender and the visible From: header are separate things, and the
+# relay only checks the first one - both maps exist so mail from any local
+# user (root, cron, the invoking user) leaves as the ZeroSMTP account
+# instead of bouncing with "553 5.7.1 ... not owned by user".
+printf '/.+/    %s\n' "$USERNAME" > /etc/postfix/sender_canonical
+postmap /etc/postfix/sender_canonical
+printf '/^From:.*/  REPLACE From: %s\n' "$USERNAME" > /etc/postfix/header_from
+# header_from is deliberately not run through postmap - regexp:/pcre: tables
+# are read as plain text and have no .db to build.
+
+postconf -e "relayhost = [mx.msgwing.com]:$ZEROSMTP_PORT"
+postconf -e "smtp_sasl_auth_enable = yes"
+postconf -e "smtp_sasl_password_maps = ${DB_TYPE}:/etc/postfix/sasl_passwd"
+postconf -e "smtp_sasl_security_options = noanonymous"
+postconf -e "smtp_tls_security_level = encrypt"
+postconf -e "smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt"
+postconf -e "sender_canonical_classes = envelope_sender, header_sender"
+postconf -e "sender_canonical_maps = regexp:/etc/postfix/sender_canonical"
+postconf -e "smtp_header_checks = regexp:/etc/postfix/header_from"
+
+if [[ "$ZEROSMTP_PORT" == "465" ]]; then
+  postconf -e "smtp_tls_wrappermode = yes"
+else
+  postconf -e "smtp_tls_wrappermode = no"
+fi
+
+echo "== Restarting Postfix =="
+systemctl restart postfix
+
+echo "== Sending a test message =="
+read -r -p "Address to send a test email to (blank to skip): " TEST_TO
+if [[ -n "$TEST_TO" ]]; then
+  echo "Test from setup-postfix-relay.sh" | mail -s "ZeroSMTP relay test" "$TEST_TO"
+  sleep 2
+  LAST_SENDER="$(grep 'from=<' /var/log/mail.log 2>/dev/null | tail -1 || true)"
+  echo "$LAST_SENDER"
+  if [[ "$LAST_SENDER" == *"from=<$USERNAME>"* ]]; then
+    echo "Sender rewrite confirmed: mail is leaving as $USERNAME."
+  else
+    echo "Could not confirm the rewrite from /var/log/mail.log yet - check it manually:"
+    echo "  grep 'from=<' /var/log/mail.log | tail -1"
+  fi
+  echo "If $TEST_TO doesn't receive it, see docs/TROUBLESHOOTING.md."
+else
+  echo "Skipped. Test later with:"
+  echo "  echo test | mail -s subject you@example.com"
+fi
+
+echo "Done. Full reference and other MTAs (msmtp, Exim4): docs/SYSTEM-MTA.md"