Add SQL backend files

sql/README.md

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+# Postfix TLS Policy Maps - SQL Proxy
+
+If you don't want to use a plain file Postfix lookup table to manage TLS policy maps, but a SQL backend, you'll very likely find the files in this directory helpful. You can use them as follows:
+
+1. Create a `tls_policy` table in the SQL database you want to use with Postfix. You can use the provided [`scheme.sql`](scheme.sql) if you want to.
+
+2. Create a proxy configuration file (e.g. `/etc/postfix/tls_policy.cf`) to tell Postfix the SQL query to use. You can again use the provided [`postfix_proxy.cf`](postfix_proxy.cf) as a blueprint, but don't forget to change username and password.
+
+3. Configure Postfix to actually use the proxy configuration file by setting the `smtp_tls_policy_maps` parameter in Postfix's `main.cf` accordingly. Don't forget to reload/restart Postfix afterwards.
+   ```
+   smtp_tls_policy_maps = mysql:/etc/postfix/tls_policy.cf
+   ```
+
+4. Use the provided [`update_database.sh`](update_database.sh) to convert the plain file Postfix lookup table to SQL queries and execute them. You can e.g. simply pipe stdout of the script to the `mysql` command.
+   ```
+   $ ./update_database.sh ../tls_policy | mysql --user=db_user --password "db_name"
+   ```
+   You can change the built-in SQL query template by setting the environment variable `TEMPLATE`. You can use the placeholders `{domain}`, `{policy}` and `{params}` in the template. As a reference, this is the script's default template:
+   ```
+   DELETE FROM tls_policy WHERE domain = '{domain}'; INSERT INTO tls_policy (domain, policy, params) VALUES ('{domain}', '{policy}', '{params}');
+   ```
+
+5. You may want to repeat Step 4 on a regular basis (e.g. weekly) to always use the newest upstream TLS policy maps on your server. The provided `update_database.sh` always validates the policy file before converting it into SQL queries, so you can safely automatize this task with a cronjob. The following crontab line is intended to provide inspiration for you to create your own cronjob (it will work with Debian only). Most importantly, you'll have to find a way to safely pass the password of the SQL user to the cronjob.
+   ```
+   0 4	* * 7	root	curl -sS "https://raw.githubusercontent.com/csware/postfix-tls-policy/master/tls_policy" | /path/to/update_database.sh - | mysql --defaults-file="/etc/mysql/debian.cnf" --silent "db_name"
+   ```
+

sql/postfix_proxy.cf

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+user = db_user
+password = db_password
+hosts = 127.0.0.1
+dbname = db_name
+
+query =
+    SELECT      policy,
+                params
+    FROM        tls_policy
+    WHERE       domain = '%s'

sql/scheme.sql

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+CREATE TABLE tls_policy (
+    domain varchar(255) NOT NULL,
+    policy enum('none', 'may', 'encrypt', 'dane', 'dane-only', 'fingerprint', 'verify', 'secure') NOT NULL,
+    params varchar(255),
+    PRIMARY KEY (domain)
+);

sql/update_database.sh

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+#!/bin/sh
+if [ -z "$1" ]; then
+    echo "Usage: $(basename "$0") POLICY_FILE" >&2
+    exit 1
+fi
+if [ -z "$TEMPLATE" ]; then
+    TEMPLATE="DELETE FROM tls_policy WHERE domain = '{domain}';"
+    TEMPLATE="$TEMPLATE INSERT INTO tls_policy (domain, policy, params) VALUES ('{domain}', '{policy}', '{params}');"
+fi
+
+DATABASE="$1"
+[ "$DATABASE" = "-" ] && DATABASE="/dev/stdin"
+
+EXIT_CODE=0
+while read -r LINE || [ -n "$LINE" ]; do
+    [ -n "$LINE" ] || continue
+    [ "$(echo "$LINE" | cut -c 1)" = "#" ] && continue
+
+    DATA="$(echo "$LINE" | sed -e 's/\s\+/ /')"
+    DOMAIN="$(echo "$DATA" | cut -s -d ' ' -f 1)"
+    POLICY="$(echo "$DATA" | cut -s -d ' ' -f 2)"
+    PARAMS="$(echo "$DATA" | cut -s -d ' ' -f 3-)"
+
+    if [ -z "$(echo "$DOMAIN" | sed -ne '/^[a-zA-Z0-9._-]\{1,255\}$/p')" ] \
+        || ( case "$POLICY" in none|may|encrypt|dane|dane-only|fingerprint|verify|secure) false ;; esac ) \
+        || ( case "$POLICY" in fingerprint|verify|secure) false ;; esac && [ -n "$PARAMS" ] ) \
+        || ( [ "$POLICY" = "fingerprint" ] && [ -z "$(echo "$PARAMS" | sed -ne '/^match=[a-fA-F0-9:|]\+$/p')" ] ) \
+        || ( ! case "$POLICY" in verify|secure) false ;; esac && [ -z "$(echo "$PARAMS" | sed -ne '/^match=[a-zA-Z0-9:._-]\+$/p')" ] )
+    then
+        echo "Invalid row: $LINE" >&2
+        EXIT_CODE=1
+        continue
+    fi
+
+    echo "$TEMPLATE" | sed \
+        -e "s/{domain}/$(echo "$DOMAIN" | sed -e 's/[\/&]/\\&/g')/g" \
+        -e "s/{policy}/$(echo "$POLICY" | sed -e 's/[\/&]/\\&/g')/g" \
+        -e "s/{params}/$(echo "$PARAMS" | sed -e 's/[\/&]/\\&/g')/g"
+done < "$DATABASE"
+
+exit $EXIT_CODE
+