dnsmasq: store leases in sqlite database

To avoid dnsmasq writing out the whole leasefile on each request
before replying. This gets slow on high‐latency storage.

Also tweak DNS updates a bit.

roles/dnsmasq/tasks/main.yml

@@ -2,9 +2,10 @@
   package:
     name:
     - dnsmasq
-    - bind-tools
+    - bind-tools # for DNS updates
-    - krb5
+    - krb5 # for DNS updates
-    - py3-pexpect
+    - py3-pexpect # for creating kerberos keytab
+    - sqlite # for lease DB
 
 - name: Configure kerberos
   template:
@@ -23,11 +24,11 @@
   args:
     creates: /etc/krb5.keytab
 
-- name: Copy DNS updater script
+- name: Copy DHCP lease script
   template:
-    dest: "/usr/local/bin/dns-update"
+    dest: "/usr/local/bin/dnsmasq-script"
-    src: "dns-update.j2"
+    src: "dnsmasq-script.j2"
-    mode: 0700
+    mode: 0755
 
 - name: Configure dnsmasq
   template:

roles/dnsmasq/templates/00-options.conf.j2

@@ -18,4 +18,8 @@ interface = {{ interfaces | map(attribute='name') | join(',') }}
 dhcp-option = option:dns-server,{{ dns | join(',') }}
 dhcp-option = option:ntp-server,{{ ntp | join(',') }}
 
-dhcp-script = /usr/local/bin/dns-update
+dhcp-script = /usr/local/bin/dnsmasq-script
+dhcp-scriptuser = dnsmasq
+
+# track leases with dnsmasq-script instead
+leasefile-ro

roles/dnsmasq/templates/dns-update.j2

@@ -1,34 +0,0 @@
-#!/bin/sh
-
-# don’t spam the DNS server when starting dnsmasq
-if [ -n "${DNSMASQ_INTERFACE}" ] ; then
-	exit
-fi
-
-domain={{ domain }}
-ldap_user={{ password.ldap_user }}
-ttl=3600
-
-address="${3}"
-
-case "${1}" in
-add)
-	host="${4}"
-	kinit -k "${ldap_user}"
-	nsupdate -g <<EOF
-update add ${host}.${domain} ${ttl} A ${address}
-send
-EOF
-	;;
-old)
-	if [ -n "${DNSMASQ_OLD_HOSTNAME}" -a -n "${DNSMASQ_SUPPLIED_HOSTNAME}" ] ; then
-		kinit -k "${ldap_user}"
-		nsupdate -g <<EOF
-update del ${DNSMASQ_OLD_HOSTNAME}.${domain}
-update add ${DNSMASQ_SUPPLIED_HOSTNAME}.${domain} ${ttl} A ${address}
-send
-EOF
-	fi
-	;;
-# TODO del, probably
-esac

roles/dnsmasq/templates/dnsmasq-script.j2

@@ -0,0 +1,76 @@
+#!/bin/sh
+
+# This script replaces the dnsmasq leasefile with an sqlite database
+# for performance on slow storage, and updates DNS records for
+# assigned addresses.
+
+cmd="${1}"
+
+# dnsmasq calls this script on startup for each known lease, ignore it
+[ "${cmd}" != "init" ] && [ -z "${DNSMASQ_INTERFACE}" ] && exit 0
+
+# parameters
+db=/var/lib/misc/dnsmasq.leases.db
+domain={{ domain }}
+ldap_user={{ password.ldap_user }}
+ttl=3600
+
+# sanitize input
+mac="${2//[![:xdigit:]:]/}"
+ipv4="${3//[![:digit:].]/}"
+hostname="${4//[![:alnum:]_-]/}"
+client_id="${DNSMASQ_CLIENT_ID//[![:xdigit:]:]/}"
+
+# construct SQL to query or update leases, and nsupdate script to update DNS records
+dns=""
+sql=""
+case "${cmd}" in
+"init")
+	# init runs as root, so ensure the dnsmasq user can write to database later
+	touch "${db}"
+	chown dnsmasq:dnsmasq "${db}"
+	setfacl -m u:dnsmasq:rwx "$(dirname ${db})"
+	setfacl -m u:dnsmasq:r "/etc/krb5.keytab" # needed for DNS updates
+
+	# ensure the leases table exists, and get all leases from it
+	sql="CREATE TABLE IF NOT EXISTS leases (
+			ipv4 TEXT PRIMARY KEY NOT NULL,
+			mac TEXT NOT NULL,
+			hostname TEXT NOT NULL,
+			client_id TEXT NOT NULL,
+			renewed INTEGER NOT NULL);
+		SELECT renewed, mac, ipv4, hostname, client_id FROM leases;"
+	;;
+
+"add" | "old")
+	# add or update the lease
+	sql="INSERT INTO leases
+			VALUES ('${ipv4}', '${mac}', '${hostname:-*}', '${client_id:-*}', unixepoch())
+			ON CONFLICT(ipv4) DO UPDATE SET renewed = unixepoch();"
+
+	# add or update the DNS record
+	if [ -n "${hostname}" ] ; then
+		dns="update add ${hostname}.${domain} ${ttl} A ${ipv4}\n"
+	fi
+	if [ -n "${DNSMASQ_OLD_HOSTNAME}" ] ; then
+		dns="${dns}update del ${DNSMASQ_OLD_HOSTNAME}.${domain}\n"
+	fi
+	;;
+
+"del")
+	# delete the lease
+	sql="DELETE FROM leases WHERE ipv4 = '${ipv4}';"
+	# TODO probably delete the DNS record
+	;;
+esac
+
+# update lease database
+if [ -n "${sql}" ]; then
+	sqlite3 -separator " " "${db}" "${sql}"
+fi
+
+# update DNS records
+if [ -n "${dns}" ]; then
+	kinit -k "${ldap_user}"
+	echo -e "${dns}send" | nsupdate -g
+fi