Make the minimum cache TTL configurable (CACHE_MIN_TTL)

README.md

@@ -221,6 +221,21 @@ dig +https @dns1.slowb.ro . NS         # compare against opennic.hints
 dig +https @dns1.slowb.ro . DNSKEY     # compare the KSK (flags 257) against opennic.key
 ```
 
+## Cache TTL
+
+To reduce the load on authoritative servers and keep responses fast, answers
+are cached for at least one hour, even when their TTL is shorter. The downside
+is that changes to records with short TTLs can take an hour or more to be
+visible to clients.
+
+The minimum can be changed with the `CACHE_MIN_TTL` environment variable, in
+seconds, from `0` to `86400` (add `-e CACHE_MIN_TTL=60` to the `docker run`
+command). `CACHE_MIN_TTL=0` honors the TTLs set by the zone owners. Without
+the variable, the one-hour default is used.
+
+The value applies to both the Unbound cache and the encrypted-dns cache, and is
+applied every time the container starts.
+
 # Join the network
 
 If you want to help against DNS centralization and surveillance,

entrypoint.sh

@@ -167,8 +167,29 @@ ensure_initialized() {
     fi
 }
 
+apply_cache_min_ttl() {
+    cache_min_ttl=$(printf '%s\n' "${CACHE_MIN_TTL:-3600}" | sed 's/^0*\([0-9]\)/\1/')
+    case "$cache_min_ttl" in
+    *[!0-9]* | ??????*) cache_min_ttl_valid=0 ;;
+    *) cache_min_ttl_valid=1 ;;
+    esac
+    if [ "$cache_min_ttl_valid" = 0 ] || [ "$cache_min_ttl" -gt 86400 ]; then
+        echo "CACHE_MIN_TTL must be a number of seconds between 0 and 86400, not [${CACHE_MIN_TTL}]" >&2
+        exit 1
+    fi
+    if grep -qE '^[[:space:]]*cache_ttl_min[[:space:]]*=' "$CONFIG_FILE"; then
+        if ! grep -qE "^cache_ttl_min = ${cache_min_ttl}\$" "$CONFIG_FILE"; then
+            sed -i -E "s/^[[:space:]]*cache_ttl_min[[:space:]]*=.*/cache_ttl_min = ${cache_min_ttl}/" "$CONFIG_FILE"
+        fi
+    elif [ -n "${CACHE_MIN_TTL:-}" ]; then
+        echo "No cache_ttl_min setting in [${CONFIG_FILE}], leaving the encrypted-dns cache unchanged" >&2
+    fi
+    CACHE_MIN_TTL=$cache_min_ttl
+}
+
 start() {
     ensure_initialized
+    apply_cache_min_ttl
     fix_key_ownership
     if [ -f "${KEYS_DIR}/secret.key" ]; then
         echo "Importing the previous secret key [${KEYS_DIR}/secret.key]"
@@ -187,8 +208,9 @@ start() {
     done
 
     # runit wipes the environment before running services, so persist
-    # settings that service scripts need to a file.
+    # settings that service scripts need to files.
     printf '%s\n' "${OPENNIC:-}" >/etc/opennic-env
+    printf '%s\n' "${CACHE_MIN_TTL:-}" >/etc/cache-min-ttl-env
 
     exec /etc/runit/2 </dev/null >/dev/null 2>/dev/null
 }

unbound.sh

@@ -27,9 +27,18 @@ case "${OPENNIC:-}" in
 1 | true | yes | on) opennic_enabled=1 ;;
 esac
 
+if [ -z "${CACHE_MIN_TTL:-}" ] && [ -f /etc/cache-min-ttl-env ]; then
+    CACHE_MIN_TTL=$(cat /etc/cache-min-ttl-env)
+fi
+case "${CACHE_MIN_TTL:-}" in
+"" | *[!0-9]*) cache_min_ttl=3600 ;;
+*) cache_min_ttl=$CACHE_MIN_TTL ;;
+esac
+
 provider_name=$(cat "$KEYS_DIR/provider_name")
 
 sed \
+    -e "s/@CACHE_MIN_TTL@/${cache_min_ttl}/" \
     -e "s/@MSG_CACHE_SIZE@/${msg_cache_size}/" \
     -e "s/@PROVIDER_NAME@/${provider_name}/" \
     -e "s/@RR_CACHE_SIZE@/${rr_cache_size}/" \
@@ -43,7 +52,7 @@ server:
   so-reuseport: yes
   edns-buffer-size: 1232
   delay-close: 10000
-  cache-min-ttl: 3600
+  cache-min-ttl: @CACHE_MIN_TTL@
   cache-max-ttl: 86400
   do-daemonize: no
   username: "_unbound"