Generalize reverse proxy source allowlist

.env.example

@@ -3,7 +3,8 @@ IMAGE_PORT=8191
 IMAGE_REPOSITORY_PATH=/home/letrhee/sam_rebuild/image
 IMAGE_UID=1000
 IMAGE_GID=1000
-CADDY_SOURCE_CIDR=172.30.1.75/32
+# Comma- or space-separated source CIDRs of hosts that reverse-proxy to port 8191.
+TRUSTED_PROXY_CIDRS=172.30.1.75/32
 
 IMAGE_REMOTE_URL=https://gitea.hided.net/devsam/image.git
 IMAGE_REPOSITORY_FULL_NAME=devsam/image

README.md

@@ -39,11 +39,14 @@ remote branch tip, rejects dirty or non-fast-forward updates, and serializes all
 Git changes. The initial and only allowed branch is `master` unless
 `IMAGE_ALLOWED_BRANCHES` is explicitly expanded.
 
-The current Caddy host reaches this server from `172.30.1.75`. Keep the observed
-source in the untracked `.env`; re-check it whenever Caddy networking changes.
-The published port binds all server interfaces, so production also needs a
-host `DOCKER-USER` (or equivalent) firewall rule allowing that source CIDR to
-TCP 8191 and rejecting other sources. Nginx applies the same source allowlist.
+The service is reverse-proxy agnostic. Its current reverse proxy happens to
+reach this server from `172.30.1.75`, but Caddy, Nginx, HAProxy, or another
+proxy can be used. Keep every observed proxy source CIDR in the untracked
+`TRUSTED_PROXY_CIDRS` value, separated by commas or spaces, and re-check the
+value whenever proxy networking changes. The published port binds all server
+interfaces, so production also needs a host `DOCKER-USER` (or equivalent)
+firewall rule allowing those source CIDRs to TCP 8191 and rejecting other
+sources. The static Nginx edge applies the same source allowlist.
 
 ### Prepare
 
@@ -57,7 +60,7 @@ docker compose build
 Apply and inspect the dedicated Docker ingress chain with root privileges:
 
 ```sh
-sudo env CADDY_SOURCE_CIDR=172.30.1.75/32 IMAGE_PORT=8191 \
+sudo env TRUSTED_PROXY_CIDRS=172.30.1.75/32 IMAGE_PORT=8191 \
   ./deploy/scripts/firewall-8191.sh apply
 sudo ./deploy/scripts/firewall-8191.sh check
 ```
@@ -109,7 +112,7 @@ signed administration command:
 ./deploy/scripts/admin-deploy.sh <branch> [expected-commit]
 ```
 
-The administration route is not proxied through Nginx or Caddy.
+The administration route is not exposed through the public reverse proxy.
 
 ### Tests and rollback
 

compose.yaml

@@ -54,7 +54,7 @@ services:
         condition: service_healthy
     read_only: true
     environment:
-      CADDY_SOURCE_CIDR: ${CADDY_SOURCE_CIDR:?Set CADDY_SOURCE_CIDR to the direct Caddy source CIDR}
+      TRUSTED_PROXY_CIDRS: ${TRUSTED_PROXY_CIDRS:?Set TRUSTED_PROXY_CIDRS to the direct reverse-proxy source CIDR list}
     ports:
       - "${IMAGE_BIND_ADDRESS:-0.0.0.0}:${IMAGE_PORT:-8191}:8080"
     volumes:

deploy/nginx/entrypoint.sh

@@ -1,13 +1,29 @@
 #!/bin/sh
 set -eu
 
-if [ -z "${CADDY_SOURCE_CIDR:-}" ]; then
-    echo "CADDY_SOURCE_CIDR is required" >&2
+trusted_proxy_cidrs=${TRUSTED_PROXY_CIDRS:-${CADDY_SOURCE_CIDR:-}}
+
+if [ -z "$trusted_proxy_cidrs" ]; then
+    echo "TRUSTED_PROXY_CIDRS is required" >&2
     exit 1
 fi
 
-envsubst '${CADDY_SOURCE_CIDR}' \
-    < /etc/image/default.conf.template \
-    > /tmp/nginx.conf
+: > /tmp/trusted-proxy-allow.conf
+for trusted_proxy_cidr in $(printf '%s' "$trusted_proxy_cidrs" | tr ',' ' '); do
+    case "$trusted_proxy_cidr" in
+        *[!0-9A-Fa-f:./]*)
+            echo "Invalid trusted proxy CIDR: $trusted_proxy_cidr" >&2
+            exit 2
+            ;;
+    esac
+    printf 'allow %s;\n' "$trusted_proxy_cidr" >> /tmp/trusted-proxy-allow.conf
+done
+
+if [ ! -s /tmp/trusted-proxy-allow.conf ]; then
+    echo "TRUSTED_PROXY_CIDRS must contain at least one CIDR" >&2
+    exit 2
+fi
+
+cp /etc/image/default.conf.template /tmp/nginx.conf
 
 exec nginx -c /tmp/nginx.conf -g 'daemon off;'

deploy/nginx/templates/default.conf.template

@@ -24,7 +24,7 @@ http {
 
     allow 127.0.0.1;
     allow ::1;
-    allow ${CADDY_SOURCE_CIDR};
+    include /tmp/trusted-proxy-allow.conf;
     deny all;
 
     location = /healthz {

deploy/scripts/firewall-8191.sh

@@ -2,7 +2,7 @@
 set -eu
 
 action=${1:-check}
-source_cidr=${CADDY_SOURCE_CIDR:-}
+trusted_proxy_cidrs=${TRUSTED_PROXY_CIDRS:-${CADDY_SOURCE_CIDR:-}}
 image_port=${IMAGE_PORT:-8191}
 chain=SAM_IMAGE_INGRESS
 
@@ -24,15 +24,31 @@ case "$action" in
         ;;
     apply)
         require_root
-        if [ -z "$source_cidr" ]; then
-            echo "CADDY_SOURCE_CIDR is required" >&2
+        if [ -z "$trusted_proxy_cidrs" ]; then
+            echo "TRUSTED_PROXY_CIDRS is required" >&2
+            exit 2
+        fi
+        rule_count=0
+        for trusted_proxy_cidr in $(printf '%s' "$trusted_proxy_cidrs" | tr ',' ' '); do
+            case "$trusted_proxy_cidr" in
+                *[!0-9A-Fa-f:./]*)
+                    echo "Invalid trusted proxy CIDR: $trusted_proxy_cidr" >&2
+                    exit 2
+                    ;;
+            esac
+            rule_count=$((rule_count + 1))
+        done
+        if [ "$rule_count" -eq 0 ]; then
+            echo "TRUSTED_PROXY_CIDRS must contain at least one CIDR" >&2
             exit 2
         fi
         iptables -n -L DOCKER-USER >/dev/null
         iptables -n -L "$chain" >/dev/null 2>&1 || iptables -N "$chain"
         iptables -F "$chain"
-        iptables -A "$chain" -s "$source_cidr" -j ACCEPT
         iptables -A "$chain" -j DROP
+        for trusted_proxy_cidr in $(printf '%s' "$trusted_proxy_cidrs" | tr ',' ' '); do
+            iptables -I "$chain" 1 -s "$trusted_proxy_cidr" -j ACCEPT
+        done
         iptables -C DOCKER-USER -p tcp -m conntrack --ctorigdstport "$image_port" -j "$chain" 2>/dev/null \
             || iptables -I DOCKER-USER 1 -p tcp -m conntrack --ctorigdstport "$image_port" -j "$chain"
         ;;