contrib/sni-router: use host networking for HAProxy to preserve client IPs

contrib/sni-router/Caddyfile

@@ -10,14 +10,14 @@
 	# to Caddy's access log.  The `tls` wrapper must follow so that TLS
 	# is terminated on the unwrapped connection.
 	#
-	# `allow` lists the networks permitted to send PROXY headers.  These
-	# ranges cover docker compose's default bridge networks; tighten
-	# them if you pin a specific subnet in docker-compose.yml.
+	# `allow` lists the networks permitted to send PROXY headers.
+	# HAProxy runs in the host netns and reaches Caddy via host loopback
+	# (see docker-compose.yml), so the only legitimate peer is loopback.
 	servers :8443 {
 		listener_wrappers {
 			proxy_protocol {
 				timeout 5s
-				allow 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16
+				allow 127.0.0.0/8 ::1/128
 			}
 			tls
 		}

contrib/sni-router/README.md

@@ -58,6 +58,30 @@ must stay in sync:
 If you disable one, disable all four, otherwise the backend will fail
 to parse the connection.
 
+## Why host networking for HAProxy
+
+HAProxy runs in the host network namespace (`network_mode: host` in
+`docker-compose.yml`) so it sees the real client source IP on every
+inbound connection.  With the default bridge networking + published
+ports the source IP is rewritten to the bridge gateway — by Docker's
+userland proxy (`docker-proxy`), by rootless Podman's `slirp4netns`
+or `pasta`, or by NAT on the Docker host — and the PROXY v2 header
+HAProxy then sends to mtg and Caddy carries that useless address.
+Host networking lifts HAProxy out of the rewrite path; mtg and Caddy
+stay on the compose bridge and HAProxy dials them via host loopback
+(`127.0.0.1`).
+
+Trade-off: HAProxy occupies the host's `:443` and `:80` directly, so
+nothing else on the host may listen on those ports.  For a dedicated
+mtg/SNI-router host that is the intended layout.
+
+Rootless Podman users binding the privileged ports `:80`/`:443` need
+the host-side sysctl once (rootful Docker handles this implicitly):
+
+```bash
+sudo sysctl -w net.ipv4.ip_unprivileged_port_start=80
+```
+
 ## Fronting loop (why `[domain-fronting]` is set explicitly)
 
 When mtg sees TLS that isn't valid Telegram (a probe or a browser

contrib/sni-router/docker-compose.yml

@@ -24,9 +24,12 @@ x-domain-env: &domain-env
 services:
   haproxy:
     image: haproxy:lts-alpine
-    ports:
-      - "443:443"
-      - "80:80"
+    # Host networking so HAProxy sees the real client source IP on
+    # inbound (bridge networking with published ports rewrites it to
+    # the bridge gateway under both Docker's userland-proxy and
+    # rootless Podman's slirp4netns/pasta).  See "Why host networking"
+    # in README.md.
+    network_mode: host
     volumes:
       - ./haproxy.cfg:/usr/local/etc/haproxy/haproxy.cfg:ro,Z
     environment:
@@ -35,15 +38,14 @@ services:
       - mtg
       - web
     restart: unless-stopped
-    sysctls:
-      - net.ipv4.ip_unprivileged_port_start=80
 
   mtg:
     image: nineseconds/mtg:2
     volumes:
       - ./mtg-config.toml:/config/config.toml:ro,Z
-    expose:
-      - "3128"
+    # Publish on host loopback so the host-mode HAProxy can reach it.
+    ports:
+      - "127.0.0.1:3128:3128"
     restart: unless-stopped
     extra_hosts:
       - "host.containers.internal:host-gateway"
@@ -54,9 +56,12 @@ services:
       - ./Caddyfile:/etc/caddy/Caddyfile:ro,Z
       - caddy_data:/data
       - ./www:/srv:ro,Z
-    expose:
-      - "80"
-      - "8443"
+    # Publish on host loopback so the host-mode HAProxy can reach it.
+    # Caddy's HTTP listener is mapped off :80 (occupied by HAProxy)
+    # to :8080; haproxy.cfg dials it on 127.0.0.1:8080 for ACME.
+    ports:
+      - "127.0.0.1:8080:80"
+      - "127.0.0.1:8443:8443"
     environment:
       <<: *domain-env
     restart: unless-stopped

contrib/sni-router/haproxy.cfg

@@ -50,14 +50,19 @@ backend mtg
     # send-proxy-v2 prepends a PROXY protocol v2 header so mtg sees the
     # real client IP instead of HAProxy's.  mtg must have
     # `proxy-protocol-listener = true` in its config.
-    server mtg mtg:3128 send-proxy-v2
+    #
+    # HAProxy runs in the host network namespace (see docker-compose.yml)
+    # and mtg publishes :3128 on host loopback, so we dial 127.0.0.1.
+    server mtg 127.0.0.1:3128 send-proxy-v2
 
 backend web
     # send-proxy-v2 prepends a PROXY protocol v2 header so Caddy logs the
     # real client IP instead of HAProxy's.  Caddy must enable the
     # proxy_protocol listener wrapper on :8443 (see Caddyfile).
-    server web web:8443 send-proxy-v2
+    server web 127.0.0.1:8443 send-proxy-v2
 
 backend web_acme
     mode http
-    server web web:80
+    # Caddy's :80 is published on host 127.0.0.1:8080 (HAProxy occupies
+    # the host's :80 itself).
+    server web 127.0.0.1:8080