fix(gateway): move localhost routing behind an insecure debug flag

dstack/gateway/dstack-app/builder/entrypoint.sh

@@ -114,7 +114,6 @@ tls_versions = ["1.2"]
 listen_addr = "0.0.0.0"
 listen_port = "${PROXY_LISTEN_PORT:-443}"
 connect_top_n = 3
-localhost_enabled = false
 app_address_ns_compat = true
 workers = ${PROXY_WORKERS:-32}
 max_connections_per_app = ${MAX_CONNECTIONS_PER_APP:-0}

dstack/gateway/gateway.toml

@@ -56,6 +56,11 @@ insecure_no_auth = false
 
 [core.debug]
 insecure_enable_debug_rpc = false
+# Route the app address "localhost" to 127.0.0.1 on the gateway host. Off by
+# default: the app address also comes from the _dstack-app-address TXT record of
+# arbitrary custom domains, so enabling this lets any DNS zone owner reach the
+# gateway's own loopback on a port of their choosing, bypassing port_policy.
+insecure_localhost_backend = false
 insecure_skip_attestation = false
 key_file = "debug_key.json"
 address = "127.0.0.1:8012"
@@ -80,7 +85,6 @@ agent_port = 8090
 buffer_size = 65536
 # number of hosts to try to connect to
 connect_top_n = 3
-localhost_enabled = false
 app_address_ns_prefix = "_dstack-app-address"
 app_address_ns_compat = true
 workers = 32

dstack/gateway/src/config.rs

@@ -129,7 +129,6 @@ pub struct ProxyConfig {
     /// 1 MiB pipe fed without the syscall rate 8 KiB imposed.
     pub buffer_size: usize,
     pub connect_top_n: usize,
-    pub localhost_enabled: bool,
     pub workers: usize,
     /// Run one single-threaded runtime per worker, each with its own
     /// `SO_REUSEPORT` listener, instead of one accept thread feeding a shared
@@ -522,6 +521,19 @@ pub struct DebugConfig {
     pub insecure_enable_debug_rpc: bool,
     #[serde(default)]
     pub insecure_skip_attestation: bool,
+    /// Let the app-address `localhost` resolve to 127.0.0.1, so a hostname can
+    /// be routed to a service on the gateway host itself.
+    ///
+    /// This lives under `debug` and carries the `insecure_` prefix because the
+    /// app address is not only read from the platform's own `<id>.<base_domain>`
+    /// grammar: it also comes from the `_dstack-app-address` TXT record of an
+    /// arbitrary custom domain. With this on, anyone who controls any DNS zone
+    /// can point the gateway at its own loopback -- where the admin and debug
+    /// listeners bind precisely because being unreachable is their access
+    /// control -- and pick the port, since the `localhost` shortcut is not a
+    /// registered instance and so bypasses `port_policy` entirely.
+    #[serde(default)]
+    pub insecure_localhost_backend: bool,
     /// Path to pre-generated debug key data file (JSON format containing key, quote, event_log, and vm_config)
     #[serde(default)]
     pub key_file: String,

dstack/gateway/src/main.rs

@@ -21,7 +21,7 @@ use rocket::{
 };
 use serde::{Deserialize, Serialize};
 use std::sync::Arc;
-use tracing::info;
+use tracing::{info, warn};
 
 use admin_service::AdminRpcHandler;
 use main_service::{Proxy, ProxyOptions, RpcHandler};
@@ -245,6 +245,15 @@ async fn main() -> Result<()> {
     if config.sync.enabled && config.sync.node_id == 0 {
         anyhow::bail!("node_id must be greater than 0");
     }
+    if config.debug.insecure_localhost_backend {
+        warn!(
+            "core.debug.insecure_localhost_backend = true; the app address \"localhost\" now \
+             resolves to 127.0.0.1 on this host. App addresses also come from the \
+             _dstack-app-address TXT record of arbitrary custom domains, so any DNS zone owner \
+             can reach this host's loopback on a port of their choosing, bypassing port_policy. \
+             Never use this outside local development"
+        );
+    }
     // Before anything reads `proxy.ktls`: the acceptor built later decides
     // whether to extract session secrets from it.
     proxy::disable_ktls_if_unsupported(&mut config.proxy);

dstack/gateway/src/main_service.rs

@@ -1143,7 +1143,7 @@ impl ProxyState {
     }
 
     pub(crate) fn select_top_n_hosts(&mut self, id: &str) -> Result<AddressGroup> {
-        if self.config.proxy.localhost_enabled && id == "localhost" {
+        if self.config.debug.insecure_localhost_backend && id == "localhost" {
             return Ok(smallvec![AddressInfo {
                 ip: Ipv4Addr::new(127, 0, 0, 1),
                 counter: Default::default(),

dstack/gateway/test-run/proxy/gwconfig.py

@@ -58,6 +58,7 @@ def main():
 rpc_domain = ""
 set_ulimit = false
 [core.debug]
+insecure_localhost_backend = true
 insecure_skip_attestation = true
 insecure_enable_debug_rpc = false
 [core.admin]
@@ -81,7 +82,6 @@ def main():
 [core.proxy]
 listen_addr = "127.0.0.1"
 listen_port = {o["proxy_port"]}
-localhost_enabled = true
 base_domain = "{o["base_domain"]}"
 cert_chain = "{cert}"
 cert_key = "{key}"

dstack/gateway/test-run/test_proxy.sh

@@ -35,8 +35,8 @@ ORIGIN_PLAIN=$((BASE_PORT + 4))
 ORIGIN_TLS=$((BASE_PORT + 5))
 
 BASE_DOMAIN="gwtest.local"
-# `localhost_enabled` routes `localhost-<port>[s]` to 127.0.0.1:<port>, which is
-# what lets these tests run without registering a CVM.
+# `insecure_localhost_backend` routes `localhost-<port>[s]` to 127.0.0.1:<port>,
+# which is what lets these tests run without registering a CVM.
 SNI_TERMINATE="localhost-$ORIGIN_PLAIN.$BASE_DOMAIN"
 SNI_PASSTHROUGH="localhost-${ORIGIN_TLS}s.$BASE_DOMAIN"
 ADMIN_TOKEN="proxy-integration-test"