httpd: make --insecure only listen to localhost by default

README.md

@@ -69,7 +69,8 @@ curl 7.82 or newer; older versions take
 
 For demo purposes, let's first start the service *without authentication*.
 This mode is NOT SECURE! See below how to set up authentication.
-`--insecure` also turns TLS off, hence the plain `http://` below.
+`--insecure` also turns TLS off, hence the plain `http://` below. It
+listens on `127.0.0.1:1031` only. Use `--bind=` if more is needed.
 
 ```console
 $ systemd-run --user ./target/debug/varlink-httpd --insecure
@@ -297,7 +298,9 @@ authenticate the client; without mTLS the bridge refuses to start.
 mTLS is enabled separately (see below) and applies on top of whatever
 `--auth=` selects, so `--auth=ssh` together with mTLS requires both to
 pass. `--insecure` is the one way to serve with no authentication at
-all, and implies `--auth=none`.
+all, and implies `--auth=none`. Because nothing authenticates the
+client, it defaults to listening on `127.0.0.1:1031` instead of the
+usual `0.0.0.0:1031`. Use `--bind=` if more is needed.
 
 ### TLS / mTLS
 
@@ -308,7 +311,8 @@ TLS flag names follow the systemd convention.
 --key=PATH     path to TLS private key PEM file
 --trust=PATH   path to CA certificate PEM for client verification (mTLS)
 --require-mtls require a verified client certificate
---insecure     run over plain HTTP without any authentication (DANGEROUS)
+--insecure     run over plain HTTP without any authentication (DANGEROUS,
+               listens on localhost only unless --bind= says otherwise)
 ```
 
 `--require-mtls` makes every client present a certificate signed by the

src/bin/varlink-httpd/main.rs

@@ -1620,7 +1620,8 @@ fn print_help() {
           VARLINK_SOCKETS_PATH              directory of sockets or a single socket
                                             (default: /run/varlink/registry)
           --bind=ADDR                       address to bind to (repeatable;
-                                            default: 0.0.0.0:{DEFAULT_PORT})
+                                            default: 0.0.0.0:{DEFAULT_PORT}, or
+                                            127.0.0.1:{DEFAULT_PORT} with --insecure)
                                             use vsock::PORT for vsock (e.g. vsock::{DEFAULT_PORT})
           --auth=MECHANISMS                 comma-separated per-request authentication
                                             ({auth}); required unless --insecure.
@@ -1638,7 +1639,9 @@ fn print_help() {
           --authorized-keys=PATH            authorized SSH public keys file
           --api-keys=PATH                   API key hashes file (see gen-api-key)
           --insecure                        run over plain HTTP without any
-                                            authentication (DANGEROUS)
+                                            authentication (DANGEROUS); listens
+                                            on localhost only unless --bind= says
+                                            otherwise
           --help                            display this help and exit
     ",
             auth = AuthMechanism::names(),
@@ -1683,6 +1686,20 @@ fn check_mechanism_flags(
     Ok(())
 }
 
+fn default_bind(insecure: bool) -> (String, Option<String>) {
+    if insecure {
+        (
+            format!("127.0.0.1:{DEFAULT_PORT}"),
+            Some(format!(
+                "--insecure listens on 127.0.0.1:{DEFAULT_PORT} only, \
+                 pass --bind= if needed"
+            )),
+        )
+    } else {
+        (format!("0.0.0.0:{DEFAULT_PORT}"), None)
+    }
+}
+
 fn parse_cli() -> anyhow::Result<Command> {
     use lexopt::prelude::*;
 
@@ -1734,7 +1751,11 @@ fn parse_cli() -> anyhow::Result<Command> {
     }
 
     if bind_strs.is_empty() {
-        bind_strs.push(format!("0.0.0.0:{DEFAULT_PORT}"));
+        let (bind, note) = default_bind(insecure);
+        if let Some(note) = note {
+            warn!("{note}");
+        }
+        bind_strs.push(bind);
     }
     let binds: Vec<BindAddr> = bind_strs
         .iter()

src/bin/varlink-httpd/tests.rs

@@ -1689,6 +1689,21 @@ fn test_format_x509_subject_multiple_fields() {
 
 // --- bind address parsing tests ---
 
+#[test]
+fn test_default_bind_insecure_is_localhost_only() {
+    let (bind, note) = crate::default_bind(true);
+    assert_eq!(bind, format!("127.0.0.1:{DEFAULT_PORT}"));
+    let note = note.expect("--insecure should explain the narrowed default");
+    assert!(note.contains("--bind="), "note: {note}");
+}
+
+#[test]
+fn test_default_bind_authenticated_is_wildcard() {
+    let (bind, note) = crate::default_bind(false);
+    assert_eq!(bind, format!("0.0.0.0:{DEFAULT_PORT}"));
+    assert_eq!(note, None);
+}
+
 #[test]
 fn test_bind_addr_parse_defaults() {
     // "vsock" and "vsock:" both mean CID_ANY + default port