diff --git a/app/sovran_systemsos_web/ddns_update.py b/app/sovran_systemsos_web/ddns_update.py
new file mode 100644
index 0000000..72c69b6
--- /dev/null
+++ b/app/sovran_systemsos_web/ddns_update.py
@@ -0,0 +1,177 @@
+#!/usr/bin/env python3
+"""Sovran DDNS update runner (sovran-ddns-update.service).
+
+For every stored Njal.la update URL this asks Njal.la to point the record at
+the address the request came from ("&auto"), then reads back the address
+Njal.la says it recorded.  That address is saved to /var/lib/secrets/external-ip,
+where livekit-turn-setup and the Hub read it.
+
+Njal.la is the only party involved.  It has to learn the address to publish
+it, so nothing else -- no STUN server, no public resolver, no "what is my IP"
+service -- is ever asked for it.
+
+Kept from the previous runner:
+  * every URL goes through _validate_ddns_url() (https, njal.la only, /update/)
+  * curl is run directly: no shell, no redirects
+  * the update key is never printed or logged
+
+The module is installed next to security_helpers.py (/etc/sovran/) and run as a
+script by the service; it is also importable as
+sovran_systemsos_web.ddns_update so the tests can exercise it.
+"""
+import ipaddress
+import json
+import os
+import subprocess
+import sys
+import tempfile
+
+try:
+    from .security_helpers import _validate_ddns_url  # imported as part of the Hub package
+except ImportError:  # run as a script from /etc/sovran, next to security_helpers.py
+    sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+    from security_helpers import _validate_ddns_url
+
+URLS_FILE = "/var/lib/njalla/ddns_urls.json"
+IP_FILE = "/var/lib/secrets/external-ip"
+
+_CGNAT = ipaddress.ip_network("100.64.0.0/10")
+
+
+def is_public_ipv4(value) -> bool:
+    """True for a globally routable IPv4 literal (not private, loopback, CGNAT ...)."""
+    try:
+        ip = ipaddress.ip_address(str(value).strip())
+    except ValueError:
+        return False
+    if ip.version != 4 or ip in _CGNAT:
+        return False
+    # is_global alone is not enough: CPython reports multicast as global.
+    return ip.is_global and not (
+        ip.is_multicast or ip.is_reserved or ip.is_loopback
+        or ip.is_link_local or ip.is_unspecified or ip.is_private
+    )
+
+
+def normalise_url(raw: str) -> str:
+    """Return the URL to call for a stored (or freshly pasted) update URL.
+
+    * Older Hubs stored "...&a=${IP}": the address was looked up locally and
+      substituted.  Njal.la can use the address the request came from, so that
+      placeholder becomes "&auto".
+    * "&quiet" is dropped: the reply is how we learn the address Njal.la recorded.
+    """
+    return raw.replace("&a=${IP}", "&auto").replace("&quiet", "")
+
+
+def parse_reply(body: str):
+    """Return the public IPv4 address Njal.la says it recorded, or None.
+
+    A successful update replies with JSON of the form
+        {"status": 200, "message": "record updated", "value": {"A": "203.0.113.7", ...}}
+    """
+    try:
+        data = json.loads(body)
+    except (TypeError, ValueError):
+        return None
+    if not isinstance(data, dict) or str(data.get("status")) != "200":
+        return None
+    value = data.get("value")
+    ip = value.get("A") if isinstance(value, dict) else None
+    return str(ip).strip() if is_public_ipv4(ip) else None
+
+
+def read_ip_file(path: str = None):
+    """The address recorded by the last successful update, or None."""
+    try:
+        with open(path or IP_FILE) as f:
+            return f.read().strip() or None
+    except OSError:
+        return None
+
+
+def write_ip_file(ip: str, path: str = None) -> None:
+    """Replace the file atomically so a path watcher never sees a partial write."""
+    path = path or IP_FILE
+    directory = os.path.dirname(path)
+    os.makedirs(directory, exist_ok=True)
+    fd, tmp = tempfile.mkstemp(dir=directory, prefix=".external-ip-")
+    try:
+        with os.fdopen(fd, "w") as f:
+            f.write(ip)
+        os.chmod(tmp, 0o644)
+        os.replace(tmp, path)
+    except BaseException:
+        try:
+            os.unlink(tmp)
+        except OSError:
+            pass
+        raise
+
+
+def update_all(urls, *, run=None, validate=_validate_ddns_url):
+    """Call every update URL once; return the address Njal.la reported, or None.
+
+    Nothing secret is printed: URLs are only ever referred to by position.
+    """
+    run = run or subprocess.run  # resolved per call so tests can substitute it
+    reported = None
+    seen = set()
+    todo = []
+    for raw in urls:
+        url = normalise_url(raw)
+        if url not in seen:  # an old "&a=${IP}" entry and its "&auto" twin are one record
+            seen.add(url)
+            todo.append(url)
+    for number, url in enumerate(todo, 1):
+        try:
+            validate(url)
+            proc = run(
+                ["curl", "--silent", "--ipv4", "--max-time", "15", "--fail", "--no-location", url],
+                capture_output=True, text=True, timeout=20, check=False,
+            )
+        except Exception:
+            print(f"DDNS update {number}/{len(todo)}: skipped (invalid URL or curl unavailable)")
+            continue
+        if proc.returncode != 0:
+            print(f"DDNS update {number}/{len(todo)}: failed (curl exit {proc.returncode})")
+            continue
+        ip = parse_reply(proc.stdout)
+        if ip is None:
+            print(f"DDNS update {number}/{len(todo)}: Njal.la did not report a public IPv4 address")
+            continue
+        print(f"DDNS update {number}/{len(todo)}: ok")
+        if reported is None:
+            reported = ip
+        elif ip != reported:
+            print("DDNS: Njal.la reported different addresses for different records; using the first")
+    return reported
+
+
+def main() -> int:
+    try:
+        with open(URLS_FILE) as f:
+            urls = json.load(f)
+        if not isinstance(urls, list):
+            raise ValueError("not a list")
+    except Exception:
+        return 0  # no URLs configured -- nothing to do
+    urls = [u for u in urls if isinstance(u, str)]
+    if not urls:
+        return 0
+
+    ip = update_all(urls)
+    if ip is None:
+        print("DDNS: no address reported by Njal.la; keeping the last known one")
+        return 0
+    previous = read_ip_file()
+    if ip == previous:
+        print(f"DDNS: public IP unchanged ({ip})")
+        return 0
+    write_ip_file(ip)
+    print(f"DDNS: public IP is now {ip} (was {previous or 'unknown'})")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/app/sovran_systemsos_web/server.py b/app/sovran_systemsos_web/server.py
index 1a4bd1f..b7423be 100644
--- a/app/sovran_systemsos_web/server.py
+++ b/app/sovran_systemsos_web/server.py
@@ -21,7 +21,6 @@ import subprocess
 import tempfile
 import threading
 import time
-import sys
 import urllib.error
 import urllib.parse
 import urllib.request
@@ -41,6 +40,7 @@ from .config import load_config, load_versions
 from . import systemctl as sysctl
 from sovran_nwc import nwc_hub_manager as _nwc_mgr
 from . import support_ops as _support_ops
+from .ddns_update import normalise_url as _normalise_ddns_url
 from .security_helpers import (
     _nix_escape,
     NPUB_RE,
@@ -1008,44 +1008,21 @@ def _save_internal_ip(ip: str):
             pass
 
 
-def _save_external_ip(ip: str):
-    """Write the external IP to a file so other services (e.g. LiveKit) can
-    reference it without running their own detection."""
-    if ip and ip != "unavailable":
-        try:
-            os.makedirs(os.path.dirname(EXTERNAL_IP_FILE), exist_ok=True)
-            with open(EXTERNAL_IP_FILE, "w") as f:
-                f.write(ip)
-        except OSError:
-            pass
-
-
 def _get_external_ip() -> str:
-    """Public IP via the shared detector (/var/lib/sovran/public-ip.py).
+    """Public IP as recorded by the Njal.la DDNS runner (ddns_update.py).
 
-    The detector owns discovery (STUN -> DNS -> opt-in HTTPS echo), caches the
-    result in /var/lib/secrets/external-ip, and contacts at most one third
-    party per refresh interval. This function only reads the cache and asks
-    the detector to refresh when it is missing or stale — it performs no
-    per-call external queries of its own.
+    Nothing here looks the address up or contacts anyone. The DDNS update asks
+    Njal.la to use the address the request came from, Njal.la reports it back,
+    and the runner saves it to EXTERNAL_IP_FILE. Returns "unavailable" until
+    the first successful update (and on machines with no DDNS URL, e.g. Desktop).
     """
-    try:
-        r = subprocess.run(
-            [sys.executable, "/var/lib/sovran/public-ip.py", "check"],
-            capture_output=True, text=True, timeout=20,
-        )
-        if r.returncode == 0 and r.stdout.strip():
-            return r.stdout.strip().splitlines()[0]
-    except Exception:
-        pass
     try:
         with open(EXTERNAL_IP_FILE) as f:
             ip = f.read().strip()
-        if ip:
-            return ip
-    except OSError:
-        pass
-    return "unavailable"
+        ipaddress.ip_address(ip)
+        return ip
+    except (OSError, ValueError):
+        return "unavailable"
 
 
 # ── Port status helpers (local-only, no external calls) ──────────
@@ -3796,9 +3773,6 @@ async def api_network():
     # Keep the internal-ip file in sync for credential lookups
     _save_internal_ip(internal)
     _cached_external_ip = external
-    # Persist the external IP so other services (e.g. LiveKit) can reuse the
-    # Hub's detection instead of running their own.
-    _save_external_ip(external)
     return {"internal_ip": internal, "external_ip": external}
 
 
@@ -4723,51 +4697,26 @@ def _save_ddns_urls(urls: list[str]) -> None:
 
 
 def _run_njalla_ddns() -> None:
-    """Update Njal.la DDNS records immediately (best-effort).
+    """Ask the DDNS runner to update Njal.la right away (best-effort, non-blocking).
 
-    Resolves the current public IP once, then invokes ``curl`` directly as a
-    subprocess for each stored DDNS update URL.  No shell interpolation is
-    performed and no user-controlled value is interpreted as shell syntax.
-    Each URL is revalidated through ``_validate_ddns_url()`` after ``${IP}``
-    substitution; URLs that fail validation are silently skipped.
+    The runner (modules/core/njalla.nix -> ddns_update.py) is the only code that
+    talks to Njal.la. It validates every stored URL, calls it with "&auto" so
+    Njal.la uses the address the request came from, and records the address
+    Njal.la reports back for LiveKit and the Hub (EXTERNAL_IP_FILE).
 
-    Called when a domain/DDNS entry is saved and when a DDNS-backed feature
-    is enabled, so DNS is refreshed right away instead of waiting for the
-    15-minute timer tick (see modules/core/njalla.nix).
+    Called when a domain/DDNS entry is saved and when a DDNS-backed feature is
+    enabled, so DNS is refreshed right away instead of waiting for the
+    15-minute timer tick.
     """
-    urls = _load_ddns_urls()
-    if not urls:
+    if not _load_ddns_urls():
         return
-    # Resolve current public IP (best-effort; skip if unavailable)
-    public_ip = ""
     try:
-        ip_result = subprocess.run(
-            ["dig", "@resolver4.opendns.com", "myip.opendns.com", "+short", "-4"],
-            capture_output=True, text=True, timeout=10, check=False,
+        subprocess.run(
+            ["systemctl", "start", "--no-block", "sovran-ddns-update.service"],
+            capture_output=True, timeout=10, check=False,
         )
-        raw_ip = ip_result.stdout.strip().splitlines()[0] if ip_result.stdout.strip() else ""
-        # Validate strictly as a proper IPv4/IPv6 address before substitution
-        ipaddress.ip_address(raw_ip)
-        public_ip = raw_ip
     except Exception:
-        public_ip = ""
-
-    if not public_ip:
-        return  # skip to avoid sending bare ${IP} to curl
-
-    for raw_url in urls:
-        try:
-            # Replace the placeholder with the validated IP (safe string replacement)
-            url = raw_url.replace("${IP}", public_ip)
-            # Revalidate after substitution — enforces /update/ path, no $, etc.
-            _validate_ddns_url(url)
-            subprocess.run(
-                ["curl", "--silent", "--max-time", "15", "--fail", "--no-location", url],
-                timeout=20, check=False,
-                stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL,
-            )
-        except Exception:
-            pass
+        pass
 
 
 def _reload_caddy_for_domain_change() -> None:
@@ -4906,18 +4855,17 @@ async def api_domains_set(req: DomainSetRequest):
         # Strip surrounding quotes
         if len(ddns_url) >= 2 and ddns_url[0] in ('"', "'") and ddns_url[-1] == ddns_url[0]:
             ddns_url = ddns_url[1:-1]
-        # Replace trailing &auto with the IP placeholder used by _run_njalla_ddns
-        if ddns_url.endswith("&auto"):
-            ddns_url = ddns_url[:-5] + "&a=${IP}"
+        # Keep Njal.la's "&auto": Njal.la then uses the address the request comes
+        # from, so nothing on this machine has to look the address up. Old
+        # "&a=${IP}" pastes and "&quiet" are normalised exactly as the runner does.
+        ddns_url = _normalise_ddns_url(ddns_url)
         # Validate URL strictly — reject injection attempts before persisting.
-        # The placeholder ${IP} is replaced temporarily so the validator sees a
-        # real address; the original URL (with the placeholder) is kept for storage.
         try:
-            _validate_ddns_url(ddns_url.replace("${IP}", "127.0.0.1"))
+            _validate_ddns_url(ddns_url)
         except ValueError as exc:
             raise HTTPException(status_code=400, detail=f"Invalid DDNS URL: {exc}")
         # Persist the URL in the JSON store (never in executable shell source)
-        existing_urls = _load_ddns_urls()
+        existing_urls = list(dict.fromkeys(_normalise_ddns_url(u) for u in _load_ddns_urls()))
         if ddns_url not in existing_urls:
             existing_urls.append(ddns_url)
         try:
@@ -6308,13 +6256,12 @@ async def _background_domain_reachability_checker():
     consecutive_failures = 0
     while True:
         try:
-            # Keep the persisted external IP fresh (dynamic WAN IPs), so
-            # services like LiveKit can read /var/lib/secrets/external-ip.
+            # Pick up the address the Njal.la DDNS runner last recorded (a plain
+            # file read; nothing is looked up from here).
             loop = asyncio.get_event_loop()
             external = await loop.run_in_executor(None, _get_external_ip)
             if external != "unavailable":
                 _cached_external_ip = external
-                _save_external_ip(external)
 
             cfg = load_config()
             services = cfg.get("services", [])
diff --git a/modules/core/njalla.nix b/modules/core/njalla.nix
index c6f017f..5af7476 100644
--- a/modules/core/njalla.nix
+++ b/modules/core/njalla.nix
@@ -1,43 +1,64 @@
 { config, pkgs, lib, ... }:
 
 {
+  # The public-IP detector (STUN / OpenDNS / HTTPS echo) is gone: the public
+  # address is whatever Njal.la reports back for the DDNS update below, and
+  # nothing else on the system looks it up. Fail with a pointer, instead of
+  # silently ignoring them, if a custom.nix still sets one of its old options.
+  imports = map (opt:
+    lib.mkRemovedOptionModule [ "sovran_systemsOS" "publicIP" opt ]
+      "Sovran no longer looks up the public IP: the Njal.la DDNS update reports it (modules/core/njalla.nix). To force an address for Element Calling, set sovran_systemsOS.elementCalling.externalIP."
+  ) [ "stunServer" "stunPort" "dnsResolver" "httpsEcho" "cacheTTL" ];
+
   # ── Ensure njalla directory exists on every build ────────────────────────
   systemd.tmpfiles.rules = [
     "d /var/lib/njalla 0750 root root -"
   ];
 
-  # ── Install the shared validation helper so the DDNS runner can import it ─
-  # The exact same _validate_ddns_url() function used by the Hub web application
-  # is installed here as a read-only system file.  The DDNS runner imports it
-  # directly so the two code paths share one validator — no weaker inline copy.
+  # ── Install the DDNS runner and the validator it shares with the Hub ─────
+  # Both files come straight from the Hub's source tree and are installed side
+  # by side as read-only system files. The runner imports the exact same
+  # _validate_ddns_url() the Hub API uses — no weaker inline copy.
   environment.etc."sovran/security_helpers.py" = {
     source = ../../app/sovran_systemsos_web/security_helpers.py;
     mode   = "0444";
     user   = "root";
     group  = "root";
   };
+  environment.etc."sovran/ddns-update.py" = {
+    source = ../../app/sovran_systemsos_web/ddns_update.py;
+    mode   = "0444";
+    user   = "root";
+    group  = "root";
+  };
 
   # ── Safe DDNS update service ─────────────────────────────────────────────
   # Reads DDNS update URLs from the JSON store written by the Hub API and
   # invokes curl directly — no shell interpolation, no script execution.
-  # Replaces the legacy root cron job that ran /var/lib/njalla/njalla.sh.
+  # Njal.la is asked to use the address the request came from ("&auto") and
+  # reports it back; the runner saves it to /var/lib/secrets/external-ip,
+  # where LiveKit and the Hub read it. See app/sovran_systemsos_web/ddns_update.py.
   systemd.services.sovran-ddns-update = {
     description = "Sovran Njal.la DDNS update (safe JSON-based runner)";
     wants = [ "network-online.target" ];
     after = [ "network-online.target" ];
+    # curl is not in a NixOS unit's default PATH (coreutils, findutils, grep,
+    # sed, systemd): without this the runner cannot start it.
+    path = [ pkgs.curl ];
     serviceConfig = {
       Type        = "oneshot";
       User        = "root";
-      ExecStart   = "${pkgs.python3}/bin/python3 /var/lib/sovran/ddns-update.py";
-      # Harden the service — it only needs network access and read access to
-      # /var/lib/njalla/ddns_urls.json.
+      ExecStart   = "${pkgs.python3}/bin/python3 /etc/sovran/ddns-update.py";
+      # Harden the service — it needs network access, the URL store, and the
+      # file that receives the reported address.
       NoNewPrivileges    = true;
       ProtectSystem      = "strict";
       ReadWritePaths     = [ "/var/lib/njalla" "/var/lib/secrets" ];
       ReadOnlyPaths      = [ "/etc/sovran" ];
       ProtectHome        = true;
       PrivateTmp         = true;
-      RestrictAddressFamilies = [ "AF_INET" "AF_INET6" ];
+      # AF_UNIX: name lookups can go through nscd / systemd-resolved sockets.
+      RestrictAddressFamilies = [ "AF_UNIX" "AF_INET" "AF_INET6" ];
     };
   };
 
@@ -52,86 +73,9 @@
     };
   };
 
-  # Install the Python runner script at build time so the service can find it.
-  # The script is owned by root and not world-writable.
-  # Uses _validate_ddns_url() from /etc/sovran/security_helpers.py — the same
-  # production validator used by the Hub API — before executing any curl call.
-  # No shell is used; no redirects; no script execution.
-  # ${IP} placeholder is preserved in stored URLs and substituted at runtime;
-  # the URL is validated after substitution so any remaining $ is rejected.
+  # The runner used to be written to /var/lib/sovran by this activation script,
+  # next to the old public-ip.py detector. Remove those stale copies.
   system.activationScripts.sovran-ddns-update-script = ''
-    install -d -m 0755 /var/lib/sovran
-    cat > /var/lib/sovran/ddns-update.py <<'PYEOF'
-#!/usr/bin/env python3
-"""Sovran safe DDNS update runner.
-
-Reads ddns_urls.json, substitutes the public IP for the ''${IP} placeholder,
-validates each URL using the production _validate_ddns_url() from
-/etc/sovran/security_helpers.py, then calls curl per URL.
-No shell interpolation.  No redirects.  No script execution.
-"""
-import ipaddress, json, os, subprocess, sys
-
-sys.path.insert(0, '/etc/sovran')
-try:
-    from security_helpers import _validate_ddns_url
-except ImportError:
-    sys.exit(1)  # validator missing — fail so systemd logs the misconfiguration
-
-URLS_FILE = "/var/lib/njalla/ddns_urls.json"
-
-try:
-    with open(URLS_FILE) as f:
-        urls = json.load(f)
-    if not isinstance(urls, list):
-        raise ValueError("not a list")
-except Exception:
-    sys.exit(0)  # no URLs configured — nothing to do
-
-# Resolve current public IP via the shared detector — one script, one cache
-# (STUN -> DNS -> opt-in HTTPS echo; see /var/lib/sovran/public-ip.py).
-# The detector refreshes /var/lib/secrets/external-ip, which the Hub and
-# LiveKit read as well, so the whole system shares a single detected value.
-public_ip = ""
-try:
-    r = subprocess.run(
-        [sys.executable, "/var/lib/sovran/public-ip.py", "check"],
-        capture_output=True, text=True, timeout=20,
-    )
-    raw = r.stdout.strip().splitlines()[0] if r.stdout.strip() else ""
-    ipaddress.ip_address(raw)  # validates — raises if not a real IP
-    public_ip = raw
-except Exception:
-    pass
-
-if not public_ip:
-    # Last resort: the shared cache file, if the detector is unavailable.
-    try:
-        with open("/var/lib/secrets/external-ip") as f:
-            raw = f.read().strip()
-        ipaddress.ip_address(raw)
-        public_ip = raw
-    except Exception:
-        pass
-
-if not public_ip:
-    sys.exit(0)  # no IP resolved — skip to avoid sending bare ''${IP}
-
-for raw_url in urls:
-    try:
-        # Substitute ''${IP} placeholder then validate through production validator.
-        # After substitution there must be no $ left; _validate_ddns_url rejects
-        # any remaining $ expression.
-        url = raw_url.replace("''${IP}", public_ip)
-        _validate_ddns_url(url)
-        subprocess.run(
-            ["curl", "--silent", "--max-time", "15", "--fail", "--no-location", url],
-            timeout=20, check=False,
-            stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL,
-        )
-    except Exception:
-        pass
-PYEOF
-    chmod 0500 /var/lib/sovran/ddns-update.py
+    rm -f /var/lib/sovran/ddns-update.py /var/lib/sovran/public-ip.py
   '';
 }
diff --git a/modules/core/public-ip.nix b/modules/core/public-ip.nix
deleted file mode 100644
index 18c521b..0000000
--- a/modules/core/public-ip.nix
+++ /dev/null
@@ -1,323 +0,0 @@
-# ── Unified public-IP detection (privacy-first) ─────────────────────────────
-#
-# One script, one cache file, every consumer on the system reads the same
-# value. Previously the public IP was detected independently in three places,
-# each phoning home to a different third party:
-#   * the Hub (server.py _get_external_ip) → api.ipify.org / ifconfig.me /
-#     icanhazip.com over HTTPS on every /api/network call and every
-#     background-loop tick
-#   * DDNS (ddns-update.py) → myip.opendns.com via OpenDNS
-#   * LiveKit → STUN (its own embedded detection)
-#
-# This module replaces all of that with a single script
-# (/var/lib/sovran/public-ip.py) that detects the IP once per TTL using the
-# least-exposing mechanism available, and caches it in
-# /var/lib/secrets/external-ip. Consumers (Hub, DDNS, LiveKit) read the cache
-# and only invoke the script when it is missing or stale.
-#
-# Detection chain (first success wins, stops immediately):
-#   1. pin         — sovran_systemsOS.elementCalling.externalIP (baked in)
-#   2. cache       — /var/lib/secrets/external-ip if newer than cacheTTL
-#   3. STUN        — UDP binding request (one packet, no application data,
-#                    no HTTP metadata; the same protocol every WebRTC client
-#                    uses). Server configurable via publicIP.stunServer.
-#   4. DNS         — "myip.opendns.com" A query via publicIP.dnsResolver
-#                    (single DNS query, no HTTP headers)
-#   5. HTTPS echo  — ONLY endpoints listed in publicIP.httpsEcho (empty by
-#                    default → never contacted)
-#
-# Privacy property: while the cache is fresh, zero third parties are
-# contacted. When detection runs, at most ONE party learns the IP per
-# refresh interval (default 5 minutes), and the STUN/DNS mechanisms expose
-# nothing beyond the bare address.
-{
-  config,
-  pkgs,
-  lib,
-  ...
-}:
-
-let
-  stunServer  = config.sovran_systemsOS.publicIP.stunServer;
-  stunPort    = config.sovran_systemsOS.publicIP.stunPort;
-  dnsResolver = config.sovran_systemsOS.publicIP.dnsResolver;
-  httpsEcho   = config.sovran_systemsOS.publicIP.httpsEcho;
-  cacheTTL    = config.sovran_systemsOS.publicIP.cacheTTL;
-
-  # Optional pin shared with element-calling (baked in at build time).
-  pin = if config.sovran_systemsOS.elementCalling.externalIP != null then config.sovran_systemsOS.elementCalling.externalIP else "";
-
-  echoList = lib.concatStringsSep "," (map (u: "'${u}'") httpsEcho);
-in
-{
-  options.sovran_systemsOS.publicIP = {
-    stunServer = lib.mkOption {
-      type = lib.types.str;
-      default = "stun.l.google.com";
-      description = ''
-        STUN server used to discover the public IP over UDP. STUN is the most
-        privacy-preserving detection mechanism: a single stateless packet,
-        no HTTP metadata. Only used when the cache is stale.
-      '';
-    };
-    stunPort = lib.mkOption {
-      type = lib.types.port;
-      default = 19302;
-    };
-    dnsResolver = lib.mkOption {
-      type = lib.types.str;
-      default = "resolver4.opendns.com";
-      description = ''
-        DNS resolver used as fallback (myip.opendns.com trick) when STUN is
-        unavailable (e.g. ISP blocks UDP egress). A single DNS query, no
-        HTTP headers.
-      '';
-    };
-    httpsEcho = lib.mkOption {
-      type = lib.types.listOf lib.types.str;
-      default = [ ];
-      example = [ "https://api.ipify.org" ];
-      description = ''
-        OPT-IN HTTPS endpoints that return the caller's public IP as a bare
-        IPv4 literal. Each listed endpoint observes this server's public IP
-        and HTTP metadata every time detection runs. Empty by default — no
-        HTTPS echo service is ever contacted unless you add one here. This is
-        the last-resort fallback after STUN and DNS.
-      '';
-    };
-    cacheTTL = lib.mkOption {
-      type = lib.types.int;
-      default = 300;
-      description = "Seconds the detected public IP is cached before re-detection.";
-    };
-  };
-
-  # ── Install the unified detector ──────────────────────────────────────────
-  # This module declares `options` above, so ALL configuration must go under
-  # the `config` attribute: NixOS forbids mixing bare top-level settings
-  # (like `system.*`) with the `options`/`config` keyword attributes in the
-  # same module. (Fixes: "Module ... has an unsupported attribute `system'".)
-  config.system.activationScripts.sovranPublicIpInstall = lib.stringAfter [ "users" ] ''
-    install -d -m 0755 /var/lib/sovran
-    cat > /var/lib/sovran/public-ip.py <<'PYEOF'
-#!/usr/bin/env python3
-"""sovran-public-ip — one detector, one cache, every consumer reads the same IP.
-
-Privacy-first detection chain (first success wins):
-  1. pin     — baked in from sovran_systemsOS.elementCalling.externalIP
-  2. cache   — /var/lib/secrets/external-ip if newer than CACHE_TTL seconds
-  3. STUN    — UDP binding request (one packet, no application data)
-  4. DNS     — myip.opendns.com A query via the configured resolver
-  5. HTTPS   — ONLY endpoints baked in from publicIP.httpsEcho (opt-in)
-
-Usage:
-  public-ip.py check     print current public IP (cache first; refresh if stale)
-  public-ip.py refresh   force re-detection, update the cache file, print IP
-
-Exit status: 0 with the IP on stdout on success; 1 if no IP is available
-(cached value, if any, is still printed to stdout with a warning on stderr).
-"""
-import ipaddress
-import os
-import random
-import socket
-import struct
-import sys
-import time
-import urllib.request
-
-CACHE_FILE = "/var/lib/secrets/external-ip"
-PIN        = "${pin}"
-STUN_SERVER = "${stunServer}"
-STUN_PORT   = ${toString stunPort}
-DNS_RESOLVER = "${dnsResolver}"
-DNS_HOST     = "myip.opendns.com"
-ECHO_URLS    = [ ${echoList} ]
-CACHE_TTL    = ${toString cacheTTL}
-TIMEOUT      = 3.0
-
-# ---------------------------------------------------------------------------
-# Detection primitives
-# ---------------------------------------------------------------------------
-
-def is_usable_ip(text: str) -> bool:
-    """True if text is a globally routable IPv4 that LiveKit may advertise."""
-    try:
-        ip = ipaddress.ip_address(text)
-    except ValueError:
-        return False
-    if ip.version != 4:
-        return False
-    if (ip.is_private or ip.is_loopback or ip.is_link_local or ip.is_multicast
-            or ip.is_reserved or ip.is_unspecified or not ip.is_global):
-        return False
-    # RFC 6598 shared (CGNAT) space — not reachable from the internet.
-    if ip in ipaddress.ip_network("100.64.0.0/10"):
-        return False
-    return True
-
-
-def stun_public_ip() -> str | None:
-    """RFC 5389 Binding request over UDP; returns the mapped (public) IPv4."""
-    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
-    sock.settimeout(TIMEOUT)
-    try:
-        txid = random.randbytes(12)
-        req = struct.pack("!HHI", 0x0001, 0, 0) + txid  # Binding request
-        sock.sendto(req, (STUN_SERVER, STUN_PORT))
-        data, _ = sock.recvfrom(2048)
-    except OSError:
-        return None
-    finally:
-        sock.close()
-
-    if len(data) < 20:
-        return None
-    mtype, _mlen = struct.unpack("!HH", data[:4])
-    if mtype != 0x0101:  # Binding success response
-        return None
-
-    cookie = data[4:8]
-    i = 20
-    while i + 4 <= len(data):
-        atype, alen = struct.unpack("!HH", data[i : i + 4])
-        aval = data[i + 4 : i + 4 + alen]
-        if atype in (0x0001, 0x0020) and len(aval) >= 8:  # MAPPED / XOR-MAPPED
-            family = aval[1]
-            if family == 0x01:  # IPv4
-                raw = aval[4:8]
-                if atype == 0x0020:  # XOR with magic cookie + txid prefix
-                    raw = bytes(b ^ c for b, c in zip(raw, cookie + txid[:4]))
-                return socket.inet_ntop(socket.AF_INET, raw)
-        i += 4 + ((alen + 3) // 4) * 4
-    return None
-
-
-def dns_public_ip() -> str | None:
-    """Minimal DNS A query for myip.opendns.com against the given resolver."""
-    qid = random.randint(0, 0xFFFF)
-    qname = b"".join(bytes([len(p)]) + p.encode() for p in DNS_HOST.split(".")) + b"\x00"
-    query = struct.pack("!HHHHHH", qid, 0x0100, 1, 0, 0, 0) + qname + struct.pack("!HH", 1, 1)
-    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
-    sock.settimeout(TIMEOUT)
-    try:
-        sock.sendto(query, (DNS_RESOLVER, 53))
-        data, _ = sock.recvfrom(4096)
-    except OSError:
-        return None
-    finally:
-        sock.close()
-
-    try:
-        if len(data) < 12:
-            return None
-        rid, _flags, _qd, an, _ns, _ar = struct.unpack("!HHHHHH", data[:12])
-        if rid != qid or an == 0:
-            return None
-        i = 12
-        for _ in range(_qd):  # skip question
-            while data[i] != 0:
-                i += 1 + data[i]
-            i += 5
-        for _ in range(an):
-            if data[i] & 0xC0 == 0xC0:
-                i += 2
-            else:
-                while data[i] != 0:
-                    i += 1 + data[i]
-                i += 1
-            rtype, _rclass, _ttl, rdlen = struct.unpack("!HHIH", data[i : i + 10])
-            i += 10
-            if rtype == 1 and rdlen == 4:
-                return socket.inet_ntop(socket.AF_INET, data[i : i + 4])
-            i += rdlen
-    except (IndexError, struct.error):
-        return None
-    return None
-
-
-def echo_public_ip() -> str | None:
-    """Opt-in HTTPS echo endpoints (baked in at build time; empty by default)."""
-    for url in ECHO_URLS:
-        try:
-            req = urllib.request.Request(url, headers={"User-Agent": "sovran-public-ip"})
-            with urllib.request.urlopen(req, timeout=TIMEOUT) as resp:
-                text = resp.read().decode().strip()
-            if is_usable_ip(text):
-                return text
-        except Exception:
-            continue
-    return None
-
-
-# ---------------------------------------------------------------------------
-# Cache handling
-# ---------------------------------------------------------------------------
-
-def read_cache() -> str:
-    try:
-        with open(CACHE_FILE) as f:
-            return f.read().strip()
-    except OSError:
-        return ""
-
-
-def write_cache(ip: str) -> None:
-    try:
-        os.makedirs(os.path.dirname(CACHE_FILE), exist_ok=True)
-        tmp = f"{CACHE_FILE}.tmp"
-        with open(tmp, "w") as f:
-            f.write(ip + "\n")
-        os.replace(tmp, CACHE_FILE)
-    except OSError:
-        pass
-
-
-def cache_fresh() -> bool:
-    try:
-        return time.time() - os.path.getmtime(CACHE_FILE) < CACHE_TTL
-    except OSError:
-        return False
-
-
-def detect() -> str:
-    """Run the chain; returns usable IP or an empty string."""
-    if PIN and is_usable_ip(PIN):
-        return PIN
-    for fn in (stun_public_ip, dns_public_ip, echo_public_ip):
-        try:
-            cand = fn()
-        except Exception:
-            continue
-        if cand and is_usable_ip(cand):
-            return cand
-    return ""
-
-
-def main() -> int:
-    force = len(sys.argv) > 1 and sys.argv[1] == "refresh"
-    ip = ""
-    if not force and cache_fresh():
-        ip = read_cache()
-    if not ip:
-        ip = detect()
-        if ip:
-            write_cache(ip)
-        else:
-            stale = read_cache()
-            if stale:
-                print(stale)
-                print("WARNING: detection failed; using last known public IP", file=sys.stderr)
-                return 0
-            print("ERROR: could not determine a public IP (STUN/DNS unreachable)", file=sys.stderr)
-            return 1
-    print(ip)
-    return 0
-
-
-if __name__ == "__main__":
-    sys.exit(main())
-PYEOF
-    chmod 0555 /var/lib/sovran/public-ip.py
-  '';
-}
diff --git a/modules/core/roles.nix b/modules/core/roles.nix
index d89407d..7d2da4b 100755
--- a/modules/core/roles.nix
+++ b/modules/core/roles.nix
@@ -107,9 +107,11 @@
         description = ''
           Optional pin: force LiveKit to advertise this public IPv4 in its
           host/TURN ICE candidates. Not required in normal operation — the
-          module auto-detects the public IP at runtime (HTTPS egress
-          detection, falling back to STUN). Set it only to override a
-          mis-detected address (e.g. multi-WAN/VPN setups).
+          address is the one Njal.la reports for the DDNS update (set up in
+          the Hub's Domains page), and nothing on this system looks it up
+          anywhere else. Set it for a fixed public address with no Njal.la
+          DDNS entry, or to override the reported one (e.g. multi-WAN/VPN
+          setups).
         '';
       };
     };
diff --git a/modules/element-calling.nix b/modules/element-calling.nix
index 7ae83e9..6d8c471 100755
--- a/modules/element-calling.nix
+++ b/modules/element-calling.nix
@@ -204,35 +204,37 @@ EOF
       # NAT with port-forwarding. It does not need to be assigned to this box,
       # and it may be dynamic.
       #
-      # Reuse the shared detector (/var/lib/sovran/public-ip.py — see
-      # modules/core/public-ip.nix) instead of running our own: one script,
-      # one cache, privacy-first (STUN -> DNS -> opt-in HTTPS echo). Priority:
+      # Nothing here looks the address up. Priority:
       #   1. sovran_systemsOS.elementCalling.externalIP (explicit pin, if set)
-      #   2. /var/lib/secrets/external-ip (the shared cache)
-      #   3. run the detector now (it refreshes the cache)
-      #   4. STUN auto-detection (use_external_ip) as the fallback, with a
-      #      warning — this is where broken installs used to silently end up
-      #      advertising a private IP, causing "call connects but no video".
+      #   2. /var/lib/secrets/external-ip — the address Njal.la reported for the
+      #      last DDNS update (modules/core/njalla.nix). The runner rewrites that
+      #      file only when the address changes, and livekit-external-ip.path
+      #      then re-runs this script.
+      # With neither, or with an address that is not public, this unit fails with
+      # a clear message instead of guessing: advertising a wrong or private
+      # address is what produces "call connects but no video".
       EXTERNAL_IP='${if config.sovran_systemsOS.elementCalling.externalIP != null then config.sovran_systemsOS.elementCalling.externalIP else ""}'
 
       PUBLIC_IP="$EXTERNAL_IP"
       if [ -z "$PUBLIC_IP" ] && [ -f /var/lib/secrets/external-ip ]; then
         PUBLIC_IP=$(tr -d '[:space:]' < /var/lib/secrets/external-ip 2>/dev/null)
       fi
-      if [ -z "$PUBLIC_IP" ] && [ -x /var/lib/sovran/public-ip.py ]; then
-        PUBLIC_IP=$(python3 /var/lib/sovran/public-ip.py check 2>/dev/null | head -n1)
+
+      if [ -z "$PUBLIC_IP" ]; then
+        echo "ERROR: no public IP is known for LiveKit yet." >&2
+        echo "ERROR: It is recorded after the first successful Njal.la DDNS update (Hub, Domains)." >&2
+        echo "ERROR: To use a fixed address instead, set sovran_systemsOS.elementCalling.externalIP." >&2
+        exit 1
       fi
 
       # Reject non-routable addresses (loopback, private, link-local, CGNAT).
-      # A detected/pinned address like this must never be advertised.
-      if [ -n "$PUBLIC_IP" ] && printf '%s' "$PUBLIC_IP" | grep -qE \
-        '^(0\.|127\.|10\.|100\.64\.|169\.254\.|172\.(1[6-9]|2[0-9]|3[01])\.|192\.168\.)'; then
-        echo "WARNING: external IP '$PUBLIC_IP' is not routable; falling back to STUN auto-detection." >&2
-        PUBLIC_IP=""
+      if printf '%s' "$PUBLIC_IP" | grep -qE \
+        '^(0\.|127\.|10\.|100\.(6[4-9]|[7-9][0-9]|1[01][0-9]|12[0-7])\.|169\.254\.|172\.(1[6-9]|2[0-9]|3[01])\.|192\.168\.)'; then
+        echo "ERROR: $PUBLIC_IP is not a public address, so remote peers cannot reach LiveKit there." >&2
+        exit 1
       fi
 
-      if [ -n "$PUBLIC_IP" ]; then
-        cat > /run/livekit/livekit.yaml <<EOF
+      cat > /run/livekit/livekit.yaml <<EOF
 port: 7880
 rtc:
   use_external_ip: false
@@ -244,22 +246,7 @@ rtc:
     includes:
       - $IFACE
 EOF
-        echo "LiveKit will advertise public IP: $PUBLIC_IP"
-      else
-        cat > /run/livekit/livekit.yaml <<EOF
-port: 7880
-rtc:
-  use_external_ip: true
-  skip_external_ip_validation: true
-  advertise_internal_ip: true
-  tcp_port: 7881
-  udp_port: 7882
-  interfaces:
-    includes:
-      - $IFACE
-EOF
-        echo "WARNING: could not determine a public IP for LiveKit; using STUN auto-detection. If calls connect without media, check STUN egress or set sovran_systemsOS.elementCalling.externalIP." >&2
-      fi
+      echo "LiveKit will advertise public IP: $PUBLIC_IP"
 
       # Webhooks → lk-jwt-service. The JWT service validates the HMAC
       # signature against the same key file it issues tokens with, and uses
@@ -414,15 +401,32 @@ EOF
   # Restart LiveKit / lk-jwt-service when a rebuild regenerates their runtime
   # configs (new domains, externalIP, full-access list), mirroring the domain
   # change flow.
-  # Re-run the config generator and restart LiveKit when a rebuild regenerates
-  # the runtime config, or when the Hub persists a new external IP (dynamic
-  # WAN IPs), so the advertised ICE candidate stays current without a manual
-  # restart. The trigger chain: external-ip change → livekit-turn-setup
-  # re-runs → rewrites livekit.yaml → livekit restarts with the new config.
-  systemd.services.livekit-turn-setup.restartTriggers = [ "/var/lib/secrets/external-ip" ];
   systemd.services.livekit.restartTriggers = [ "/run/livekit/livekit.yaml" ];
   systemd.services.lk-jwt-service.restartTriggers = [ "/run/lk-jwt-service/env" ];
 
+  # Follow a changing public IP. ddns-update.py rewrites
+  # /var/lib/secrets/external-ip only when Njal.la reports a different address;
+  # this path unit then re-runs livekit-turn-setup (new node_ip and TURN
+  # address) and starts LiveKit if it is not running, e.g. because no address
+  # was known yet at boot. restartTriggers cannot do this: it is evaluated when
+  # the system is built, so it cannot watch a file that changes at runtime.
+  systemd.paths.livekit-external-ip = {
+    description = "Watch the public IP recorded by the Njal.la DDNS runner";
+    wantedBy = [ "multi-user.target" ];
+    pathConfig.PathChanged = "/var/lib/secrets/external-ip";
+  };
+  systemd.services.livekit-external-ip = {
+    description = "Re-run LiveKit setup for a changed public IP";
+    serviceConfig.Type = "oneshot";
+    unitConfig.ConditionPathExists = "/var/lib/domains/element-calling";
+    script = ''
+      # livekit.service requires livekit-turn-setup, so it restarts with it.
+      systemctl restart livekit-turn-setup.service
+      # No-op if LiveKit is already running; starts it after an earlier failure.
+      systemctl start livekit.service
+    '';
+  };
+
   ####### PUBLIC REACHABILITY SELF-CHECK #######
   # Diagnostic only — never a hard dependency of livekit/caddy. Catches the
   # classic "call connects but no media" setup errors at boot instead of at
diff --git a/modules/modules.nix b/modules/modules.nix
index 74e0eb6..b7b8098 100755
--- a/modules/modules.nix
+++ b/modules/modules.nix
@@ -17,7 +17,6 @@
     ./core/no-sleep.nix
     ./core/cpu-performance.nix
     ./core/local-domain-loopback.nix
-    ./core/public-ip.nix
 
     # ── Always on (no flag) ───────────────────────────────────
     ./php.nix
diff --git a/tests/test_ddns_update.py b/tests/test_ddns_update.py
new file mode 100644
index 0000000..07be34d
--- /dev/null
+++ b/tests/test_ddns_update.py
@@ -0,0 +1,249 @@
+"""Tests for the DDNS runner (sovran_systemsos_web.ddns_update).
+
+The runner asks Njal.la to use the address the request came from ("&auto"),
+reads back the address Njal.la recorded and saves it for LiveKit and the Hub.
+
+Tests must never:
+  - access the network (curl is replaced by a fake ``run``)
+  - write to system paths (the URL and IP files live in a temp dir)
+"""
+
+import contextlib
+import io
+import json
+import os
+import subprocess
+import sys
+import tempfile
+import unittest
+from unittest import mock
+
+_REPO_ROOT = os.path.normpath(os.path.join(os.path.dirname(__file__), ".."))
+_APP_PARENT = os.path.join(_REPO_ROOT, "app")
+if _APP_PARENT not in sys.path:
+    sys.path.insert(0, _APP_PARENT)
+
+from sovran_systemsos_web import ddns_update as d  # noqa: E402
+
+KEY = "SECRETKEY123"
+AUTO_URL = f"https://njal.la/update/?h=sub.example.com&k={KEY}&auto"
+LEGACY_URL = f"https://njal.la/update/?h=sub.example.com&k={KEY}&a=${{IP}}"
+PUBLIC_IP = "93.184.216.34"
+OTHER_IP = "8.8.4.4"
+
+
+def reply(ip=PUBLIC_IP, status=200):
+    return json.dumps({"status": status, "message": "record updated", "value": {"A": ip}})
+
+
+class FakeRun:
+    """Stands in for subprocess.run; records every command it is given."""
+
+    def __init__(self, stdout=None, returncode=0, raises=None):
+        self.stdout = reply() if stdout is None else stdout
+        self.returncode = returncode
+        self.raises = raises
+        self.calls = []
+
+    def __call__(self, cmd, **kwargs):
+        self.calls.append(cmd)
+        if self.raises:
+            raise self.raises
+        return subprocess.CompletedProcess(cmd, self.returncode, stdout=self.stdout, stderr="")
+
+
+class NormaliseUrlTests(unittest.TestCase):
+    def test_legacy_placeholder_becomes_auto(self):
+        self.assertEqual(d.normalise_url(LEGACY_URL), AUTO_URL)
+
+    def test_quiet_is_dropped_so_the_reply_can_be_read(self):
+        self.assertEqual(d.normalise_url(AUTO_URL + "&quiet"), AUTO_URL)
+
+    def test_plain_auto_is_unchanged(self):
+        self.assertEqual(d.normalise_url(AUTO_URL), AUTO_URL)
+
+    def test_explicit_address_is_unchanged(self):
+        url = "https://njal.la/update/?h=a.example.com&k=K&a=93.184.216.34"
+        self.assertEqual(d.normalise_url(url), url)
+
+
+class IsPublicIpv4Tests(unittest.TestCase):
+    def test_public_addresses(self):
+        for ip in ("93.184.216.34", "8.8.8.8", " 1.1.1.1\n"):
+            self.assertTrue(d.is_public_ipv4(ip), ip)
+
+    def test_everything_else_is_rejected(self):
+        for ip in ("10.0.0.1", "192.168.1.5", "172.16.0.9", "127.0.0.1", "169.254.1.1",
+                   "100.64.0.1", "0.0.0.0", "224.0.0.1", "::1", "2001:4860:4860::8888",
+                   "not-an-ip", "", None):
+            self.assertFalse(d.is_public_ipv4(ip), ip)
+
+
+class ParseReplyTests(unittest.TestCase):
+    def test_success_returns_the_recorded_address(self):
+        self.assertEqual(d.parse_reply(reply()), PUBLIC_IP)
+
+    def test_status_may_be_a_string(self):
+        self.assertEqual(d.parse_reply(reply(status="200")), PUBLIC_IP)
+
+    def test_error_status_is_rejected(self):
+        body = json.dumps({"status": 401, "message": "invalid host or key"})
+        self.assertIsNone(d.parse_reply(body))
+
+    def test_garbage_is_rejected(self):
+        for body in ("", "not json", "[]", "null", "{}", json.dumps({"status": 200})):
+            self.assertIsNone(d.parse_reply(body), body)
+
+    def test_non_public_or_non_ipv4_address_is_rejected(self):
+        for ip in ("10.1.2.3", "100.64.9.9", "127.0.0.1", "::1", "2001:4860:4860::8888", "x"):
+            self.assertIsNone(d.parse_reply(reply(ip)), ip)
+
+
+class IpFileTests(unittest.TestCase):
+    def test_write_then_read(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            path = os.path.join(tmp, "secrets", "external-ip")
+            d.write_ip_file(PUBLIC_IP, path)
+            self.assertEqual(d.read_ip_file(path), PUBLIC_IP)
+            self.assertEqual(open(path).read(), PUBLIC_IP)  # no trailing newline
+            self.assertEqual(os.stat(path).st_mode & 0o777, 0o644)
+
+    def test_replace_is_atomic_and_leaves_no_temp_files(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            path = os.path.join(tmp, "external-ip")
+            d.write_ip_file(PUBLIC_IP, path)
+            d.write_ip_file(OTHER_IP, path)
+            self.assertEqual(d.read_ip_file(path), OTHER_IP)
+            self.assertEqual(os.listdir(tmp), ["external-ip"])
+
+    def test_missing_file_reads_as_none(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            self.assertIsNone(d.read_ip_file(os.path.join(tmp, "nope")))
+
+
+class UpdateAllTests(unittest.TestCase):
+    def run_update(self, urls, fake):
+        out = io.StringIO()
+        with contextlib.redirect_stdout(out):
+            result = d.update_all(urls, run=fake)
+        return result, out.getvalue()
+
+    def test_curl_is_called_directly_with_ipv4_and_no_redirects(self):
+        fake = FakeRun()
+        result, _ = self.run_update([AUTO_URL], fake)
+        self.assertEqual(result, PUBLIC_IP)
+        self.assertEqual(len(fake.calls), 1)
+        cmd = fake.calls[0]
+        self.assertEqual(cmd[0], "curl")
+        for flag in ("--ipv4", "--no-location", "--fail", "--silent"):
+            self.assertIn(flag, cmd)
+        self.assertEqual(cmd[-1], AUTO_URL)
+
+    def test_legacy_entry_and_its_auto_twin_are_one_call(self):
+        fake = FakeRun()
+        self.run_update([LEGACY_URL, AUTO_URL], fake)
+        self.assertEqual(fake.calls[0][-1], AUTO_URL)
+        self.assertEqual(len(fake.calls), 1)
+
+    def test_url_for_another_host_is_never_called(self):
+        fake = FakeRun()
+        result, _ = self.run_update([f"https://evil.example/update/?h=x&k={KEY}&auto"], fake)
+        self.assertIsNone(result)
+        self.assertEqual(fake.calls, [])
+
+    def test_failed_curl_yields_nothing(self):
+        result, _ = self.run_update([AUTO_URL], FakeRun(returncode=22))
+        self.assertIsNone(result)
+
+    def test_reply_without_an_address_yields_nothing(self):
+        result, _ = self.run_update([AUTO_URL], FakeRun(stdout=json.dumps({"status": 200})))
+        self.assertIsNone(result)
+
+    def test_missing_curl_is_survived(self):
+        result, out = self.run_update([AUTO_URL], FakeRun(raises=FileNotFoundError("curl")))
+        self.assertIsNone(result)
+        self.assertIn("skipped", out)
+
+    def test_first_reported_address_wins(self):
+        calls = iter([reply(PUBLIC_IP), reply(OTHER_IP)])
+
+        def fake(cmd, **kwargs):
+            return subprocess.CompletedProcess(cmd, 0, stdout=next(calls), stderr="")
+
+        other = f"https://njal.la/update/?h=other.example.com&k={KEY}&auto"
+        out = io.StringIO()
+        with contextlib.redirect_stdout(out):
+            result = d.update_all([AUTO_URL, other], run=fake)
+        self.assertEqual(result, PUBLIC_IP)
+
+    def test_the_key_is_never_printed(self):
+        for fake in (FakeRun(), FakeRun(returncode=22), FakeRun(stdout="junk"),
+                     FakeRun(raises=FileNotFoundError("curl"))):
+            _, out = self.run_update([AUTO_URL, LEGACY_URL], fake)
+            self.assertNotIn(KEY, out)
+
+
+class MainTests(unittest.TestCase):
+    def setUp(self):
+        self.tmp = tempfile.TemporaryDirectory()
+        self.addCleanup(self.tmp.cleanup)
+        self.urls_file = os.path.join(self.tmp.name, "ddns_urls.json")
+        self.ip_file = os.path.join(self.tmp.name, "secrets", "external-ip")
+        for patch in (mock.patch.object(d, "URLS_FILE", self.urls_file),
+                      mock.patch.object(d, "IP_FILE", self.ip_file)):
+            patch.start()
+            self.addCleanup(patch.stop)
+
+    def store(self, urls):
+        with open(self.urls_file, "w") as f:
+            json.dump(urls, f)
+
+    def main(self, fake):
+        out = io.StringIO()
+        with mock.patch.object(d.subprocess, "run", fake), contextlib.redirect_stdout(out):
+            code = d.main()
+        self.assertEqual(code, 0)
+        return out.getvalue()
+
+    def test_first_update_records_the_address(self):
+        self.store([LEGACY_URL])
+        out = self.main(FakeRun())
+        self.assertEqual(d.read_ip_file(self.ip_file), PUBLIC_IP)
+        self.assertIn("now " + PUBLIC_IP, out)
+        self.assertNotIn(KEY, out)
+
+    def test_unchanged_address_does_not_touch_the_file(self):
+        # A path unit restarts LiveKit whenever the file is written, so an
+        # unchanged address must not rewrite it.
+        self.store([AUTO_URL])
+        self.main(FakeRun())
+        before = os.stat(self.ip_file)
+        out = self.main(FakeRun())
+        after = os.stat(self.ip_file)
+        self.assertEqual((before.st_ino, before.st_mtime_ns), (after.st_ino, after.st_mtime_ns))
+        self.assertIn("unchanged", out)
+
+    def test_changed_address_is_recorded(self):
+        self.store([AUTO_URL])
+        self.main(FakeRun(stdout=reply(PUBLIC_IP)))
+        out = self.main(FakeRun(stdout=reply(OTHER_IP)))
+        self.assertEqual(d.read_ip_file(self.ip_file), OTHER_IP)
+        self.assertIn(f"now {OTHER_IP} (was {PUBLIC_IP})", out)
+
+    def test_failed_update_keeps_the_last_known_address(self):
+        self.store([AUTO_URL])
+        self.main(FakeRun(stdout=reply(PUBLIC_IP)))
+        self.main(FakeRun(returncode=7))
+        self.assertEqual(d.read_ip_file(self.ip_file), PUBLIC_IP)
+
+    def test_nothing_configured_does_nothing(self):
+        fake = FakeRun()
+        self.main(fake)  # no URL file at all
+        self.store([])
+        self.main(fake)  # empty list
+        self.assertEqual(fake.calls, [])
+        self.assertFalse(os.path.exists(self.ip_file))
+
+
+if __name__ == "__main__":
+    unittest.main()