fix: the web service works in every state of the router.

ADR.md

@@ -29,7 +29,7 @@ restores the rules. The rules cover IPv4 only.
 
 ## ADR-2: A status web service of the standard library, bound to the internal address
 
-Status: Accepted
+Status: Accepted; amended by ADR-7
 
 Context: The router has no way to see its state without a shell. A check from the protected
 network, by a person or by a monitor, needs a page or an endpoint. The router is an
@@ -55,7 +55,7 @@ address is picked up at the next run of `vpn-router-setup`.
 
 ## ADR-3: /config is authenticated with a short-lived JWT signed with a shared secret
 
-Status: Accepted
+Status: Accepted; amended by ADR-7
 
 Context: A client such as manage-ui needs the configuration and the resolved state of the
 router, which `/` and `/status` must not show. The service has only the standard library, listens
@@ -148,3 +148,29 @@ Consequences: The web service still imports nothing outside the standard library
 program serves the command line and the API. Each request starts a process, which is fine for
 user administration. The API is protected by the token of `/config`, so whoever has the secret
 can manage users.
+
+## ADR-7: The web service starts on its own and reads its inputs at each request
+
+Status: Accepted
+
+Context: The web service took the address to listen on, the port, the token secret and the
+features from `/run/vpn-router/web.json`, written by `vpn-router-setup`, and ended with a failure
+while the file or the internal address was missing. It was ordered after `vpn-router-setup`, so a
+problem in the setup kept the status page, the tool to see that problem, from running.
+
+Decision: The service depends on nothing to start and does not end on its own. A discovery
+thread works out, every 30 seconds, the internal address from `vpn-router.json` as
+`general.mode` directs, with the discovery code in `vpnrouter_net.py`, which uses only the
+standard library and which `vpnrouter.py` uses too, and `web.port` and `web.enabled` from the
+same file. It runs the HTTP server in a thread of its own at that address and port, stops it and
+starts a new one when they change, and runs none while the service is disabled, the address is
+not known or it cannot be bound. Each request reads the token secret from `vpn-router.json` and
+the features and resolved facts from `web.json`, which setup still writes, because they need the
+configuration code; while that file is missing the router shows no features. The unit is not
+ordered after `vpn-router-setup`, and setup does not restart it.
+
+Consequences: The status page answers as soon as the internal address exists, whatever state
+setup is in, and follows a changed address, port or `web.enabled` within 30 seconds without a
+restart. `web.json` no longer holds the address, the port or the secret, so the secret is only in
+`vpn-router.json`. A change of the secret applies to the next request. The interfaces are read
+every 30 seconds, which costs a few `ip` commands.

README.md

@@ -493,11 +493,14 @@ curl -H "Authorization: Bearer $TOKEN" -H 'Content-Type: application/json' \
      -d '{"name": "alice", "password": "s3cr3t-pass"}' http://10.0.0.4:8080/users
 ```
 
-`vpn-router-setup` opens `web.port` on the internal interface in the firewall, writes the address,
-the port, the secret and the facts for `/config` to `/run/vpn-router/web.json` (readable by root
-only) and restarts the service, so a changed `web` section
-applies with `systemctl restart vpn-router-setup`. The service ends without an error when
-`web.enabled` is `false`, and keeps trying to start until the router has an internal address.
+The service starts on its own, without waiting for `vpn-router-setup`, and keeps running. Every
+30 seconds it works out the internal address from `vpn-router.json` as `general.mode` directs,
+and reads `web.port` and `web.enabled`; it listens there, moves when the address or the port
+changes, and stops listening while `web.enabled` is `false` or the address is not known. Each
+request reads the token secret from `vpn-router.json`, and the features and the facts for
+`/config` from `/run/vpn-router/web.json` (readable by root only), which `vpn-router-setup` writes;
+before that file exists the page shows no features. `vpn-router-setup` opens `web.port` on the
+internal interface in the firewall.
 
 ## Removal
 

build-and-publish.pl

@@ -0,0 +1,87 @@
+#!/usr/bin/env python3
+"""Commit everything, tag the version of the newest changelog entry and push main with the tags.
+
+The tag is the upstream version of the first entry of debian-package/debian/changelog,
+without the Debian revision: vpn-router (0.5.1-1) is tagged 0.5.1, which starts the
+release workflow.
+"""
+
+import argparse
+import pathlib
+import shlex
+import subprocess
+import sys
+
+ROOT = pathlib.Path(__file__).resolve().parent
+CHANGELOG = ROOT / 'debian-package' / 'debian' / 'changelog'
+
+
+#: Print the git commands instead of running them; set by -n/--dry-run.
+DRY_RUN = False
+
+
+def git(*args):
+    """Run git in the repository, or print the command on a dry run; stop with git's exit status when it fails."""
+    command = ['git', *args]
+    if DRY_RUN:
+        print(shlex.join(command))
+        return
+    result = subprocess.run(command, cwd=ROOT)
+    if result.returncode != 0:
+        sys.exit(result.returncode)
+
+
+def summary():
+    """The summary line of the first changelog entry, or None when it has none.
+
+    The summary is the entry's first text line: indented by two spaces, not a "*" item,
+    with a blank line between it and the header and a blank line after it.
+    """
+    lines = CHANGELOG.read_text(encoding='utf-8').splitlines()
+    if len(lines) < 4 or lines[1].strip() or lines[3].strip():
+        return None
+    line = lines[2]
+    text = line.strip()
+    if not line.startswith('  ') or line.startswith('   ') or not text or text.startswith('*') or line.startswith(' -- '):
+        return None
+    return text
+
+
+def version():
+    """The upstream version of the first changelog entry."""
+    first = CHANGELOG.read_text(encoding='utf-8').splitlines()[0]
+    start, end = first.find('('), first.find(')')
+    if start == -1 or end < start:
+        sys.exit(f'{CHANGELOG}: the first line has no version: {first}')
+    full = first[start + 1:end]
+    return full.rsplit('-', 1)[0] if '-' in full else full
+
+
+def main():
+    global DRY_RUN
+    parser = argparse.ArgumentParser(description=__doc__.splitlines()[0])
+    parser.add_argument('-m', '--message', help='commit message; default: the summary line of the newest changelog entry, a line without "*" between blank lines right after its header')
+    parser.add_argument('-n', '--dry-run', action='store_true', help='print the git commands instead of running them')
+    parser.add_argument('--no-push', action='store_true', help='commit and tag, but do not push')
+    args = parser.parse_args()
+    DRY_RUN = args.dry_run
+
+    message = args.message or summary()
+    if not message:
+        sys.exit(f'no commit message: pass -m, or start the newest entry of {CHANGELOG} with a summary line '
+                 'without "*", indented by two spaces, with a blank line above and below it')
+    tag = version()
+    if subprocess.run(['git', 'rev-parse', '-q', '--verify', f'refs/tags/{tag}'], cwd=ROOT,
+                      capture_output=True).returncode == 0:
+        sys.exit(f'tag {tag} exists; add a new changelog entry first')
+
+    git('add', '-A')
+    git('commit', '-m', message)
+    git('tag', tag)
+    if not args.no_push:
+        git('push', 'origin', 'main', '--tags')
+    return 0
+
+
+if __name__ == '__main__':
+    sys.exit(main())

debian-package/debian/changelog

@@ -1,3 +1,21 @@
+vpn-router (0.5.1-1) unstable; urgency=medium
+
+  fix: the web service works in every state of the router.
+
+  * vpn-router-web works whether /etc/vpn-router/pki and
+    /etc/swanctl/x509crl exist or not; they exist only while OpenVPN or
+    road-warrior access is configured. The paths the unit may write are
+    used when they exist and ignored when they do not.
+  * vpn-router-web starts on its own and keeps running. Every 30 seconds a
+    discovery thread works out the internal address from vpn-router.json as
+    general.mode directs, with web.port and web.enabled, and starts, moves
+    or stops the server thread to match. Each request reads the token
+    secret, the features and the facts it shows. The service is no longer
+    ordered after vpn-router-setup, which no longer restarts it, and
+    web.json no longer holds the address, the port or the secret.
+
+ -- Slawomir Koszewski <slawek@koszewscy.waw.pl>  Sat, 24 Oct 2026 10:00:00 +0200
+
 vpn-router (0.5.0-1) unstable; urgency=medium
 
   * OpenVPN P2S access, set in the openvpn section (enabled, address_pool,

debian-package/debian/install

@@ -1,6 +1,7 @@
 src/etc/sysctl.d/99-vpn-router.conf                   etc/sysctl.d/
 src/usr/lib/vpn-router/vpnrouter.py                   usr/lib/vpn-router/
 src/usr/lib/vpn-router/vpnrouter_ldap.py              usr/lib/vpn-router/
+src/usr/lib/vpn-router/vpnrouter_net.py               usr/lib/vpn-router/
 src/usr/lib/vpn-router/openvpn-same-user              usr/lib/vpn-router/
 src/usr/sbin/vpn-router-user                          usr/sbin/
 src/usr/lib/vpn-router/simple-ca                      usr/lib/vpn-router/

debian-package/debian/vpn-router.vpn-router-web.service

@@ -1,7 +1,7 @@
 [Unit]
 Description=VPN Router status web service
 Documentation=file:/usr/share/doc/vpn-router/README.md
-After=network-online.target vpn-router-setup.service
+After=network-online.target
 Wants=network-online.target
 
 [Service]
@@ -11,7 +11,7 @@ Restart=on-failure
 RestartSec=15
 NoNewPrivileges=yes
 ProtectSystem=strict
-ReadWritePaths=/run/vpn-router /etc/vpn-router/pki /etc/swanctl/x509crl
+ReadWritePaths=-/run/vpn-router -/etc/vpn-router/pki -/etc/swanctl/x509crl
 ProtectHome=yes
 PrivateTmp=yes
 

debian-package/src/usr/lib/vpn-router/setup

@@ -378,15 +378,13 @@ def refresh_docker_rules():
 
 
 def refresh_web(cfg, ctx):
-    """Hand the web service what it needs and restart it.
+    """Hand the web service the features and facts it shows.
 
-    The service takes the address to bind to, the port, the features of the
-    router, the secret that signs its tokens, the configuration with its
-    secrets masked and the facts resolved from the system from a file here, so
-    that it needs nothing but the standard library and the configuration code.
-    The file is readable by root only. The service ends quietly when
-    web.enabled is false. The restart is queued, not waited for, because the
-    unit is ordered after vpn-router-setup.
+    The service reads the file at each request: the features of the router,
+    the configuration with its secrets masked and the facts resolved from the
+    system, which it cannot work out with the standard library alone. The file
+    is readable by root only. The service follows a changed address or port
+    itself.
     """
     masked = {
         section: {option: '********' if SECRET_OPTION.search(option) and value else value
@@ -397,10 +395,6 @@ def refresh_web(cfg, ctx):
     WEB_STATE.touch(mode=0o600)
     WEB_STATE.chmod(0o600)
     WEB_STATE.write_text(json.dumps({
-        'enabled': bool(ctx['web_enabled']),
-        'bind': ctx['int_addr'],
-        'port': ctx['web_port'],
-        'jwt_secret': cfg['web']['jwt_secret'],
         'site_to_site': bool(vpnrouter.s2s_ready(ctx)),
         'p2s': bool(vpnrouter.p2s_ready(ctx)),
         'wireguard': bool(vpnrouter.wg_ready(ctx)),
@@ -419,7 +413,6 @@ def refresh_web(cfg, ctx):
             'remote_cidrs': ctx['remote_cidr_list'],
         },
     }) + '\n', encoding='utf-8')
-    run(['systemctl', 'restart', '--no-block', 'vpn-router-web.service'], check=False)
 
 
 def main():

debian-package/src/usr/lib/vpn-router/vpnrouter.py

@@ -21,6 +21,7 @@ if str(LIB_DIR) not in sys.path:
 
 import vpnrouter_ldap
 import vpnrouter_platforms
+from vpnrouter_net import ConfigError, detect_interfaces, interface_addresses, ip_output
 
 CONFIG_FILE  = pathlib.Path('/etc/vpn-router/vpn-router.json')
 TEMPLATE_DIR = pathlib.Path('/usr/share/vpn-router/templates')
@@ -92,10 +93,6 @@ MODES = ('manual', 'interfaces', 'auto')
 PLATFORM_NONE = 'none'
 
 
-class ConfigError(Exception):
-    """A setting is present but malformed."""
-
-
 def warn(message):
     print(f'vpn-router: warning: {message}', file=sys.stderr)
 
@@ -327,55 +324,6 @@ def deferred(cfg):
     return (_get(cfg, 'general', 'platform') or PLATFORM_NONE) == PLATFORM_NONE
 
 
-def _ip_route(*args):
-    result = subprocess.run(['ip', *args], capture_output=True, text=True)
-    return result.stdout if result.returncode == 0 else ''
-
-
-def detect_interfaces():
-    """Work out the external and internal interface names.
-
-    Only reached in auto mode, where the operator has accepted that a wrong
-    guess is possible. The external interface is whichever one carries the
-    default route; the internal one is the only other addressed interface.
-    """
-    tokens = _ip_route('route', 'get', '1.1.1.1').split()
-    external = ''
-    for index, token in enumerate(tokens):
-        if token == 'dev' and index + 1 < len(tokens):
-            external = tokens[index + 1]
-            break
-    if not external:
-        raise ConfigError('auto mode: cannot determine the external interface')
-
-    candidates = [name for name in _addressed_interfaces() if name != external]
-    if len(candidates) != 1:
-        raise ConfigError(
-            f'auto mode: expected one internal interface, found {len(candidates)}; '
-            'use mode = interfaces and name them')
-    return external, candidates[0]
-
-
-def _is_virtual(name):
-    """True for an interface the kernel creates without a device behind it.
-
-    Loopback, bridges, veth pairs and tunnels live under /sys/devices/virtual,
-    which is where the Docker bridge is; a NIC of the virtual machine does not.
-    """
-    link = pathlib.Path('/sys/class/net') / name
-    return '/devices/virtual/' in str(link.resolve())
-
-
-def _addressed_interfaces():
-    """Names of the NICs holding at least one IPv4 address; virtual interfaces are left out."""
-    names = []
-    for line in _ip_route('-o', '-4', 'addr', 'show').splitlines():
-        fields = line.split()
-        if len(fields) >= 4 and fields[1] not in names and not _is_virtual(fields[1]):
-            names.append(fields[1])
-    return names
-
-
 def detect_gateway(iface, address):
     """Derive the next hop on the internal side.
 
@@ -383,12 +331,12 @@ def detect_gateway(iface, address):
     back to the first host address of the connected subnet, which is a
     convention rather than a fact - so say so.
     """
-    for line in _ip_route('-o', 'route', 'show', 'dev', iface).splitlines():
+    for line in ip_output('-o', 'route', 'show', 'dev', iface).splitlines():
         fields = line.split()
         if 'via' in fields:
             return fields[fields.index('via') + 1]
 
-    for line in _ip_route('-o', '-4', 'addr', 'show', 'dev', iface).splitlines():
+    for line in ip_output('-o', '-4', 'addr', 'show', 'dev', iface).splitlines():
         fields = line.split()
         if len(fields) >= 4 and fields[3].split('/')[0] == address:
             network = ipaddress.ip_interface(fields[3]).network
@@ -399,34 +347,6 @@ def detect_gateway(iface, address):
     return ''
 
 
-def interface_addresses(name, setting):
-    """IPv4 addresses configured on an interface.
-
-    The interface name is configuration; its addresses are read from the
-    system. Looking up the addresses of a named interface is deterministic -
-    unlike working out which interface to use, which is a guess the package
-    deliberately does not make.
-    """
-    if not name:
-        return []
-    if not (pathlib.Path('/sys/class/net') / name).exists():
-        raise ConfigError(f'{setting}: interface {name!r} does not exist')
-
-    result = subprocess.run(
-        ['ip', '-o', '-4', 'addr', 'show', 'dev', name],
-        capture_output=True, text=True,
-    )
-    if result.returncode != 0:
-        raise ConfigError(f'{setting}: cannot read addresses of {name!r}')
-
-    addresses = []
-    for line in result.stdout.splitlines():
-        fields = line.split()
-        if len(fields) >= 4:
-            addresses.append(fields[3].split('/')[0])
-    return addresses
-
-
 def resolve_interfaces(cfg):
     """Return (ext_iface, int_iface, local_addrs, int_addr, int_gateway_ip).
 

debian-package/src/usr/lib/vpn-router/vpnrouter_net.py

@@ -0,0 +1,109 @@
+"""Interfaces and addresses of the router, read from the system.
+
+Only the Python standard library is used, so the web service can use this module too.
+"""
+
+import pathlib
+import subprocess
+
+
+class ConfigError(Exception):
+    """A setting is present but malformed."""
+
+
+def ip_output(*args):
+    """The output of an ip command, or '' when it fails."""
+    try:
+        result = subprocess.run(['ip', *args], capture_output=True, text=True)
+    except OSError:
+        return ''
+    return result.stdout if result.returncode == 0 else ''
+
+
+def detect_interfaces():
+    """Work out the external and internal interface names.
+
+    Only reached in auto mode, where the operator has accepted that a wrong
+    guess is possible. The external interface is whichever one carries the
+    default route; the internal one is the only other addressed interface.
+    """
+    tokens = ip_output('route', 'get', '1.1.1.1').split()
+    external = ''
+    for index, token in enumerate(tokens):
+        if token == 'dev' and index + 1 < len(tokens):
+            external = tokens[index + 1]
+            break
+    if not external:
+        raise ConfigError('auto mode: cannot determine the external interface')
+
+    candidates = [name for name in _addressed_interfaces() if name != external]
+    if len(candidates) != 1:
+        raise ConfigError(
+            f'auto mode: expected one internal interface, found {len(candidates)}; '
+            'use mode = interfaces and name them')
+    return external, candidates[0]
+
+
+def _is_virtual(name):
+    """True for an interface the kernel creates without a device behind it.
+
+    Loopback, bridges, veth pairs and tunnels live under /sys/devices/virtual,
+    which is where the Docker bridge is; a NIC of the virtual machine does not.
+    """
+    link = pathlib.Path('/sys/class/net') / name
+    return '/devices/virtual/' in str(link.resolve())
+
+
+def _addressed_interfaces():
+    """Names of the NICs holding at least one IPv4 address; virtual interfaces are left out."""
+    names = []
+    for line in ip_output('-o', '-4', 'addr', 'show').splitlines():
+        fields = line.split()
+        if len(fields) >= 4 and fields[1] not in names and not _is_virtual(fields[1]):
+            names.append(fields[1])
+    return names
+
+
+def interface_addresses(name, setting):
+    """IPv4 addresses configured on an interface.
+
+    The interface name is configuration; its addresses are read from the
+    system. Looking up the addresses of a named interface is deterministic -
+    unlike working out which interface to use, which is a guess the package
+    deliberately does not make.
+    """
+    if not name:
+        return []
+    if not (pathlib.Path('/sys/class/net') / name).exists():
+        raise ConfigError(f'{setting}: interface {name!r} does not exist')
+
+    result = subprocess.run(
+        ['ip', '-o', '-4', 'addr', 'show', 'dev', name],
+        capture_output=True, text=True,
+    )
+    if result.returncode != 0:
+        raise ConfigError(f'{setting}: cannot read addresses of {name!r}')
+
+    addresses = []
+    for line in result.stdout.splitlines():
+        fields = line.split()
+        if len(fields) >= 4:
+            addresses.append(fields[3].split('/')[0])
+    return addresses
+
+
+def internal_address(mode, internal, int_addr):
+    """The router's address on the internal network, as general.mode directs, or '' when it is not known yet.
+
+    manual takes local.int_addr; interfaces reads the address of interfaces.internal; auto also
+    works out which interface that is.
+    """
+    if mode == 'manual':
+        return int_addr
+    try:
+        if mode == 'auto':
+            internal = detect_interfaces()[1]
+        addresses = interface_addresses(internal, 'interfaces.internal')
+    except ConfigError:
+        return ''
+    return addresses[0] if addresses else ''

debian-package/src/usr/lib/vpn-router/web

@@ -21,12 +21,14 @@ web.jwt_secret is not set.
 Docker is installed, and the state of site-to-site, road-warrior (IPsec P2S), OpenVPN and
 WireGuard. No configuration data, key or address is shown.
 
-The address to bind to, the port, the secret and the configuration shown by /config
-are not read from the configuration here: vpn-router-setup resolves them, with the
-interfaces, and writes them to /run/vpn-router/web.json, readable by root only, before
-it restarts this service. The service ends with
-success when the file says the service is disabled, and with failure, to be
-started again, when the file is missing or the router has no internal address yet.
+The service needs nothing else to start and does not end on its own. A discovery thread works
+out the internal address from /etc/vpn-router/vpn-router.json as general.mode directs, with
+web.port and web.enabled, every DISCOVERY_SECONDS, and runs the server thread there: it starts
+it, moves it when the address or the port changes, and stops it while the service is disabled
+or the address is not known. Every request reads what it needs when it is made: the token secret
+from the configuration, and the features of the router and the facts shown by /config from
+/run/vpn-router/web.json, which vpn-router-setup writes. Before setup has written that file
+the router has no features, and /config shows no configuration or interfaces.
 """
 
 import base64
@@ -42,7 +44,18 @@ import time
 from html import escape
 from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
 
+sys.path.insert(0, os.path.dirname(os.path.realpath(__file__)))
+
+import vpnrouter_net
+
 STATE_FILE = '/run/vpn-router/web.json'
+CONFIG_FILE = '/etc/vpn-router/vpn-router.json'
+
+#: The port when web.port is not set.
+DEFAULT_PORT = 8080
+
+#: Seconds between two discoveries of where the service should listen.
+DISCOVERY_SECONDS = 30
 COMMAND_TIMEOUT = 5
 CACHE_SECONDS = 2
 
@@ -73,6 +86,33 @@ _cache = {'at': 0.0, 'data': None}
 _lock = threading.Lock()
 
 
+def read_json(path):
+    """The object in a JSON file, or {} when the file is missing or holds no object."""
+    try:
+        with open(path, encoding='utf-8') as handle:
+            data = json.load(handle)
+    except (OSError, ValueError):
+        return {}
+    return data if isinstance(data, dict) else {}
+
+
+def section(configuration, name):
+    """A section of the configuration, or {} when it is missing or not an object."""
+    value = configuration.get(name)
+    return value if isinstance(value, dict) else {}
+
+
+def current():
+    """What a request needs, read now: the features and facts setup resolved, and the token secret.
+
+    Before setup has run the router has no features, and without a secret /config and /users are
+    not served.
+    """
+    state = read_json(STATE_FILE)
+    secret = section(read_json(CONFIG_FILE), 'web').get('jwt_secret')
+    return dict(state, jwt_secret=secret if isinstance(secret, str) else '')
+
+
 def run(*args):
     """The finished process of a command, or None when it cannot be run."""
     try:
@@ -433,13 +473,12 @@ class Handler(BaseHTTPRequestHandler):
     def status(self):
         with _lock:
             if _cache['data'] is None or time.monotonic() - _cache['at'] > CACHE_SECONDS:
-                _cache['data'] = collect(self.server.state)
+                _cache['data'] = collect(current())
                 _cache['at'] = time.monotonic()
             return _cache['data']
 
-    def authorized(self):
+    def authorized(self, secret):
         """Whether the request carries a token signed with the secret; False also while no secret is set."""
-        secret = self.server.state.get('jwt_secret')
         header = self.headers.get('Authorization', '')
         return bool(secret) and header.startswith('Bearer ') and verify(header[7:].strip(), secret) is not None
 
@@ -449,16 +488,17 @@ class Handler(BaseHTTPRequestHandler):
         # A body that is not read must not be taken for the next request.
         if self.command not in ('GET', 'HEAD'):
             self.close_connection = True
-        protected =path == '/config' or path == '/users' or path.startswith('/users/')
-        if protected and not self.server.state.get('jwt_secret'):
+        protected = path == '/config' or path == '/users' or path.startswith('/users/')
+        state = current() if protected else {}
+        if protected and not state['jwt_secret']:
             body, kind, code = b'Not found\n', 'text/plain; charset=utf-8', 404
-        elif protected and not self.authorized():
+        elif protected and not self.authorized(state['jwt_secret']):
             body, kind, code = b'Unauthorized\n', 'text/plain; charset=utf-8', 401
             extra.append(('WWW-Authenticate', 'Bearer'))
         elif path == '/config' and self.command in ('GET', 'HEAD'):
-            body, kind, code = json.dumps(details(self.server.state), indent=2).encode() + b'\n', 'application/json', 200
+            body, kind, code = json.dumps(details(state), indent=2).encode() + b'\n', 'application/json', 200
         elif protected and path != '/config':
-            code, document = users(self.server.state, self.command, path, self.request_body())
+            code, document = users(state, self.command, path, self.request_body())
             body, kind = json.dumps(document, indent=2).encode() + b'\n', 'application/json'
         elif self.command not in ('GET', 'HEAD'):
             body, kind, code = b'Method not allowed\n', 'text/plain; charset=utf-8', 405
@@ -515,31 +555,65 @@ class Handler(BaseHTTPRequestHandler):
         sys.stderr.write('%s - - [%s] %s\n' % (self.client_address[0], self.log_date_time_string(), format % args))
 
 
+def wanted():
+    """Where the service should listen now, as (address, port), or the reason it should not listen."""
+    configuration = read_json(CONFIG_FILE)
+    web = section(configuration, 'web')
+    if web.get('enabled', True) is False:
+        return None, 'the web service is disabled in the configuration'
+    port = web.get('port', DEFAULT_PORT)
+    if isinstance(port, bool) or not isinstance(port, int) or not 1 <= port <= 65535:
+        port = DEFAULT_PORT
+    general, interfaces, local = (section(configuration, name) for name in ('general', 'interfaces', 'local'))
+    address = vpnrouter_net.internal_address(str(general.get('mode') or 'manual'), str(interfaces.get('internal') or ''),
+                                             str(local.get('int_addr') or ''))
+    if not address:
+        return None, 'the internal address is not known yet'
+    return (address, port), ''
+
+
+def log(message):
+    print(f'vpn-router-web: {message}', file=sys.stderr, flush=True)
+
+
+def discover():
+    """Work out where to listen every DISCOVERY_SECONDS, and run the server thread there.
+
+    When the address or the port changes, the server thread is stopped and a new one listens at
+    the new place; while the service is disabled, the address is not known or it cannot be bound,
+    no server runs and the next round tries again.
+    """
+    server, thread, listening, reported = None, None, None, None
+    while True:
+        target, reason = wanted()
+        if target != listening and server is not None:
+            server.shutdown()
+            server.server_close()
+            thread.join()
+            server, listening = None, None
+        if target is not None and server is None:
+            try:
+                server = ThreadingHTTPServer(target, Handler)
+            except OSError as exc:
+                reason = f'cannot listen on {target[0]}:{target[1]}: {exc}'
+            else:
+                thread = threading.Thread(target=server.serve_forever, name='server', daemon=True)
+                thread.start()
+                listening, reason = target, f'serving on http://{target[0]}:{target[1]}'
+        if reason and reason != reported:
+            log(reason)
+            reported = reason
+        time.sleep(DISCOVERY_SECONDS)
+
+
 def main():
-    try:
-        with open(STATE_FILE, encoding='utf-8') as handle:
-            state = json.load(handle)
-    except (OSError, ValueError) as exc:
-        print(f'vpn-router-web: cannot read {STATE_FILE}: {exc}', file=sys.stderr)
-        return 1
-
-    if not state.get('enabled'):
-        print('vpn-router-web: the web service is disabled in the configuration')
-        return 0
-    if not state.get('bind'):
-        print('vpn-router-web: the router has no internal address yet', file=sys.stderr)
-        return 1
-
-    server = ThreadingHTTPServer((state['bind'], state['port']), Handler)
-    server.state = state
     signal.signal(signal.SIGTERM, lambda *_: sys.exit(0))
-    print(f'vpn-router-web: serving on http://{state["bind"]}:{state["port"]}')
+    discovery = threading.Thread(target=discover, name='discovery', daemon=True)
+    discovery.start()
     try:
-        server.serve_forever()
+        discovery.join()
     except KeyboardInterrupt:
         pass
-    finally:
-        server.server_close()
     return 0