Add helpers for resolve1 and nameservers

detect_resolve1_resolv_conf() detects if systemd-resolved is enabled and
manages /etc/resolv.conf.

get_resolve1_nameservers() gets upstream DNS servers from
systemd-resolved's D-Bus interface.

get_dnspython_nameservers() gets upstream DNS servers from
/etc/resolv.conf via dns.python.

get_nameservers() gets a list of unique, non-loopback DNS server IP
addresses.

Also fixes setup.py to include D-Bus for ipalib instead of ipapython.

See: https://pagure.io/freeipa/issue/8275
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

ipalib/install/dnsforwarders.py

@@ -0,0 +1,155 @@
+#
+# Copyright (C) 2020  FreeIPA Contributors see COPYING for license
+#
+"""DNS forwarder and systemd-resolve1 helpers
+"""
+import ipaddress
+import logging
+import os
+import socket
+
+import dbus
+
+from ipaplatform.paths import paths
+from ipapython.dnsutil import get_ipa_resolver
+
+
+logger = logging.getLogger(__name__)
+
+
+_SYSTEMD_RESOLV_CONF = {
+    "/run/systemd/resolve/stub-resolv.conf",
+    "/run/systemd/resolve/resolv.conf",
+    "/lib/systemd/resolv.conf",
+    "/usr/lib/systemd/resolv.conf",
+}
+
+_DBUS_RESOLVE1_NAME = "org.freedesktop.resolve1"
+_DBUS_RESOLVE1_PATH = "/org/freedesktop/resolve1"
+_DBUS_RESOLVE1_MANAGER_IF = "org.freedesktop.resolve1.Manager"
+_DBUS_PROPERTY_IF = "org.freedesktop.DBus.Properties"
+
+# netlink interface index for resolve1 global settings and loopback
+IFINDEX_GLOBAL = 0
+IFINDEX_LOOPBACK = 1
+
+
+def detect_resolve1_resolv_conf():
+    """Detect if /etc/resolv.conf is managed by systemd-resolved
+
+    See man(5) NetworkManager.conf
+    """
+    try:
+        dest = os.readlink(paths.RESOLV_CONF)
+    except OSError:
+        # not a link
+        return False
+    # convert path relative to /etc/resolv.conf to abs path
+    dest = os.path.normpath(
+        os.path.join(os.path.dirname(paths.RESOLV_CONF), dest)
+    )
+    return dest in _SYSTEMD_RESOLV_CONF
+
+
+def get_resolve1_nameservers(*, with_ifindex=False):
+    """Get list of DNS nameservers from systemd-resolved
+
+    :return: list of tuples (ifindex, ipaddress_obj)
+    """
+    bus = dbus.SystemBus()
+    try:
+        resolve1 = bus.get_object(_DBUS_RESOLVE1_NAME, _DBUS_RESOLVE1_PATH)
+        prop_if = dbus.Interface(resolve1, _DBUS_PROPERTY_IF)
+        dns_prop = prop_if.Get(_DBUS_RESOLVE1_MANAGER_IF, "DNSEx")
+    finally:
+        bus.close()
+
+    results = []
+    for ifindex, af, dns_arr, port, sniname in dns_prop:
+        if port not in {0, 53} or sniname:
+            # non-default port, non-standard port, or SNI name configuration
+            # for DNS over TLS, e.g. 1.2.3.4:9953#example.com
+            continue
+        # convert packed format to IPAddress object (like inet_ntop)
+        if af == socket.AF_INET:
+            dnsip = ipaddress.IPv4Address(bytes(dns_arr))
+        elif af == socket.AF_INET6:
+            dnsip = ipaddress.IPv6Address(bytes(dns_arr))
+        else:
+            # neither IPv4 nor IPv6
+            continue
+        if with_ifindex:
+            # netlink interface index, see socket.if_nameindex()
+            ifindex = int(ifindex)
+            results.append((ifindex, dnsip))
+        else:
+            results.append(dnsip)
+
+    return results
+
+
+def get_dnspython_nameservers(*, with_ifindex=False):
+    """Get list of DNS nameservers from dnspython
+
+    On Linux dnspython parses /etc/resolv.conf for us
+
+    :return: list of tuples (ifindex, ipaddress_obj)
+    """
+    results = []
+    for nameserver in get_ipa_resolver().nameservers:
+        nameserver = ipaddress.ip_address(nameserver)
+        if with_ifindex:
+            results.append((IFINDEX_GLOBAL, nameserver))
+        else:
+            results.append(nameserver)
+    return results
+
+
+def get_nameservers():
+    """Get list of unique, non-loopback DNS nameservers
+
+    :return: list of strings
+    """
+    if detect_resolve1_resolv_conf():
+        logger.debug(
+            "systemd-resolved detected, fetching nameservers from D-Bus"
+        )
+        nameservers = get_resolve1_nameservers(with_ifindex=True)
+    else:
+        logger.debug(
+            "systemd-resolved not detected, parsing %s", paths.RESOLV_CONF
+        )
+        nameservers = get_dnspython_nameservers(with_ifindex=True)
+
+    logger.debug("Detected nameservers: %r", nameservers)
+
+    result = []
+    seen = set()
+    for ifindex, ip in nameservers:
+        # unique entries
+        if ip in seen:
+            continue
+        seen.add(ip)
+        # skip loopback
+        if ifindex == IFINDEX_LOOPBACK or ip.is_loopback:
+            continue
+        result.append(str(ip))
+
+    logger.debug("Use nameservers %r", result)
+
+    return result
+
+
+if __name__ == "__main__":
+    from pprint import pprint
+
+    print("systemd-resolved detected:", detect_resolve1_resolv_conf())
+    print("Interfaces:", socket.if_nameindex())
+    print("dnspython nameservers:")
+    pprint(get_dnspython_nameservers(with_ifindex=True))
+    print("resolve1 nameservers:")
+    try:
+        pprint(get_resolve1_nameservers(with_ifindex=True))
+    except Exception as e:
+        print(e)
+    print("nameservers:", get_nameservers())

ipalib/setup.py

@@ -44,6 +44,6 @@ if __name__ == '__main__':
             "six",
         ],
         extras_require={
-            "install": ["ipaplatform"],
+            "install": ["dbus-python"],  # for certmonger and resolve1
         },
     )

ipapython/setup.py

@@ -46,7 +46,6 @@ if __name__ == '__main__':
             "six",
         ],
         extras_require={
-            "install": ["dbus-python"],  # for certmonger
             "ldap": ["python-ldap"],  # ipapython.ipaldap
             # CheckedIPAddress.get_matching_interface
             "netifaces": ["netifaces"],