NM: Make DNS settings the maximum to avoid DNS leak fallback

NM has atrocious defaults. In the current situation, if you're
connected to OpenVPN/WireGuard (full tunnel, default gateway), DNS may
go outside of the VPN if the hostname only resolves in e.g. a DHCP
provided dns server. This is determined with the ipv4/ipv6
dns-priority setting and the dns-search setting.

We have to ensure that the dns-priority gets a negative value, from
nm-settings:

DNS servers priority. The relative priority for DNS servers specified
by this setting. A lower numerical value is better (higher
priority). Negative values have the special effect of excluding other
configurations with a greater numerical priority value; so in presence
of at least one negative priority, only DNS servers from connections
with the lowest priority value will be used. To avoid all DNS leaks,
set the priority of the profile that should be used to the most
negative value of all active connections profiles

We thus make the following change if full tunnel/default gateway:

- Set the DNS priority:
nmcli con modify eduVPN ipv4.dns-priority -2147483648 (int32 min)
nmcli con modify eduVPN ipv6.dns-priority -2147483648 (int32 min)

- Include the ~. DNS search domain:
nmcli con modify eduVPN ipv4.dns-search "~."
nmcli con modify eduVPN ipv6.dns-search "~."

Modifying the DNS search domain to ~. is needed according to
https://systemd.io/RESOLVED-VPNS/. This doc also states it's good to
set never-default to no (even if it doesn't do much), which we already
did for OpenVPN. Let's do the same for WireGuard.

Note that this only seems to happen with systemd-resolved which is the
default on Fedora. E.g. Openresolv resolvconf is not affected

eduvpn/nm.py

@@ -11,7 +11,7 @@
 from shutil import rmtree
 from socket import AF_INET, AF_INET6
 from tempfile import mkdtemp
-from typing import Any, Callable, List, Optional, TextIO, Tuple
+from typing import Any, Callable, Optional, TextIO, Tuple
 
 from eduvpn.ovpn import Ovpn
 from eduvpn.storage import get_uuid, set_uuid, write_ovpn
@@ -394,12 +394,7 @@ def set_connection(
         self,
         new_connection: "NM.SimpleConnection",
         callback: Callable,
-        default_gateway: Optional[bool],
-        dns_search_domains: List[str] = [],
     ):
-        new_connection = self.set_setting_ip_config(
-            new_connection, default_gateway, dns_search_domains
-        )
         new_connection = self.set_setting_ensure_permissions(new_connection)
         if self.uuid:
             old_con = self.client.get_connection_by_uuid(self.uuid)
@@ -408,28 +403,6 @@ def set_connection(
                 return
         self.add_connection(new_connection, callback)
 
-    def set_setting_ip_config(
-        self,
-        con: "NM.SimpleConnection",
-        default_gateway: Optional[bool],
-        dns_search_domains: List[str] = [],
-    ) -> "NM.SimpleConnection":
-        "Set IP config settings like default gateway and search domains."
-        _logger.debug(
-            f"setting ip config, default gateway: {default_gateway}, dns_search_domains: {dns_search_domains}"
-        )
-        ipv4_setting = con.get_setting_ip4_config()
-        ipv6_setting = con.get_setting_ip6_config()
-        if default_gateway is not None:
-            ipv4_setting.set_property("never-default", not default_gateway)
-            ipv6_setting.set_property("never-default", not default_gateway)
-        if dns_search_domains:
-            ipv4_setting.set_property("dns-search", dns_search_domains)
-            ipv6_setting.set_property("dns-search", dns_search_domains)
-        con.add_setting(ipv4_setting)
-        con.add_setting(ipv6_setting)
-        return con
-
     def set_setting_ensure_permissions(
         self, con: "NM.SimpleConnection"
     ) -> "NM.SimpleConnection":
@@ -443,11 +416,31 @@ def start_openvpn_connection(
     ) -> None:
         _logger.debug("writing ovpn configuration to Network Manager")
         new_con = self.import_ovpn(ovpn)
-        self.set_connection(new_con, callback, default_gateway, dns_search_domains)  # type: ignore
+        s_ip4 = new_con.get_setting_ip4_config()
+        s_ip6 = new_con.get_setting_ip6_config()
+
+        # avoid DNS leaks in default gateway
+        # see man nm-settings for dns-priority
+        # and https://systemd.io/RESOLVED-VPNS/
+        if default_gateway:
+            s_ip4.set_property(NM.SETTING_IP_CONFIG_DNS_PRIORITY, -2147483648)
+            s_ip6.set_property(NM.SETTING_IP_CONFIG_DNS_PRIORITY, -2147483648)
+            s_ip4.add_dns_search("~.")
+            s_ip6.add_dns_search("~.")
+        for i in dns_search_domains:
+            s_ip4.add_dns_search(i)
+            s_ip6.add_dns_search(i)
+        s_ip4.set_property("never-default", not default_gateway)
+        s_ip6.set_property("never-default", not default_gateway)
+        new_con.add_setting(s_ip4)
+        new_con.add_setting(s_ip6)
+
+        self.set_connection(new_con, callback)  # type: ignore
 
     def start_wireguard_connection(  # noqa: C901
         self,
         config: ConfigParser,
+        default_gateway,
         *,
         allow_wg_lan=False,
         callback=None,
@@ -517,10 +510,22 @@ def start_wireguard_connection(  # noqa: C901
             s_ip4.add_dns(i)
         for i in dns6:
             s_ip6.add_dns(i)
+
+        # avoid DNS leaks in default gateway
+        # see man nm-settings for dns-priority
+        # and https://systemd.io/RESOLVED-VPNS/
+        if default_gateway:
+            s_ip4.set_property(NM.SETTING_IP_CONFIG_DNS_PRIORITY, -2147483648)
+            s_ip6.set_property(NM.SETTING_IP_CONFIG_DNS_PRIORITY, -2147483648)
+            s_ip4.add_dns_search("~.")
+            s_ip6.add_dns_search("~.")
         for i in dns_hostnames:
             s_ip4.add_dns_search(i)
             s_ip6.add_dns_search(i)
 
+        s_ip4.set_property("never-default", not default_gateway)
+        s_ip6.set_property("never-default", not default_gateway)
+
         s_ip4.set_property(NM.SETTING_IP_CONFIG_METHOD, "manual")
         s_ip6.set_property(NM.SETTING_IP_CONFIG_METHOD, "manual")
 
@@ -586,7 +591,7 @@ def start_wireguard_connection(  # noqa: C901
         profile.add_setting(s_con)
         profile.add_setting(w_con)
 
-        self.set_connection(profile, callback, None)  # type: ignore
+        self.set_connection(profile, callback)  # type: ignore
 
     @run_in_glib_thread
     def activate_connection(self, callback: Optional[Callable] = None) -> None: