network/dhcp4: disable IPv6OnlyMode= by default

As explained in #30891, IPv6OnlyMode= should be enabled with 464XLAT
support, but we do not support it yet. Let's disable by default.

Fixes #30891.

man/systemd.network.xml

@@ -2683,8 +2683,7 @@ NFTSet=prefix:netdev:filter:eth_ipv4_prefix</programlisting>
           <para>When true, the DHCPv4 configuration will be delayed by the timespan provided by the DHCP
           server and skip to configure dynamic IPv4 network connectivity if IPv6 connectivity is provided
           within the timespan. See <ulink url="https://tools.ietf.org/html/rfc8925">RFC 8925</ulink>.
-          Defaults to true when <varname>IPv6AcceptRA=</varname> is enabled or DHCPv6 client is enabled
-          (i.e., <varname>DHCP=yes</varname>), and false otherwise.</para>
+          Defaults to false.</para>
 
           <xi:include href="version-info.xml" xpointer="v255"/>
         </listitem>

src/network/networkd-dhcp4.c

@@ -1455,10 +1455,12 @@ static bool link_dhcp4_ipv6_only_mode(Link *link) {
         assert(link);
         assert(link->network);
 
+        /* If it is explicitly specified, then honor the setting. */
         if (link->network->dhcp_ipv6_only_mode >= 0)
                 return link->network->dhcp_ipv6_only_mode;
 
-        return link_dhcp6_enabled(link) || link_ipv6_accept_ra_enabled(link);
+        /* Defaults to false, until we support 464XLAT. See issue #30891. */
+        return false;
 }
 
 static int dhcp4_configure(Link *link) {

test/test-network/conf/25-dhcp-client-ipv6-only.network

@@ -7,6 +7,9 @@ Name=veth99
 DHCP=yes
 IPv6Token=::1a:2b:3c:4d
 
+[DHCPv4]
+IPv6OnlyMode=yes
+
 [Route]
 Gateway=_ipv6ra
 Destination=2001:1234:5:9fff:ff:ff:ff:ff/128