From 7dc431839eeeffe6ed65acbe9bfe2a6e89422086 Mon Sep 17 00:00:00 2001
From: Yu Watanabe <watanabe.yu+github@gmail.com>
Date: Sun, 4 Feb 2024 16:37:33 +0900
Subject: [PATCH] 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                                 | 3 +--
 src/network/networkd-dhcp4.c                            | 4 +++-
 test/test-network/conf/25-dhcp-client-ipv6-only.network | 3 +++
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/man/systemd.network.xml b/man/systemd.network.xml
index dfae8119e7..cf4456ef0c 100644
--- a/man/systemd.network.xml
+++ b/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>
diff --git a/src/network/networkd-dhcp4.c b/src/network/networkd-dhcp4.c
index f9baf0d535..b34120f997 100644
--- a/src/network/networkd-dhcp4.c
+++ b/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) {
diff --git a/test/test-network/conf/25-dhcp-client-ipv6-only.network b/test/test-network/conf/25-dhcp-client-ipv6-only.network
index 4aba206cb4..1ec63b1fa6 100644
--- a/test/test-network/conf/25-dhcp-client-ipv6-only.network
+++ b/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