network: always enable sysctl property promote_secondaries

systemd-sysctl already enables promote_secondaries for all interface.
So, networkd also enables it unconditionally.

1 files changed, 9 insertions, 0 deletions

diff --git a/src/network/networkd-sysctl.c b/src/network/networkd-sysctl.c
index b6839f86ee..dc95d01934 100644
--- a/src/network/networkd-sysctl.c
+++ b/src/network/networkd-sysctl.c
@@ -212,6 +212,15 @@ int link_set_sysctl(Link *link) {
         if (r < 0)
                 log_link_warning_errno(link, r, "Cannot set IPv4 accept_local flag for interface, ignoring: %m");
 
+        /* If promote_secondaries is not set, DHCP will work only as long as the IP address does not
+         * changes between leases. The kernel will remove all secondary IP addresses of an interface
+         * otherwise. The way systemd-networkd works is that the new IP of a lease is added as a
+         * secondary IP and when the primary one expires it relies on the kernel to promote the
+         * secondary IP. See also https://github.com/systemd/systemd/issues/7163 */
+        r = sysctl_write_ip_property_boolean(AF_INET, link->ifname, "promote_secondaries", true);
+        if (r < 0)
+                log_link_warning_errno(link, r, "Cannot enable promote_secondaries for interface, ignoring: %m");
+
         return 0;
 }