+static bool promote_secondaries_enabled(const char *ifname) {
+ _cleanup_free_ char *promote_secondaries_sysctl = NULL;
+ char *promote_secondaries_path;
+ int r;
+
+ promote_secondaries_path = strjoina("net/ipv4/conf/", ifname, "/promote_secondaries");
+ r = sysctl_read(promote_secondaries_path, &promote_secondaries_sysctl);
+ if (r < 0) {
+ log_debug_errno(r, "Cannot read sysctl %s", promote_secondaries_path);
+ return false;
+ }
+
+ truncate_nl(promote_secondaries_sysctl);
+ r = parse_boolean(promote_secondaries_sysctl);
+ if (r < 0)
+ log_warning_errno(r, "Cannot parse sysctl %s with content %s as boolean", promote_secondaries_path, promote_secondaries_sysctl);
+ return r > 0;
+}
+
+/* dhcp4_set_promote_secondaries will ensure this interface has
+ * the "promote_secondaries" option in the kernel set. If this sysctl
+ * 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-network 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
+ */
+int dhcp4_set_promote_secondaries(Link *link) {
+ int r;
+
+ assert(link);
+ assert(link->network);
+ assert(link->network->dhcp & ADDRESS_FAMILY_IPV4);
+
+ /* check if the kernel has promote_secondaries enabled for our
+ * interface. If it is not globally enabled or enabled for the
+ * specific interface we must either enable it.
+ */
+ if (!(promote_secondaries_enabled("all") || promote_secondaries_enabled(link->ifname))) {
+ char *promote_secondaries_path = NULL;
+
+ log_link_debug(link, "promote_secondaries is unset, setting it");
+ promote_secondaries_path = strjoina("net/ipv4/conf/", link->ifname, "/promote_secondaries");
+ r = sysctl_write(promote_secondaries_path, "1");
+ if (r < 0)
+ log_link_warning_errno(link, r, "cannot set sysctl %s to 1", promote_secondaries_path);
+ return r > 0;
+ }
+
+ return 0;
+}
+