networkd: dhcpv4 - add notion of 'CriticalConnection'

These connections are never torn down, even when the DHCP specifications say that
they should be. This is useful/necessary when the rootfs (or another critical fs)
is mounted over this network connection, and dataloss would result if the connection
is lost.

This option defaults to off, but our initrd generator (TBD) will enable it when
applicable.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/network/networkd-link.c b/src/network/networkd-link.c
index d9622ac4b7..9b5ebeda20 100644
--- a/src/network/networkd-link.c
+++ b/src/network/networkd-link.c
@@ -443,6 +443,12 @@ static void dhcp_handler(sd_dhcp_client *client, int event, void *userdata) {
 
         if (event == DHCP_EVENT_IP_CHANGE || event == DHCP_EVENT_EXPIRED ||
             event == DHCP_EVENT_STOP) {
+                if (link->network->dhcp_critical) {
+                        log_warning_link(link, "DHCPv4 connection considered system critical, "
+                                         "ignoring request to reconfigure it down.");
+                        return;
+                }
+
                 if (link->dhcp_address) {
                         address_drop(link->dhcp_address, link, address_drop_handler);