networkd: dhcp client should be more lenient

The RFC requires the "Server Identifier" option, however in practice
most clients that ship with modern distributions are more lenient. This
change allows that option to be missing.

src/libsystemd-network/sd-dhcp-client.c

@@ -1250,10 +1250,16 @@ static int client_handle_offer(sd_dhcp_client *client, DHCPMessage *offer, size_
         lease->next_server = offer->siaddr;
         lease->address = offer->yiaddr;
 
+        /*
+         * The RFC requires this "option", but most clients are more lenient
+         */
+        if (lease->server_address == 0) {
+                log_dhcp_client(client, "Missing server identifier option, non-conforming DHCP");
+        }
+
         if (lease->address == 0 ||
-            lease->server_address == 0 ||
             lease->lifetime == 0) {
-                log_dhcp_client(client, "received lease lacks address, server address or lease lifetime, ignoring");
+                log_dhcp_client(client, "received lease lacks address, or lease lifetime, ignoring");
                 return -ENOMSG;
         }
 
@@ -1320,11 +1326,17 @@ static int client_handle_ack(sd_dhcp_client *client, DHCPMessage *ack, size_t le
 
         lease->address = ack->yiaddr;
 
+        /*
+         * The RFC requires this "option", but most clients are more lenient
+         */
+        if (lease->server_address == 0) {
+                log_dhcp_client(client, "Missing server identifier option, non-conforming DHCP");
+        }
+
         if (lease->address == INADDR_ANY ||
-            lease->server_address == INADDR_ANY ||
             lease->lifetime == 0) {
-                log_dhcp_client(client, "received lease lacks address, server "
-                                "address or lease lifetime, ignoring");
+                log_dhcp_client(client, "received lease lacks address, "
+                                "or lease lifetime, ignoring");
                 return -ENOMSG;
         }