Only bring down the interface at startup time if it is not already up and

running.  ndhc does its job entirely with raw sockets before a lease is
established, so it will work just fine, and this change allows for the dhcp
client to be re-run without causing downtime on an interface.

ndhc/ndhc.c

@@ -304,7 +304,8 @@ int main(int argc, char **argv)
             "cap_net_bind_service,cap_net_broadcast,cap_net_raw=ep");
     drop_root(uid, gid);
 
-    ifchange(NULL, IFCHANGE_DECONFIG);
+    if (cs.ifsPrevState != IFS_UP)
+        ifchange(NULL, IFCHANGE_DECONFIG);
 
     do_work();
     return EXIT_SUCCESS; // Never reached.

ndhc/netlink.c

@@ -40,14 +40,6 @@
 #include "arp.h"
 #include "log.h"
 
-enum {
-    IFS_NONE = 0,
-    IFS_UP,
-    IFS_DOWN,
-    IFS_SHUT,
-    IFS_REMOVED
-};
-
 static struct mnl_socket *mls;
 
 static void nl_close(struct client_state_t *cs)

ndhc/netlink.h

@@ -22,6 +22,14 @@
 
 #include "state.h"
 
+enum {
+    IFS_NONE = 0,
+    IFS_UP,
+    IFS_DOWN,
+    IFS_SHUT,
+    IFS_REMOVED
+};
+
 int nl_open(struct client_state_t *cs);
 void handle_nl_message(struct client_state_t *cs);
 int nl_getifdata(const char *ifname, struct client_state_t *cs);