commit e6b500d56c4ffed25a49c49612039969a9a1f8f4
Author: Ryota Ozaki <ozaki-r@iij.ad.jp>
Date:   Mon Jun 30 13:04:10 2014 +0900

    Stop using callout randomly
    
    nd6_dad_start uses callout when xtick > 0 while doesn't when
    xtick == 0. So if we pass a random value ranging from 0 to N,
    nd6_dad_start uses callout randomly. This behavior makes
    debugging difficult.

diff --git a/sys/netinet6/in6.c b/sys/netinet6/in6.c
index 9397bfe..2302274 100644
--- a/sys/netinet6/in6.c
+++ b/sys/netinet6/in6.c
@@ -1333,7 +1333,8 @@ in6_update_ifa1(struct ifnet *ifp, struct in6_aliasreq *ifra,
 				    mindelay;
 			}
 		}
-		nd6_dad_start(&ia->ia_ifa, dad_delay);
+		/* +1 ensures callout is always used */
+		nd6_dad_start(&ia->ia_ifa, dad_delay + 1);
 	}
 
 	return error;
@@ -2157,15 +2158,17 @@ in6_if_link_up(struct ifnet *ifp)
 		}
 
 		if (ia->ia6_flags & IN6_IFF_TENTATIVE) {
+			int delay;
 			/*
 			 * The TENTATIVE flag was likely set by hand
 			 * beforehand, implicitly indicating the need for DAD.
 			 * We may be able to skip the random delay in this
 			 * case, but we impose delays just in case.
 			 */
-			nd6_dad_start(ifa,
-			    cprng_fast32() %
-				(MAX_RTR_SOLICITATION_DELAY * hz));
+			delay = cprng_fast32() %
+			    (MAX_RTR_SOLICITATION_DELAY * hz);
+			/* +1 ensures callout is always used */
+			nd6_dad_start(ifa, delay + 1);
 		}
 	}
 
diff --git a/sys/netinet6/nd6_nbr.c b/sys/netinet6/nd6_nbr.c
index f00b218..0b7a086 100644
--- a/sys/netinet6/nd6_nbr.c
+++ b/sys/netinet6/nd6_nbr.c
@@ -1097,6 +1097,8 @@ nd6_newaddrmsg(struct ifaddr *ifa)
 /*
  * Start Duplicate Address Detection (DAD) for specified interface address.
  *
+ * Note that callout is used when xtick > 0 and not when xtick == 0.
+ *
  * xtick: minimum delay ticks for IFF_UP event
  */
 void