diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index b3345d0fe0a6..77f4de59dc9c 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -1680,6 +1680,9 @@ accept_dad - INTEGER
 	2: Enable DAD, and disable IPv6 operation if MAC-based duplicate
 	   link-local address has been found.
 
+	DAD operation and mode on a given interface will be selected according
+	to the maximum value of conf/{all,interface}/accept_dad.
+
 force_tllao - BOOLEAN
 	Enable sending the target link-layer address option even when
 	responding to a unicast neighbor solicitation.
@@ -1727,16 +1730,23 @@ suppress_frag_ndisc - INTEGER
 
 optimistic_dad - BOOLEAN
 	Whether to perform Optimistic Duplicate Address Detection (RFC 4429).
-		0: disabled (default)
-		1: enabled
+	0: disabled (default)
+	1: enabled
+
+	Optimistic Duplicate Address Detection for the interface will be enabled
+	if at least one of conf/{all,interface}/optimistic_dad is set to 1,
+	it will be disabled otherwise.
 
 use_optimistic - BOOLEAN
 	If enabled, do not classify optimistic addresses as deprecated during
 	source address selection.  Preferred addresses will still be chosen
 	before optimistic addresses, subject to other ranking in the source
 	address selection algorithm.
-		0: disabled (default)
-		1: enabled
+	0: disabled (default)
+	1: enabled
+
+	This will be enabled if at least one of
+	conf/{all,interface}/use_optimistic is set to 1, disabled otherwise.
 
 stable_secret - IPv6 address
 	This IPv6 address will be used as a secret to generate IPv6
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index d7dbcc8eda10..96861c702c06 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -1399,10 +1399,18 @@ static inline int ipv6_saddr_preferred(int type)
 	return 0;
 }
 
-static inline bool ipv6_use_optimistic_addr(struct inet6_dev *idev)
+static bool ipv6_use_optimistic_addr(struct net *net,
+				     struct inet6_dev *idev)
 {
 #ifdef CONFIG_IPV6_OPTIMISTIC_DAD
-	return idev && idev->cnf.optimistic_dad && idev->cnf.use_optimistic;
+	if (!idev)
+		return false;
+	if (!net->ipv6.devconf_all->optimistic_dad && !idev->cnf.optimistic_dad)
+		return false;
+	if (!net->ipv6.devconf_all->use_optimistic && !idev->cnf.use_optimistic)
+		return false;
+
+	return true;
 #else
 	return false;
 #endif
@@ -1472,7 +1480,7 @@ static int ipv6_get_saddr_eval(struct net *net,
 		/* Rule 3: Avoid deprecated and optimistic addresses */
 		u8 avoid = IFA_F_DEPRECATED;
 
-		if (!ipv6_use_optimistic_addr(score->ifa->idev))
+		if (!ipv6_use_optimistic_addr(net, score->ifa->idev))
 			avoid |= IFA_F_OPTIMISTIC;
 		ret = ipv6_saddr_preferred(score->addr_type) ||
 		      !(score->ifa->flags & avoid);
@@ -2460,7 +2468,8 @@ int addrconf_prefix_rcv_add_addr(struct net *net, struct net_device *dev,
 		int max_addresses = in6_dev->cnf.max_addresses;
 
 #ifdef CONFIG_IPV6_OPTIMISTIC_DAD
-		if (in6_dev->cnf.optimistic_dad &&
+		if ((net->ipv6.devconf_all->optimistic_dad ||
+		     in6_dev->cnf.optimistic_dad) &&
 		    !net->ipv6.devconf_all->forwarding && sllao)
 			addr_flags |= IFA_F_OPTIMISTIC;
 #endif
@@ -3051,7 +3060,8 @@ void addrconf_add_linklocal(struct inet6_dev *idev,
 	u32 addr_flags = flags | IFA_F_PERMANENT;
 
 #ifdef CONFIG_IPV6_OPTIMISTIC_DAD
-	if (idev->cnf.optimistic_dad &&
+	if ((dev_net(idev->dev)->ipv6.devconf_all->optimistic_dad ||
+	     idev->cnf.optimistic_dad) &&
 	    !dev_net(idev->dev)->ipv6.devconf_all->forwarding)
 		addr_flags |= IFA_F_OPTIMISTIC;
 #endif
@@ -3810,6 +3820,7 @@ static void addrconf_dad_begin(struct inet6_ifaddr *ifp)
 		goto out;
 
 	if (dev->flags&(IFF_NOARP|IFF_LOOPBACK) ||
+	    dev_net(dev)->ipv6.devconf_all->accept_dad < 1 ||
 	    idev->cnf.accept_dad < 1 ||
 	    !(ifp->flags&IFA_F_TENTATIVE) ||
 	    ifp->flags & IFA_F_NODAD) {
@@ -3841,7 +3852,7 @@ static void addrconf_dad_begin(struct inet6_ifaddr *ifp)
 	 */
 	if (ifp->flags & IFA_F_OPTIMISTIC) {
 		ip6_ins_rt(ifp->rt);
-		if (ipv6_use_optimistic_addr(idev)) {
+		if (ipv6_use_optimistic_addr(dev_net(dev), idev)) {
 			/* Because optimistic nodes can use this address,
 			 * notify listeners. If DAD fails, RTM_DELADDR is sent.
 			 */
@@ -3897,7 +3908,9 @@ static void addrconf_dad_work(struct work_struct *w)
 		action = DAD_ABORT;
 		ifp->state = INET6_IFADDR_STATE_POSTDAD;
 
-		if (idev->cnf.accept_dad > 1 && !idev->cnf.disable_ipv6 &&
+		if ((dev_net(idev->dev)->ipv6.devconf_all->accept_dad > 1 ||
+		     idev->cnf.accept_dad > 1) &&
+		    !idev->cnf.disable_ipv6 &&
 		    !(ifp->flags & IFA_F_STABLE_PRIVACY)) {
 			struct in6_addr addr;