make TLLAO option for NA packets configurable

On Friday 02 October 2009 20:53:51 you wrote:

> This is good although I would have shortened the name.

Ah, I knew I forgot something :) Here is v4.

tavi

>From 24d96d825b9fa832b22878cc6c990d5711968734 Mon Sep 17 00:00:00 2001
From: Octavian Purdila <opurdila@ixiacom.com>
Date: Fri, 2 Oct 2009 00:51:15 +0300
Subject: [PATCH] ipv6: new sysctl for sending TLLAO with unicast NAs

Neighbor advertisements responding to unicast neighbor solicitations
did not include the target link-layer address option. This patch adds
a new sysctl option (disabled by default) which controls whether this
option should be sent even with unicast NAs.

The need for this arose because certain routers expect the TLLAO in
some situations even as a response to unicast NS packets.

Moreover, RFC 2461 recommends sending this to avoid a race condition
(section 4.4, Target link-layer address)

Signed-off-by: Cosmin Ratiu <cratiu@ixiacom.com>
Signed-off-by: Octavian Purdila <opurdila@ixiacom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1086,6 +1086,24 @@ accept_dad - INTEGER
 	2: Enable DAD, and disable IPv6 operation if MAC-based duplicate
 	   link-local address has been found.
 
+force_tllao - BOOLEAN
+	Enable sending the target link-layer address option even when
+	responding to a unicast neighbor solicitation.
+	Default: FALSE
+
+	Quoting from RFC 2461, section 4.4, Target link-layer address:
+
+	"The option MUST be included for multicast solicitations in order to
+	avoid infinite Neighbor Solicitation "recursion" when the peer node
+	does not have a cache entry to return a Neighbor Advertisements
+	message.  When responding to unicast solicitations, the option can be
+	omitted since the sender of the solicitation has the correct link-
+	layer address; otherwise it would not have be able to send the unicast
+	solicitation in the first place. However, including the link-layer
+	address in this case adds little overhead and eliminates a potential
+	race condition where the sender deletes the cached link-layer address
+	prior to receiving a response to a previous solicitation."
+
 icmp/*:
 ratelimit - INTEGER
 	Limit the maximal rates for sending ICMPv6 packets.

include/linux/ipv6.h

@@ -167,6 +167,7 @@ struct ipv6_devconf {
 #endif
 	__s32		disable_ipv6;
 	__s32		accept_dad;
+	__s32		force_tllao;
 	void		*sysctl;
 };
 

net/ipv6/addrconf.c

@@ -4352,6 +4352,14 @@ static struct addrconf_sysctl_table
 			.mode		=	0644,
 			.proc_handler	=	proc_dointvec,
 		},
+		{
+			.ctl_name       = CTL_UNNUMBERED,
+			.procname       = "force_tllao",
+			.data           = &ipv6_devconf.force_tllao,
+			.maxlen         = sizeof(int),
+			.mode           = 0644,
+			.proc_handler   = proc_dointvec
+		},
 		{
 			.ctl_name	=	0,	/* sentinel */
 		}

net/ipv6/ndisc.c

@@ -598,6 +598,7 @@ static void ndisc_send_na(struct net_device *dev, struct neighbour *neigh,
 	icmp6h.icmp6_solicited = solicited;
 	icmp6h.icmp6_override = override;
 
+	inc_opt |= ifp->idev->cnf.force_tllao;
 	__ndisc_send(dev, neigh, daddr, src_addr,
 		     &icmp6h, solicited_addr,
 		     inc_opt ? ND_OPT_TARGET_LL_ADDR : 0);