Subject: Re: [PATCH iproute2 2/2] ip: remove NLM_F_EXCL in case of ECMPv6 routes

From: Nicolas Dichtel <[email-masked]>
Date: Thu, 25 Oct 2012 18:48:07 +0200

Le 25/10/2012 18:25, Stephen Hemminger a écrit :
> On Thu, 25 Oct 2012 18:20:49 +0200
> Nicolas Dichtel <[email-masked]> wrote:
>
>> Le 25/10/2012 18:06, Stephen Hemminger a écrit :
>>> On Tue, 23 Oct 2012 14:42:56 +0200
>>> Nicolas Dichtel <[email-masked]> wrote:
>>>
>>>> ECMPv6 routes are added each one after the other by the kernel, so we should
>>>> avoid to set the flag NLM_F_EXCL.
>>>>
>>>> Signed-off-by: Nicolas Dichtel <[email-masked]>
>>>> ---
>>>>    ip/iproute.c | 5 ++++-
>>>>    1 file changed, 4 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/ip/iproute.c b/ip/iproute.c
>>>> index c60156f..799a70e 100644
>>>> --- a/ip/iproute.c
>>>> +++ b/ip/iproute.c
>>>> @@ -694,8 +694,11 @@ int parse_nexthops(struct nlmsghdr *n, struct rtmsg *r, int argc, char **argv)
>>>>    		rtnh = RTNH_NEXT(rtnh);
>>>>    	}
>>>>
>>>> -	if (rta->rta_len > RTA_LENGTH(0))
>>>> +	if (rta->rta_len > RTA_LENGTH(0)) {
>>>>    		addattr_l(n, 1024, RTA_MULTIPATH, RTA_DATA(rta), RTA_PAYLOAD(rta));
>>>> +		if (r->rtm_family == AF_INET6)
>>>> +			n->nlmsg_flags &= ~NLM_F_EXCL;
>>>> +	}
>>>>    	return 0;
>>>>    }
>>>>
>>>
>>> Shouldn't this be true for multipath IPv4 as well?
>>>
>> In IPv4, the message is treating in one shot, because all nexthops are added in
>> the route. In IPv6, each nexthop is added like a single route and then they are
>> linked together.
>
> So it is a fundamental design flaw in how either v4 or v6 was implemented in
> the kernel?
>
The way to manage route is just different. Maybe a patch in the kernel is more 
appropriate:

 From b4979c97f33bc41a0fa095751bfcc05de074afec Mon Sep 17 00:00:00 2001
From: Nicolas Dichtel <[email-masked]>
Date: Thu, 25 Oct 2012 18:45:47 +0200
Subject: [PATCH] ipv6/multipath: remove flag NLM_F_EXCL after the first
  nexthop

fib6_add_rt2node() will reject the nexthop if this flag is set, so
we perform the check only for the first nexthop.

Signed-off-by: Nicolas Dichtel <[email-masked]>
---
  net/ipv6/route.c | 6 ++++++
  1 file changed, 6 insertions(+)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index c42650c..9c7b5d8 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -2449,6 +2449,12 @@ beginning:
  				goto beginning;
  			}
  		}
+		/* Because each route is added like a single route we remove
+		 * this flag after the first nexthop (if there is a collision,
+		 * we have already fail to add the first nexthop:
+		 * fib6_add_rt2node() has reject it).
+		 */
+		cfg->fc_nlinfo.nlh->nlmsg_flags &= ~NLM_F_EXCL;
  		rtnh = rtnh_next(rtnh, &remaining);
  	}

-- 
1.7.12