netdev - Re: [RFC PATCH] xfrm: Add option to reset oif in xfrm lookup

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Message-ID: <3c59303e-f393-ab2f-733e-0769a615e685@cumulusnetworks.com>
Date:	Wed, 3 Aug 2016 22:52:49 -0600
From:	David Ahern <dsa@...ulusnetworks.com>
To:	subashab@...eaurora.org
Cc:	steffen.klassert@...unet.com, netdev@...r.kernel.org,
	herbert@...dor.apana.org.au, netdev-owner@...r.kernel.org
Subject: Re: [RFC PATCH] xfrm: Add option to reset oif in xfrm lookup

On 8/3/16 5:02 PM, subashab@...eaurora.org wrote:
>> I can't explain the iptables output but from a FIB lookup perspective
>> it is using table 8 per the FIB rules, the xfrm is hit and packets
>> shift to 192.168.77.1 and go out what you have as eth0.
>>
>> Take a look at:
>>   perf record -e fib:* -a -g
>>   perf script
>>
>> And then run tcpdump on both eth0 and eth1. For me on "eth0" (which is
>> really eth11 for my VM setup) I see this on the ping:
>>
> 
> You can try running these commands as is on UML.
> We tried these out on 3.18 as well as on 4.4.
> 
>> 20:50:11.389837 ARP, Request who-has 192.168.77.2 tell 192.168.77.1, length 28
>> 20:50:11.390079 ARP, Reply 192.168.77.2 is-at 02:00:12:34:02:0a, length 28
>> 20:50:11.390101 IP 192.168.77.1 > 192.168.77.2: ICMP 192.168.77.1 udp
>> port 4500 unreachable, length 168
>>
>> So the packets are going out "eth0" as expected.
>>
>> That said, the commands you have given do not totally transfer to
>> another setup. In my case I have 2 VMs with eth11 and eth12 directly
>> connected (VM1 eth11 <--> VM2 eth11 and ditto for eth12). You have
>> given one side of the commands and I have configured the other side
>> with the .1 addresses but not bothered to translate the xfrm commands.
>>
>> That said, this seems like a contrived example -- you pin ping to
>> device eth1 (-I eth1), you are pinging a host on the network for eth1
>> but want packets to go out eth0 via the xfrm. Can you elaborate on the
>> real use case and problem here?
> 
> Applications may be bound to a specific interface but would try to send data over multiple types of networks.
> Our use case here is wifi calling. In this case, we try to force packets to go over wifi after encryption.
> The rules which we were using worked on 3.18 but we ran into issues on 4.4.
> Debugging narrowed us down to this oif preservation through xfrm.

I need to do some additional testing next week (taking PTO the next 2 days), but this should fix your problem. Can you confirm? This is better than a sysctl to handle the known use cases, but it does not handle a combination of the 2 known use cases (e.g., throw your use case into a VRF).

diff --git a/net/ipv4/xfrm4_policy.c b/net/ipv4/xfrm4_policy.c
index b644a23c3db0..41f5b504a782 100644
--- a/net/ipv4/xfrm4_policy.c
+++ b/net/ipv4/xfrm4_policy.c
@@ -29,7 +29,7 @@ static struct dst_entry *__xfrm4_dst_lookup(struct net *net, struct flowi4 *fl4,
        memset(fl4, 0, sizeof(*fl4));
        fl4->daddr = daddr->a4;
        fl4->flowi4_tos = tos;
-       fl4->flowi4_oif = oif;
+       fl4->flowi4_oif = l3mdev_master_ifindex_by_index(net, oif);
        if (saddr)
                fl4->saddr = saddr->a4;