netdev - Re: v6/sit tunnels and VRFs

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Date:   Fri, 13 Apr 2018 16:23:19 -0400
From:   Jeff Barnhill <0xeffeff@...il.com>
To:     David Ahern <dsahern@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: v6/sit tunnels and VRFs

Thanks for the response, David. I'm not questioning the need to stop
the fib lookup once the end of the VRF table is reached - I agree that
is needed.  I'm concerned with the difference in the response/error
returned from the failed lookup.

For instance, with vrf "unreachable default" route, I get this:
# ip route get 1.1.1.1 vrf vrf_258
RTNETLINK answers: No route to host

Without it (and assuming no match for 1.1.1.1 in local/main/default
tables), I get this:
# ip route get 1.1.1.1 vrf vrf_258
RTNETLINK answers: Network is unreachable

Which is also what happens when not using VRFs at all.

It seems that the ENETUNREACH response is still desirable in the VRF
case since the only difference (when using VRF vs. not) is that the
lookup should be restrained to a specific VRF.

Jeff



On Thu, Apr 12, 2018 at 10:25 PM, David Ahern <dsahern@...il.com> wrote:
> On 4/12/18 10:54 AM, Jeff Barnhill wrote:
>> Hi David,
>>
>> In the slides referenced, you recommend adding an "unreachable
>> default" route to the end of each VRF route table.  In my testing (for
>> v4) this results in a change to fib lookup failures such that instead
>> of ENETUNREACH being returned, EHOSTUNREACH is returned since the fib
>> finds the unreachable route, versus failing to find a route
>> altogether.
>>
>> Have the implications of this been considered?  I don't see a
>> clean/easy way to achieve the old behavior without affecting non-VRF
>> routing (eg. remove the unreachable route and delete the non-VRF
>> rules).  I'm guessing that programmatically, it may not make much
>> difference, ie. lookup fails, but for debugging or to a user looking
>> at it, the difference matters.  Do you (or anyone else) have any
>> thoughts on this?
>
> We have recommended moving the local table down in the FIB rules:
>
> # ip ru ls
> 1000:   from all lookup [l3mdev-table]
> 32765:  from all lookup local
> 32766:  from all lookup main
> 32767:  from all lookup default
>
> and adding a default route to VRF tables:
>
> # ip ro ls vrf red
> unreachable default  metric 4278198272
> 172.16.2.0/24  proto bgp  metric 20
>         nexthop via 169.254.0.1  dev swp3 weight 1 onlink
>         nexthop via 169.254.0.1  dev swp4 weight 1 onlink
>
> # ip -6 ro ls vrf red
> 2001:db8:2::/64  proto bgp  metric 20
>         nexthop via fe80::202:ff:fe00:e  dev swp3 weight 1
>         nexthop via fe80::202:ff:fe00:f  dev swp4 weight 1
> anycast fe80:: dev lo  proto kernel  metric 0  pref medium
> anycast fe80:: dev lo  proto kernel  metric 0  pref medium
> fe80::/64 dev swp3  proto kernel  metric 256  pref medium
> fe80::/64 dev swp4  proto kernel  metric 256  pref medium
> ff00::/8 dev swp3  metric 256  pref medium
> ff00::/8 dev swp4  metric 256  pref medium
> unreachable default dev lo  metric 4278198272  error -101 pref medium
>
> Over the last 2 years we have not seen any negative side effects to this
> and is what you want to have proper VRF separation.
>
> Without a default route lookups will proceed to the next fib rule which
> means a lookup in the next table and barring other PBR rules will be the
> main table. It will lead to wrong lookups.
>
> Here is an example:
>   ip netns add foo
>   ip netns exec foo bash
>   ip li set lo up
>   ip li add red type vrf table 123
>   ip li set red up
>   ip li add dummy1 type dummy
>   ip addr add 10.100.1.1/24 dev dummy1
>   ip li set dummy1 master red
>   ip li set dummy1 up
>   ip li add dummy2 type dummy
>   ip addr add 10.100.1.1/24 dev dummy2
>   ip li set dummy2 up
>   ip ro get 10.100.2.2
>   ip ro get 10.100.2.2 vrf red
>
> # ip ru ls
> 0:      from all lookup local
> 1000:   from all lookup [l3mdev-table]
> 32766:  from all lookup main
> 32767:  from all lookup default
>
> # ip ro ls
> 10.100.1.0/24 dev dummy2 proto kernel scope link src 10.100.1.1
> 10.100.2.0/24 via 10.100.1.2 dev dummy2
>
> # ip ro ls vrf red
> 10.100.1.0/24 dev dummy1 proto kernel scope link src 10.100.1.1
>
> That's the setup. What happens on route lookups?
> # ip ro get vrf red 10.100.2.1
> 10.100.2.1 via 10.100.1.2 dev dummy2 src 10.100.1.1 uid 0
>     cache
>
> which is clearly wrong. Let's look at the lookup sequence
>
> # perf record -e fib:* ip ro get vrf red 10.100.2.1
> 10.100.2.1 via 10.100.1.2 dev dummy2 src 10.100.1.1 uid 0
>     cache
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.003 MB perf.data (4 samples) ]
>
> #  perf script --fields trace:trace
> table 255 oif 2 iif 1 src 0.0.0.0 dst 10.100.2.1 tos 0 scope 0 flags 4
> table 123 oif 2 iif 1 src 0.0.0.0 dst 10.100.2.1 tos 0 scope 0 flags 4
> table 254 oif 2 iif 1 src 0.0.0.0 dst 10.100.2.1 tos 0 scope 0 flags 4
> nexthop dev dummy2 oif 4 src 10.100.1.1
>
> The first one is because I did not move the local table down.
> The second one is the correct vrf lookup
> The third one is the continuation to the next table - the main table.
>
> Adding a default route:
> # ip ro add vrf red unreachable default
>
> And the lookup is proper:
> # ip ro get vrf red 10.100.2.1
> RTNETLINK answers: No route to host