Date:   Sat, 18 Nov 2017 17:56:32 +0100
From:   Alexander Zubkov <zubkov318@...il.com>
To:     netdev@...r.kernel.org
Subject: Re: [PATCH iproute2] ip route: broken logic when using default word
 and family not specified

I also opened earlier a ticket in bugzilla:
https://bugzilla.kernel.org/show_bug.cgi?id=197899
And Stephen Hemminger had couple of comments there which I want to argue:

> $ ip route list default
> Means list all routes in any address family (ie same as any)
> but
>
> $ ip route list 0/0
> Means list all routes for IPv4 default route.

This is not correct, because first command do not show routes in any
address family. Now it do so only when table 0 is specified, otherwise
only IPv4 routes are showed. Here is the code from iproute.c:

        if (do_ipv6 == AF_UNSPEC && filter.tb)
                do_ipv6 = AF_INET;

> It probably is worth a man page warning, but changing semantics
> that have existed for many years is more likely to break some existing user.

Yes, backward compatibility is a reason. But as I remember, that
sematics already have changed earlier. Probably it was showing IPv4
and IPv6 routes together without family specified - I do not remember
exactly. And I have doubts that such feature could be lied on
reliably.
I as a end user would prefer to make the behaviour more consistent and
without such excetptions. But of course there may be other opinions.

On Sat, Nov 18, 2017 at 2:12 PM, Alexander Zubkov <zubkov318@...il.com> wrote:
> Hello,
>
> I have found odd behaviour when using "ip route list" (and other bound
> commands) with prefix "default".
>
> When family not specified, its value is completely ignored and "ip
> route list default" shows all inet4 prefixes. Same do "ip route list
> exact default" and "ip route list match default". Examples are at the
> end of the message.
>
> When family is specified, the behaviour changes and default works as
> expected (=0.0.0.0/0 for inet4 and =::/0 for inet6). The above
> commands all shows only default prefix in the output and only "root
> default" shows all prefixes.
>
> I tried to dig into the code and found that when default is using with
> unspecified family - the resulting structures filter.[mr]dst will
> actually become all-zeroes as in the case when nothing is specified.
>
> I propose to change this in such a way (see attached patch). When
> default prefix is parsed, the flag PREFIXLEN_SPECIFIED is attached to
> it too, like for prefixes with "/<masklen>". It seems logical to me,
> because "/0" is really implied by "default" and even directly set up
> in the code:
>
> dst->bitlen = 0;
>
> Then during filtering there is additional logic for unspecified family
> and specified prefix.
>
> With this patch ip route list commands shown above are working as
> expected. And it also works with unspecified table when routes are
> printed from different families.
>
> Examples after applying the patch:
>
> # ./ip route list
> default via 192.168.0.2 dev eth0
> 192.168.0.0/24 dev eth0 proto kernel scope link src 192.168.0.1
> 192.168.1.0/24 via 192.168.0.3 dev eth0
> # ./ip -6 route list
> fe80::/64 dev eth0 proto kernel metric 256 pref medium
> fe80:1::/64 via fe80::3 dev eth0 metric 1024 pref medium
> default via fe80::2 dev eth0 metric 1024 pref medium
> # ./ip route list default
> default via 192.168.0.2 dev eth0
> # ./ip route list exact default
> default via 192.168.0.2 dev eth0
> # ./ip route list match default
> default via 192.168.0.2 dev eth0
> # ./ip route list root default
> default via 192.168.0.2 dev eth0
> 192.168.0.0/24 dev eth0 proto kernel scope link src 192.168.0.1
> 192.168.1.0/24 via 192.168.0.3 dev eth0
> # ./ip route list default table all
> default via 192.168.0.2 dev eth0
> unreachable default dev lo proto kernel metric 4294967295 error -101 pref medium
> default via fe80::2 dev eth0 metric 1024 pref medium
> unreachable default dev lo proto kernel metric 4294967295 error -101 pref medium
> unreachable default dev lo proto kernel metric 4294967295 error -101 pref medium
> # ./ip route list root default table all
> default via 192.168.0.2 dev eth0
> 192.168.0.0/24 dev eth0 proto kernel scope link src 192.168.0.1
> ...
> ff00::/8 dev eth0 table local metric 256 pref medium
> unreachable default dev lo proto kernel metric 4294967295 error -101 pref medium
>
> And before patch:
>
> # ip route list default
> default via 192.168.0.2 dev eth0
> 192.168.0.0/24 dev eth0  proto kernel  scope link  src 192.168.0.1
> 192.168.1.0/24 via 192.168.0.3 dev eth0
> # ip route list match default
> default via 192.168.0.2 dev eth0
> 192.168.0.0/24 dev eth0  proto kernel  scope link  src 192.168.0.1
> 192.168.1.0/24 via 192.168.0.3 dev eth0
> # ip route list exact default
> default via 192.168.0.2 dev eth0
> 192.168.0.0/24 dev eth0  proto kernel  scope link  src 192.168.0.1
> 192.168.1.0/24 via 192.168.0.3 dev eth0
> # ip -4 route list exact default
> default via 192.168.0.2 dev eth0
> # ip -6 route list exact default
> default via fe80::2 dev eth0  metric 1024
> # ip route list exact default table all
> default via 192.168.0.2 dev eth0
> 192.168.0.0/24 dev eth0  proto kernel  scope link  src 192.168.0.1
> ...
> local fe80::3c09:19ff:feee:9866 dev lo  table local  proto none  metric 0
> ff00::/8 dev eth0  table local  metric 256
> unreachable default dev lo  table unspec  proto kernel  metric
> 4294967295  error -101

Powered by blists - more mailing lists