Re: RTM_DELROUTE not sent anymore when deleting (last) nexthop of routes in 6.1

[Ido Schimmel <idosch@idosch.org>]

On Thu, Nov 24, 2022 at 03:40:19PM +0100, Jonas Gorski wrote:
> On Thu, 24 Nov 2022 at 15:15, Jonas Gorski <jonas.gorski@gmail.com> wrote:
> >
> > Hi Ido,
> >
> > On Thu, 24 Nov 2022 at 13:41, Ido Schimmel <idosch@idosch.org> wrote:
> > >
> > > On Thu, Nov 24, 2022 at 10:20:00AM +0100, Jonas Gorski wrote:
> > > > Hello,
> > > >
> > > > when an IPv4 route gets removed because its nexthop was deleted, the
> > > > kernel does not send a RTM_DELROUTE netlink notifications anymore in
> > > > 6.1. A bisect lead me to 61b91eb33a69 ("ipv4: Handle attempt to delete
> > > > multipath route when fib_info contains an nh reference"), and
> > > > reverting it makes it work again.
> > > >
> > >
> > > Are you running an upstream kernel?
> >
> > Okay, after having a second look, you are right, and I got myself
> > confused by IPv6 generating RTM_DELROUTE notifications, but which is
> > besides the point.
> >
> > The point where it fails is that FRR tries to delete its route(s), and
> > fails to do so with this commit applied (=> RTM_DELROUTE goes
> > missing), then does the RTM_DELNEXTHOP.
> >
> > So while there is indeed no RTM_DELROUTE generated in response to the
> > kernel, it was generated when FRR was successfully deleting its routes
> > before.
> >
> > Not sure if this already qualifies as breaking userspace though, but
> > it's definitely something that used to work with 6.0 and before, and
> > does not work anymore now.
> >
> > The error in FRR log is:
> >
> > [YXPF5-B2CE0] netlink_route_multipath_msg_encode: RTM_DELROUTE
> > 10.0.1.0/24 vrf 0(254)
> > [HYEHE-CQZ9G] nl_batch_send: netlink-dp (NS 0), batch size=44, msg cnt=1
> > [XS99C-X3KS5] netlink-dp (NS 0): error: No such process
> > type=RTM_DELROUTE(25), seq=22, pid=2419702167
> >
> > with the revert it succeeds.
> >
> > I'll see if I can get a better idea of the actual netlink message sent.
> 
> Okay, found the knob:
> 
> nlmsghdr [len=44 type=(25) DELROUTE flags=(0x0401)
> {REQUEST,(ATOMIC|CREATE)} seq=22 pid=2185212923]
>   rtmsg [family=(2) AF_INET dstlen=24 srclen=0 tos=0 table=254
> protocol=(186) UNKNOWN scope=(0) UNIVERSE type=(0) UNSPEC flags=0x0000
> {}]
>     rta [len=8 (payload=4) type=(1) DST]
>       10.0.1.0
>     rta [len=8 (payload=4) type=(6) PRIORITY]
>       20

The route is deleted with only prefix information (NH_ID not specified).
Matches this comment and the code:
https://github.com/FRRouting/frr/blob/master/zebra/rt_netlink.c#L2091

> netlink-dp (NS 0): error: No such process type=RTM_DELROUTE(25),
> seq=22, pid=2185212923
> 
> The route was created via
> 
> nlmsghdr [len=52 type=(24) NEWROUTE flags=(0x0501)
> {REQUEST,DUMP,(ROOT|REPLACE|CAPPED),(ATOMIC|CREATE)} seq=18
> pid=2185212923]
>  rtmsg [family=(2) AF_INET dstlen=24 srclen=0 tos=0 table=254
> protocol=(186) UNKNOWN scope=(0) UNIVERSE type=(1) UNICAST
> flags=0x0000 {}]
>     rta [len=8 (payload=4) type=(1) DST]
>       10.0.1.0
>     rta [len=8 (payload=4) type=(6) PRIORITY]
>        20
>      rta [len=8 (payload=4) type=(30) NH_ID]
>      18

Here the nexthop ID is obviously present.

Let me try to fix it and add a test for this flow.

Thanks for all the details!

> 
> and for completion the nexthop is created via:
> 
> nlmsghdr [len=48 type=(104) NEWNEXTHOP flags=(0x0501)
> {REQUEST,DUMP,(ROOT|REPLACE|CAPPED),(ATOMIC|CREATE)} seq=17
> pid=2185212923]
>    nhm [family=(2) AF_INET scope=(0) UNIVERSE protocol=(11) ZEBRA
> flags=0x00000000 {}]
>     rta [len=8 (payload=4) type=(1) ID]
>       18
>     rta [len=8 (payload=4) type=(6) GATEWAY]
>       10.0.0.1
>     rta [len=8 (payload=4) type=(5) OIF]
>       62
> 
> 
> Regards
> Jonas