linux-kernel - Re: [PATCH v4 net-next 1/1] Allow user to set metric on default route learned via Router Advertisement.

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Date:   Tue, 26 Jan 2021 18:43:00 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     David Ahern <dsahern@...il.com>,
        Praveen Chaudhary <praveen5582@...il.com>
Cc:     davem@...emloft.net, corbet@....net, kuznet@....inr.ac.ru,
        yoshfuji@...ux-ipv6.org, netdev@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Zhenggen Xu <zxu@...kedin.com>,
        David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH v4 net-next 1/1] Allow user to set metric on default
 route learned via Router Advertisement.

On Mon, 25 Jan 2021 20:34:56 -0700 David Ahern wrote:
> On 1/25/21 2:44 PM, Praveen Chaudhary wrote:
> > For IPv4, default route is learned via DHCPv4 and user is allowed to change
> > metric using config etc/network/interfaces. But for IPv6, default route can
> > be learned via RA, for which, currently a fixed metric value 1024 is used.
> > 
> > Ideally, user should be able to configure metric on default route for IPv6
> > similar to IPv4. This fix adds sysctl for the same.
> > 
> > Signed-off-by: Praveen Chaudhary <pchaudhary@...kedin.com>
> > Signed-off-by: Zhenggen Xu <zxu@...kedin.com>
> > 
> > Reviewed-by: David Ahern <dsahern@...nel.org>
> > 
> > Changes in v1.
> > 1.) Correct the call to rt6_add_dflt_router.
> > 
> > Changes in v2.
> > 1.) Replace accept_ra_defrtr_metric to ra_defrtr_metric.
> > 2.) Change Type to __u32 instead of __s32.
> > 3.) Change description in Documentation/networking/ip-sysctl.rst.
> > 4.) Use proc_douintvec instead of proc_dointvec.
> > 5.) Code style in ndisc_router_discovery().
> > 6.) Change Type to u32 instead of unsigned int.
> > 
> > Changes in v3:
> > 1.) Removed '---' and '```' from description.
> > 2.) Remove stray ' after accept_ra_defrtr.
> > 3.) Fix tab in net/ipv6/addrconf.c.
> > 
> > Changes in v4:
> > 1.) Remove special case of 0 and use IP6_RT_PRIO_USER as default.
> > 2.) Do not allow 0.
> > 3.) Change Documentation accordingly.
> > 4.) Remove extra brackets and compare with zero in ndisc_router_discovery().
> > 5.) Remove compare with zero in rt6_add_dflt_router().
> > 
> > Logs:
> > 
> > For IPv4:
> > 
> > Config in etc/network/interfaces:
> > auto eth0
> > iface eth0 inet dhcp
> >     metric 4261413864
> > 
> > IPv4 Kernel Route Table:
> > $ ip route list
> > default via 172.21.47.1 dev eth0 metric 4261413864
> > 
> > FRR Table, if a static route is configured:
> > [In real scenario, it is useful to prefer BGP learned default route over DHCPv4 default route.]
> > Codes: K - kernel route, C - connected, S - static, R - RIP,
> >        O - OSPF, I - IS-IS, B - BGP, P - PIM, E - EIGRP, N - NHRP,
> >        T - Table, v - VNC, V - VNC-Direct, A - Babel, D - SHARP,  
> >        > - selected route, * - FIB route  
> >   
> > S>* 0.0.0.0/0 [20/0] is directly connected, eth0, 00:00:03  
> > K   0.0.0.0/0 [254/1000] via 172.21.47.1, eth0, 6d08h51m
> > 
> > i.e. User can prefer Default Router learned via Routing Protocol in IPv4.
> > Similar behavior is not possible for IPv6, without this fix.
> > 
> > After fix [for IPv6]:
> > sudo sysctl -w net.ipv6.conf.eth0.net.ipv6.conf.eth0.ra_defrtr_metric=1996489705
> > 
> > IP monitor: [When IPv6 RA is received]
> > default via fe80::xx16:xxxx:feb3:ce8e dev eth0 proto ra metric 1996489705  pref high
> > 
> > Kernel IPv6 routing table
> > $ ip -6 route list
> > default via fe80::be16:65ff:feb3:ce8e dev eth0 proto ra metric 1996489705 expires 21sec hoplimit 64 pref high
> > 
> > FRR Table, if a static route is configured:
> > [In real scenario, it is useful to prefer BGP learned default route over IPv6 RA default route.]
> > Codes: K - kernel route, C - connected, S - static, R - RIPng,
> >        O - OSPFv3, I - IS-IS, B - BGP, N - NHRP, T - Table,
> >        v - VNC, V - VNC-Direct, A - Babel, D - SHARP,  
> >        > - selected route, * - FIB route  
> >   
> > S>* ::/0 [20/0] is directly connected, eth0, 00:00:06  
> > K   ::/0 [119/1001] via fe80::xx16:xxxx:feb3:ce8e, eth0, 6d07h43m
> > 
> > If the metric is changed later, the effect will be seen only when next IPv6
> > RA is received, because the default route must be fully controlled by RA msg.
> > Below metric is changed from 1996489705 to 1996489704.
> > 
> > $ sudo sysctl -w net.ipv6.conf.eth0.ra_defrtr_metric=1996489704
> > net.ipv6.conf.eth0.ra_defrtr_metric = 1996489704
> > 
> > IP monitor:
> > [On next IPv6 RA msg, Kernel deletes prev route and installs new route with updated metric]
> > 
> > Deleted default via fe80::xx16:xxxx:feb3:ce8e dev eth0 proto ra metric 1996489705  expires 3sec hoplimit 64 pref high
> > default via fe80::xx16:xxxx:feb3:ce8e dev eth0 proto ra metric 1996489704  pref high
> > ---
> >  Documentation/networking/ip-sysctl.rst | 10 ++++++++++
> >  include/linux/ipv6.h                   |  1 +
> >  include/net/ip6_route.h                |  3 ++-
> >  include/uapi/linux/ipv6.h              |  1 +
> >  include/uapi/linux/sysctl.h            |  1 +
> >  net/ipv6/addrconf.c                    | 11 +++++++++++
> >  net/ipv6/ndisc.c                       | 12 ++++++++----
> >  net/ipv6/route.c                       |  5 +++--
> >  8 files changed, 37 insertions(+), 7 deletions(-)
> >   
> 
> Reviewed-by: David Ahern <dsahern@...nel.org>

Did my best to untangle the commit message and applied.

Thanks!