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  1. Jan 28, 2021
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    • Praveen Chaudhary's avatar
      net: allow user to set metric on default route learned via Router Advertisement · 6b2e04bc
      Praveen Chaudhary authored
      
      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 patch adds sysctl for the same.
      
      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
      
      Signed-off-by: default avatarPraveen Chaudhary <pchaudhary@linkedin.com>
      Signed-off-by: default avatarZhenggen Xu <zxu@linkedin.com>
      Reviewed-by: default avatarDavid Ahern <dsahern@kernel.org>
      Link: https://lore.kernel.org/r/20210125214430.24079-1-pchaudhary@linkedin.com
      
      
      Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
      6b2e04bc
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