[PATCH v0 net-next 0/1] Allow user to set metric on default route learned via Router Advertisement.

[Praveen Chaudhary]

Allow user to set metric on default route learned via Router Advertisement.

Note: RFC 4191 does not say anything for metric for IPv6 default route.

Fix:
For IPv4, default route is learned via DHCPv4 and user is allowed to change
metric using config in 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.

Logs:
----------------------------------------------------------------
For IPv4:
----------------------------------------------------------------

Config in etc/network/interfaces
----------------------------------------------------------------
```
auto eth0
iface eth0 inet dhcp
    metric 4261413864
```

IPv4 Kernel Route Table:
----------------------------------------------------------------
```
$ sudo route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         172.11.44.1     0.0.0.0         UG    -33553432 0        0 eth0
```

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,
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.accept_ra_defrtr_metric=0x770003e9
```

IP monitor:
----------------------------------------------------------------
```
default via fe80::xx16:xxxx:feb3:ce8e dev eth0 proto ra metric 1996489705  pref high
```

Kernel IPv6 routing table
----------------------------------------------------------------
```
Destination                    Next Hop                   Flag Met Ref Use If
::/0                           fe80::xx16:xxxx:feb3:ce8e  UGDAe 1996489705 0
 0 eth0
```

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 IPv6 RA is received, because the default route must be fully controlled by RA msg.
```
admin@lnos-x1-a-asw03:~$ sudo sysctl -w net.ipv6.conf.eth0.accept_ra_defrtr_metric=0x770003e8
net.ipv6.conf.eth0.accept_ra_defrtr_metric = 0x770003e8

```

IP monitor: when metric is changed after learning Default Route from previous IPv6 RA msg:
```
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
```
Praveen Chaudhary (1):
  Allow user to set metric on default route learned via Router
    Advertisement.

 Documentation/networking/ip-sysctl.rst | 18 ++++++++++++++++++
 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                    | 10 ++++++++++
 net/ipv6/ndisc.c                       | 12 +++++++++---
 net/ipv6/route.c                       |  5 +++--
 8 files changed, 45 insertions(+), 6 deletions(-)


base-commit: 139711f033f636cc78b6aaf7363252241b9698ef
-- 
2.29.0