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

[Praveen Chaudhary <praveen5582@gmail.com>]

Allow user to set metric on default route learned via Router Advertisement.
Not: 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 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 default route is learned via routing protocol too.
----------------------------------------------------------------
```
# show ip 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::be16:65ff:feb3:ce8e dev eth0 proto ra metric 1996489705  pref high
```

Kernel IPv6 routing table
----------------------------------------------------------------
```
Destination                    Next Hop                   Flag Met Ref Use If
::/0                           fe80::be16:65ff:feb3:ce8e  UGDAe 1996489705 0    
 0 eth0
```

FRR Routing Table, if default route is learned via routing protocol.
----------------------------------------------------------------
# show ipv6 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::be16:65ff:feb3:ce8e, eth0, 6d07h43m
----------------------------------------------------------------

Praveen Chaudhary (1):
  Allow user to set metric on default route learned via Router
    Advertisement.

 Documentation/networking/ip-sysctl.rst |  8 ++++++++
 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                       | 15 +++++++++++----
 net/ipv6/route.c                       |  8 +++++---
 8 files changed, 39 insertions(+), 8 deletions(-)

-- 
2.7.4