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

[praveen-li]

This PR is merged in Linux 5.12, we can keep it as patch till we reach Linux 5.12.

https://github.com/torvalds/linux/commit/6b2e04bc240fe9be9e690059f710e9f95346d34d

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 default route is learned via routing protocol too.

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.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.

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

---

[prsunny]

@praveen-li , you can create it as a "draft" PR :)

[praveen-li]

@praveen-li , you can create it as a "draft" PR :)

@prsunny, Yeah, For next time, I will remember abt draft PRs :)

[lguohan]

you can convert to draft pr.

[praveen-li]

you can convert to draft pr.

Okies, did it. Thx.

[lguohan]

is this back port fix?

[praveen-li]

is this back port fix?

We will raise it with the Linux kernel community, if they merge, we will backport till 4.9.

Yeah this patch is applicable only till 4.9.

[lguohan]

can you add commit message in the patch?

[jarias-lfx]

/easycla