dhcpcd-discuss

Hi Mike

On 07/07/2020 14:26, Mike Crowe wrote:
> I'm having some trouble with a particular configuration on an embedded
> Linux device with a v4.9 kernel after upgrading to dhcpcd-9.1.4. This
> configuration is unusual, but unfortunately one that I need to deal with.
>
> I have three interfaces all managed by dhcpcd with all options passed on
> the command line (I don't believe that it's significant that the interfaces
> represent VLANs):
>
> * wlan0: Receives a real IP address and default route via DHCP. This is the
>    interface I expect my Internet-packets to go out through.
> * eth0.6: Has link but nothing else is on the VLAN so the interface
>    receives a link-local IPv4 address after failing to receive a DHCP
>    response.
> * eth0.15: As eth0.6.
>
> When using dhcpcd-6.11.1 the kernel routing table ends up containing:
>
>   default via 192.168.147.1 dev wlan0 src 192.168.147.207 metric 306
>   169.254.0.0/16 dev eth0.6 proto kernel scope link src 169.254.138.7 metric 204
>   169.254.0.0/16 dev eth0.15 proto kernel scope link src 169.254.138.7 metric 205
>   192.168.147.0/24 dev wlan0 proto kernel scope link src 192.168.147.207 metric 306
>
> which works correctly and Internet-bound packets go out via wlan0.
>
> With dhcpcd-9.1.2 and dhcpcd-9.1.4 the kernel routing table looks like:
>
>   default dev eth0.6 scope link src 169.254.138.7 metric 204
>   default dev eth0.15 scope link src 169.254.138.7 metric 205
>   default via 192.168.147.1 dev wlan0 proto dhcp src 192.168.147.207 metric 306
>   169.254.0.0/16 dev eth0.6 scope link src 169.254.138.7 metric 204
>   169.254.0.0/16 dev eth0.15 scope link src 169.254.138.7 metric 205
>   192.168.147.0/24 dev wlan0 proto dhcp scope link src 192.168.147.207 metric 306
>
> and this means that the kernel unfortunately wants to send Internet-bound
> packets out via one of the VLAN interfaces:
>
>   # traceroute -n roy.marples.name
>   traceroute to roy.marples.name (81.108.15.115), 30 hops max, 46 byte packets
>    1  169.254.138.7  3152.914 ms !H  3119.855 ms !H  3119.976 ms !H
>
> If I delete the eth0.6 and eth0.15 routes:
>
>   # ip route del default dev eth0.6
>   # ip route del default dev eth0.15
>
> then routing seems to be correct again as packets go out via wlan0.
>
> I notice that the IPv6 routing table does not contain any equivalent
> routes, and I can connect successfully to IPv6 hosts out on the Internet,
> so this problem relates only to IPv4.
>
> So, the question now is whether this is the kernel's fault for attempting
> to use the VLAN default routes despite them having link scope or dhcpcd's
> fault for adding them? Any enlightenment gratefully received.

This is dhcpcd's fault.
I've just added a patch to bump the route metric up for IPv4LL routes and ensure 
it's evaluated last on other OS's.

https://roy.marples.name/cgit/dhcpcd.git/commit/?id=b7ede4d38fab12fd093dcb51416d83764de1557e

Does this fix it for you?

Roy