networkd: avoid managing MAC addresses for veth devices #33
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When a veth device is created, the CNI in charge of bringing the device up will set a MAC address, if `MACAddressPolicy=permanent` is set, systemd will change it to a different one, causing dropped packets due to mismatches. With this change, the address set when the device is created will remain untouched by systemd. See flatcar/Flatcar#278 for more information.
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This change adds a default network configuration .link file that `systemd-udev` will use when configuring new interfaces. It contains the default list of policies that are used when naming interfaces, as well as the policy by which the MAC address should be set. Bottlerocket packages its own version of this file rather than the default from systemd for a few reasons. 1) Bottlerocket does not create/use a udev hwdb (we disable the option in systemd compile flags), so we remove this option from the NamePolicy list, 2) CNI plugins can be confused when MAC addresses change for virtual interfaces, so Bottlerocket sets the default MACAddress Policy to "none" which directs systemd not to attempt to manage the MAC. Hardware usually has a MAC, and veth devices used by CNI generally get a MAC generated by the plugin. Additional information about the MAC address issue: systemd/systemd#3374 (comment) flatcar/Flatcar#278 flatcar/init#33
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When a veth device is created, the CNI in charge of bringing the device up will set a MAC address. If
MACAddressPolicy=permanentis set, systemd will change it to a different one, causing dropped packets due to mismatches.With this change, the address set when the device is created will remain untouched by systemd.
See flatcar/Flatcar#278 for more information.
Testing done
I've tested this manually, by creating the equivalent /etc file. I haven't tested it using a generated image. We currently don't have any kola tests that catch this, because we aren't testing any CNIs that use this. We already have a rule in place for flannel, the only one that we do test.