[dns] Add support for static DNS configuration.

[oleksandrivantsiv]

Why I did it

Add support for static DNS configuration. According to sonic-net/SONiC#1262 HLD.

How I did it

Add a new resolv-config.service that is responsible for transferring configuration from Config DB into /etc/resolv.conf file that is consumed by various subsystems in Linux to resolve domain names into IP addresses.

How to verify it

• Run the image compilation. Each component related to the static DNS feature is covered with the unit tests.
• Run sonic-mgmt tests. Static DNS feature will be covered with the system tests.
• Install the image and run manual tests.

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211

Description for the changelog

Ensure to add label/tag for the feature raised. example - PR#2174 under sonic-utilities repo. where, Generic Config and Update feature has been labelled as GCU.

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[ganglyu]

Does this feature cover DNS configuration inside container? For example, "docker exec telemetry cat /etc/resolv.conf"

[oleksandrivantsiv]

Does this feature cover DNS configuration inside container? For example, "docker exec telemetry cat /etc/resolv.conf"

Yes. During the container start the docker daemon copies "/etc/resolv.conf" from the host OS rootfs to the docker container rootfs.
But if the DNS configuration is changed in the run time when the container is running this change won't be reflected inside the docker container. In this case config reload will be required.

[qiluo-msft]

config reload will impact data plane, which is not necessary for DNS config. Could you improve this PR to update DNS config inside each container?

[qiluo-msft]

-d 4

If your intention is to check if the table is empty, no need to dump.

4 is magic number, you may consider sonic-db-cli with CONFIG_DB.

@judyjoseph to check multi-asic and T2.

[qiluo-msft]

echo 'KUBELET_EXTRA_ARGS="--resolv-conf=/etc/resolv.conf --cgroup-driver=cgroupfs --node-ip=::"' | sudo tee -a $FILESYSTEM_ROOT/etc/default/kubelet

Wondering if KUBELET_EXTRA_ARGS is impacted by this new feature?

@lixiaoyuner could you help review?

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Refers to: files/build_templates/sonic_debian_extension.j2:477 in 4fd0180. [](commit_id = 4fd01802a87f2edfe5ac75651ac5e3c7dacf146e, deletion_comment = False)

[qiluo-msft]

nameserver

In some use case, people are using "/etc/resolv.conf" to manually config DNS entries. In this PR, the content will be overwrite by j2 rendering.

Could you still allow old use case?

[oleksandrivantsiv]

Those are mutually exclusive approaches. If the user wants to edit /etc/resolv.conf manually it should not configure DNS via config DB and vice-versa. The default behavior is preserved if no DNS entries are specified in the config DB.

[qiluo-msft]

DNS_NAMESERVER

Need new yang model for this new ConfigDB table.
Also need yang unit test.

[oleksandrivantsiv]

Yang model and tests are added: #13834