[deployments-k8s#2412] Fix DNS

[Bolodya1997]

Description

Fixes DNS logic:

Old logic:

resolv.conf
---
nameserver a.a.a.a -> nameserver <default nameserver>
...

Corefile
---
. {
    forward . a.a.a.a
    log
    reload
}
...

This is incorrect, because:

1. User cannot both use and DNS configs passed from Endpoints, because there is no 127.0.0.1 nameserver entry in resolv.conf in such case.
2. In cmd-nsc-init + cmd-nsc case we have a.a.a.a == 127.0.0.1 at the cmd-nsc startup, so we have such files:

resolv.conf
---
nameserver 127.0.0.1
...

Corefile
---
. {
    forward . 127.0.0.1
    log
    reload
}
...

In such case we have no access to any OS default NS servers (we even cannot ping google.com) and it leads to loop DNS queries.

New logic:

resolv.conf
---
nameserver 127.0.0.1
nameserver a.a.a.a
...

Corefile
---
. {
    forward . <default nameserver> # <-- or just without this line if it is not set
    log
    reload
}
...

Issue link

Closes networkservicemesh/deployments-k8s#2412.

How Has This Been Tested?

• Added unit testing to cover
• Tested manually
• Tested by integration testing
• Have not tested

Types of changes

• Bug fix
• New functionallity
• Documentation
• Refactoring
• CI

[denis-tingaikin]

This looks wrong.

As I can see you're ignoring queries for any domains and it is totally incorrect.

[Bolodya1997]

Here are such cases:

• Do we have some nameserver entry in resolv.conf before running NSM client?
• Do we have some default name server set in NSM client options?

1. No (nameserver), no (default name server) - such container initially has no name server, we are adding name servers only for NSM resources, other resources like google.com are not accessible by domain name:

resolv.conf
---
nameserver 127.0.0.1
...

Corefile
---
. {
    log
    reload # to reload coredns on Corefile changes
}
my-coredns-service . {
    ...
}
...

2. No (nameserver), yes (default name server) - such container initially has no name server, we are adding name servers for NSM resources + default name server for other resources like google.com.

resolv.conf
---
nameserver 127.0.0.1
...

Corefile
---
. {
    forward 1.1.1.1
    log
    reload
}
my-coredns-service . {
    ...
}
...

3. Yes (nameserver), no (default name server) - such container initially has a name server, so it should still have access to google.com after all NSM configurations made.

resolv.conf
---
127.0.0.1  # <-- first tries this
10.96.0.10 # <-- after tries this
...

Corefile
---
. {
    log
    reload
}
my-coredns-service . {
    ...
}
...

4. Yes (nameserver), yes (default name server) - such container initially has a name server, but we want to additionally configure it with NSM for using another name server as default.

resolv.conf
---
127.0.0.1
10.96.0.10
...

Corefile
---
. {
    forward 1.1.1.1
    log
    reload
}
my-coredns-service . {
    ...
}
...