Make multiple Netbird client instances cooperate on the ip route table level on Linux (possibly other OS too?)

[nazarewk]

Is your feature request related to a problem? Please describe.
While debugging issue with missing routes (turned out to work just fine, but uses additional table I was not aware of) and digging into the setup I have discovered my multiple Netbird instances at some point started interfering with each other's ip route tables.

Basically the last instance started seem to take over the 7120 aka netbird route table deleting all entries not owned by it, seems to happen here:

netbird/client/internal/routemanager/manager.go

Lines 76 to 83 in 50201d6

	func (m *DefaultManager) Init() (peer.BeforeAddPeerHookFunc, peer.AfterRemovePeerHookFunc, error) {
	if nbnet.CustomRoutingDisabled() {
	return nil, nil, nil
	}
	if err := cleanupRouting(); err != nil {
	log.Warnf("Failed cleaning up routing: %v", err)
	}

Describe the solution you'd like
either of:

1. The simple fix is to make those 2 values configurable:
   

   netbird/client/internal/routemanager/systemops_linux.go

   Lines 26 to 29 in 50201d6

   	// NetbirdVPNTableID is the ID of the custom routing table used by Netbird.
   	NetbirdVPNTableID = 0x1BD0
   	// NetbirdVPNTableName is the name of the custom routing table used by Netbird.
   	NetbirdVPNTableName = "netbird"

2. The longer fix would be to use the same routing table, but don't interfere with network interfaces owned by other instances (having other interfce names)

Describe alternatives you've considered
see above

Additional context
I am running 2 Netbird instances (private and work-related) at the same time on NixOS (Linux) through NixOS/nixpkgs#287236

[mlsmaycon]

Thanks for the request @nazarewk, we will discuss it and give you an feedback soon