Add CustomDialer option for LeafNodes

[chezgi]

custom dialer is needed when we want to do custom operations like fwmark of connection packets.

Signed-off-by: abbas ali chezgi [email protected]

[neilalexander]

At the very least this needs tests, but also wondering more about this use-case. This must be an embedded server?

[chezgi]

Its use case is for embedded mode.
some situations that this is used:

• when used for control connection of network elements like routers, it must be able to bind to specific device for using VRF in linux
• when used for control connection of vpn elements, it must be able to use fwmark for its outgoing packets, to not pass inside tunnel.

[lschmierer]

I like the effort, we have a similar use case.
A few notes though:

• It does not handle the timeout. It might be worthwhile to rewrite the natsDialTimeout function to call DialContext with a context.WithTimeout and let a custom DialContext function be passed to that.
• If you want to capture all traffic (including DNS) I think you would to also provide your own resolver LookupHost function. Using DNS host names within a user-space VPN would require this too