From the man pages
-S, --local, --server=[/[]/[domain/]][[#][@|[#]]
The optional string after the @ character tells dnsmasq how to set the source of the queries to this nameserver. It can either be an ip-address, an interface name or both. Forcing queries to an interface is not implemented on all platforms supported by dnsmasq.
on my dhcp file, I have added the below lines to the dns section.
option server '209.222.18.222@tun0'
option server '209.222.18.218@tun0'
It does not seem to be working. Is the feature supported on LEDE?
Thanks