upstream

type Opt struct {
	// DialAddr specifies the address the upstream will
	// actually dial to.
	DialAddr string

	// Socks5 specifies the socks5 proxy server that the upstream
	// will connect though.
	// Not implemented for udp upstreams and doh upstreams with http/3.
	Socks5 string

	// SoMark specifies the mark for each packet sent through this upstream.
	SoMark int

	// Interface specifies the system network interface for this upstream to send
	// data.
	Interface string

	// IdleTimeout specifies the idle timeout for long-connections.
	// Available for TCP, DoT, DoH.
	// If negative, TCP, DoT will not reuse connections.
	// Default: TCP, DoT: 10s , DoH: 30s.
	IdleTimeout time.Duration

	// EnablePipeline enables query pipelining support as RFC 7766 6.2.1.1 suggested.
	// Available for TCP, DoT upstream with IdleTimeout >= 0.
	EnablePipeline bool

	// EnableHTTP3 enables HTTP/3 protocol for DoH upstream.
	EnableHTTP3 bool

	// MaxConns limits the total number of connections, including connections
	// in the dialing states.
	// Implemented for TCP/DoT pipeline enabled upstreams and DoH upstreams.
	// Default is 1.
	MaxConns int

	// TLSConfig specifies the tls.Config that the TLS client will use.
	// Available for DoT, DoH upstreams.
	TLSConfig *tls.Config

	// Logger specifies the logger that the upstream will use.
	Logger *zap.Logger
}

type Upstream interface {
	// ExchangeContext exchanges query message m to the upstream, and returns
	// response. It MUST NOT keep or modify m.
	ExchangeContext(ctx context.Context, m *dns.Msg) (*dns.Msg, error)

	// CloseIdleConnections closes any connections in the Upstream which
	// now sitting idle. It does not interrupt any connections currently in use.
	CloseIdleConnections()

	io.Closer
}