feat: preshared key, client port and DoH

Closes #2, Closes #3

internal/resolver/doh.go

@@ -0,0 +1,81 @@
+package resolver
+
+import (
+	"bytes"
+	"io"
+	"net"
+	"net/http"
+	"net/url"
+	"sync"
+	"time"
+)
+
+var _ net.Conn = &dohConn{}
+
+type dohConn struct {
+	addr string
+
+	once    sync.Once
+	onceErr error
+
+	in, ret bytes.Buffer
+}
+
+func (c *dohConn) Close() error                       { return nil }
+func (c *dohConn) LocalAddr() net.Addr                { return nil }
+func (c *dohConn) RemoteAddr() net.Addr               { return nil }
+func (c *dohConn) SetDeadline(t time.Time) error      { return nil }
+func (c *dohConn) SetReadDeadline(t time.Time) error  { return nil }
+func (c *dohConn) SetWriteDeadline(t time.Time) error { return nil }
+
+func (c *dohConn) Write(b []byte) (int, error) { return c.in.Write(b) }
+
+func (c *dohConn) Read(b []byte) (int, error) {
+	c.once.Do(func() {
+		url, err := url.Parse(c.addr)
+		if err != nil {
+			c.onceErr = err
+			return
+		}
+		// RFC 8484
+		url.Path = "/dns-query"
+
+		// Skip 2 bytes which are length
+		reqBody := bytes.NewReader(c.in.Bytes()[2:])
+		req, err := http.NewRequest("POST", url.String(), reqBody)
+		if err != nil {
+			c.onceErr = err
+			return
+		}
+		req.Header.Set("content-type", "application/dns-message")
+		req.Header.Set("accept", "application/dns-message")
+		resp, err := http.DefaultClient.Do(req)
+		if err != nil {
+			c.onceErr = err
+			return
+		}
+		defer resp.Body.Close()
+		respBody, err := io.ReadAll(resp.Body)
+		if err != nil {
+			c.onceErr = err
+			return
+		}
+
+		l := uint16(len(respBody))
+		_, err = c.ret.Write([]byte{uint8(l >> 8), uint8(l & ((1 << 8) - 1))})
+		if err != nil {
+			c.onceErr = err
+			return
+		}
+
+		_, err = c.ret.Write(respBody)
+		if err != nil {
+			c.onceErr = err
+			return
+		}
+	})
+	if c.onceErr != nil {
+		return 0, c.onceErr
+	}
+	return c.ret.Read(b)
+}