ARES-899: Fixes DoH client as system resolver. Fixes #91

tunneldns/https_upstream.go

@@ -6,6 +6,7 @@ import (
 	"crypto/tls"
 	"fmt"
 	"io/ioutil"
+	"net"
 	"net/http"
 	"net/url"
 	"time"
@@ -22,33 +23,18 @@ const (
 
 // UpstreamHTTPS is the upstream implementation for DNS over HTTPS service
 type UpstreamHTTPS struct {
-	client   *http.Client
-	endpoint *url.URL
+	client     *http.Client
+	endpoint   *url.URL
+	bootstraps []string
 }
 
-// NewUpstreamHTTPS creates a new DNS over HTTPS upstream from hostname
-func NewUpstreamHTTPS(endpoint string) (Upstream, error) {
+// NewUpstreamHTTPS creates a new DNS over HTTPS upstream from endpoint
+func NewUpstreamHTTPS(endpoint string, bootstraps []string) (Upstream, error) {
 	u, err := url.Parse(endpoint)
 	if err != nil {
 		return nil, err
 	}
-
-	// Update TLS and HTTP client configuration
-	tls := &tls.Config{ServerName: u.Hostname()}
-	transport := &http.Transport{
-		TLSClientConfig:    tls,
-		DisableCompression: true,
-		MaxIdleConns:       1,
-		Proxy:              http.ProxyFromEnvironment,
-	}
-	http2.ConfigureTransport(transport)
-
-	client := &http.Client{
-		Timeout:   defaultTimeout,
-		Transport: transport,
-	}
-
-	return &UpstreamHTTPS{client: client, endpoint: u}, nil
+	return &UpstreamHTTPS{client: configureClient(u.Hostname()), endpoint: u, bootstraps: bootstraps}, nil
 }
 
 // Exchange provides an implementation for the Upstream interface
@@ -58,34 +44,55 @@ func (u *UpstreamHTTPS) Exchange(ctx context.Context, query *dns.Msg) (*dns.Msg,
 		return nil, errors.Wrap(err, "failed to pack DNS query")
 	}
 
+	if len(query.Question) > 0 && query.Question[0].Name == fmt.Sprintf("%s.", u.endpoint.Hostname()) {
+		for _, bootstrap := range u.bootstraps {
+			endpoint, client, err := configureBootstrap(bootstrap)
+			if err != nil {
+				log.WithError(err).Errorf("failed to configure boostrap upstream %s", bootstrap)
+				continue
+			}
+			msg, err := exchange(queryBuf, query.Id, endpoint, client)
+			if err != nil {
+				log.WithError(err).Errorf("failed to connect to a boostrap upstream %s", bootstrap)
+				continue
+			}
+			return msg, nil
+		}
+		return nil, fmt.Errorf("failed to reach any bootstrap upstream: %v", u.bootstraps)
+	}
+
+	return exchange(queryBuf, query.Id, u.endpoint, u.client)
+}
+
+func exchange(msg []byte, queryID uint16, endpoint *url.URL, client *http.Client) (*dns.Msg, error) {
 	// No content negotiation for now, use DNS wire format
-	buf, backendErr := u.exchangeWireformat(queryBuf)
+	buf, backendErr := exchangeWireformat(msg, endpoint, client)
 	if backendErr == nil {
 		response := &dns.Msg{}
 		if err := response.Unpack(buf); err != nil {
 			return nil, errors.Wrap(err, "failed to unpack DNS response from body")
 		}
 
-		response.Id = query.Id
+		response.Id = queryID
 		return response, nil
 	}
 
-	log.WithError(backendErr).Errorf("failed to connect to an HTTPS backend %q", u.endpoint)
+	log.WithError(backendErr).Errorf("failed to connect to an HTTPS backend %q", endpoint)
 	return nil, backendErr
 }
 
 // Perform message exchange with the default UDP wireformat defined in current draft
 // https://datatracker.ietf.org/doc/draft-ietf-doh-dns-over-https
-func (u *UpstreamHTTPS) exchangeWireformat(msg []byte) ([]byte, error) {
-	req, err := http.NewRequest("POST", u.endpoint.String(), bytes.NewBuffer(msg))
+func exchangeWireformat(msg []byte, endpoint *url.URL, client *http.Client) ([]byte, error) {
+	req, err := http.NewRequest("POST", endpoint.String(), bytes.NewBuffer(msg))
 	if err != nil {
 		return nil, errors.Wrap(err, "failed to create an HTTPS request")
 	}
 
 	req.Header.Add("Content-Type", "application/dns-message")
-	req.Host = u.endpoint.Host
+	req.Host = endpoint.Host
 
-	resp, err := u.client.Do(req)
+	resp, err := client.Do(req)
 	if err != nil {
 		return nil, errors.Wrap(err, "failed to perform an HTTPS request")
 	}
@@ -104,3 +111,33 @@ func (u *UpstreamHTTPS) exchangeWireformat(msg []byte) ([]byte, error) {
 
 	return buf, nil
 }
+
+func configureBootstrap(bootstrap string) (*url.URL, *http.Client, error) {
+	b, err := url.Parse(bootstrap)
+	if err != nil {
+		return nil, nil, err
+	}
+	if ip := net.ParseIP(b.Hostname()); ip == nil {
+		return nil, nil, fmt.Errorf("bootstrap address of %s must be an IP address", b.Hostname())
+	}
+
+	return b, configureClient(b.Hostname()), nil
+}
+
+// configureClient will configure a HTTPS client for upstream DoH requests
+func configureClient(hostname string) *http.Client {
+	// Update TLS and HTTP client configuration
+	tls := &tls.Config{ServerName: hostname}
+	transport := &http.Transport{
+		TLSClientConfig:    tls,
+		DisableCompression: true,
+		MaxIdleConns:       1,
+		Proxy:              http.ProxyFromEnvironment,
+	}
+	http2.ConfigureTransport(transport)
+
+	return &http.Client{
+		Timeout:   defaultTimeout,
+		Transport: transport,
+	}
+}