feat: add WithHTTPClient option

request.go

@@ -19,7 +19,7 @@ const (
 
 var log = logging.Logger("doh")
 
-func doRequest(ctx context.Context, url string, m *dns.Msg) (*dns.Msg, error) {
+func doRequest(ctx context.Context, client *http.Client, url string, m *dns.Msg) (*dns.Msg, error) {
 	data, err := m.Pack()
 	if err != nil {
 		return nil, err
@@ -35,7 +35,7 @@ func doRequest(ctx context.Context, url string, m *dns.Msg) (*dns.Msg, error) {
 
 	req = req.WithContext(ctx)
 
-	resp, err := http.DefaultClient.Do(req)
+	resp, err := client.Do(req)
 	if err != nil {
 		return nil, err
 	}
@@ -62,13 +62,13 @@ func doRequest(ctx context.Context, url string, m *dns.Msg) (*dns.Msg, error) {
 	return r, nil
 }
 
-func doRequestA(ctx context.Context, url string, domain string) ([]net.IPAddr, uint32, error) {
+func doRequestA(ctx context.Context, client *http.Client, url string, domain string) ([]net.IPAddr, uint32, error) {
 	fqdn := dns.Fqdn(domain)
 
 	m := new(dns.Msg)
 	m.SetQuestion(fqdn, dns.TypeA)
 
-	r, err := doRequest(ctx, url, m)
+	r, err := doRequest(ctx, client, url, m)
 	if err != nil {
 		return nil, 0, err
 	}
@@ -90,13 +90,13 @@ func doRequestA(ctx context.Context, url string, domain string) ([]net.IPAddr, u
 	return result, ttl, nil
 }
 
-func doRequestAAAA(ctx context.Context, url string, domain string) ([]net.IPAddr, uint32, error) {
+func doRequestAAAA(ctx context.Context, client *http.Client, url string, domain string) ([]net.IPAddr, uint32, error) {
 	fqdn := dns.Fqdn(domain)
 
 	m := new(dns.Msg)
 	m.SetQuestion(fqdn, dns.TypeAAAA)
 
-	r, err := doRequest(ctx, url, m)
+	r, err := doRequest(ctx, client, url, m)
 	if err != nil {
 		return nil, 0, err
 	}
@@ -119,13 +119,13 @@ func doRequestAAAA(ctx context.Context, url string, domain string) ([]net.IPAddr
 	return result, ttl, nil
 }
 
-func doRequestTXT(ctx context.Context, url string, domain string) ([]string, uint32, error) {
+func doRequestTXT(ctx context.Context, client *http.Client, url string, domain string) ([]string, uint32, error) {
 	fqdn := dns.Fqdn(domain)
 
 	m := new(dns.Msg)
 	m.SetQuestion(fqdn, dns.TypeTXT)
 
-	r, err := doRequest(ctx, url, m)
+	r, err := doRequest(ctx, client, url, m)
 	if err != nil {
 		return nil, 0, err
 	}

resolver.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"math"
 	"net"
+	"net/http"
 	"strings"
 	"sync"
 	"time"
@@ -15,8 +16,9 @@ import (
 )
 
 type Resolver struct {
-	mx  sync.Mutex
-	url string
+	mx         sync.Mutex
+	url        string
+	httpClient *http.Client
 
 	// RR cache
 	ipCache     map[string]ipAddrEntry
@@ -52,6 +54,19 @@ func WithCacheDisabled() Option {
 	}
 }
 
+// WithHTTPClient sets the http.Client used for DoH requests, letting callers
+// control transport, timeouts, proxy, or TLS settings. When unset, the resolver
+// uses http.DefaultClient.
+func WithHTTPClient(client *http.Client) Option {
+	return func(tr *Resolver) error {
+		if client == nil {
+			return errors.New("http client must not be nil")
+		}
+		tr.httpClient = client
+		return nil
+	}
+}
+
 func NewResolver(url string, opts ...Option) (*Resolver, error) {
 	if strings.HasPrefix(url, "http:") &&
 		!strings.HasPrefix(url, "http://localhost") &&
@@ -66,6 +81,7 @@ func NewResolver(url string, opts ...Option) (*Resolver, error) {
 
 	r := &Resolver{
 		url:         url,
+		httpClient:  http.DefaultClient,
 		ipCache:     make(map[string]ipAddrEntry),
 		txtCache:    make(map[string]txtEntry),
 		maxCacheTTL: time.Duration(math.MaxUint32) * time.Second,
@@ -96,12 +112,12 @@ func (r *Resolver) LookupIPAddr(ctx context.Context, domain string) (result []ne
 
 	resch := make(chan response, 2)
 	go func() {
-		ip4, ttl, err := doRequestA(ctx, r.url, domain)
+		ip4, ttl, err := doRequestA(ctx, r.httpClient, r.url, domain)
 		resch <- response{ip4, ttl, err}
 	}()
 
 	go func() {
-		ip6, ttl, err := doRequestAAAA(ctx, r.url, domain)
+		ip6, ttl, err := doRequestAAAA(ctx, r.httpClient, r.url, domain)
 		resch <- response{ip6, ttl, err}
 	}()
 
@@ -129,7 +145,7 @@ func (r *Resolver) LookupTXT(ctx context.Context, domain string) ([]string, erro
 		return result, nil
 	}
 
-	result, ttl, err := doRequestTXT(ctx, r.url, domain)
+	result, ttl, err := doRequestTXT(ctx, r.httpClient, r.url, domain)
 	if err != nil {
 		return nil, err
 	}

resolver_test.go

@@ -6,6 +6,7 @@ import (
 	"net"
 	"net/http"
 	"net/http/httptest"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -216,6 +217,46 @@ func TestCleartextLocalhostEndpoint(t *testing.T) {
 	}
 }
 
+// countingRoundTripper counts the requests passing through it before delegating
+// to the wrapped transport.
+type countingRoundTripper struct {
+	rt    http.RoundTripper
+	count atomic.Int64
+}
+
+func (c *countingRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	c.count.Add(1)
+	return c.rt.RoundTrip(req)
+}
+
+func TestWithHTTPClient(t *testing.T) {
+	domain := "example.com"
+	resolver := mockDoHResolver(t, map[uint16]*dns.Msg{
+		dns.TypeTXT: mockDNSAnswerTXT(dns.Fqdn(domain), []string{"dnslink=/ipns/example.com"}),
+	})
+	defer resolver.Close()
+
+	rt := &countingRoundTripper{rt: http.DefaultTransport}
+	r, err := NewResolver(resolver.URL, WithHTTPClient(&http.Client{Transport: rt}))
+	if err != nil {
+		t.Fatal("resolver cannot be initialised")
+	}
+
+	if _, err := r.LookupTXT(context.Background(), domain); err != nil {
+		t.Fatal(err)
+	}
+	if rt.count.Load() == 0 {
+		t.Fatal("expected the custom http client to be used")
+	}
+}
+
+func TestWithHTTPClientNil(t *testing.T) {
+	_, err := NewResolver("https://cloudflare-dns.com/dns-query", WithHTTPClient(nil))
+	if err == nil {
+		t.Fatal("expected an error when passing a nil http client")
+	}
+}
+
 func sameIPs(a, b []net.IPAddr) bool {
 	if len(a) != len(b) {
 		return false