contrib/internal/httptrace: handle DD_CLIENT_IP_HEADER env var

contrib/internal/httptrace/httptrace.go

@@ -36,7 +36,7 @@ func StartRequestSpan(r *http.Request, opts ...ddtrace.StartSpanOption) (tracer.
 			tracer.Tag("http.host", r.Host),
 		}, opts...)
 	}
-	if ip := getClientIP(r.RemoteAddr, r.Header); ip.IsValid() {
+	if ip := getClientIP(r.RemoteAddr, r.Header, cfg.ipHeader); ip.IsValid() {
 		opts = append(opts, tracer.Tag(ext.HTTPClientIP, ip.String()))
 	}
 	if spanctx, err := tracer.Extract(tracer.HTTPHeadersCarrier(r.Header)); err == nil {
@@ -73,10 +73,26 @@ var (
 	ipv6SpecialNetworks = []*netaddr.IPPrefix{
 		ippref("fec0::/10"), // site local
 	}
-	ipHeaders = []string{"x-forwarded-for", "x-real-ip", "x-client-ip", "x-forwarded", "x-cluster-client-ip", "forwarded-for", "forwarded", "via", "true-client-ip"}
+	ipHeaders = []string{
+		"x-forwarded-for",
+		"x-real-ip",
+		"x-client-ip",
+		"x-forwarded",
+		"x-cluster-client-ip",
+		"forwarded-for",
+		"forwarded",
+		"via",
+		"true-client-ip",
+	}
 )
 
-func getClientIP(remoteAddr string, headers http.Header) netaddr.IP {
+// getClientIP uses the request headers to resolve the client IP. If a specific header to check is provided through
+// DD_CLIENT_IP_HEADER, then only this header is checked.
+func getClientIP(remoteAddr string, headers http.Header, clientIPHeader string) netaddr.IP {
+	ipHeaders := ipHeaders
+	if len(clientIPHeader) > 0 {
+		ipHeaders = []string{clientIPHeader}
+	}
 	check := func(value string) netaddr.IP {
 		for _, ip := range strings.Split(value, ",") {
 			ipStr := strings.Trim(ip, " ")

contrib/internal/httptrace/httptrace_test.go

@@ -31,10 +31,11 @@ func TestStartRequestSpan(t *testing.T) {
 }
 
 type IPTestCase struct {
-	name       string
-	remoteAddr string
-	headers    map[string]string
-	expectedIP netaddr.IP
+	name         string
+	remoteAddr   string
+	headers      map[string]string
+	expectedIP   netaddr.IP
+	userIPHeader string
 }
 
 func genIPTestCases() []IPTestCase {
@@ -151,7 +152,7 @@ func TestIPHeaders(t *testing.T) {
 			for k, v := range tc.headers {
 				header.Add(k, v)
 			}
-			require.Equal(t, tc.expectedIP.String(), getClientIP(tc.remoteAddr, header).String())
+			require.Equal(t, tc.expectedIP.String(), getClientIP(tc.remoteAddr, header, tc.userIPHeader).String())
 		})
 	}
 

contrib/internal/httptrace/options.go

@@ -0,0 +1,13 @@
+package httptrace
+
+import "os"
+
+type config struct {
+	ipHeader string
+}
+
+var cfg = config{}
+
+func init() {
+	cfg.ipHeader = os.Getenv("DD_CLIENT_IP_HEADER")
+}