Forward headers to upstream

client.go

@@ -78,6 +78,14 @@ func (c client) Do(ctx context.Context, req *http.Request) (*http.Response, []by
 		}
 		req.URL.RawQuery = reqParams.Encode()
 	}
+	if header, ok := getForwardedHeader(ctx); ok {
+		if req.Header == nil {
+			req.Header = make(http.Header)
+		}
+		for key, fh := range header {
+			req.Header[key] = fh
+		}
+	}
 	if c.authz != "" {
 		if req.Header == nil {
 			req.Header = make(http.Header)

flag.go

@@ -0,0 +1,20 @@
+package main
+
+import (
+	"strings"
+)
+
+// StringSliceVar is a custom type that implements the flag.Value interface
+// to store a list of strings.
+type StringSliceVar []string
+
+// String returns a string representation of the StringSliceVar type.
+func (ss *StringSliceVar) String() string {
+	return strings.Join(*ss, ", ")
+}
+
+// Set appends a value to the StringSliceVar.
+func (ss *StringSliceVar) Set(value string) error {
+	*ss = append(*ss, value)
+	return nil
+}

header.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"context"
+	"net/http"
+)
+
+type headerKey int
+
+// addHeader adds forwarded headers to the context
+func addForwardedHeader(ctx context.Context, h *http.Header, forwardHeaders *StringSliceVar) context.Context {
+	if forwardHeaders == nil {
+		return ctx
+	}
+	newH := make(http.Header)
+	for _, fh := range *forwardHeaders {
+		if values := (*h).Values(fh); values != nil {
+			for _, v := range values {
+				newH.Add(fh, v)
+			}
+		}
+	}
+	return context.WithValue(ctx, headerKey(0), newH)
+}
+
+// getForwardedHeader extracts from context the header
+func getForwardedHeader(ctx context.Context) (http.Header, bool) {
+	if ctxValue := ctx.Value(headerKey(0)); ctxValue != nil {
+		if header, ok := ctxValue.(http.Header); ok {
+			return header, true
+		}
+	}
+	return nil, false
+}

main.go

@@ -25,6 +25,7 @@ var (
 	tlsSkipVerify         bool
 	bearerFile            string
 	forceGet              bool
+	forwardHeaders        StringSliceVar
 )
 
 func parseFlag() {
@@ -33,6 +34,7 @@ func parseFlag() {
 	flag.BoolVar(&tlsSkipVerify, "tlsSkipVerify", false, "Skip TLS Verification")
 	flag.StringVar(&bearerFile, "bearer-file", "", "File containing bearer token for API requests")
 	flag.BoolVar(&forceGet, "force-get", false, "Force api.Client to use GET by rejecting POST requests")
+	flag.Var(&forwardHeaders, "forward-header", "A header that will be forwarded to upstream")
 	flag.Parse()
 }
 
@@ -79,6 +81,9 @@ func main() {
 		klog.Infof("Forcing api,Client to use GET requests")
 		options = append(options, withGet)
 	}
+	if forwardHeaders != nil {
+		klog.Infof("Following headers will be forwarded upstream: %v", forwardHeaders.String())
+	}
 	if c, err = newClient(c, options...); err != nil {
 		klog.Fatalf("error building custom API client:", err)
 	}
@@ -106,6 +111,7 @@ func federate(ctx context.Context, w http.ResponseWriter, r *http.Request, apiCl
 	if params.Del("match[]"); len(params) > 0 {
 		nctx = addValues(nctx, params)
 	}
+	nctx = addForwardedHeader(nctx, &r.Header, &forwardHeaders)
 	start := time.Now()
 	val, _, err := apiClient.Query(nctx, matchQuery, time.Now()) // Ignoring warnings for now.
 	responseTime := time.Since(start).Seconds()