Added command line option to set arbitrary headers on upstream query …

…to prometheus

Added option to set host header on upstream query to prometheus - addresses #135

injectproxy/routes.go

@@ -18,6 +18,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"log"
 	"net/http"
 	"net/http/httputil"
 	"net/url"
@@ -261,7 +262,7 @@ func (sle StaticLabelEnforcer) ExtractLabel(next http.HandlerFunc) http.Handler
 	})
 }
 
-func NewRoutes(upstream *url.URL, label string, extractLabeler ExtractLabeler, opts ...Option) (*routes, error) {
+func NewRoutes(upstream *url.URL, label string, extractLabeler ExtractLabeler, extraHttpHeaders []string, rewriteHostHeader string, opts ...Option) (*routes, error) {
 	opt := options{}
 	for _, o := range opts {
 		o.apply(&opt)
@@ -271,7 +272,24 @@ func NewRoutes(upstream *url.URL, label string, extractLabeler ExtractLabeler, o
 		opt.registerer = prometheus.NewRegistry()
 	}
 
-	proxy := httputil.NewSingleHostReverseProxy(upstream)
+	proxy := &httputil.ReverseProxy{
+		Rewrite: func(r *httputil.ProxyRequest) {
+			r.SetURL(upstream)
+			if len(strings.TrimSpace(rewriteHostHeader)) == 0 {
+				r.Out.Host = r.In.Host
+			} else {
+				r.Out.Host = strings.TrimSpace(rewriteHostHeader)
+			}
+			for _, headerArg := range extraHttpHeaders {
+				header, val, found := strings.Cut(headerArg, ":")
+				if !found {
+					log.Printf("Header %s specified but ':' delimited not found", headerArg)
+					continue
+				}
+				r.Out.Header[strings.TrimSpace(header)] = []string{strings.TrimSpace(val)}
+			}
+		},
+	}
 
 	r := &routes{
 		upstream:       upstream,

main.go

@@ -64,6 +64,8 @@ func main() {
 		enableLabelAPIs        bool
 		unsafePassthroughPaths string // Comma-delimited string.
 		errorOnReplace         bool
+		extraHttpHeaders       arrayFlags
+		rewriteHostHeader      string
 	)
 
 	flagset := flag.NewFlagSet(os.Args[0], flag.ExitOnError)
@@ -81,6 +83,8 @@ func main() {
 		"This option is checked after Prometheus APIs, you cannot override enforced API endpoints to be not enforced with this option. Use carefully as it can easily cause a data leak if the provided path is an important "+
 		"API (like /api/v1/configuration) which isn't enforced by prom-label-proxy. NOTE: \"all\" matching paths like \"/\" or \"\" and regex are not allowed.")
 	flagset.BoolVar(&errorOnReplace, "error-on-replace", false, "When specified, the proxy will return HTTP status code 400 if the query already contains a label matcher that differs from the one the proxy would inject.")
+	flagset.Var(&extraHttpHeaders, "extra-http-header", "Additional HTTP headers to add to the upstream prometheus query in the format 'header: value'. Can be repeated multiple times for additional headers.")
+	flagset.StringVar(&rewriteHostHeader, "rewrite-host-header-to", "", "Rewrite host header to supplied value when sending the query to the upstream URL.")
 
 	//nolint: errcheck // Parse() will exit on error.
 	flagset.Parse(os.Args[1:])
@@ -109,6 +113,13 @@ func main() {
 		log.Fatalf("Invalid scheme for upstream URL %q, only 'http' and 'https' are supported", upstream)
 	}
 
+	for _, headerArg := range extraHttpHeaders {
+		header, val, found := strings.Cut(headerArg, ":")
+		if !found || len(strings.TrimSpace(header)) == 0 || len(strings.TrimSpace(val)) == 0 {
+			log.Fatalf("extra-http-header %s is not in the format 'key:value'", headerArg)
+		}
+	}
+
 	reg := prometheus.NewRegistry()
 	reg.MustRegister(
 		collectors.NewGoCollector(),
@@ -140,7 +151,7 @@ func main() {
 
 	{
 		// Run the insecure HTTP server.
-		routes, err := injectproxy.NewRoutes(upstreamURL, label, extractLabeler, opts...)
+		routes, err := injectproxy.NewRoutes(upstreamURL, label, extractLabeler, extraHttpHeaders, rewriteHostHeader, opts...)
 		if err != nil {
 			log.Fatalf("Failed to create injectproxy Routes: %v", err)
 		}