From dcc66161743315466f999f3299b0d304bb73ead8 Mon Sep 17 00:00:00 2001
From: Roman Iuvshyn <roman.iuvshyn@transferwise.com>
Date: Fri, 17 Apr 2020 11:01:58 +0300
Subject: [PATCH] Add use_proxy_dns to bypass local DNS resolution

Changes picked up from original PR https://github.com/prometheus/blackbox_exporter/pull/554 which was rejected by maintainer.
---
 CONFIGURATION.md |  3 +++
 config/config.go |  1 +
 prober/http.go   | 42 +++++++++++++++++++++++++++---------------
 3 files changed, 31 insertions(+), 15 deletions(-)

diff --git a/CONFIGURATION.md b/CONFIGURATION.md
index 7610bd4c..8cde55be 100644
--- a/CONFIGURATION.md
+++ b/CONFIGURATION.md
@@ -92,6 +92,9 @@ The other placeholders are specified separately.
   # HTTP proxy server to use to connect to the targets.
   [ proxy_url: <string> ]
 
+  # Skip local DNS resolution when using an HTTP proxy server.
+  [ use_proxy_dns: <boolean> | default = false ]
+
   # The IP protocol of the HTTP probe (ip4, ip6).
   [ preferred_ip_protocol: <string> | default = "ip6" ]
   [ ip_protocol_fallback: <boolean> | default = true ]
diff --git a/config/config.go b/config/config.go
index 3f3daead..e15565d4 100644
--- a/config/config.go
+++ b/config/config.go
@@ -138,6 +138,7 @@ type HTTPProbe struct {
 	FailIfHeaderMatchesRegexp    []HeaderMatch           `yaml:"fail_if_header_matches,omitempty"`
 	FailIfHeaderNotMatchesRegexp []HeaderMatch           `yaml:"fail_if_header_not_matches,omitempty"`
 	Body                         string                  `yaml:"body,omitempty"`
+	UseProxyDNS                  bool                    `yaml:"use_proxy_dns,omitempty"`
 	HTTPClientConfig             config.HTTPClientConfig `yaml:"http_client_config,inline"`
 }
 
diff --git a/prober/http.go b/prober/http.go
index f4abdbe1..82381efc 100644
--- a/prober/http.go
+++ b/prober/http.go
@@ -285,7 +285,13 @@ func ProbeHTTP(ctx context.Context, target string, module config.Module, registr
 		})
 	)
 
-	for _, lv := range []string{"resolve", "connect", "tls", "processing", "transfer"} {
+	// Check the option to send the url unresolved to the proxy
+	skipDNS := module.HTTP.UseProxyDNS && module.HTTP.HTTPClientConfig.ProxyURL.URL != nil
+
+	if !skipDNS {
+		durationGaugeVec.WithLabelValues("resolve")
+	}
+	for _, lv := range []string{"connect", "tls", "processing", "transfer"} {
 		durationGaugeVec.WithLabelValues(lv)
 	}
 
@@ -315,12 +321,17 @@ func ProbeHTTP(ctx context.Context, target string, module config.Module, registr
 		targetHost = targetURL.Host
 	}
 
-	ip, lookupTime, err := chooseProtocol(ctx, module.HTTP.IPProtocol, module.HTTP.IPProtocolFallback, targetHost, registry, logger)
-	if err != nil {
-		level.Error(logger).Log("msg", "Error resolving address", "err", err)
-		return false
+	// Resolve the host unless letting the proxy do the job.
+	var ip *net.IPAddr
+	if !skipDNS {
+		var lookupTime float64
+		ip, lookupTime, err = chooseProtocol(ctx, module.HTTP.IPProtocol, module.HTTP.IPProtocolFallback, targetHost, registry, logger)
+		if err != nil {
+			level.Error(logger).Log("msg", "Error resolving address", "err", err)
+			return false
+		}
+		durationGaugeVec.WithLabelValues("resolve").Add(lookupTime)
 	}
-	durationGaugeVec.WithLabelValues("resolve").Add(lookupTime)
 
 	httpClientConfig := module.HTTP.HTTPClientConfig
 	if len(httpClientConfig.TLSConfig.ServerName) == 0 {
@@ -367,18 +378,19 @@ func ProbeHTTP(ctx context.Context, target string, module config.Module, registr
 		httpConfig.Method = "GET"
 	}
 
-	// Replace the host field in the URL with the IP we resolved.
+	// Replace the host field in the URL with the IP we resolved unless letting a proxy server do the name resolution.
 	origHost := targetURL.Host
-	if targetPort == "" {
-		if strings.Contains(ip.String(), ":") {
-			targetURL.Host = "[" + ip.String() + "]"
+	if !skipDNS {
+		if targetPort == "" {
+			if strings.Contains(ip.String(), ":") {
+				targetURL.Host = "[" + ip.String() + "]"
+			} else {
+				targetURL.Host = ip.String()
+			}
 		} else {
-			targetURL.Host = ip.String()
+			targetURL.Host = net.JoinHostPort(ip.String(), targetPort)
 		}
-	} else {
-		targetURL.Host = net.JoinHostPort(ip.String(), targetPort)
 	}
-
 	var body io.Reader
 	var respBodyBytes int64
 
@@ -515,7 +527,7 @@ func ProbeHTTP(ctx context.Context, target string, module config.Module, registr
 			"end", trace.end,
 		)
 		// We get the duration for the first request from chooseProtocol.
-		if i != 0 {
+		if i != 0 && !skipDNS {
 			durationGaugeVec.WithLabelValues("resolve").Add(trace.dnsDone.Sub(trace.start).Seconds())
 		}
 		// Continue here if we never got a connection because a request failed.