Add hostname parameter

Signed-off-by: anemyte <anemyte@gmail.com>

README.md

@@ -101,6 +101,33 @@ scrape_configs:
         replacement: 127.0.0.1:9115  # The blackbox exporter's real hostname:port.
 ```
 
+HTTP probes can accept an additional `hostname` parameter that will set `Host` header and TLS SNI. This can be especially useful with `dns_sd_config`:
+```yaml
+scrape_configs:
+  - job_name: blackbox_all
+    metrics_path: /probe
+    params:
+      module: [ http_2xx ]  # Look for a HTTP 200 response.
+    dns_sd_configs:
+      - names:
+          - example.com
+          - prometheus.io
+        type: A
+        port: 443
+    relabel_configs:
+      - source_labels: [__address__]
+        target_label: __param_target
+        replacement: https://$1/  # Make probe URL be like https://1.2.3.4:443/
+      - source_labels: [__param_target]
+        target_label: instance
+      - target_label: __address__
+        replacement: 127.0.0.1:9115  # The blackbox exporter's real hostname:port.
+      - source_labels: [__meta_dns_name]
+        target_label: __param_hostname  # Make domain name become 'Host' header for probe requests
+      - source_labels: [__meta_dns_name]
+        target_label: vhost  # and store it in 'vhost' label
+```
+
 ## Permissions
 
 The ICMP probe requires elevated privileges to function:

main.go

@@ -120,6 +120,23 @@ func probeHandler(w http.ResponseWriter, r *http.Request, c *config.Config, logg
 		return
 	}
 
+	if module.Prober == "http" {
+		paramHost := params.Get("hostname")
+		if paramHost != "" {
+			if module.HTTP.Headers == nil {
+				module.HTTP.Headers = make(map[string]string)
+			} else {
+				for name, value := range module.HTTP.Headers {
+					if strings.Title(name) == "Host" && value != paramHost {
+						http.Error(w, fmt.Sprintf("Host header defined both in module configuration (%s) and with parameter 'hostname' (%s)", value, paramHost), http.StatusBadRequest)
+						return
+					}
+				}
+			}
+			module.HTTP.Headers["Host"] = paramHost
+		}
+	}
+
 	sl := newScrapeLogger(logger, moduleName, target)
 	level.Info(sl).Log("msg", "Beginning probe", "probe", module.Prober, "timeout_seconds", timeoutSeconds)
 

main_test.go

@@ -15,6 +15,7 @@ package main
 
 import (
 	"bytes"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -190,3 +191,68 @@ func TestComputeExternalURL(t *testing.T) {
 		}
 	}
 }
+
+func TestHostnameParam(t *testing.T) {
+	headers := map[string]string{}
+	c := &config.Config{
+		Modules: map[string]config.Module{
+			"http_2xx": config.Module{
+				Prober:  "http",
+				Timeout: 10 * time.Second,
+				HTTP: config.HTTPProbe{
+					Headers:            headers,
+					IPProtocolFallback: true,
+				},
+			},
+		},
+	}
+
+	// check that 'hostname' parameter make its way to Host header
+	hostname := "foo.example.com"
+
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Host != hostname {
+			t.Errorf("Unexpected Host: expected %q, got %q.", hostname, r.Host)
+		}
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer ts.Close()
+
+	requrl := fmt.Sprintf("?debug=true&hostname=%s&target=%s", hostname, ts.URL)
+
+	req, err := http.NewRequest("GET", requrl, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	rr := httptest.NewRecorder()
+
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		probeHandler(w, r, c, log.NewNopLogger(), &resultHistory{})
+	})
+
+	handler.ServeHTTP(rr, req)
+
+	if status := rr.Code; status != http.StatusOK {
+		t.Errorf("probe request handler returned wrong status code: %v, want %v", status, http.StatusOK)
+	}
+
+	// check that ts got the request to perform header check
+	if !strings.Contains(rr.Body.String(), "probe_success 1") {
+		t.Errorf("probe failed, response body: %v", rr.Body.String())
+	}
+
+	// check that host header both in config and in parameter will result in 400
+	c.Modules["http_2xx"].HTTP.Headers["Host"] = hostname + ".something"
+
+	handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		probeHandler(w, r, c, log.NewNopLogger(), &resultHistory{})
+	})
+
+	rr = httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if status := rr.Code; status != http.StatusBadRequest {
+		t.Errorf("probe request handler returned wrong status code: %v, want %v", status, http.StatusBadRequest)
+	}
+}

prober/http.go

@@ -341,9 +341,20 @@ func ProbeHTTP(ctx context.Context, target string, module config.Module, registr
 
 	httpClientConfig := module.HTTP.HTTPClientConfig
 	if len(httpClientConfig.TLSConfig.ServerName) == 0 {
-		// If there is no `server_name` in tls_config, use
-		// the hostname of the target.
-		httpClientConfig.TLSConfig.ServerName = targetHost
+		// If there is no `server_name` in tls_config it makes
+		// sense to use the host header value to avoid possible
+		// TLS handshake problems.
+		changed := false
+		for name, value := range httpConfig.Headers {
+			if strings.Title(name) == "Host" {
+				httpClientConfig.TLSConfig.ServerName = value
+				changed = true
+			}
+		}
+		if !changed {
+			// Otherwise use the hostname of the target.
+			httpClientConfig.TLSConfig.ServerName = targetHost
+		}
 	}
 	client, err := pconfig.NewClientFromConfig(httpClientConfig, "http_probe", pconfig.WithKeepAlivesDisabled())
 	if err != nil {