services: Set Nomad's User-Agent by default on HTTP checks for nomad services (#16248)

.changelog/16248.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+services: Set Nomad's User-Agent by default on HTTP checks in Nomad services
+```

client/serviceregistration/checks/client.go

@@ -15,6 +15,7 @@ import (
 	"github.com/hashicorp/go-cleanhttp"
 	"github.com/hashicorp/go-hclog"
 	"github.com/hashicorp/nomad/client/serviceregistration"
+	"github.com/hashicorp/nomad/helper/useragent"
 	"github.com/hashicorp/nomad/nomad/structs"
 	"oss.indeed.com/go/libtime"
 )
@@ -163,14 +164,18 @@ func (c *checker) checkHTTP(ctx context.Context, qc *QueryContext, q *Query) *st
 		qr.Status = structs.CheckFailure
 		return qr
 	}
+
 	for header, values := range q.Headers {
 		for _, value := range values {
 			request.Header.Add(header, value)
 		}
 	}
 
-	request.Host = request.Header.Get("Host")
+	if len(request.Header.Get(useragent.Header)) == 0 {
+		request.Header.Set(useragent.Header, useragent.String())
+	}
 
+	request.Host = request.Header.Get("Host")
 	request.Body = io.NopCloser(strings.NewReader(q.Body))
 	request = request.WithContext(ctx)
 

client/serviceregistration/checks/client_test.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/hashicorp/nomad/ci"
 	"github.com/hashicorp/nomad/helper/testlog"
+	"github.com/hashicorp/nomad/helper/useragent"
 	"github.com/hashicorp/nomad/nomad/mock"
 	"github.com/hashicorp/nomad/nomad/structs"
 	"github.com/shoenig/test/must"
@@ -239,7 +240,7 @@ func TestChecker_Do_HTTP_extras(t *testing.T) {
 	}
 
 	encoding := [2]string{"Accept-Encoding", "gzip"}
-	agent := [2]string{"User-Agent", "Go-http-client/1.1"}
+	agent := [2]string{useragent.Header, useragent.String()}
 
 	cases := []struct {
 		name    string
@@ -270,6 +271,13 @@ func TestChecker_Do_HTTP_extras(t *testing.T) {
 				[2]string{"Authorization", "Basic ZWxhc3RpYzpjaGFuZ2VtZQ=="},
 			),
 		},
+		{
+			name:   "user agent header",
+			method: "GET",
+			headers: makeHeaders(encoding,
+				[2]string{"User-Agent", "my-custom-agent"},
+			),
+		},
 		{
 			name:   "host header",
 			method: "GET",
@@ -348,7 +356,7 @@ func TestChecker_Do_HTTP_extras(t *testing.T) {
 				}
 			}
 			if !hostSent {
-				must.Eq(t, nil, tc.headers["Host"])
+				must.Nil(t, tc.headers["Host"])
 			}
 
 			must.Eq(t, tc.headers, headers)

helper/useragent/useragent.go

@@ -8,6 +8,12 @@ import (
 	"github.com/hashicorp/nomad/version"
 )
 
+const (
+	// Header is the User-Agent header key
+	// https://www.rfc-editor.org/rfc/rfc7231#section-5.5.3
+	Header = `User-Agent`
+)
+
 var (
 	// projectURL is the project URL.
 	projectURL = "https://www.nomadproject.io/"
@@ -37,6 +43,6 @@ type HeaderSetter interface {
 // SetHeaders configures the User-Agent http.Header for the client.
 func SetHeaders(client HeaderSetter) {
 	client.SetHeaders(http.Header{
-		"User-Agent": []string{String()},
+		Header: []string{String()},
 	})
 }