feat(client): OAuth2 Client credential support (#259)

* Initial implementation

* Added OAuth2 support to `client` config

* Revert "Initial implementation"

This reverts commit 7f2f3a603a.

* Restore vendored clientcredentials

* configureOAuth2 is now a func (including tests)

* README update

* Use the same OAuth2Config in all related tests

* Cleanup & comments

README.md

@@ -273,11 +273,16 @@ See [examples/docker-compose-postgres-storage](.examples/docker-compose-postgres
 In order to support a wide range of environments, each monitored endpoint has a unique configuration for 
 the client used to send the request.
 
-| Parameter                | Description                                                             | Default |
-|:-------------------------|:------------------------------------------------------------------------|:--------|
-| `client.insecure`        | Whether to skip verifying the server's certificate chain and host name. | `false` |
-| `client.ignore-redirect` | Whether to ignore redirects (true) or follow them (false, default).     | `false` |
-| `client.timeout`         | Duration before timing out.                                             | `10s`   |
+| Parameter                     | Description                                                                | Default         |
+|:------------------------------|:---------------------------------------------------------------------------|:----------------|
+| `client.insecure`             | Whether to skip verifying the server's certificate chain and host name.    | `false`         |
+| `client.ignore-redirect`      | Whether to ignore redirects (true) or follow them (false, default).        | `false`         |
+| `client.timeout`              | Duration before timing out.                                                | `10s`           |
+| `client.oauth2`               | OAuth2 client configuration.                                               | `{}`            |
+| `client.oauth2.token-url`     | The token endpoint URL                                                     | required `""`   |
+| `client.oauth2.client-id`     | The client id which should be used for the `Client credentials flow`       | required `""`   |
+| `client.oauth2.client-secret` | The client secret which should be used for the `Client credentials flow`   | required `""`   |
+| `client.oauth2.scopes[]`      | A list of `scopes` which should be used for the `Client credentials flow`. | required `[""]` |
 
 Note that some of these parameters are ignored based on the type of endpoint. For instance, there's no certificate involved
 in ICMP requests (ping), therefore, setting `client.insecure` to `true` for an endpoint of that type will not do anything.
@@ -304,6 +309,20 @@ endpoints:
       - "[STATUS] == 200"
 ```
 
+This example shows how you can use the `client.oauth2` configuration to query a backend API with `Bearer token`:
+```yaml
+endpoints:
+  - name: website
+    url: "https://your.health.api/getHealth"
+    client:
+      oauth2:
+        token-url: https://your-token-server/token
+        client-id: 00000000-0000-0000-0000-000000000000
+        client-secret: your-client-secret
+        scopes: ['https://your.health.api/.default']
+    conditions:
+      - "[STATUS] == 200"
+```
 
 ### Alerting
 Gatus supports multiple alerting providers, such as Slack and PagerDuty, and supports different alerts for each

client/client_test.go

@@ -1,8 +1,13 @@
 package client
 
 import (
+	"bytes"
+	"io/ioutil"
+	"net/http"
 	"testing"
 	"time"
+
+	"github.com/TwiN/gatus/v3/test"
 )
 
 func TestGetHTTPClient(t *testing.T) {
@@ -10,6 +15,12 @@ func TestGetHTTPClient(t *testing.T) {
 		Insecure:       false,
 		IgnoreRedirect: false,
 		Timeout:        0,
+		OAuth2Config: &OAuth2Config{
+			ClientID:     "00000000-0000-0000-0000-000000000000",
+			ClientSecret: "secretsauce",
+			TokenURL:     "https://token-server.local/token",
+			Scopes:       []string{"https://application.local/.default"},
+		},
 	}
 	cfg.ValidateAndSetDefaults()
 	if GetHTTPClient(cfg) == nil {
@@ -146,3 +157,71 @@ func TestCanCreateTCPConnection(t *testing.T) {
 		t.Error("should've failed, because there's no port in the address")
 	}
 }
+
+// This test checks if a HTTP client configured with `configureOAuth2()` automatically
+// performs a Client Credentials OAuth2 flow and adds the obtained token as a `Authorization`
+// header to all outgoing HTTP calls.
+func TestHttpClientProvidesOAuth2BearerToken(t *testing.T) {
+
+	defer InjectHTTPClient(nil)
+
+	oAuth2Config := &OAuth2Config{
+		ClientID:     "00000000-0000-0000-0000-000000000000",
+		ClientSecret: "secretsauce",
+		TokenURL:     "https://token-server.local/token",
+		Scopes:       []string{"https://application.local/.default"},
+	}
+
+	mockHttpClient := &http.Client{
+		Transport: test.MockRoundTripper(func(r *http.Request) *http.Response {
+
+			// if the mock HTTP client tries to get a token from the `token-server`
+			// we provide the expected token response
+			if r.Host == "token-server.local" {
+				return &http.Response{
+					StatusCode: http.StatusOK,
+					Body: ioutil.NopCloser(bytes.NewReader(
+						[]byte(
+							`{"token_type":"Bearer","expires_in":3599,"ext_expires_in":3599,"access_token":"secret-token"}`,
+						),
+					)),
+				}
+			}
+
+			// to verify the headers were sent as expected, we echo them back in the
+			// `X-Org-Authorization` header and check if the token value matches our
+			// mocked `token-server` response
+			return &http.Response{
+				StatusCode: http.StatusOK,
+				Header: map[string][]string{
+					"X-Org-Authorization": {r.Header.Get("Authorization")},
+				},
+				Body: http.NoBody,
+			}
+		}),
+	}
+
+	mockHttpClientWithOAuth := configureOAuth2(mockHttpClient, *oAuth2Config)
+	InjectHTTPClient(mockHttpClientWithOAuth)
+
+	request, err := http.NewRequest(http.MethodPost, "http://127.0.0.1:8282", http.NoBody)
+	if err != nil {
+		t.Error("expected no error, got", err.Error())
+	}
+
+	response, err := mockHttpClientWithOAuth.Do(request)
+	if err != nil {
+		t.Error("expected no error, got", err.Error())
+	}
+
+	if response.Header == nil {
+		t.Error("expected response headers, but got nil")
+	}
+
+	// the mock response echos the Authorization header used in the request back
+	// to us as `X-Org-Authorization` header, we check here if the value matches
+	// our expected token `secret-token`
+	if response.Header.Get("X-Org-Authorization") != "Bearer secret-token" {
+		t.Error("exptected `secret-token` as Bearer token in the mocked response header `X-Org-Authorization`, but got", response.Header.Get("X-Org-Authorization"))
+	}
+}

client/config.go

@@ -1,9 +1,14 @@
 package client
 
 import (
+	"context"
 	"crypto/tls"
+	"errors"
 	"net/http"
 	"time"
+
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/clientcredentials"
 )
 
 const (
@@ -17,6 +22,10 @@ var (
 		IgnoreRedirect: false,
 		Timeout:        defaultHTTPTimeout,
 	}
+
+	ErrInvalidClientOAuth2Config = errors.New(
+		"invalid OAuth2 configuration, all fields are required",
+	)
 )
 
 // GetDefaultConfig returns a copy of the default configuration
@@ -36,14 +45,40 @@ type Config struct {
 	// Timeout for the client
 	Timeout time.Duration `yaml:"timeout"`
 
+	// OAuth2 configuration for the client
+	OAuth2Config *OAuth2Config `yaml:"oauth2,omitempty"`
+
 	httpClient *http.Client
 }
 
+// OAuth2Config is the configuration for the OAuth2 client credentials flow
+type OAuth2Config struct {
+	TokenURL     string   `yaml:"token-url"` // e.g. https://dev-12345678.okta.com/token
+	ClientID     string   `yaml:"client-id"`
+	ClientSecret string   `yaml:"client-secret"`
+	Scopes       []string `yaml:"scopes"` // e.g. ["openid"]
+}
+
 // ValidateAndSetDefaults validates the client configuration and sets the default values if necessary
-func (c *Config) ValidateAndSetDefaults() {
+func (c *Config) ValidateAndSetDefaults() error {
 	if c.Timeout < time.Millisecond {
 		c.Timeout = 10 * time.Second
 	}
+	if c.HasOAuth2Config() && !c.OAuth2Config.isValid() {
+		return ErrInvalidClientOAuth2Config
+	}
+
+	return nil
+}
+
+// HasOAuth2Config returns true if the client has OAuth2 configuration parameters
+func (c *Config) HasOAuth2Config() bool {
+	return c.OAuth2Config != nil
+}
+
+// isValid() returns true if the OAuth2 configuration is valid
+func (c *OAuth2Config) isValid() bool {
+	return len(c.TokenURL) > 0 && len(c.ClientID) > 0 && len(c.ClientSecret) > 0 && len(c.Scopes) > 0
 }
 
 // GetHTTPClient return an HTTP client matching the Config's parameters.
@@ -68,6 +103,22 @@ func (c *Config) getHTTPClient() *http.Client {
 				return nil
 			},
 		}
+		if c.HasOAuth2Config() {
+			c.httpClient = configureOAuth2(c.httpClient, *c.OAuth2Config)
+		}
 	}
 	return c.httpClient
 }
+
+// configureOAuth2 returns an HTTP client that will obtain and refresh tokens as necessary.
+// The returned Client and its Transport should not be modified.
+func configureOAuth2(httpClient *http.Client, c OAuth2Config) *http.Client {
+	oauth2cfg := clientcredentials.Config{
+		ClientID:     c.ClientID,
+		ClientSecret: c.ClientSecret,
+		Scopes:       c.Scopes,
+		TokenURL:     c.TokenURL,
+	}
+	ctx := context.WithValue(context.Background(), oauth2.HTTPClient, httpClient)
+	return oauth2cfg.Client(ctx)
+}

core/endpoint.go

@@ -111,7 +111,10 @@ func (endpoint *Endpoint) ValidateAndSetDefaults() error {
 	if endpoint.ClientConfig == nil {
 		endpoint.ClientConfig = client.GetDefaultConfig()
 	} else {
-		endpoint.ClientConfig.ValidateAndSetDefaults()
+		err := endpoint.ClientConfig.ValidateAndSetDefaults()
+		if err != nil {
+			return err
+		}
 	}
 	if endpoint.UIConfig == nil {
 		endpoint.UIConfig = ui.GetDefaultConfig()

vendor/golang.org/x/oauth2/clientcredentials/clientcredentials.go

@@ -0,0 +1,120 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package clientcredentials implements the OAuth2.0 "client credentials" token flow,
+// also known as the "two-legged OAuth 2.0".
+//
+// This should be used when the client is acting on its own behalf or when the client
+// is the resource owner. It may also be used when requesting access to protected
+// resources based on an authorization previously arranged with the authorization
+// server.
+//
+// See https://tools.ietf.org/html/rfc6749#section-4.4
+package clientcredentials // import "golang.org/x/oauth2/clientcredentials"
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"net/url"
+	"strings"
+
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/internal"
+)
+
+// Config describes a 2-legged OAuth2 flow, with both the
+// client application information and the server's endpoint URLs.
+type Config struct {
+	// ClientID is the application's ID.
+	ClientID string
+
+	// ClientSecret is the application's secret.
+	ClientSecret string
+
+	// TokenURL is the resource server's token endpoint
+	// URL. This is a constant specific to each server.
+	TokenURL string
+
+	// Scope specifies optional requested permissions.
+	Scopes []string
+
+	// EndpointParams specifies additional parameters for requests to the token endpoint.
+	EndpointParams url.Values
+
+	// AuthStyle optionally specifies how the endpoint wants the
+	// client ID & client secret sent. The zero value means to
+	// auto-detect.
+	AuthStyle oauth2.AuthStyle
+}
+
+// Token uses client credentials to retrieve a token.
+//
+// The provided context optionally controls which HTTP client is used. See the oauth2.HTTPClient variable.
+func (c *Config) Token(ctx context.Context) (*oauth2.Token, error) {
+	return c.TokenSource(ctx).Token()
+}
+
+// Client returns an HTTP client using the provided token.
+// The token will auto-refresh as necessary.
+//
+// The provided context optionally controls which HTTP client
+// is returned. See the oauth2.HTTPClient variable.
+//
+// The returned Client and its Transport should not be modified.
+func (c *Config) Client(ctx context.Context) *http.Client {
+	return oauth2.NewClient(ctx, c.TokenSource(ctx))
+}
+
+// TokenSource returns a TokenSource that returns t until t expires,
+// automatically refreshing it as necessary using the provided context and the
+// client ID and client secret.
+//
+// Most users will use Config.Client instead.
+func (c *Config) TokenSource(ctx context.Context) oauth2.TokenSource {
+	source := &tokenSource{
+		ctx:  ctx,
+		conf: c,
+	}
+	return oauth2.ReuseTokenSource(nil, source)
+}
+
+type tokenSource struct {
+	ctx  context.Context
+	conf *Config
+}
+
+// Token refreshes the token by using a new client credentials request.
+// tokens received this way do not include a refresh token
+func (c *tokenSource) Token() (*oauth2.Token, error) {
+	v := url.Values{
+		"grant_type": {"client_credentials"},
+	}
+	if len(c.conf.Scopes) > 0 {
+		v.Set("scope", strings.Join(c.conf.Scopes, " "))
+	}
+	for k, p := range c.conf.EndpointParams {
+		// Allow grant_type to be overridden to allow interoperability with
+		// non-compliant implementations.
+		if _, ok := v[k]; ok && k != "grant_type" {
+			return nil, fmt.Errorf("oauth2: cannot overwrite parameter %q", k)
+		}
+		v[k] = p
+	}
+
+	tk, err := internal.RetrieveToken(c.ctx, c.conf.ClientID, c.conf.ClientSecret, c.conf.TokenURL, v, internal.AuthStyle(c.conf.AuthStyle))
+	if err != nil {
+		if rErr, ok := err.(*internal.RetrieveError); ok {
+			return nil, (*oauth2.RetrieveError)(rErr)
+		}
+		return nil, err
+	}
+	t := &oauth2.Token{
+		AccessToken:  tk.AccessToken,
+		TokenType:    tk.TokenType,
+		RefreshToken: tk.RefreshToken,
+		Expiry:       tk.Expiry,
+	}
+	return t.WithExtra(tk.Raw), nil
+}

vendor/modules.txt

@@ -116,6 +116,7 @@ golang.org/x/net/ipv6
 # golang.org/x/oauth2 v0.0.0-20210514164344-f6687ab2804c
 ## explicit; go 1.11
 golang.org/x/oauth2
+golang.org/x/oauth2/clientcredentials
 golang.org/x/oauth2/internal
 # golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
 ## explicit