ACME HTTP: Allow customizing HTTP client x509.CertPool (#571)

This commit updates `acme/http.go` to allow customizing the
`*x509.CertPool` used by the `HTTPClient` by specifying the filepath of
a custom CA certificate via the `CA_CERTIFICATE` environment variable.

This allows developers to easily trust a non-standard CA when
interacting with an ACME test server (e.g. Pebble):

```
CA_CERTIFICATE=~/go/src/github.com/letsencrypt/pebble/test/certs/pebble.minica.pem \
lego \
  --server https://localhost:14000/dir \
  --email foo@bar.com \
  -d example.com \
  run
```

acme/http.go

@@ -1,22 +1,29 @@
 package acme
 
 import (
+	"crypto/tls"
+	"crypto/x509"
 	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"net"
 	"net/http"
+	"os"
 	"runtime"
 	"strings"
 	"time"
 )
 
+var (
 	// UserAgent (if non-empty) will be tacked onto the User-Agent string in requests.
-var UserAgent string
+	UserAgent string
 
-// HTTPClient is an HTTP client with a reasonable timeout value.
-var HTTPClient = http.Client{
+	// HTTPClient is an HTTP client with a reasonable timeout value and
+	// potentially a custom *x509.CertPool based on the caCertificatesEnvVar
+	// environment variable (see the `initCertPool` function)
+	HTTPClient = http.Client{
 		Transport: &http.Transport{
 			Proxy: http.ProxyFromEnvironment,
 			Dial: (&net.Dialer{
@@ -26,8 +33,12 @@ var HTTPClient = http.Client{
 			TLSHandshakeTimeout:   15 * time.Second,
 			ResponseHeaderTimeout: 15 * time.Second,
 			ExpectContinueTimeout: 1 * time.Second,
+			TLSClientConfig: &tls.Config{
+				RootCAs: initCertPool(),
+			},
 		},
 	}
+)
 
 const (
 	// defaultGoUserAgent is the Go HTTP package user agent string. Too
@@ -36,8 +47,36 @@ const (
 
 	// ourUserAgent is the User-Agent of this underlying library package.
 	ourUserAgent = "xenolf-acme"
+
+	// caCertificatesEnvVar is the environment variable name that can be used to
+	// specify the path to PEM encoded CA Certificates that can be used to
+	// authenticate an ACME server with a HTTPS certificate not issued by a CA in
+	// the system-wide trusted root list.
+	caCertificatesEnvVar = "LEGO_CA_CERTIFICATES"
 )
 
+// initCertPool creates a *x509.CertPool populated with the PEM certificates
+// found in the filepath specified in the caCertificatesEnvVar OS environment
+// variable. If the caCertificatesEnvVar is not set then initCertPool will
+// return nil. If there is an error creating a *x509.CertPool from the provided
+// caCertificatesEnvVar value then initCertPool will panic.
+func initCertPool() *x509.CertPool {
+	if customCACertsPath := os.Getenv(caCertificatesEnvVar); customCACertsPath != "" {
+		customCAs, err := ioutil.ReadFile(customCACertsPath)
+		if err != nil {
+			panic(fmt.Sprintf("error reading %s=%q: %v",
+				caCertificatesEnvVar, customCACertsPath, err))
+		}
+		certPool := x509.NewCertPool()
+		if ok := certPool.AppendCertsFromPEM(customCAs); !ok {
+			panic(fmt.Sprintf("error creating x509 cert pool from %s=%q: %v",
+				caCertificatesEnvVar, customCACertsPath, err))
+		}
+		return certPool
+	}
+	return nil
+}
+
 // httpHead performs a HEAD request with a proper User-Agent string.
 // The response body (resp.Body) is already closed when this function returns.
 func httpHead(url string) (resp *http.Response, err error) {

acme/http_test.go

@@ -1,8 +1,10 @@
 package acme
 
 import (
+	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"strings"
 	"testing"
 )
@@ -98,3 +100,111 @@ func TestUserAgent(t *testing.T) {
 		t.Errorf("Expected custom UA to contain %s, got '%s'", UserAgent, ua)
 	}
 }
+
+// TestInitCertPool tests the http.go initCertPool function for customizing the
+// HTTP Client *x509.CertPool with an environment variable.
+func TestInitCertPool(t *testing.T) {
+	// writeTemp creates a temp file with the given contents & prefix and returns
+	// the file path. If an error occurs, t.Fatalf is called to end the test run.
+	writeTemp := func(t *testing.T, contents, prefix string) string {
+		t.Helper()
+		tmpFile, err := ioutil.TempFile("", prefix)
+		if err != nil {
+			t.Fatalf("Unable to create tempfile: %v", err)
+		}
+		err = ioutil.WriteFile(tmpFile.Name(), []byte(contents), 0700)
+		if err != nil {
+			t.Fatalf("Unable to write tempfile contents: %v", err)
+		}
+		return tmpFile.Name()
+	}
+
+	invalidFileContents := "not a certificate"
+	invalidFile := writeTemp(t, invalidFileContents, "invalid.pem")
+
+	// validFileContents is lifted from Pebble[0]. Generate your own CA cert with
+	// MiniCA[1].
+	// [0]: https://github.com/letsencrypt/pebble/blob/de6fa233ea1f283eeb9751d42c8e1ae72718c44e/test/certs/pebble.minica.pem
+	// [1]: https://github.com/jsha/minica
+	validFileContents := `
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
+	`
+	validFile := writeTemp(t, validFileContents, "valid.pem")
+
+	testCases := []struct {
+		Name        string
+		EnvVar      string
+		ExpectPanic bool
+		ExpectNil   bool
+	}{
+		// Setting the env var to a file that doesn't exist should panic
+		{
+			Name:        "Env var with missing file",
+			EnvVar:      "not.a.real.file.pem",
+			ExpectPanic: true,
+		},
+		// Setting the env var to a file that contains invalid content should panic
+		{
+			Name:        "Env var with invalid content",
+			EnvVar:      invalidFile,
+			ExpectPanic: true,
+		},
+		// Setting the env var to the empty string should not panic and should
+		// return nil
+		{
+			Name:        "No env var",
+			EnvVar:      "",
+			ExpectPanic: false,
+			ExpectNil:   true,
+		},
+		// Setting the env var to a file that contains valid content should not
+		// panic and should not return nil
+		{
+			Name:        "Env var with valid content",
+			EnvVar:      validFile,
+			ExpectPanic: false,
+			ExpectNil:   false,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.Name, func(t *testing.T) {
+			os.Setenv(caCertificatesEnvVar, tc.EnvVar)
+			defer os.Setenv(caCertificatesEnvVar, "")
+
+			defer func() {
+				if r := recover(); r == nil && tc.ExpectPanic {
+					t.Errorf("expected initCertPool() to panic, it did not")
+				} else if r != nil && !tc.ExpectPanic {
+					t.Errorf("expected initCertPool() to not panic, but it did")
+				}
+			}()
+
+			result := initCertPool()
+
+			if result == nil && !tc.ExpectNil {
+				t.Errorf("initCertPool() returned nil, expected non-nil")
+			} else if result != nil && tc.ExpectNil {
+				t.Errorf("initCertPool() returned non-nil, expected nil")
+			}
+		})
+	}
+}