Add option to specify additional CA certificates for SSL peer validation

main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"crypto/tls"
+	"crypto/x509"
 	"encoding/pem"
 	"flag"
 	"fmt"
@@ -62,6 +63,7 @@ var (
 	clientCertFile  string
 	fourOnly        bool
 	sixOnly         bool
+	cacert          string
 
 	// number of redirects followed
 	redirectsFollowed int
@@ -84,6 +86,7 @@ func init() {
 	flag.StringVar(&clientCertFile, "E", "", "client cert file for tls config")
 	flag.BoolVar(&fourOnly, "4", false, "resolve IPv4 addresses only")
 	flag.BoolVar(&sixOnly, "6", false, "resolve IPv6 addresses only")
+	flag.StringVar(&cacert, "cacert", "", "CA certificate to verify peer against (SSL)")
 
 	flag.Usage = usage
 }
@@ -140,6 +143,23 @@ func main() {
 	visit(url)
 }
 
+// readCACerts - helper function to load additional CA certificates
+func readCACerts(filename string) (*x509.CertPool, error) {
+	if filename == "" {
+		return nil, nil
+	}
+	certFileBytes, err := ioutil.ReadFile(filename)
+
+	if err != nil {
+		return nil, fmt.Errorf("failed to read CA certificate file: %v", err)
+	}
+
+	certPool := x509.NewCertPool()
+	certPool.AppendCertsFromPEM(certFileBytes)
+
+	return certPool, nil
+}
+
 // readClientCert - helper function to read client certificate
 // from pem formatted file
 func readClientCert(filename string) []tls.Certificate {
@@ -269,10 +289,16 @@ func visit(url *url.URL) {
 			host = req.Host
 		}
 
+		rootCAs, err := readCACerts(cacert)
+		if err != nil {
+			log.Printf("warning: failed to read CA certificates: %s\n", err)
+		}
+
 		tr.TLSClientConfig = &tls.Config{
 			ServerName:         host,
 			InsecureSkipVerify: insecure,
 			Certificates:       readClientCert(clientCertFile),
+			RootCAs:            rootCAs,
 		}
 
 		// Because we create a custom TLSClientConfig, we have to opt-in to HTTP/2.