GelfWriter: actually verify TLS server certificates

And add a new option insecure_noverify to explicitly disable it if desired.

doc/09-object-types.md

@@ -1326,6 +1326,7 @@ Configuration Attributes:
   enable\_send\_perfdata    | Boolean               | **Optional.** Enable performance data for 'CHECK RESULT' events.
   enable\_ha                | Boolean               | **Optional.** Enable the high availability functionality. Only valid in a [cluster setup](06-distributed-monitoring.md#distributed-monitoring-high-availability-features). Defaults to `false`.
   enable\_tls               | Boolean               | **Optional.** Whether to use a TLS stream. Defaults to `false`.
+  insecure\_noverify        | Boolean               | **Optional.** Disable TLS peer verification.
   ca\_path                  | String                | **Optional.** Path to CA certificate to validate the remote host. Requires `enable_tls` set to `true`.
   cert\_path                | String                | **Optional.** Path to host certificate to present to the remote host for mutual verification. Requires `enable_tls` set to `true`.
   key\_path                 | String                | **Optional.** Path to host key to accompany the cert\_path. Requires `enable_tls` set to `true`.

lib/perfdata/gelfwriter.cpp

@@ -206,6 +206,18 @@ void GelfWriter::ReconnectInternal()
 				<< "TLS handshake with host '" << GetHost() << " failed.'";
 			throw;
 		}
+
+		if (!GetInsecureNoverify()) {
+			if (!tlsStream.GetPeerCertificate()) {
+				BOOST_THROW_EXCEPTION(std::runtime_error("Graylog Gelf didn't present any TLS certificate."));
+			}
+
+			if (!tlsStream.IsVerifyOK()) {
+				BOOST_THROW_EXCEPTION(std::runtime_error(
+					"TLS certificate validation failed: " + std::string(tlsStream.GetVerifyError())
+				));
+			}
+		}
 	}
 
 	SetConnected(true);

lib/perfdata/gelfwriter.ti

@@ -34,6 +34,9 @@ class GelfWriter : ConfigObject
     [config] bool enable_tls {
         default {{{ return false; }}}
     };
+    [config] bool insecure_noverify {
+        default {{{ return false; }}}
+    };
     [config] String ca_path;
     [config] String cert_path;
     [config] String key_path;