Added the "OpenVPNPushDummyIPv4AddressOnL2Mode" option for the OpenVPN L2 mode.

To fix the bug of OpenVPN 2.4.6 and particular version of kernel mode TAP driver on Linux, the TAP device must be up after the OpenVPN client is connected. However there is no direct push instruction to do so to OpenVPN client. Therefore we push the dummy IPv4 address (RFC7600) to the OpenVPN client to enforce the TAP driver UP state.

src/Cedar/Cedar.c

@@ -1554,6 +1554,8 @@ CEDAR *NewCedar(X *server_x, K *server_k)
 
 	StrCpy(c->OpenVPNDefaultClientOption, sizeof(c->OpenVPNDefaultClientOption), OVPN_DEF_CLIENT_OPTION_STRING);
 
+	c->OpenVPNPushDummyIPv4AddressOnL2Mode = true; // Default true. Override by the config file.
+
 #ifdef	BETA_NUMBER
 	c->Beta = BETA_NUMBER;
 #endif	// BETA_NUMBER

src/Cedar/Cedar.h

@@ -1083,6 +1083,7 @@ typedef struct CEDAR
 	char OpenVPNDefaultClientOption[MAX_SIZE];	// OpenVPN: Default Client Option String
 	bool OpenVPNObfuscation;					// OpenVPN: Obfuscation mode
 	char OpenVPNObfuscationMask[MAX_SIZE];		// OpenVPN: String (mask) for XOR obfuscation
+	bool OpenVPNPushDummyIPv4AddressOnL2Mode;	// OpenVPN: Push a dummy IPv4 address on L2 mode
 } CEDAR;
 
 // Type of CEDAR

src/Cedar/Proto_OpenVPN.c

@@ -2351,6 +2351,18 @@ void OvsRecvPacket(OPENVPN_SERVER *s, LIST *recv_packet_list, UINT protocol)
 									OvsLog(s, se, c, "LP_SET_IPV4_PARAM",
 										ip_client, ip_subnet_mask, ip_defgw, ip_dns1, ip_dns2, ip_wins1, ip_wins2);
 								}
+								else
+								{
+									// OpenVPN L2 mode. To fix the bug of OpenVPN 2.4.6 and particular version of kernel mode TAP driver
+									// on Linux, the TAP device must be up after the OpenVPN client is connected.
+									// However there is no direct push instruction to do so to OpenVPN client.
+									// Therefore we push the dummy IPv4 address (RFC7600) to the OpenVPN client.
+
+									if (s->Cedar->OpenVPNPushDummyIPv4AddressOnL2Mode)
+									{
+										StrCat(option_str, sizeof(option_str), ",ifconfig 192.0.0.8 255.255.255.240");
+									}
+								}
 
 								WriteFifo(c->SslPipe->SslInOut->SendFifo, option_str, StrSize(option_str));
 

src/Cedar/Server.c

@@ -5872,6 +5872,17 @@ void SiLoadServerCfg(SERVER *s, FOLDER *f)
 			}
 		}
 
+		// OpenVPN Push a dummy IPv4 address on L2 mode
+		if (CfgIsItem(f, "OpenVPNPushDummyIPv4AddressOnL2Mode") == false)
+		{
+			// Default enable
+			c->OpenVPNPushDummyIPv4AddressOnL2Mode = true;
+		}
+		else
+		{
+			c->OpenVPNPushDummyIPv4AddressOnL2Mode = CfgGetBool(f, "OpenVPNPushDummyIPv4AddressOnL2Mode");
+		}
+
 		// Disable the NAT-traversal feature
 		s->DisableNatTraversal = CfgGetBool(f, "DisableNatTraversal");
 
@@ -6289,6 +6300,8 @@ void SiWriteServerCfg(FOLDER *f, SERVER *s)
 
 		CfgAddStr(f, "OpenVPNDefaultClientOption", c->OpenVPNDefaultClientOption);
 
+		CfgAddBool(f, "OpenVPNPushDummyIPv4AddressOnL2Mode", c->OpenVPNPushDummyIPv4AddressOnL2Mode);
+
 		if (c->Bridge == false)
 		{
 			OPENVPN_SSTP_CONFIG config;