libreCMC/docs/Layer_2_OpenVPN.md

Layer 2 OpenVPN

Introduction

Librecmc can operate as an OpenVPN server. OpenVPN technology connects two networks via an encrypted tunnel. With proper server, network, and client configuration, OpenVPN allows a client outside of your LAN to see the LAN as though it were physically connected to the LAN.

OpenVPN can run in layer 2 or layer 3 mode. In layer 3 mode, the remote client sees your LAN as though it is on the other side of an IP router. In layer 2 mode, the remote client sees your LAN as though they are both on the same Data Link segment (e.g., the same Ethernet link). Layer 3 mode is easier to set up, but layer 2 mode is sometimes desired to give clients a more direct exposure to services on the LAN.

The scenario we are targeting here is to have:

• An OpenVPN server running on a libreCMC system which can receive traffic from the Internet.
• An OpenVPN client running on a libreCMC system somewhere else in the Internet. The client can be hidden behind a NAT.

Warnings

This information is provided for educational purposes only and is not meant to be a guide to best network security practices. Readers are advised to study all relevant OpenVPN and network security documentation.

Server Setup and Configuration

Required LibreCMC packages

• openvpn-openssl
• openvpn-easy-rsa
• luci-app-openvpn

Interface Setup

In LuCi, select Network >> Interfaces and then Add New Interface.

• Set Name of the new interface to l2server.
• Set Protocol of the new interface to unmanaged.
• Set Cover the following interface to Custom Interface: vpn0.

In my working configuration, I added tap0 into the LAN bridge interface, and deleted the WAN interface. However, my vpn server is a separate unit on my network, intended to operate in "bridge mode", where if you server is your gateway router, a different configuration might be necessary.

Certificate and Key Setup Instructions

cd /etc/easy-rsa
source vars
clean-all
build-ca
build-dh
build-key-server l2server
openvpn --genkey --secret /etc/easy-rsa/keys/ta.key
mkdir -m 700 /etc/openvpn/keys
mv ca.crt l2server.crt l2server.key dh2018.pem /etc/openvpn/keys

N.B.: Using easy-rsa is a straightforward approach, but it may be possible to produce more secure certificates using openssl directly.

Generating a Client Key

build-key l2client

In the server-client configuration we are aiming for here, your client key needs to be unencrypted (i.e., not password protected).

Server configuration

For the server bridge option: The first two parameters are the ip and netmask of the gateway on the bridged subnet. The next two parameters indicate the pool-start-IP and pool-end-IP, which is the part of your IP address pool that you have reserved just for VPN clients. You must to make sure that the DHCP server for your LAN is not leasing out those IP addresses to local (non-vpn) clients.

uci set openvpn.l2server=openvpn
uci set openvpn.l2server.enabled='1'
uci set openvpn.l2server.dev='tap0'
uci set openvpn.l2server.port='1194'
uci set openvpn.l2server.proto='udp'
uci set openvpn.l2server.keepalive='10 120'
uci set openvpn.l2server.persist_key='1'
uci set openvpn.l2server.persist_tun='1'
uci set openvpn.l2server.user='nobody'
uci set openvpn.l2server.group='nogroup'
uci set openvpn.l2server.ca='/etc/openvpn/keys/ca.crt'
uci set openvpn.l2server.cert='/etc/openvpn/keys/l2server.crt'
uci set openvpn.l2server.key='/etc/openvpn/keys/l2server.key'
uci set openvpn.l2server.dh='/etc/openvpn/keys/dh2048.pem'
uci set openvpn.l2server.tls_server='1'
uci set openvpn.l2server.tls_auth='/etc/openvpn/keys/ta.key 0'
uci set openvpn.l2server.server_bridge='192.168.1.1 255.255.255.0 192.168.1.201 192.168.1.220'
uci set openvpn.l2server.client_to_client='1'
uci set openvpn.l2server.push='persist-key' 'persist-tun' 'redirect-gateway def1'
uci set 'route 192.168.1.0 255.255.255.0' 'dhcp-option DNS 192.168.1.1'
uci set openvpn.l2server.mute='15'
uci set openvpn.l2server.verb='3'
uci commit

Firewall

I do not want to describe firewall adjustments in this document, but the important point is that your server needs to be able to receive UDP packages from the Internet on port 1194.

Client Setup and Configuration

This assumes the above configuration for the server.

Required LibreCMC packages

• openvpn-openssl
• luci-app-openvpn

Interface setup

In LuCi, select Network >> Interfaces and then Add New Interface.

• Set Name of the new interface to l2client.
• Set Protocol of the new interface to unmanaged.
• Set Cover the following interface to Custom Interface: vpn0.

Adjust the LAN interface so that it bridges over the vpn0 physical interface as well as the default eth0 and wlan0 interfaces. This is done from the Network >> Interfaces menu, pressing the Edit button next to LAN, and selecting the Physical Settings tab.

Certificate and key storage

mkdir -m 700 /etc/openvpn/keys

Client will the need ca.crt, l2client.crt, l2client.key', and ta.keyyou generated in the server section, stored in the /etc/openvpn/keys` directory.

Client configuration

uci set openvpn.l2client=openvpn
uci set openvpn.l2client.float='1'
uci set openvpn.l2client.client='1'
uci set openvpn.l2client.dev='tap'
uci set openvpn.l2client.reneg_sec='0'
uci set openvpn.l2client.persist_key='1'
uci set openvpn.l2client.nobind='1'
uci set openvpn.l2client.remote_cert_tls='server'
uci set openvpn.l2client.remote='remote.alaskasi.com'
uci set openvpn.l2client.proto='udp'
uci set openvpn.l2client.rport='1194'
uci set openvpn.l2client.resolv_retry='infinite'
uci set openvpn.l2client.mute_replay_warnings='1'
uci set openvpn.l2client.key_direction='1'
uci set openvpn.l2client.redirect_gateway='def1'
uci set openvpn.l2client.enabled='1'
uci set openvpn.l2client.ca='/etc/openvpn/keys/ca.crt'
uci set openvpn.l2client.cert='/etc/openvpn/keys/l2client.crt'
uci set openvpn.l2client.key='/etc/openvpn/keys/l2client.key'
uci set openvpn.l2client.tls_auth='/etc/openvpn/keys/ta.key 1'
uci set openvpn.l2client.mute='15'
uci set openvpn.l2client.verb='3'
uci commit

Troubleshooting

You are likely to run into one of two issues:

• Either your client or your server is not receiving UDP packets from the other.
• You have an error in the server or client configurations.
• There is some problem with the keys or certificates, or they are in the wrong location.

These are some useful tools:

• Without OpenVPN even running, you can use the nc program (netcat) to send UDP packets from the client to the server, and then use the tcpdump program on the server to see if the UDP packets are arriving at port 1194. The syntax of these programs will not be covered in this document.

• The log output on the server and on the client is very helpful. Run logread to view the log or logread && logread -f to monitor for log messages. If you find an OpenVPN error, use that in conjunction with the OpenVPN manual page, to figure out what needs to be tweaked.[1]

[1] [https://openvpn.net/index.php/open-source/documentation/manuals.html]