With the advance of cheap MikroTik routers and ready to use CHR instances, setting up a VPN concentrator for remote access has become an easy task. Moving even further, a single router could provide VPN access and dynamic routing to integrate remote networks to the backbone.
I have started a gig as a consultant and sysadmin for a logistics insurance company, and one of my first proposals was to improve the network acess for road warriors and remote workers.
There was a Proxmox hypervisor, with some Windows 2012 R2 servers, providing Terminal Services, to execute a locally installed client for an ERP system. Proxmox was also using iptables on its the Debian backend of the to masquerade the VM networks with a public IP address, for Internet connectivity, dstnat rules for a NGINX reverse proxy, and RDP for the Windows servers..
I guess we all know having internet-exposed RDP is not a good idea, even if it is running in a non default port, so the former sysadmin transitioned to a SSH tunnel system, where the users connected to the hypervisor via SSH to establish tunnel to the desired server.
This solution, which I considered not elegant, was the only available at the moment due to networking constraints of the VPS provider, so really it was the best they were able to do, and it worked fine for them.
Networks behind the tunnel endpoints were routed with static routers, so I configured a quick multi-area OSPF routing system, with the directly connected networks on area 0, along with the /30 network of the tunnel. I added an additional area on both ends, for the future VPN networks. Once OSPF was working as expected, I remove the static routes.
Securing the tunnels
This interconnection via the L2TP tunnel was just plain ol’ L2TP, without IPsec. This is no bueno, and could be improved. Fortunately, IPsec configuration on MikroTik is trivial. Just select “Use IPsec” on both ends, and use the same IPsec pre-shared key.
This can of course be configured via CLI. Would you like some RouterOS configuration Ansible on next posts? Let me know in the comments.
/interface l2tp-server server set authentication=mschap1,mschap2 default-profile=VPN enabled=yes ipsec-secret="PUT_A_SECRET_HERE" use-ipsec=yes
It’s always a good idea to copy the default-encryption profile, and create a new one based on that template. I set up a local address which was of course, part of the networks announced in a separate area by the OSPF process. I also added a IP pool to be able to provide dynamic addresses for the VPN users.
Maybe you are aware that in the Cisco world, you have to use
tcp adjust-mss to adjust the maximum TCP segment size, to advoid fragmentation of packets over the tunnel. Fortunately, this is configured by default on RouterOS.
Finally, to be able to redirect the dial-in to a RADIUS server, we need to instruct the PPP AAA system to use RADIUS, as shown next.
Setting up RADIUS authentication
RADIUS servers are very simple to set up on RouterOS.
Under the RADIUS submenu, add a new server for PPP service, and configure the following parameters.
- IP address of the radius server
- RADIUS secret
- Authentication and accounting ports, usually 1812 and 1813. Some servers use 1645 for accounting. Those are all UDP.
- REALM if your server supports that extension
- Which source address should the router use for its NAS-IP-Address
Using Windows NPS as a RADIUS server
NPS can work without a Certificate Authority but if you are working in an Active Directory environment, you’ll save a lot of headaches by installing the CA role.
In my particular scenario, the server was not part of a domain so the certificate generation and association was skipped.
Once the roles have been configured, I headed to the NPS service configuration, and add new RADIUS client.
Make sure to match the RADIUS secret and the source IP address as you configured on the MikroTik side.
Next, the network access policies. I wanted to match the NAS IPv4 address, and the authentication types. If you are not familiar with the RADIUS lang, NAS stands for Network Access Server, which in this case, is the MikroTik router which provides the VPN service.
I had to use a CHAP fallback due to some legacy devices withuout MSCHAP support.
Next, I added a new Access condition, matched the NAS address once again, and selected the local server as point of authentication.
Once everything was properly configured, I set up the VPN client on my side, which looks like as follows. The idea of using NPS as RADIUS was to be able to use my Windows account credentials for the VPN.
I verified the successful authentication on the router logs, and the VPN was sucessfully connected.