Connecting to external services with IP block lists via Tailscale

If you’re migrating from a traditional office networks or a centralized VPN concentrator, you might find you have external servers that don’t run Tailscale but still need to have their connections secured.

Third-party services, or internal services running on “serverless” cloud providers such as Heroku, might have configured an IP block list (sometimes known as an IP whitelist) that are expecting all your user traffic to originate from a single IP address or a small number of IP addresses. Since Tailscale doesn’t need to send all traffic through a central concentrator, your user traffic will suddenly start arriving from all over the Internet, running into the IP block list protections operated by your service provider.

What is an IP block list?

Traditionally, it has been hard to set up secure links between machines in different locations. As a result, people would operate “internal use” services that still use Internet-facing ports and IP addresses, then connect to those servers from their office locations. We often see this behaviour with ssh, RDP (remote desktop protocol), and various web services.

The exposed port allows attacks from anywhere on the Internet, such as port scanning, password guessing, infrastructure zerodays, and so on. Even if the service itself requires encryption and 2-factor authentication, a bug anywhere in the stack could result in a complete bypass of security protections.

To make these kinds of attacks harder, traditional best practice is to restrict, usually at the firewall level, which IP addresses are even allowed to initiate connections to your service. This makes your server “invisible” to most people on the Internet, so they don’t even know where to attack.

Unfortunately, IP block lists have a few flaws:

  • IP block lists are, essentially, “security by obscurity.” If they can learn which IP addresses are allowed to initiate connections - and this is not always too hard to guess, if it’s the IP address of your office network for example - then a sufficiently dedicated attacker can forge packets from that address, or take control of a machine located at that address (for example using a botnet), to talk to the server.

  • Users are not all at a single location. Remote workers, multiple offices, or your own servers running in various datacenters will all generate traffic coming from different IP addresses. IP block lists need to get longer and longer in order to make sure all those locations can access the service, and the longer the block list, the more ways an attacker can get in.

  • Cloud VPCs make IP address propagation even worse. If you have internal services running in a cloud provider and accessing the Internet via NAT gateways and elastic IPs, then your traffic might come from a variety of IPs, possibly entire subnets. This can require very wide IP block lists to cover, for example, entire AWS address blocks just in case your services get allocated one of those blocks. Needless to say, this makes an attacker’s job much easier, since all they need to do is run their attack from one of those cloud providers, and wait until they get allocated an IP address in one of your permitted subnets.

  • To avoid this problem, some companies route all their internal traffic through a single VPN concentrator or central Internet gateway/NAT. This makes all traffic seem to come from a single address, but adds a lot of latency for people whose devices are not geographically near the NAT. It also adds a single point of failure and a choke point for Internet bandwidth.

Nevertheless, IP block listed services are a fact of life. If you can’t make a secure VPN tunnel to your service provider, for example because you don’t have rights to install any software at the hosting provider or because the software is operated by a third party, IP block lists are still considered a best practice, in addition to other security measures.

Using Tailscale to improve on IP block lists

In a pure Tailscale network, you don’t need IP block lists because you have something better: Tailscale’s secure IP addresses, which aren’t allowed over the physical network, plus Tailscale role-based ACLs, which let you configure exactly which groups of users are allowed to see a particular server. This allows you to build something like a central IP block list, but using encryption keys instead of IP addresses for protection.

However, most networks are not pure Tailscale, and you eventually need to integrate with a third-party service. Luckily, Tailscale can help you at greatly reduce the size of your IP block list (and thus the surface area for attack), without centralizing your network access and while letting your users access the service from anywhere. Plus, you can still use Tailscale role-based ACLs to control access.

What we’ll do is set up a Tailscale relay (also known as an “exit node”) that can route traffic for just the servers with the block list.

The necessary steps are:

  1. Set up a Linux machine or VM node, ideally somewhere near the external service you want to reach. (For example, if the service is in us-west, start a Linux machine in a datacenter or cloud provider in us-west.) Make sure your VM has a public-facing, static IP address.

  2. Make sure the VM’s public-facing IP address is in the IP block list of the service it needs to access. Eventually, you might want to remove all other IP addresses and subnets from the list.

  3. Install tailscaled on the Linux machine.

  4. Tell the Linux tailscaled to advertise that it can provide access to the IP address of the service in question. For example, let’s say you want to access the service at 8.8.8.8 and 8.8.4.4. You would use

    tailscale up --advertise-routes=8.8.8.8,8.8.4.4
  5. Ensure that your ACL policies are configured to allow the correct users or groups to be able to access the server IPs in question.

  6. As a Tailscale domain administrator, enable the advertised routes using the admin panel.

That’s it! Now all your organization’s traffic to the external service will be routed through your new relay node, as long as it is permitted by your ACL policy.

Note

--advertise-routes requires IP forwarding to be enabled. If you see an error about IP forwarding, follow the steps here.

How does it work?

Tailscale is a multi-point VPN, which means every client device (node) can be connected to multiple servers and/or relays (other nodes) at the same time. Because Tailscale’s WireGuard connections are so lightweight, each node can connect to as many other nodes as it wants, simultaneously.

In this example, what we’ve done is create a node that advertises a subnet route to your external service (in our example, 8.8.8.8/32 and 8.8.4.4/32). Normally, a subnet relay node is used to provide access to an office network or VPC without having to install Tailscale on every node on the subnet. But there’s no particular reason a subnet route has to include more than one or two machines, and there’s no reason you can’t provide a route to an IP address that’s on an external network.

The advertised route gets shared with the Tailscale coordination server. Then the admin needs to approve it (this is to prevent abuse, since otherwise any user could set up a node that tries to advertise subnet routes), and the routing and public key information are distributed automatically to every node in your network.

The Tailscale role-based ACL policies are compiled into packet filters and then distributed automatically to each node as needed. The packet filter provided to your Linux relay node lets it automatically restrict which users it will allow to access the external service.

You can read more information about Tailscale and subnet relays.

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