Caius Theory

Now with even more cowbell…

Tailscale, RFC1918, and DNS Rebinding Protection

Edit: Originally this post was written to be a workaround for Tailscale routing all DNS traffic over its own link when you configured it to push out existing DNS Server IPs. This turned out to be a bad assumption on my part. Thanks to apenwarr for helping me understand that shouldn’t be the case, and encouraging me to debug it properly rather than making assumptions.

Naturally it turned out to be a PEBKAC. I’d pushed out as the DNS Server IP which is a nameserver rather than a forwarder. This in turn meant people were getting empty answers back to DNS queries, which stopped once they quit tailscale. (Go figure, Tailscale removes the resolver from the network stack when it quits.) The post has been updated to remove that invalid assumption. 🤦🏻‍♂️

Imagine we have a fleet of machines sat in a private network somewhere on a IP range, with entries pointing at them published on public DNS servers. Eg, dig +short workhorse.fake.tld returns

Initially this all works swimmingly, until someone comes along that is using a DNS forwarder that with DNS rebinding protection enabled. Daniel Miessler has a wonderfully succinct explanation on his blog about DNS Rebinding attacks, but to protect against it you stop your resolver returning answers to DNS queries from public servers which resolve to IP addresses within standard internal network ranges. (ie, rfc1918.)

This means for those users they can successfully connect to our Tailscale network and access everything by IPs directly, but can’t access any of the internal infrastructure by hostname. eg, dig +short workhorse.fake.tld will return an empty answer for them.

Once we figured out the root cause of that, for workarounds we figured we could either run a DNS forwarder within our own infrastructure, or get all our staff to change their home DNS settings and hope they were never on locked down networks ever again.

We chose the former, and thankfully dnsmasq is really easy to configure in this fashion and we already have a node which is acting as the tailscale subnet relay, so we dropped the following config in /etc/dnsmasq.conf on there:

# Only listen for requests from VPN/local for debugging

# Google DNS
# Quad9
# Cloudflare
# Race all servers to see which wins

# Try and stop DNS rebinding, except where we expect it to happen


One quick puppet run later, and our Tailscale subnet relays are happily running both tailscale and dnsmasq, serving out answers as fast as they can to other Tailscale nodes. Add port 53 to the Tailscale ACL and away we went.