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pgproxy
The pgproxy server is a proxy for the Postgres wire protocol. Read more in our blog post about it!
The proxy runs an in-process Tailscale instance, accepts postgres client connections over Tailscale only, and proxies them to the configured upstream postgres server.
This proxy exists because postgres clients default to very insecure connection settings: either they "prefer" but do not require TLS; or they set sslmode=require, which merely requires that a TLS handshake took place, but don't verify the server's TLS certificate or the presented TLS hostname. In other words, sslmode=require enforces that a TLS session is created, but that session can trivially be machine-in-the-middled to steal credentials, data, inject malicious queries, and so forth.
Because this flaw is in the client's validation of the TLS session, you have no way of reliably detecting the misconfiguration server-side. You could fix the configuration of all the clients you know of, but the default makes it very easy to accidentally regress.
Instead of trying to verify client configuration over time, this proxy removes the need for postgres clients to be configured correctly: the upstream database is configured to only accept connections from the proxy, and the proxy is only available to clients over Tailscale.
Therefore, clients must use the proxy to connect to the database. The client<>proxy connection is secured end-to-end by Tailscale, which the proxy enforces by verifying that the connecting client is a known current Tailscale peer. The proxy<>server connection is established by the proxy itself, using strict TLS verification settings, and the client is only allowed to communicate with the server once we've established that the upstream connection is safe to use.
A couple side benefits: because clients can only connect via Tailscale, you can use Tailscale ACLs as an extra layer of defense on top of the postgres user/password authentication. And, the proxy can maintain an audit log of who connected to the database, complete with the strongly authenticated Tailscale identity of the client.