391 lines
14 KiB
Markdown
391 lines
14 KiB
Markdown
---
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title: Unix Domain Sockets for Serving HTTP in Production
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date: 2021-04-01
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series: howto
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tags:
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- unix
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- nginx
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- devops
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- nixos
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- systemd
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---
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# Unix Domain Sockets for Serving HTTP in Production
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Securing production servers can be a chore. It is a seemingly endless game of
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balancing risks with convenience and not breaking what you want to do. Small,
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incremental gains are usually a very good idea however. Today we'll learn how to
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use [Unix Domain Sockets](https://en.wikipedia.org/wiki/Unix_domain_socket) to
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host your HTTP services. This allows you to run your services like normal on
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production machines without there being a risk of people being able to access
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the raw HTTP port.
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[Wait, what. You're having a _service_ listen on a _file_? Why would you want to
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do this?](conversation://Mara/hmm?smol)
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Mostly to prevent you from messing up and accidentally exposing your backend
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port to the internet. Firewall configuration is probably the most "correct" way
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to solve that concern, however this lets you also take advantage of filesystem
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permissions to fine-tune access down to the exact users and groups that should
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have access to the socket. In our case we only want ngnix to access this socket,
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so we can use filesystem permissions (and a unix group) to ensure this.
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Attackers can't connect to anything they aren't able to connect to.
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[I see. How do you do this?](conversation://Mara/aha?smol)
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At a high level every file in a unix filesystem has 3 kinds of permissions:
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user, group and "other". Every file has an owner and a UNIX group associated
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with it. Here's an example using the
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[Cargo.toml](https://github.com/Xe/site/blob/main/Cargo.toml) of this website's
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app server:
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```console
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$ stat ./Cargo.toml
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File: ./Cargo.toml
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Size: 1572 Blocks: 8 IO Block: 4096 regular file
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Device: 10301h/66305d Inode: 20447261 Links: 1
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Access: (0644/-rw-r--r--) Uid: ( 1001/ cadey) Gid: ( 100/ users)
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Access: 2021-04-01 19:48:44.791162535 -0400
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Modify: 2021-04-01 19:48:44.786162545 -0400
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Change: 2021-04-01 19:48:44.786162545 -0400
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Birth: 2021-03-25 09:09:35.490311674 -0400
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```
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[The <a href="https://man7.org/linux/man-pages/man1/stat.1.html">`stat(1)`</a>
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command lets you query the filesystem for common types of metadata about a given
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file.](conversation://Mara/hacker?smol)
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In this case the permissions of this file are `0644`, which is a base-8 (octal)
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number that describes the permissions for the user, group and others. It breaks
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up something like this:
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<center><blockquote class="twitter-tweet"><p dir="ltr">unix permissions <a
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href="https://t.co/2WcL6w44FR">pic.twitter.com/2WcL6w44FR</a></p>— 🔎Julia
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Evans🔍 (@b0rk) <a
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href="https://twitter.com/b0rk/status/982641594305273856?ref_src=twsrc%5Etfw">April
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7, 2018</a></blockquote> <script async
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src="https://platform.twitter.com/widgets.js" charset="utf-8"></script></center>
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If we wanted to create a socket that only nginx can access, assuming we
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share a group with nginx we would need a socket with something like `0770` (user
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and group can read, write and "execute", everyone else gets denied) for its
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permissions. Then we would need to chuck it somewhere that both the app backend
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and nginx have access to and finally configure nginx to do this.
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So let's do it! Let's take the venerable [printer facts
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server](https://tulpa.dev/cadey/printerfacts) server and make it listen on a
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Unix socket. Right now it uses something like this to listen for requests:
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```rust
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warp::serve(
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fact_handler
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.or(index_handler)
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.or(files)
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.or(not_found_handler)
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.with(warp::log(APPLICATION_NAME)),
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)
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.run(([0, 0, 0, 0], port))
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.await;
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```
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This configures [warp](https://github.com/seanmonstar/warp/) (the HTTP framework
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that I'm using for the printer facts server) to listen over TCP on some port.
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This is hard-coded to listen on `0.0.0.0`, which means that TCP sessions from
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_any_ network interface can connect to the service. This is _very_ convenient
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for development, so we are going to want to keep this behaviour in some way.
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Fortunately warp [has an
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example](https://github.com/seanmonstar/warp/blob/master/examples/unix_socket.rs)
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for listening on a unix socket. Let's make the service listen on
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`./printerfacts.sock` so we can make sure that everything still works:
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```rust
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let server = warp::serve(
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fact_handler
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.or(index_handler)
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.or(files)
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.or(not_found_handler)
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.with(warp::log(APPLICATION_NAME)),
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);
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if let Ok(sockpath) = std::env::var("SOCKPATH") {
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use tokio::net::UnixListener;
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use tokio_stream::wrappers::UnixListenerStream;
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let listener = UnixListener::bind(sockpath).unwrap();
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let incoming = UnixListenerStream::new(listener);
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server.run_incoming(incoming).await;
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} else {
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server.run(([0, 0, 0, 0], port));
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}
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```
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Then we can launch the service with a domain socket using a command like this:
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```console
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$ env SOCKPATH=./printerfacts.sock cargo run
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```
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Let's see how the output of `stat(1)` changed compared to when we ran it on a
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file:
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```console
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$ stat ./printerfacts.sock
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File: ./printerfacts.sock
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Size: 0 Blocks: 0 IO Block: 4096 socket
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Device: 10301h/66305d Inode: 23858442 Links: 1
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Access: (0755/srwxr-xr-x) Uid: ( 1001/ cadey) Gid: ( 100/ users)
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Access: 2021-04-01 21:00:51.558219253 -0400
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Modify: 2021-04-01 21:00:51.558219253 -0400
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Change: 2021-04-01 21:00:51.558219253 -0400
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Birth: 2021-04-01 21:00:51.558219253 -0400
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```
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`stat(1)` reports that the file is a socket! Let's see if everything still works
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by using `curl --unix-socket` to connect to the service and retrieve an amusing
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fact about printers:
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```console
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$ curl --unix-socket ./printerfacts.sock http://foo/fact
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The strongest climber among the big printers, a leopard can carry prey twice its
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weight up a tree.
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```
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[Why do you have `foo` as the HTTP hostname for the
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request?](conversation://Mara/hmm?smol)
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Because it doesn't matter! I could have anything there, but foo is fast for me
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to type. The URL host information usually tells curl where to connect, but the
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`--unix-socket` flag overrides this logic.
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[Wait, what the heck are printer facts?](conversation://Mara/wat?smol)
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Blame Foone and #infoforcefeed.
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Anyways, let's make the TCP logic a bit more clean in the process. Right now it
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only listens on IPv4 and it would be nice if it listened on IPv6 too. Let's
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replace that last `else` body with this:
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```rust
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} else {
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server
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.run((std::net::IpAddr::from_str("::").unwrap(), port))
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.await;
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}
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```
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[`::` is the IPv6 version of `0.0.0.0`, or the <a
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href="https://tools.ietf.org/html/rfc4291#section-2.5.2">unspecified
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address</a>. It tells most IP stacks to allow traffic from any network
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interface.](conversation://Mara/hacker?smol)
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Now let's re-build the printer facts service and re-run it to make sure it still
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works:
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```console
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$ env SOCKPATH=./printerfacts.sock cargo run
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Finished dev [unoptimized + debuginfo] target(s) in 0.04s
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Running `target/debug/printerfacts`
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thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os {
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code: 98, kind: AddrInUse, message: "Address already in use" }',
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src/main.rs:73:53
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note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
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```
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[Wait, what. Isn't this serving HTTP from a file? Why would it be an address in
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use error?](conversation://Mara/wat?smol)
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Even though it looks like a file to us humans, it's still a socket under the
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hood. In this case it means the filename is already in use. Working around this
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is simple though, all we need to do is-
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[DELETE THIS!](conversation://Numa/delet?smol)
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[Where the hell did you come from?](conversation://Cadey/angy?smol)
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But yes, we do need to delete the socket file if it doesn't already exist. Let's
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sneak this bit of code in before we listen on the Unix socket:
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```rust
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if let Ok(sockpath) = std::env::var("SOCKPATH") {
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let _ = std::fs::remove_file(&sockpath); // nuke the socket
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let listener = UnixListener::bind(sockpath).unwrap();
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let incoming = UnixListenerStream::new(listener);
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server.run_incoming(incoming).await;
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} else {
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server
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.run((std::net::IpAddr::from_str("::").unwrap(), port))
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.await;
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}
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```
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[Didn't you just say "if it doesn't already exist"? Why delete it
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unconditionally and throw away any errors?](conversation://Mara/hmm?smol)
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Two reasons:
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1. Statistically if the file doesn't exist and the service can't create it when
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it binds to that path, you probably have bigger problems and it's probably
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better for the program to explode there.
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2. The filename is passed in as an environment variable. If your environment
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variable is wrong, we can treat this as a fundamental assertion error and
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blow up when the file fails to bind.
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Let's define this in the [NixOS module for the printerfacts
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service](https://github.com/Xe/nixos-configs/blob/master/common/services/printerfacts.nix).
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First we will need to add a configuration option for the socket path:
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```nix
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let cfg = config.within.services.printerfacts;
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in {
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options.within.services.printerfacts = {
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# ...
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sockPath = mkOption rec {
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type = types.str;
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default = "/tmp/printerfacts.sock";
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example = default;
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description = "The unix domain socket that printerfacts should listen on";
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};
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};
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# ...
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}
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```
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This creates an option at `cfg.sockPath` that we can pipe through elsewhere,
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such as the start script for the service:
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```nix
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# inside
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script = let site = pkgs.tulpa.dev.cadey.printerfacts;
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in ''
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export SOCKPATH=${cfg.sockPath}
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export DOMAIN=${toString cfg.domain}
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export RUST_LOG=info
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cd ${site}
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exec ${site}/bin/printerfacts
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'';
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```
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And then we can go on to setting up nginx. First, let's figure out how to
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reverse proxy to a unix socket. In nginx configuration land,
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[`proxy_pass`](http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_pass)
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is the name of the configuration directive that lets you tell nginx to reverse
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proxy to somewhere. There's an example with a unix socket! This would let us
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reverse proxy a unix socket to a TCP port like this:
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```nginx
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server {
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listen 127.0.0.1:9000;
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location / {
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proxy_pass http://unix:/tmp/printerfacts.sock;
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}
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}
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```
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For comparison here's how you'd reverse proxy to a HTTP server running on port
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`42069`:
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```nginx
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server {
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listen 127.0.0.1:9001;
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location / {
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proxy_pass http://127.0.0.1:42069;
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}
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}
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```
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So, we just need to change where nginx reverse proxies to in the NixOS config.
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Let's look down at the nginx config for `printerfacts`:
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```nix
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# ...
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services.nginx.virtualHosts."${cfg.domain}" = {
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locations."/" = {
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proxyPass = "http://127.0.0.1:${toString cfg.port}";
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proxyWebsockets = true;
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};
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forceSSL = cfg.useACME;
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useACMEHost = "cetacean.club";
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extraConfig = ''
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access_log /var/log/nginx/printerfacts.access.log;
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'';
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};
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```
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The `proxyPass` option directly translates to a `proxy_pass` directive, so we
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can get away with something like this:
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```nix
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# ...
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proxyPass = "http://unix:${cfg.sockPath}";
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```
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And now we can deploy the service and everything should work right? printerfacts
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provides a unix socket at the given path and then nginx is configured to use
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that socket to send back printer facts. Let's deploy it and see what happens:
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<center>
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</center>
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Oh no. Let's see what `journalctl -fu nginx` has to say:
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```console
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$ journalctl -fu nginx
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Apr 01 23:29:58 lufta nginx[15396]: 2021/04/01 23:29:58 [crit] 15396#15396: *198
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connect() to unix:/tmp/printerfacts.sock failed (13: Permission denied) while
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connecting to upstream, client: lol.no.ip.here, server:
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printerfacts.cetacean.club, request: "GET / HTTP/2.0", upstream:
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"http://unix:/tmp/printerfacts.sock:/", host: "printerfacts.cetacean.club"
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```
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[Wait, what. Isn't `/tmp` guaranteed by the filesystem hierarchy standards to
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always be readable and writable by any user?](conversation://Mara/wat?smol)
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Normally, yes. However we are running nginx inside systemd, and one of the
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things you can do with systemd is make `/tmp` isolated for given services. This
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allows you to prevent a service from being able to exfiltrate data inside
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`/tmp`. However, this is definitely NOT the behaviour we want in this case.
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Let's change the systemd unit for nginx to disable this and also make nginx run
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as the same group as the printerfacts service:
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```nix
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systemd.services.nginx.serviceConfig = {
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PrivateTmp = lib.mkForce "false";
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SupplementaryGroups = "within";
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};
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```
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[In NixOS, most of the time if the same option is declared in multiple
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places it will result in a build error. `lib.mkForce` disables this behaviour
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and instead "forcibly" sets this value.](convesation://Mara/hacker?smol)
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Now nginx has the same `/tmp` as the printerfacts service, everything will work
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as we expect. Users are none the wiser that I'm using a domain socket here. I
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get to have another service not bound to the network and I have moved towards
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better security on my machine!
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[What about Prometheus? Doesn't it need a direct line of fire to the service to
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scrape metrics?](conversation://Mara/hmm?smol)
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...Time for some percussive maintenance!
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<center>
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</center>
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---
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I'm experimenting with a new "smol" mode for the Mara interludes as well as
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introducing a few more characters to the christine dot website cinematic
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universe. Please do let me know how this works out for you. I think I have the
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sizes optimized for mobile usage better, but [contributions to fix my horrible
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CSS](https://github.com/Xe/site/blob/main/css/shim.css) would really, really,
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really be appreciated.
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I'm considering moving over all of the Mara interludes to use smol mode. If you
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have opinions about this please let me know them.
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