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//! Host-header allowlist middleware.
//!
//! The client sends the `Host:` header, so it is only a claim. Code
//! that builds absolute URLs, reset links or cache keys from it trusts
//! whoever called. This layer is the check: a request whose `Host` is
//! not on the list is rejected with a 400. It is an allowlist, not a
//! blocklist, so a host you never named is refused without you having
//! to predict it.
//!
//! Entries may be exact hosts (`example.com`), dot-prefix subdomain
//! wildcards (`.example.com` matches `api.example.com` and
//! `example.com` itself), or the catch-all `*`.
//!
//! ## Quick start
//!
//! ```ignore
//! use rustango::host_validation::{AllowedHostsLayer, AllowedHostsRouterExt};
//!
//! let app = Router::new()
//! .route("/", get(home))
//! .allowed_hosts(AllowedHostsLayer::new([
//! "example.com",
//! ".example.com", // any subdomain
//! "localhost",
//! ]));
//! ```
//!
//! A missing or unlisted `Host` gets a `400 Bad Request`. The body
//! echoes the rejected host to help ops, but never the allowed list.
//!
//! ## Settings wiring
//!
//! `Settings.security.allowed_hosts: Vec<String>` (parsed by
//! `env::list("ALLOWED_HOSTS")`) feeds
//! [`AllowedHostsLayer::from_settings_list`]. An empty list turns the
//! check off, so the operator has to opt in.
//!
//! [`AllowedHostsLayer::from_settings_list`]: crate::host_validation::AllowedHostsLayer::from_settings_list
use std::sync::Arc;
use axum::body::Body;
use axum::extract::Request;
use axum::http::{Response, StatusCode};
use axum::middleware::Next;
use axum::Router;
/// One allowed-host entry, pre-parsed so matching allocates nothing
/// per request.
#[derive(Clone, Debug)]
enum Pattern {
/// Catch-all `*`: every host matches, which switches the check
/// off. Dangerous in production — name your hosts instead.
Wildcard,
/// Exact match against the lowercased `Host` header.
Exact(String),
/// Dot-prefix wildcard `.example.com`: matches `example.com` and
/// any subdomain. Stored without the leading dot.
Subdomain(String),
}
impl Pattern {
fn parse(entry: &str) -> Option<Self> {
let entry = entry.trim();
if entry.is_empty() {
return None;
}
if entry == "*" {
return Some(Self::Wildcard);
}
if let Some(rest) = entry.strip_prefix('.') {
if rest.is_empty() {
return None;
}
return Some(Self::Subdomain(rest.to_ascii_lowercase()));
}
Some(Self::Exact(entry.to_ascii_lowercase()))
}
fn matches(&self, host: &str) -> bool {
match self {
Self::Wildcard => true,
Self::Exact(h) => host == h,
Self::Subdomain(tail) => {
// Match the base domain itself, or a host ending in
// `.<tail>`. The dot matters: without it
// `eviltail.com` would match `.tail.com`.
host == tail
|| host
.strip_suffix(tail)
.is_some_and(|prefix| prefix.ends_with('.'))
}
}
}
}
/// The parsed pattern list. Apply it with
/// [`AllowedHostsRouterExt::allowed_hosts`].
#[derive(Clone)]
pub struct AllowedHostsLayer {
patterns: Arc<Vec<Pattern>>,
}
impl AllowedHostsLayer {
/// Build a layer from allowed-host entries: exact hostnames,
/// `.example.com` subdomain wildcards, or the catch-all `*`.
/// Blank entries are dropped.
#[must_use]
pub fn new<I, S>(entries: I) -> Self
where
I: IntoIterator<Item = S>,
S: AsRef<str>,
{
let patterns: Vec<Pattern> = entries
.into_iter()
.filter_map(|s| Pattern::parse(s.as_ref()))
.collect();
Self {
patterns: Arc::new(patterns),
}
}
/// Wire from `Settings.security.allowed_hosts`. An empty list
/// disables the layer and every host passes.
#[must_use]
pub fn from_settings_list<I, S>(entries: I) -> Self
where
I: IntoIterator<Item = S>,
S: AsRef<str>,
{
Self::new(entries)
}
/// `true` when the list permits this host header. An empty list
/// passes every host: the operator opted out.
#[must_use]
pub fn permits(&self, host: &str) -> bool {
if self.patterns.is_empty() {
return true;
}
let host = strip_port(host).to_ascii_lowercase();
self.patterns.iter().any(|p| p.matches(&host))
}
}
/// Drop a trailing `:<port>` so the allowlist compares host names
/// only. Returns the input unchanged when there is no port. Handles
/// bracketed IPv6 literals.
fn strip_port(host: &str) -> &str {
if let Some(rest) = host.strip_prefix('[') {
// IPv6 literal: `[::1]:8080` → cut at the closing bracket.
if let Some(end) = rest.find(']') {
return &host[..end + 2.min(host.len())];
}
return host;
}
match host.rfind(':') {
Some(i) => &host[..i],
None => host,
}
}
/// Router extension: `.allowed_hosts(layer)`.
pub trait AllowedHostsRouterExt {
#[must_use]
fn allowed_hosts(self, layer: AllowedHostsLayer) -> Self;
}
impl<S: Clone + Send + Sync + 'static> AllowedHostsRouterExt for Router<S> {
fn allowed_hosts(self, layer: AllowedHostsLayer) -> Self {
let cfg = Arc::new(layer);
self.layer(axum::middleware::from_fn(
move |req: Request<Body>, next: Next| {
let cfg = cfg.clone();
async move { handle(cfg, req, next).await }
},
))
}
}
async fn handle(cfg: Arc<AllowedHostsLayer>, req: Request<Body>, next: Next) -> Response<Body> {
let host = req
.headers()
.get(axum::http::header::HOST)
.and_then(|h| h.to_str().ok())
.unwrap_or("");
if cfg.permits(host) {
next.run(req).await
} else {
let msg = format!(
"DisallowedHost: rejected Host header {host:?} — \
add it to Settings.security.allowed_hosts to allow"
);
let mut resp = Response::new(Body::from(msg));
*resp.status_mut() = StatusCode::BAD_REQUEST;
resp
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn empty_list_passes_every_host() {
let layer = AllowedHostsLayer::new(Vec::<String>::new());
assert!(layer.permits("anywhere.example.com"));
assert!(layer.permits(""));
}
#[test]
fn exact_match_is_case_insensitive() {
let layer = AllowedHostsLayer::new(["Example.COM"]);
assert!(layer.permits("example.com"));
assert!(layer.permits("EXAMPLE.com"));
assert!(!layer.permits("api.example.com"));
}
#[test]
fn dot_prefix_wildcard_matches_subdomains_plus_base() {
let layer = AllowedHostsLayer::new([".example.com"]);
assert!(layer.permits("example.com"));
assert!(layer.permits("api.example.com"));
assert!(layer.permits("a.b.example.com"));
// `evilexample.com` ends with "example.com" but is not a
// subdomain: the boundary character is not a dot.
assert!(!layer.permits("evilexample.com"));
}
#[test]
fn star_is_catchall() {
let layer = AllowedHostsLayer::new(["*"]);
assert!(layer.permits("anything"));
assert!(layer.permits("attacker.com"));
}
#[test]
fn port_is_stripped_before_comparison() {
let layer = AllowedHostsLayer::new(["example.com"]);
assert!(layer.permits("example.com:8080"));
assert!(layer.permits("example.com:443"));
}
#[test]
fn ipv6_with_port_is_handled() {
let layer = AllowedHostsLayer::new(["[::1]"]);
assert!(layer.permits("[::1]:8080"));
}
#[test]
fn whitespace_entries_are_ignored() {
let layer = AllowedHostsLayer::new(["", " ", "example.com"]);
// Only the real entry counts; other hosts are still rejected.
assert!(layer.permits("example.com"));
assert!(!layer.permits("attacker.com"));
}
#[test]
fn rejected_host_does_not_match_other_patterns_in_list() {
let layer = AllowedHostsLayer::new(["a.com", ".b.com", "c.com"]);
assert!(layer.permits("a.com"));
assert!(layer.permits("foo.b.com"));
assert!(layer.permits("c.com"));
assert!(!layer.permits("d.com"));
assert!(!layer.permits("malicious.a.com"));
}
}