Dns Cache Poisoning in Axum (Rust)

DNS cache poisoning is a network-layer attack where false DNS responses cause a resolver to cache incorrect IP mappings. In Axum applications written in Rust, the risk emerges not from Axum itself, which is a web framework, but from how the runtime environment and dependencies perform DNS resolution. When Axum routes requests to backend services using hostnames — for example, when calling other APIs or databases via client libraries — the operating system’s resolver or a Rust DNS library is used. If those libraries or the OS cache is compromised, an attacker can redirect traffic to malicious hosts, enabling request tampering, data exposure, or SSRF-like redirection.

Rust-based Axum services often rely on crates like trust-dns-resolver or the system resolver via crates such as reqwest or hyper with default system DNS behavior. If the resolver does not enforce strict validation (e.g., DNSSEC), or if responses are accepted from non-authoritative sources, poisoned records can be cached. This is particularly relevant when Axum services run in containerized or cloud environments where DNS is shared across network namespaces. An attacker on the same network or via a compromised upstream resolver can inject forged responses, leading the Axum application to route sensitive internal calls to attacker-controlled endpoints. This can facilitate further attacks such as credential theft or request smuggling.

Because middleBrick tests unauthenticated attack surfaces and checks for SSRF and related network injection patterns, it can detect indicators of DNS manipulation, such as inconsistent IP resolutions across repeated calls or unexpected destination IPs in outbound connections. The scan does not assume internal architecture but observes behavioral anomalies that suggest unsafe DNS handling. Developers must ensure that any DNS-dependent logic in Axum services uses validated resolvers, avoids caching untrusted responses, and applies strict network segmentation to limit exposure.

To mitigate DNS cache poisoning in Axum Rust services, control DNS resolution explicitly and avoid reliance on system defaults. Use a hardened resolver crate such as trust-dns-resolver with custom configuration that disables system caching and enforces DNSSEC validation where possible. Configure the resolver to use only trusted upstream servers and set short TTLs to minimize the impact of any poisoned entry. For outbound HTTP calls, prefer clients that allow custom resolver integration or disable system DNS caching.

The following example shows an Axum handler using reqwest with a custom trust-dns-resolver configured resolver, ensuring that each request resolves through a controlled path:

use axum::{routing::get, Router};
use reqwest::Client;
use std::net::SocketAddr;
use trust_dns_resolver::Resolver;
use trust_dns_resolver::config::{ResolverConfig, ResolverOpts, NameServerConfig};

async fn fetch_upstream() -> Result> {
    // Configure a trusted resolver
    let mut opts = ResolverOpts::default();
    opts.validate = true; // Prefer DNSSEC-enabled validation if upstream supports it
    opts.attempts = 2;
    opts.timeout = std::time::Duration::from_secs(2);

    let ns = NameServerConfig::new(
        "1.1.1.1".parse()?,
        trust_dns_proto::op::MessageType::Query,
    );
    let config = ResolverConfig::from_parts(None, vec![ns], vec![]);
    let resolver = Resolver::new(config, opts)?;

    // Resolve hostname securely
    let ip = resolver.lookup_ip("api.example.com")?.iter().next().ok_or("No IP found")?;

    // Use reqwest with a system DNS bypass by connecting via resolved IP
    let client = Client::new();
    let resp = client.get(&format("http://{}/health", ip))
        .header("Host", "api.example.com")
        .send()
        .await?;

    let body = resp.text().await?;
    Ok(body)
}

#[tokio::main]
async fn main() {
    let app = Router::new().route("/check", get(|| async { "ok" }));
    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
    axum::Server::bind(&addr)
        .serve(app.into_make_service())
        .await
        .unwrap();
}

In this setup, the resolver does not rely on the OS cache, and DNSSEC validation is enabled where supported. The resolved IP is used directly in the request, reducing exposure to poisoned cache entries. For production use, rotate upstream DNS servers and monitor resolution consistency as part of runtime security checks. middleBrick’s unauthenticated scans can help verify that no unexpected IPs are being contacted by your endpoints, supporting continuous detection of DNS-related misconfigurations.

Additionally, avoid storing sensitive routing logic in environment variables that depend on DNS. Combine these practices with network policies that restrict outbound connections to known endpoints. The Pro plan’s continuous monitoring can track resolution anomalies over time, and the CLI allows integration into pre-deployment checks to flag risky configurations before deployment.