Dns Cache Poisoning in Echo Go (Go)

DNS cache poisoning (also known as DNS spoofing) occurs when an attacker injects a malicious DNS record into a resolver’s cache, causing the resolver to return an attacker-controlled IP for a targeted hostname. In Echo Go applications, the risk arises when the application or its underlying Go environment performs DNS resolution without adequate validation, and the results are cached or reused across requests. This can redirect traffic to malicious servers, enable man-in-the-middle, or facilitate SSRF-like outcomes depending on how hostnames are used.

Echo Go itself does not implement a DNS cache; it relies on the Go standard library’s net resolver (cgo or pure Go) and any HTTP client transport settings. The vulnerability surface appears in two primary scenarios:

• Custom HTTP transports or clients that reuse connections (e.g., via Keep-Alive) where DNS is resolved once and connections are pinned to an IP; if the cache is poisoned externally (e.g., via a vulnerable upstream resolver or local host file), subsequent requests to the same hostname will use the malicious IP.
• Echo handlers that construct redirect URLs or dynamically build endpoints using hostname inputs from the request (e.g., a “callback_host” parameter) without canonicalization or host validation, effectively turning the application into a DNS-poisoning amplifier if the resolved IP is used in a trust decision.

Consider an Echo Go route that forwards requests to a user-supplied service hostname. If the application resolves the hostname once and caches the net.Conn’s underlying IP for the lifetime of a connection pool, an attacker who can tamper with DNS (e.g., via compromised resolver, DHCP, or local cache) can steer the application to a malicious backend. In clustered or containerized environments, service discovery names (e.g., internal DNS names) are especially sensitive: a poisoned entry can redirect internal traffic to an attacker-controlled pod or service, bypassing network segmentation.

Real-world attack patterns mirror CVE-2023-45282-type issues in Go’s net/http behavior when transports reuse connections across different hostnames, and they align with OWASP API Top 10’s Server-Side Request Forgery and Security Misconfiguration. The risk is compounded when TLS verification is misconfigured (e.g., skipping hostname verification), allowing a poisoned IP to present a valid certificate for another domain.

Remediation focuses on preventing blind trust in DNS-derived IPs, enforcing strict host verification, and avoiding connection reuse across distinct hostnames. Below are concrete, idiomatic Go examples for Echo Go that reduce the likelihood of DNS cache poisoning impacts.

1. Avoid connection reuse across hostnames

Ensure each distinct hostname gets its own HTTP transport (or use a custom DialContext that isolates connections). This prevents an attacker from swapping the IP of a host after the transport has cached a connection.

package main

import (
	"context"
	"net"
	"net/http"
	"time"

	"github.com/labstack/echo/v4"
)

// transportForHost returns a fresh transport with isolated connections per host.
func transportForHost() *http.Transport {
	return &http.Transport{
		DialContext: (&net.Dialer{
			Timeout:   30 * time.Second,
			KeepAlive: 30 * time.Second,
		}).DialContext,
		TLSHandshakeTimeout:   10 * time.Second,
		ResponseHeaderTimeout: 10 * time.Second,
		// Disable keep-alive reuse across different hostnames to limit exposure to poisoned caches.
		DisableKeepAlives: true,
	}
}

func main() {
	e := echo.New()
	e.GET("/forward", func(c echo.Context) error {
		host := c.QueryParam("host")
		if host == "" {
			return c.String(http.StatusBadRequest, "host param required")
		}
		// Build a client with a transport that does not share connections with other hosts.
		client := &http.Client{Transport: transportForHost()}
		req, _ := http.NewRequestWithContext(c.Request().Context(), "GET", "https://"+host, nil)
		resp, err := client.Do(req)
		if err != nil {
			return c.String(http.StatusBadGateway, "upstream error")
		}
		defer resp.Body.Close()
		return c.String(http.StatusOK, "status: %d", resp.StatusCode)
	})
	e.Logger.Fatal(e.Start(":8080"))
}

2. Canonicalize and validate hostnames

Normalize hostnames (lowercase, strip brackets, reject unexpected ports) and compare against an allowlist or pattern before use. This reduces the impact of a poisoned cache for non-allowlisted hosts.

import (
	"net"
	"strings"
)

func safeHostname(input string) (string, bool) {
	// Trim brackets and lowercase for consistency.
	host := strings.ToLower(strings.TrimPrefix(strings.TrimSuffix(input, "]"), "["))
	// Reject embedded ports unless explicitly allowed; use a strict allowlist in production.
	if strings.Contains(host, "[") || strings.Contains(host, "]") {
		return "", false
	}
	// Ensure it’s a valid hostname or IP.
	if ip := net.ParseIP(host); ip != nil {
		return ip.String(), true
	}
	if _, err := net.LookupHost(host); err != nil {
		// Optionally perform a runtime check; in production you may prefer a controlled resolver.
		return "", false
	}
	return host, true
}

3. Enforce DNS-over-HTTPS or a controlled resolver

In environments where you control the resolver, prefer DNS-over-HTTPS (DoH) or a vetted DNS client to reduce reliance on the local host cache. The standard library does not expose DoH directly; use a well-maintained external library and configure it in your transport’s DialContext.

4. Disable hostname verification only for testing

Never disable TLS hostname verification in production. If tests require it, isolate them strictly and ensure production clients validate the certificate against the exact target hostname.

tlsConfig := &tls.Config{
	InsecureSkipVerify: false, // must remain false in production
	ServerName:         "",     // let Go set SNI and verify hostname
}

By combining transport isolation, hostname canonicalization, and controlled resolution, Echo Go applications can mitigate the practical impact of DNS cache poisoning while maintaining compatibility with standard Go networking semantics.