From 11f95ff384ff8e857e6d660976b7cd3b82159f2e Mon Sep 17 00:00:00 2001
From: Nutomic <me@nutomic.com>
Date: Wed, 28 Jan 2026 13:44:39 +0000
Subject: [PATCH] Improve error message, allow local IP federation via env var
 (#158)

* Improve error message, allow local IP federation via env var (fixes #152)

* fix
---
 src/config.rs | 34 +++++++++++++++++-----------------
 src/error.rs  |  3 +++
 2 files changed, 20 insertions(+), 17 deletions(-)

diff --git a/src/config.rs b/src/config.rs
index 9eb0b97..a7cbf94 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -25,6 +25,7 @@ use async_trait::async_trait;
 use bytes::Bytes;
 use derive_builder::Builder;
 use dyn_clone::{clone_trait_object, DynClone};
+use itertools::Itertools;
 use moka::future::Cache;
 use regex::Regex;
 use reqwest::{redirect::Policy, Client, Request};
@@ -186,27 +187,26 @@ impl<T: Clone> FederationConfig<T> {
             // Resolve domain and see if it points to private IP
             // TODO: Use is_global() once stabilized
             //       https://doc.rust-lang.org/std/net/enum.IpAddr.html#method.is_global
-            let invalid_ip =
-                lookup_host((domain.to_owned(), 80))
-                    .await?
-                    .any(|addr| match addr.ip() {
-                        IpAddr::V4(addr) => {
-                            addr.is_private()
-                                || addr.is_link_local()
-                                || addr.is_loopback()
-                                || addr.is_multicast()
-                        }
-                        IpAddr::V6(addr) => {
-                            addr.is_loopback()
+            let mut ips = lookup_host((domain.to_owned(), 80)).await?;
+            let allow_local = std::env::var("DANGER_FEDERATION_ALLOW_LOCAL_IP").is_ok();
+            let invalid_ip = !allow_local
+                && ips.any(|addr| match addr.ip() {
+                    IpAddr::V4(addr) => {
+                        addr.is_private()
+                            || addr.is_link_local()
+                            || addr.is_loopback()
+                            || addr.is_multicast()
+                    }
+                    IpAddr::V6(addr) => {
+                        addr.is_loopback()
                         || addr.is_multicast()
                         || ((addr.segments()[0] & 0xfe00) == 0xfc00) // is_unique_local
                         || ((addr.segments()[0] & 0xffc0) == 0xfe80) // is_unicast_link_local
-                        }
-                    });
+                    }
+                });
             if invalid_ip {
-                return Err(Error::UrlVerificationError(
-                    "Localhost is only allowed in debug mode",
-                ));
+                let ip_addrs = ips.join(", ");
+                return Err(Error::DomainResolveError(domain.to_string(), ip_addrs));
             }
         }
 
diff --git a/src/error.rs b/src/error.rs
index 0661071..07e377b 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -28,6 +28,9 @@ pub enum Error {
     /// url verification error
     #[error("URL failed verification: {0}")]
     UrlVerificationError(&'static str),
+    /// Resolving domain points to local IP.
+    #[error("Resolving domain {0} points to local IP {1}. This may indicate an attacker attempting to access internal services. If intentional, you can ignore this error by setting DANGER_FEDERATION_ALLOW_LOCAL_IP=1")]
+    DomainResolveError(String, String),
     /// Incoming activity has invalid digest for body
     #[error("Incoming activity has invalid digest for body")]
     ActivityBodyDigestInvalid,