diff --git a/app/logical/ip_lookup.rb b/app/logical/ip_lookup.rb
new file mode 100644
index 000000000..bc78e83b4
--- /dev/null
+++ b/app/logical/ip_lookup.rb
@@ -0,0 +1,27 @@
+# API client for https://ipregistry.co/
+
+class IpLookup
+  extend Memoist
+
+  attr_reader :ip_addr, :api_key, :cache_duration
+
+  def initialize(ip_addr, api_key: Danbooru.config.ip_registry_api_key, cache_duration: 1.day)
+    @ip_addr = ip_addr
+    @api_key = api_key
+    @cache_duration = cache_duration
+  end
+
+  def info
+    return {} unless api_key.present?
+    response = Danbooru::Http.cache(cache_duration).get("https://api.ipregistry.co/#{ip_addr}?key=#{api_key}")
+    return {} if response.status != 200
+    json = response.parse.deep_symbolize_keys.with_indifferent_access
+    json
+  end
+
+  def is_proxy?
+    info[:security].present? && info[:security].values.any?
+  end
+
+  memoize :info, :is_proxy?
+end
diff --git a/app/models/ip_address.rb b/app/models/ip_address.rb
index 6d7c351ad..0fcf5a8fb 100644
--- a/app/models/ip_address.rb
+++ b/app/models/ip_address.rb
@@ -37,6 +37,10 @@ class IpAddress < ApplicationRecord
     group(:user_id).select("user_id, COUNT(*) AS count_all").reorder("count_all DESC, user_id")
   end
 
+  def lookup
+    @lookup ||= IpLookup.new(ip_addr)
+  end
+
   def to_s
     # include the subnet mask only when the IP denotes a subnet.
     (ip_addr.size > 1) ? ip_addr.to_string : ip_addr.to_s
diff --git a/config/danbooru_default_config.rb b/config/danbooru_default_config.rb
index 6a15f1c60..16f3a441d 100644
--- a/config/danbooru_default_config.rb
+++ b/config/danbooru_default_config.rb
@@ -513,6 +513,12 @@ module Danbooru
       "https://#{hostname}"
     end
 
+    # API key for https://ipregistry.co. Used for looking up IP address
+    # information and for detecting proxies during signup.
+    def ip_registry_api_key
+      nil
+    end
+
     # Cloudflare API token. Used to purge URLs from Cloudflare's cache when a
     # post is replaced. The token must have 'zone.cache_purge' permissions.
     # https://support.cloudflare.com/hc/en-us/articles/200167836-Managing-API-Tokens-and-Keys