feat: privacy mode

Adds support for hiding IP addresses & hostnames associated with clients sending
authenticated mail in to Postal over SMTP and HTTP

lib/postal/received_header.rb

@@ -0,0 +1,43 @@
+module Postal
+  class ReceivedHeader
+
+    OUR_HOSTNAMES = {
+      smtp: Postal.config.dns.smtp_server_hostname,
+      http: Postal.config.web.host
+    }
+
+    class << self
+
+      def generate(server, helo, ip_address, method)
+        our_hostname = OUR_HOSTNAMES[method]
+        if our_hostname.nil?
+          raise Error, "`method` is invalid (must be one of #{OUR_HOSTNAMES.join(', ')})"
+        end
+
+        header = "by #{our_hostname} with #{method.to_s.upcase}; #{Time.now.utc.rfc2822}"
+
+        if server.nil? || server.privacy_mode == false
+          hostname = resolve_hostname(ip_address)
+          header = "from #{helo} (#{hostname} [#{ip_address}]) #{header}"
+        end
+
+        header
+      end
+
+      private
+
+      def resolve_hostname(ip_address)
+        Resolv::DNS.open do |dns|
+          dns.timeouts = [10, 5]
+          begin
+            dns.getname(ip_address)
+          rescue StandardError
+            ip_address
+          end
+        end
+      end
+
+    end
+
+  end
+end

lib/postal/smtp_server/client.rb

@@ -103,17 +103,6 @@ module Postal
 
       private
 
-      def resolve_hostname
-        Resolv::DNS.open do |dns|
-          dns.timeouts = [10, 5]
-          @hostname = begin
-            dns.getname(@ip_address)
-          rescue StandardError
-            @ip_address
-          end
-        end
-      end
-
       def proxy(data)
         if m = data.match(/\APROXY (.+) (.+) (.+) (.+) (.+)\z/)
           @ip_address = m[2]
@@ -143,7 +132,6 @@ module Postal
       end
 
       def ehlo(data)
-        resolve_hostname
         @helo_name = data.strip.split(" ", 2)[1]
         transaction_reset
         @state = :welcomed
@@ -151,7 +139,6 @@ module Postal
       end
 
       def helo(data)
-        resolve_hostname
         @helo_name = data.strip.split(" ", 2)[1]
         transaction_reset
         @state = :welcomed
@@ -377,10 +364,10 @@ module Postal
         @headers = {}
         @receiving_headers = true
 
-        received_header_content = "from #{@helo_name} (#{@hostname} [#{@ip_address}]) by #{Postal.config.dns.smtp_server_hostname} with SMTP; #{Time.now.utc.rfc2822}".force_encoding("BINARY")
-        unless Postal.config.smtp_server.strip_received_headers?
-          @data << "Received: #{received_header_content}\r\n"
-        end
+        received_header = Postal::ReceivedHeader.generate(@credential&.server, @helo_name, @ip_address, :smtp)
+                                                .force_encoding("BINARY")
+
+        @data << "Received: #{received_header_content}\r\n"
         @headers["received"] = [received_header_content]
 
         handler = proc do |data|