Fix support for HTTP proxies (#11245)

* Disable incorrect check for hidden services in Socket

Hidden services can only be accessed with an HTTP proxy, in which
case the host seen by the Socket class will be the proxy, not the
target host.

Hidden services are already filtered in `Request#initialize`.

* Use our Socket class to connect to HTTP proxies

Avoid the timeout logic being bypassed

* Add support for IP addresses in Request::Socket

* Refactor a bit, no need to keep the DNS resolver around

app/lib/request.rb

@@ -30,7 +30,8 @@ class Request
     @verb        = verb
     @url         = Addressable::URI.parse(url).normalize
     @http_client = options.delete(:http_client)
-    @options     = options.merge(use_proxy? ? Rails.configuration.x.http_client_proxy : { socket_class: Socket })
+    @options     = options.merge(socket_class: use_proxy? ? ProxySocket : Socket)
+    @options     = @options.merge(Rails.configuration.x.http_client_proxy) if use_proxy?
     @headers     = {}
 
     raise Mastodon::HostValidationError, 'Instance does not support hidden service connections' if block_hidden_service?
@@ -177,19 +178,22 @@ class Request
   class Socket < TCPSocket
     class << self
       def open(host, *args)
-        return super(host, *args) if thru_hidden_service?(host)
-
         outer_e = nil
         port    = args.first
 
+        addresses = []
+        begin
+          addresses = [IPAddr.new(host)]
+        rescue IPAddr::InvalidAddressError
           Resolv::DNS.open do |dns|
             dns.timeouts = 5
-
             addresses = dns.getaddresses(host).take(2)
+          end
+        end
 
         addresses.each do |address|
           begin
-              raise Mastodon::HostValidationError if PrivateAddressCheck.private_address?(IPAddr.new(address.to_s))
+            check_private_address(address)
 
             sock     = ::Socket.new(address.is_a?(Resolv::IPv6) ? ::Socket::AF_INET6 : ::Socket::AF_INET, ::Socket::SOCK_STREAM, 0)
             sockaddr = ::Socket.pack_sockaddr_in(port, address.to_s)
@@ -219,7 +223,6 @@ class Request
             outer_e = e
           end
         end
-        end
 
         if outer_e
           raise outer_e
@@ -230,11 +233,21 @@ class Request
 
       alias new open
 
-      def thru_hidden_service?(host)
+      def check_private_address(address)
-        Rails.configuration.x.access_to_hidden_service && /\.(onion|i2p)$/.match(host)
+        raise Mastodon::HostValidationError if PrivateAddressCheck.private_address?(IPAddr.new(address.to_s))
       end
     end
   end
 
-  private_constant :ClientLimit, :Socket
+  class ProxySocket < Socket
+    class << self
+      def check_private_address(_address)
+        # Accept connections to private addresses as HTTP proxies will usually
+        # be on local addresses
+        nil
+      end
+    end
+  end
+
+  private_constant :ClientLimit, :Socket, :ProxySocket
 end