Windows – Why I can’t block PING.EXE inbound and outbound traffic

TL;DR, yes, it means that if you make a connection outbound, that the service you connected to can respond. They cannot connect to you unsolicited however, unless you set up port forwarding rules to allow them to.

First, you are confusing Asymetric NAT with Stateful Packet Filtering. NAT does not actually firewall anything, it just allows ports to be translated from source to destination. SPF is whats responsible for allowing only solicited responses to traffic.

TCP is a connection orriented protocol, in that both sides of the transmission negotiate and maintain a virtual connection (a flow of packets that are related, such that the order of packets can be maintained even if they arrive out of order, malformed or missing packets are easily noticed so a resend can be requested, and can perform flow control operations to make sure that traffic does not cause too much congestion).

This connection-orrientation allows the firewall to tell whether a given packet is asking for a new connection, or part of an existing and established one.

SPF is generally set up to allow all outgoing connection attempts, and to allow remote servers to whom a connection has been attempted, to reply in-band within that connection (eg the packet comes from the right source on the right port, has SYN/ACK flags set correctly, and has the correct SYN and ACK values) to come through the NAT.

What SPF generally DOESN'T allow, is a remote server initiating a connection to a service inside the NAT wall. so you can request a remote servers resources, and it can send them to you, but the remote server cannot request your resources.

Port forwarding or Destination NAT allows you to set up a service so that outside servers can initiate connections to it to request resources. you only want to do this if absolutely necessary, as it does open up the service to potential attacks.

Note that with PortForwarding, even once you have a port set up to expose your internal service, you must still allow that port through the firewall, as an incoming connection.

As for what you are seeing with Skype and windows firewall, when it says "block all incomming connections" it means that it will not allow a remote server to create an unsolicited connection to your box. it does NOT mean that it will block replies that are part of a connection initiated from your PC however, so Skype can connect to the skype servers, and those servers can respond as part of that connection.

hope that helps.

The reason for it working when "All programs" is selected and not when using the ping.exe seems to be that the firewall does not see the source/destination program for the ICMPv4 traffic, and only sees the "System" process and that the "System" process falls under the "All programs" category.

Note that

• In the field where it is asking for a full path to the executable you can just write: System
• you will need to have your rules applied to the correct network profile of Domain, Private, or Public for your current connection
• the "tracert" program is particularly usefull for diagnostics.

The default when adding a new ICMPv4 rule, allows all ICMPv4 types and codes. There are a bunch of icmp types and codes, see http://www.iana.org/assignments/icmp-parameters/icmp-parameters.xhtml for full detail. You may not want to allow all of the them.

For ping to work, it at least needs both outbound and inbound traffic for the

• Echo request (ping) ICMP type 8
• Echo reply (pong) ICMP type 0

Being MS they've tried to dumb it down a bit and hide the full detail. If you look in the "Advanced firewall area -> rule properties -> protocol and ports tab -> customise button" you will see the area for types and codes. On my system, which I believe is still in its default state, it has

• outbound - ICMPv4 "Echo Request"
• Inbound
  • ICMPv4 "Echo Request" - (I'm assuming "Echo Request" in microsoftese means either request or reply)
  • ICMP Type 3 (destination unreachable) code 4 (Fragmentation Needed and Don't Fragment was Set). Though I'm not sure why only code 4 is allowed when the other type 3 codes are quite useful to know.