Virgin Media
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The other day I got a letter about a device on my home network responding to Multicast DNS (mDNS) queries from outside my home network.
Other people have received similar letters and also letters discussing Netbios, and SSDP vulnerabilities as well.
These letters come as a result of Virgin Media being contacted by a third party organisation -shadowserver.org Who send queries out to peoples public facing IP's and see if they get a response back.
Why are Shadowserver doing this?
Quite simply to enable the closing down of exploits that others can use to mount DDOS attacks against internet users.
Devices in the DMZ
In several cases in the mDNS threads we see devices have been placed in the DMZ to facilitate some aspect of internet connectivity. In several cases these have been PS4's in my own case it was a PC. The problem is that if a device is in the DMZ - all unsolicited traffic is sent to that device - UNLESS there is a separate port forwarding rule in place. This exposes any flaw in the way a device handles incoming SSDP and mDNS queries from the internet.
False positive?
Several people have suggested that the letters are sent out based on false positives. Based on my own experience, I would say this is not correct. Contacting Shadowserver, I was sent the log of what they found from my IP. I recognised the culprit straight away as being Airserver, which allows me to stream from my Ipad to my PC. However the logs can be confusing to the average user.
Port blocking?
Advice from Virgin Media suggests blocking inbound ports in the firewall section. Unfortunately the Superhub 3 does not have any rules to do this, and some people advise that turning up the firewall breaks their gaming experience. So we need to consider an alternative method.
Using Port Fowawrding to drop the inbound traffic.
Inbound traffic is evaluated in the following order.
NAT table entry - response to outbound traffic?
Port Forwarding rule
Device in DMZ
So by setting up port forwarding rules to an IP address that doesn't have anything connected we can drop the inbound traffic from the internet side of the network.
This won't affect normal LAN traffic, so devices on the same LAN can still find each other. I've already done this with mDNS and my IPad can still happily find Airserver on my PC but Shadowserver can no longer find it.
This help article by Virgin Media describes for to Port Forward on the different Hubs
As noted above rules should be set according to the Vulnerability or you can preempt them. And set them all up.
I currently have mDNS - Port 5353 UDP forwarded to 192.168.0.253
SSDP - Port 1900 UDP forwarded to 192.168.0.253
Will this stop a device connecting to the Internet?
No - these services are meant for use on the Local network only. Devices connecting to the net use other outbound ports to do so.
Ravenstar68
Note: Windows Firewall makes it possible to limit a system to allowing inbound connections from the same LAN. I've actually done this with Airserver and a number of other mDNS listeners. However as this won't help people who are not using Windows devices, I feel port forwarding offers the easiest option.
It helps to know the EXACT wording on the letter so we can give advice tailored specifically to your needs.
All network devices use ports to communicate. They're not physical ports as such but a connection at the TCP/IP level for example needs two pieces of information.
An IP address
A port number
We also need the IP address and the port number of the device making the connection in the first place. So a connection consists of the following
Source IP - Source Port
Destination IP - Destination Port
Lets look at an example connection and reply to a website in Wireshark
The ports used are highlighted in yellow.
My fix takes the ports that should not be accessible from the internet
5353 UDP - mDNS
1900 UDP - SSDP
and forwards inbound connections to an unused IP address. This in effect causes queries from the internet to be dropped but still allows devices on the local network to respond to one another as normal.
In my case Airserver was responding on port 5353 so I port forwarded this. Now Shadowserver no longer see it, and more importantly - neither does anyone else. However my IPad can still connect to it when on the same LAN.
Ravenstar68
