Blocked that hard-coded google dns garbage.
this post was submitted on 08 Nov 2023
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I have a firewall rule to dst-nat any outgoing DNS requests not coming from piHole back to the piHole server. That way all devices on the LAN are forced to use piHole for DNS and can't bypass it. I don't have an OPNSense firewall but I would think it should be able to do that as well.
I’ve been having some issues with random IoT devices bypassing my pihole despite it being a router-level DNS for all my devices. Can you go into more detail about dst-nat and how I might be able to improve catching requests so they can routed to pihole for filtering? My router is running openwrt and pihole is on a VM in my hypervisor that’s directly connected to the router. This is the first time I’m hearing about dst-nat.
NAT TLDR
Your router is, at it's core, a very advanced traffic cop and NAT -- Network Address Translation -- is it's primary function. You have multiple devices on your local network (LAN) that need to communicate with other non-local servers via the WAN (i.e. the internet). Now you have a problem. Your ISP assigns you (usually) a single IP address on their network which is on a different subnet than your LAN. Devices on your local network and devices on the WAN are not aware of one another and cannot communicate with each other directly. So, requests have to be routed to the correct destination via your router.
SRC-NAT
Let's say you're trying to pull up a website on your computer. Your computer sends the request to the router. Your router alters the IP packet headers so that the request source address, and therefore the address that the server responds to, is your WAN IP instead of the requesting devices LAN IP. Your router then forwards the packet to the destination server, tracks the connection, and forwards the response back to your computer.
DST-NAT
Let's say you're hosting a web service on your home server that you want to make available publicly. You would set up a dst-nat (often called port forwarding) rule in your router/firewall which will tell your router to redirect any requests received at the WAN IP on port 80 or 443 to your home server's IP address. Unlike SRC-NAT, your router doesn't replace the source IP address. Just the destination. Your server knows that the requesting device is not on your LAN subnet and will forward the response back to the gateway (your router) which is already tracking the connection and will forward the response back to the requesting device via the WAN.
Routing DNS with DST-NAT
Since DST-NAT is just changing the destination IP address and routing the packet to the new destination, this can be done internally in some situations as well. To redirect DNS requests, you would set up a rule in your router/firewall to grab outbound UDP packets that originated from the LAN, do not originate from your internal dns server, and have a destination of port 53 and redirect/dst-nat them to the IP address of your choice. The new destination can be an internal or external IP address and the requesting device won't know the request was redirected. OpenWRT's documentation actually has a section that deals with DNS redirection which you can find here. The DNS redirection part is near the bottom of the page.
I suspect DoT and DoH still go through, though? I mean you can always block the port 853 for DoT, but DoH is another story.
Yeah you'd need an L7 application layer filtering firewall to catch DoH since it would detect the SSL packet signature on port 53. Unfortunately that balloons the cost of the device past a reasonable level for a home aficionado.
A workaround for now would be to block known public servers that use DoH like Google DNS, since a lot of devices are adding features to enable DoH by default at the OS level
That's correct. I block DoT in my firewall and block known DoH domains in piHole. I'm sure stuff slips through occasionally but the vast majority of my DNS requests are handled by piHole.
Traditional DNS over UDP/53 is insecure but I'm using ProtonVPN's DNS server over VPN externally so I'm not worried about that.
I see. I may try to do something similar but towards Unbound on my OPNSense router, if that's possible.
How do you block the DoH servers in the pihole? Pihole is a DNS server, devices using a third party DoH server would just bypass the pihole as they're using the IP of the DoH with no DNS lookup required. No?
To block DoH I think you need to block it at the firewall level with a list of blocked IPs for the DoH servers you want to block over 443
You're probably better off blocking it at the firewall level. It would be more thorough but also more effort. In my experience, most devices/apps that use DoH call a domain name rather than an IP. If you block the domain in piHole, the app cant resolve the DoH server IP and therefore won't be able to use DoH.
I've blocked outgoing port 53 udp/tcp for all subnet except pi-hole. If I can't trust a device to use the DNS I configured I can't trust it to go on the internet, and it goes on my list of devices to replace.