When a ping is sent from a device on a local network to a device on the internet, the router performing NAT rewrites the source address of the ping to its public IP address and rewrites the ID field of the ICMP packet to a unique value. When the response is received, the router uses the unique ID value to forward the response to the correct device on the local network.
How does Linux NAT a ping?
21–30 of 108 posts
Re: How does Linux NAT a ping?
#22When a ping is sent from a device on a local network to a device on the internet, the router performing NAT rewrites the source address of the ping to its public IP address and rewrites the ID field of the ICMP packet to a unique value. When the response is received, the router uses the unique ID value to forward the response to the correct device on the local network.
Taking this thought just a tiny bit further, this is changing a stateless protocol to a stateful one.
Re: How does Linux NAT a ping?
#23I wonder if ping could be abused to send short messages for p2p networking over UDP without a central server to handle NAT busting. Looks like someone figured the message part out:
https://stackoverflow.com/questions/31857419/how-to-send-a-m...
Unfortunately ping is handled by the OS so apps on the peer IPs wouldn't be able to read the messages.
I wonder if it's time to provide hooks to some of these services in user space to make true p2p under double-ended NAT possible. At least a readonly event stream or something. It just feels like the barriers preventing that are entirely artificial now.
Re: How does Linux NAT a ping?
#24NAT is such a trashy abstraction. IPv4 needs to die.
I have a few devices on my home internet, on a handful of 192.168 subnets The other week I moved my ISP. The AS my house belonged to obviously changed to the new ISP, and I got a new v4 IP All I had to do was update my Wan router to forward trafffic from the new Ip. Instead with ipv6 I would have to change every node on my network, update my internal DNS. Now in theory I could have my own /48 which I take with me. Th…
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Re: How does Linux NAT a ping?
#25I wonder if ping could be abused to send short messages for p2p networking over UDP without a central server to handle NAT busting. Looks like someone figured the message part out: https://stackoverflow.com/questions/31857419/how-to-send-a-m... Unfortunately ping is handled by the OS so apps on the peer IPs wouldn't be able to read the messages. I wonder if it's time to provide hooks to some of these services in user…
ping is icmp not udp
Re: How does Linux NAT a ping?
#26NAT is such a trashy abstraction. IPv4 needs to die.
be mindful of the Lindy effect, an observation on the future longevity of non-perishable things like technology or an idea being proportional to their current age, ipv4 due to its age will likely be around for quite some time to come.
Re: How does Linux NAT a ping?
#27I wonder if ping could be abused to send short messages for p2p networking over UDP without a central server to handle NAT busting. Looks like someone figured the message part out: https://stackoverflow.com/questions/31857419/how-to-send-a-m... Unfortunately ping is handled by the OS so apps on the peer IPs wouldn't be able to read the messages. I wonder if it's time to provide hooks to some of these services in user…
ping is icmp not udp
It's super confusing because you can use udp to read icmp packets (but not send, iirc), and i might be wrong, but i remember seeing tuts that did this!!
Re: How does Linux NAT a ping?
#28I wonder if ping could be abused to send short messages for p2p networking over UDP without a central server to handle NAT busting. Looks like someone figured the message part out: https://stackoverflow.com/questions/31857419/how-to-send-a-m... Unfortunately ping is handled by the OS so apps on the peer IPs wouldn't be able to read the messages. I wonder if it's time to provide hooks to some of these services in user…
As IPv6 gains more and more adoption this should become less as an issue if everyone has a publicly routable IP and can avoid NAT altogether.
Re: How does Linux NAT a ping?
#29NAT is such a trashy abstraction. IPv4 needs to die.
I have a few devices on my home internet, on a handful of 192.168 subnets The other week I moved my ISP. The AS my house belonged to obviously changed to the new ISP, and I got a new v4 IP All I had to do was update my Wan router to forward trafffic from the new Ip. Instead with ipv6 I would have to change every node on my network, update my internal DNS. Now in theory I could have my own /48 which I take with me. Th…
You could be using IPv6 ULA addresses internally on your home network to have static addressing. The real solution is moving to DNS names though with your router maintaining them based on DHCP leases or just using multicast DNS (Zeroconf).
In the future you can probably go "IPv6-mostly" with a CLAT engine to ditch dual-stack: https://blog.apnic.net/2022/11/21/deploying-ipv6-mostly-acce...
Re: How does Linux NAT a ping?
#30When a ping is sent from a device on a local network to a device on the internet, the router performing NAT rewrites the source address of the ping to its public IP address and rewrites the ID field of the ICMP packet to a unique value. When the response is received, the router uses the unique ID value to forward the response to the correct device on the local network.
Taking this thought just a tiny bit further, this is changing a stateless protocol to a stateful one.
Is or was a thing with NAT. Linux also comes with stateful modules (ip_conntrack*) to track and rewrite higher level protocols, such as FTP control connections.