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Comment by teamspirit | Hacker News Reader

https://www.cloudflare.com/cdn-cgi/trace will return your ip.

The (above) shared url leveraging the cloudflare.com domain name seems to show ip v6 address, while I've noticed that the following defaults to showing ip v4 address: https://1.1.1.1/cdn-cgi/trace

Pick your poison as you wish - either is great! :-)

I’d pick the one that supports IPv6 and then make two separate requests to it. One request over IPv4 and one over IPv6.

  curl -4 https://www.cloudflare.com/cdn-cgi/trace

  curl -6 https://www.cloudflare.com/cdn-cgi/trace

Also the reason that the 1.1.1.1 one shows only IPv4 address is because 1.1.1.1 is itself an IPv4 address. So any connection to it will have to be using IPv4.

could you not retrieve your ipv6 directly from the system?

Yes, but getting it in a response from an external server means I don’t have to be specific about which interface to get the IPv6 address of and so on.

the same specific interface used to contact the external server

Which is neatly abstracted away so you don’t have to think about it unless you want to. And therefore reaching out to an external server and having it say where the request came from is the path of least resistance for a script that can work across different hosts with minimal machine specific configuration.

Listen, if you want to check the IPv6 address from the interface list go ahead I’m not trying to stop you.

But because I anyway need to reach a third party to know my own IPv4 address then yeah when that third party can also tell me IPv6 address I’m gonna do it that way.

That requires running it on the router/device which gets the public IP address. By using the service you can update your DNS IP address on a system that is behind the router.

What do you mean?

Every device gets a public ipv6 (usually).

I have a router connected to the internet, it gets the public IP address.

The router is connected to the internal network in my home and has the IP address of 192.168.1.1.

Behind the router is my computer which has a non-public IP address, for example 192.168.1.2. My computer is the one I want to run the program to update the DNS entry. My computer does not know what the public IP address is by looking at its interfaces.

We're talking ipv6 not ipv4

I think the same situation applies for IPv6 if you are behind a router.

No, it doesn't

I'll agree to disagree :)

I.e. you agree to deny reality

https://superuser.com/questions/468727/how-to-get-the-ipv6-i...

As I've said before. The server behind the router does not have a public IPv6 address. It is NATed to the Internet. Getting the internal IPv6 address is useless for this case.

I do not like to have the servers/computers that are on the internal home network directly connected to the Internet.

Nating in the context of ipv6 is not a common thing. It is the exception, while it's the rule for ipv4

i.e., an arbitrarily-selected interface capable of reaching Cloudflare.

This is how I use it in my bash script:

  current_ip=$(curl -s -X GET https://1.1.1.1/cdn-cgi/trace | grep -Po "(?<=ip=)(.*)")

I find awk more clear for this kind of job. You can replace

  grep -Po "(?<=ip=)(.*)"

with

  awk -F= '$1 == "ip" { print $2 }'

Thanks for that, I agree.

Works on every cloudflare-MITMed domain btw.

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