A New Future for Icanhazip

Reminds me of `echo $(dig @ns1.google.com o-o.myaddr.l.google.com TXT +short | tr -d \")`. I have no idea where this DNS query came from, because searching all of Google turns up nothing but https://github.com/GoogleCloudPlatform/cloud-self-test-kit/b..., which is never referenced by anyone. I had to track it down myself for a bootstrap.sh, but I don't like using undocumented sources for critical infrastructure.

My use case was needing to set the result of `hostname -f` in /etc/hosts in an automated fashion if a VPS provider didn't already add a line for the public Internet address in that file. You need to do this so that sendmail doesn't fail on `apt install` when it attempts to read your FQDN. So I couldn't use the NGINX example posted elsewhere here.

It seems like https://checkip.amazonaws.com/ is much more "reliable" in that it is publicly documented at https://docs.aws.amazon.com/sdk-for-net/v3/developer-guide/s....

To anyone who needs to read this: please don't use "services" like icanhazip for your provisioning. Even my examples above are bad.

It does strike me as weird that there is seemingly no POSIX-compliant way to get your public Internet address, from my readings.

Edit: Oh goodness... even Amazon's documentation recommends using Google's undocumented DNS query.[1]

[1]: https://aws.amazon.com/premiumsupport/knowledge-center/route...

> It does strike me as weird that there is seemingly no POSIX-compliant way to get your public Internet address, from my readings.

There is no singular thing called a "public Internet address". Imagine you're writing paper letters to someone. You write a letter, you put your own From address, you drop it in the slot. When the mailperson comes to collect the letter, they replace your mailing address with a special other codeword. And when they receive mail, they replace that codeword back with your original address. You would never know it was intercepted unless you asked around. There's no official protocol to ask for your codeword, it's just a trick the mail service does on your behalf.

Your home router does exactly this; it's known as "Network Address Translation", or NAT. It's not an official part of IPv4, and there's no protocol to ask what it is. Your computer thinks its local IP address (typically some variety of 192.168.0.1) is its real, public address, and your router does the swap behind your back.

`ip a` will tell you your IP addresses. As far as I can tell, for IPv6 it actually does know it's public address since that is globally unique with no need for a public/private split.

The issue is that ifconfig and ip do not recognize the Private-Use Networks under any flag and thusly query DHCP for the public address, nor does DHCP have such a query. (To my knowledge.)