(Replying to PARENT post)
It is not possible to know your public IP address, except by fetching the information from a known entity on the public network.
And in some scenarios, your public IP will change frequently. There is no guarantee that it will be consistent across multiple requests.
(Replying to PARENT post)
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.
(Replying to PARENT post)
Because traditionally if you're doing things right, you're not using NAT, which is against IP specs and a nonstandard kludge. So you just take your socket and query its local endpoint address using getsockname and voila.
(Replying to PARENT post)
Genuinely asking because I've always used the query
dig +short myip.opendns.com @resolver1.opendns.com
I use Route53 and I either completely missed the checkip link or they simply don't mention it
(Replying to PARENT post)
dig... | tr... works fine without it.
also, `dig -4 ... ` to get your IPv4 address, for us dual-stacked folks. Otherwise it returns your V6 address by default.
(Replying to PARENT post)
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...