How know SSL fingerprint for some website using Linux CLI?

I’m trying update (using esp8266 microcontroller), update a DNS record hosted on DigitalOcean, using the API.

How do I do to get HTTPS fingerprint using Linux CLI, for use on my code?

Answer

If you are using ESP (arduino IDE) library WiFiClientSecure.h, you need declare the “const” variable for fingerprint, like bellow:

const char* fingerprint = "9A 8E 2F 33 95 CA 72 4F 47 26 54 52 A4 E5 00 70 F0 BD ED 85";

So anyhow, these values can be obtained running the command

$ echo | openssl s_client -connect host.example.com:443 |& openssl x509 -fingerprint -noout

Result:

SHA1 Fingerprint=9A:8E:2F:33:95:CA:72:4F:47:26:54:52:A4:E5:00:70:F0:BD:ED:85