root@prod:~/cert# openssl x509 -in my_cert.cer -out my_cert.pem -inform DER -outform PEM
Than I saw that I have stil problems with certificate
root@prod:~/cert# openssl verify my_cert.pem: [details removed]
error 20 at 0 depth lookup:unable to get local issuer certificate
I googled and found that I should get issuer certificate and combine it with my certificate.
I downloaded issuer certificate in DER format and converted it to PEM.
root@prod:~/cert# openssl x509 -in l4.cer -out l4.pem -inform DER -outform PEM
Than I checked if everything is OK:
root@prod:~/cert# openssl verify -CAfile l4.pem my_cert.pem: OK
Final step was combined my_cert.pem with l4.pem :
root@prod:~/cert# cat my_cert.pem l4.pem >> my_cert.crt
I copied my_cert.crt and my_cert.key to /etc/ssl/ and configured my nginx to use it.
No more certificate error.