Why Signing CSR need specify CA Certificate

Question

From my understanding, you use CA's private key to sign a CSR, but why all openssl CSR signing commands need to specify CA cert when signing a CSR like below:

openssl x509 -req -days 360 -in server.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out server.crt

Answer 1

The CSR is issued by adding information of the issuer (taken from the CA certificate) and signing the data (which is done using the private key of the CA certificate).