How to replicate the configuration of a self-signed certificate with OpenSSL?

Question

I have a load balancer that requires a certificate with a specific configuration, unfortunately those who created the first certificates did not document this configuration and I only have a list of commands that is not complete either.

I have these two files: example_ca.crt and example.crt

And using this OpenSSL command:

openssl x509 -in file_name.crt -text -noout

These are its properties (I will omit non-relevant information):

example_ca.crt

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            cb:0f:b8:78:38:9a:a9:da
        Signature Algorithm: sha256WithRSAEncryption
        Issuer: CN = example.org
        Validity
            Not Before: Jun 10 10:33:06 2020 GMT
            Not After : May 17 10:33:06 2120 GMT
        Subject: CN = example.org
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                RSA Public-Key: (2048 bit)
                Modulus:
                    [...]
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Subject Key Identifier: 
                81:FE:D0:6D:DE:0A:CC:10:1D:B3:74:EA:4B:C8:F3:43:37:B4:D1:FD
            X509v3 Authority Key Identifier: 
                keyid:81:FE:D0:6D:DE:0A:CC:10:1D:B3:74:EA:4B:C8:F3:43:37:B4:D1:FD

            X509v3 Basic Constraints: 
                CA:TRUE
    Signature Algorithm: sha256WithRSAEncryption
         [...]

example.crt

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            80:1d:bb:9e:9f:2c:4e:ce
        Signature Algorithm: sha256WithRSAEncryption
        Issuer: CN = example.org
        Validity
            Not Before: Jun 10 10:33:44 2020 GMT
            Not After : May 17 10:33:44 2120 GMT
        Subject: CN = example.org
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                RSA Public-Key: (2048 bit)
                Modulus:
                    [...]
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Extended Key Usage: 
                TLS Web Client Authentication, TLS Web Server Authentication
            X509v3 Authority Key Identifier: 
                keyid:81:FE:D0:6D:DE:0A:CC:10:1D:B3:74:EA:4B:C8:F3:43:37:B4:D1:FD

            X509v3 Subject Key Identifier: 
                B1:2C:74:04:EE:03:84:C9:F7:92:35:CE:6E:20:EF:C6:FE:B8:23:A7
    Signature Algorithm: sha256WithRSAEncryption
         [...]

I managed to replicate example_ca.crt with these commands and configuration (the expiration date is not relevant):

openssl genrsa -out example_ca.key 2048
openssl req -new -x509 -days 365 -key example_ca.key -out example_ca.crt -config root.cnf

root.cnf

# OpenSSL configuration for Root CA

[ req ]

prompt             = no
string_mask        = default

default_bits       = 2048
distinguished_name = req_distinguished_name
x509_extensions    = x509_ext

[ req_distinguished_name ]
commonName = example.org

[ x509_ext ]
extendedKeyUsage = clientAuth, serverAuth
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid
basicConstraints=CA:true

My problem comes at this point when I can't replicate example.crt, I have tried so many possibilities in the server.cnf and openssl.cnf file and I don't get any closer to the desired result.

For the last steps I have used these commands:

openssl genrsa -out example.key 2048
openssl req -new -out example.csr -key example.key -config server.cnf

echo extendedKeyUsage = clientAuth > openssl.cnf
openssl x509 -req -in example.csr -out example.crt -signkey example.key -CA example_ca.crt -CAkey example_ca.key -CAcreateserial -days 365 -extfile openssl.cnf

I will skip the contents of server.cnf because that is where I need help. But basically I always miss the "X509v3 extensions" session of example.crt

Feel free to force the use of a password if necessary, or to correct my replication of example_ca.crt I have simply explained the fundamentals.

UPDATE:

server.cnf

# OpenSSL configuration for end-entity cert

[ req ]
prompt             = no
string_mask        = default

default_bits       = 2048
distinguished_name = req_distinguished_name

x509_extensions    = x509_ext

[ req_distinguished_name ]
commonName = example.org

[ x509_ext ]
keyUsage=critical,digitalSignature,keyAgreement

subjectAltName = @alt_names

Multiple Alternate Names are possible
[alt_names]
DNS.1 = example.org
IP.1 = 127.0.0.1
# DNS.2 = altName.example.com

Answer 1

Create a local.cnf file with something similar to (remove my comments if you want):

[server]

# These two are expected...
subjectKeyIdentifier=hash
authorityKeyIdentifier=keyid:always,issuer

# This is wise for end-entities and SHOULD be critical:
# keyUsage = critical, digitalSignature, keyAgreement
# Choose (wisely) from: digitalSignature, nonRepudiation, keyEncipherment, dataEncipherment, keyAgreement,  encipherOnly, and decipherOnly
# but not keyCertSign or cRLSign as they are for CAs.

# This is for end-entity certificates only.
extendedKeyUsage = clientAuth, serverAuth
# Choose (wisely) from: https://www.openssl.org/docs/manmaster/man5/x509v3_config.html#Extended-Key-Usage

Then, use the following flags on your openssl x509 command to apply:

openssl x509 ... -extfile local.cnf -extensions server