We generate certificates that are intended to be used for server authentication in a couple of places:

- `bin/elasticsearch-certutil http` command  ( See `CertGenUtils.generateSignedCertificate` in `HttpCertificateCommand` )
- TLS auto-configuraion for node startup ( see [`AutoConfigureNode`](https://github.com/elastic/elasticsearch/blob/814f7f9f52cb8c05db6465b9bfc818ea5d76c575/x-pack/plugin/security/cli/src/main/java/org/elasticsearch/xpack/security/cli/AutoConfigureNode.java#L454) )

In these cases we know that the certificate is meant to be used for server authentication, so we can set Extended Key Usage extension to `1.3.6.1.5.5.7.3.1`
x-pack/plugin/security/cli/src/main/java/org/elasticsearch/xpack/security/cli/HttpCertificateCommand.java
Map.ofEntries(Map.entry("P12", p12Name), Map.entry("YML", ymlFile), Map.entry("PASSWORD", hasPassword ? "*" : ""))
                );
                writeTextFile(zip, dirName + "/README.txt", ES_README_P12, substitutions);
                writeKeyStore(zip, dirName + "/" + p12Name, certificate, keyPair.getPrivate(), password, ca.certAndKey.cert);
                writeTextFile(zip, dirName + "/" + ymlFile, ES_YML_P12, substitutions);
            }
        } catch (OperatorException | IOException | GeneralSecurityException e) {
            throw new ElasticsearchException("Failed to write certificate to ZIP file", e);
        }
    }
    private void writeCertificateAuthority(ZipOutputStream zip, String dirName, CertificateTool.CAInfo ca, Environment env) {
        assert ca != null;
        assert ca.generated;
        try {
            writeTextFile(
                zip,
                dirName + "/README.txt",
                CA_README_P12,
                buildSubstitutions(
                    env,
                    Map.of(
                        "P12",
                        "ca.p12",
                        "DN",
                        ca.certAndKey.cert.getSubjectX500Principal().getName(),
                        "PASSWORD",
                        ca.password == null || ca.password.length == 0 ? "" : "*"
                    )
                )
            );