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/AutoConfigureNode.java
private boolean isInitialClusterNode(Settings settings) {
        return DiscoveryModule.isSingleNodeDiscovery(settings)
            || (ClusterBootstrapService.INITIAL_MASTER_NODES_SETTING.get(settings).isEmpty()
                && SettingsBasedSeedHostsProvider.DISCOVERY_SEED_HOSTS_SETTING.get(settings).isEmpty()
                && DiscoveryModule.DISCOVERY_SEED_PROVIDERS_SETTING.get(settings).isEmpty())
            || ClusterBootstrapService.INITIAL_MASTER_NODES_SETTING.get(settings).equals(List.of(NODE_NAME_SETTING.get(settings)));
    }
    private static void fullyWriteFile(
        Path basePath,
        String fileName,
        boolean replace,
        @Nullable String groupOwner,
        CheckedConsumer<OutputStream, Exception> writer
    ) throws Exception {
        Path filePath = basePath.resolve(fileName);
        if (false == replace && Files.exists(filePath)) {
            throw new UserException(
                ExitCodes.IO_ERROR,
                String.format(Locale.ROOT, "Output file [%s] already exists and " + "will not be replaced", filePath)
            );
        }
        
        Set<PosixFilePermission> permission = PosixFilePermissions.fromString("rw-rw----");
        
        if (Files.exists(filePath)) {
            PosixFileAttributeView view = Files.getFileAttributeView(filePath, PosixFileAttributeView.class);
            if (view != null) {
                permission = view.readAttributes().permissions();