Apache httpd侦听端口必须大于或等于9990

Question

我介绍了CentO 7.3
httpd-2.4.6-89.el7.centos.x86_64
php-7.3.7-3.el7.remi.x86_64

默认httpd.conf，无变化。
创建vhosts.conf
    听9990

<VirtualHost *:9990>
    ServerAdmin admin@xxx.com
    DocumentRoot "/var/www/sites/localhost/httpdocs"
    ServerName localhost
    ErrorLog "logs/localhost-error.log"
    CustomLog "logs/localhost-access.log" common

    <Directory "/var/www/sites/localhost/httpdocs">
        Options FollowSymLinks
        AllowOverride All
        Require all granted
    </Directory>
</VirtualHost>

没关系。

但是9990以下的任何其他端口均失败。我尝试了9989、9980、9970 ... 8000 ... 7000 ... 6000。除了8080以外，所有都失败了！

8080还行！所有其他小于9990的端口都会失败。

我知道了

[root@localhost httpdocs]# service httpd restart
Redirecting to /bin/systemctl restart httpd.service
Job for httpd.service failed because the control process exited with error code. See "systemctl status httpd.service" and "journalctl -xe" for details.

为什么？