今天用我半年前在阿里云买的CentOS配Apache，结果一直都无法访问，后来查看 /etc/httpd/logs/ 下 error_log
[Mon Oct 22 17:07:53.919507 2018] [mpm_prefork:notice] [pid 3990] AH00170: caught SIGWINCH, shutting down gracefully
[Mon Oct 22 17:07:54.956601 2018] [suexec:notice] [pid 4456] AH01232: suEXEC mechanism enabled (wrapper: /usr/sbin/suexec)
[Mon Oct 22 17:07:54.963905 2018] [auth_digest:notice] [pid 4456] AH01757: generating secret for digest authentication ...
[Mon Oct 22 17:07:54.964368 2018] [lbmethod_heartbeat:notice] [pid 4456] AH02282: No slotmem from mod_heartmonitor
[Mon Oct 22 17:07:54.966187 2018] [mpm_prefork:notice] [pid 4456] AH00163: Apache/2.4.6 (CentOS) configured -- resuming normal operations
[Mon Oct 22 17:07:54.966201 2018] [core:notice] [pid 4456] AH00094: Command line: '/usr/sbin/httpd -D FOREGROUND'
百度后发现是端口没有开放，被防火墙拦下了。。。。。。
所以又开放80端口，然后就可以访问了：
/sbin/iptables -I INPUT -p tcp --dport 80 -j ACCEPT
