Explanation: 
Conclusions:
1. I find that it is common to add various service access limits in the exam RHCE. The exercises like: require one network segment can be accessed another network segments can not be accessed, the following are some conclusions for various service:
tcp_wrappers:/etc/hosts.allow,/etc/hosts.deny
tcp_wrappers can filter the TCP’s accessing service. TCP whether has the filtering function which depends on this service whether use the function library of tcp_wrappers, or this service whether has the xinetd process of starting function of tcp_wrappers. tcp_wrappers’s main configuration file is /etc/hosts.allow,/etc/hosts.deny.
And the priority of the documents in hosts. allow is higher than hosts. deny. Visit will be passed if no match was found.
sshd,vsftpd can use the filtering service of tcp_wrappers.
Configuration example:
Notice:
The two configuration files’ syntax can refer to hosts_access (5) and hosts_options(5) sshd_config
There are four parameters in this configuration file: DenyUsers, AllowUsers, DenyGroups, AllowGroups, they are used to limit some users or user groups to proceed Remote Login through the SSH. These parameters’ priority level is DenyUsers->AllowUsers->DenyGroups->AllowGroups
Configuration example:
httpd Service
Through the /etc/httpd/conf/httpd.conf in parameters, can add to control the url access. Just as:
Notice:
So pay attention, deny’s and allow’s priority level in order deny,allow is: the backer has the higher priority level. But here, allow’s priority has a higher priority level.
nfs Service
nfs service directly control the visits through file /etc/exports, just as:
samba Service
Parameter hosts allow in /etc/samba/smb.conf which is used as Access Control, just as:
2. Paying attention to use Mount parameters: _netdev,defaults when you are mounting ISCSI disk.
3. Stop the NetworkManager
/etc/init.d/NetworkManager stop
chkconfig NetworkManager off
4. When you are deploying ifcfg-ethX, add parameters:
PEERDNS=no
5. Empty the firewall in RHCSA、RHCE:
6. Narrow lv steps:
7. Mount the using command - swap which is newly added in /etc/fstab
8. If Verification is not passed when you are installing software, can import public key: rpm import /etc/pki/rpm…/…release and so on. In yum.repo, you also can deploy gpgkey, for example, gpgkey=/etc/pki/rpm…/…release
9. When you are using “Find” command to search and keep these files, paying attention to use cp -a to copy files if you use user name and authority as your searching methods.
