Linux Basic Learning (15) System failure recovery and DNS service configuration

System failure Recovery

One:
7.0 The boot level after the/usr/lib/systemd/system/******.target/etc/systemd/system/default.target can be changed by linking to the start level
If/etc/systemd/system/default.target is removed and the/usr/lib/systemd/system/reboot.target is linked to Default.target, the system restarts Cannot start
How to resolve:
When entering the system's selection interface, press E to enter the edit mode, remove the kernel configuration to RO, change to RW rd.break and then ctrl+x restart
Reboot will enter the command line, enter chroot/sysroot/into the real root environment, then rm-fr/etc/systemd/system/default.target Delete defaullt, then ln-s/usr/lib/ Systemd/system/graphical.target/etc/systemd/system/default.target to link the graphical interface, it's OK.

Two:
If you forget the root password, you want to change the password:
Also press E to enter the editing interface, the same as RW rd.break,ctrl+x, enter the command line. Chroot/sysroot. Then change the password. Once the modifications are complete, touch/.autorelabel the file and then restarts

Three:
Trouble Tool Kit
Test1: Automatically change root password
Test2: Automatically delete grub boot
TEST3: Removed kernel and grub boot
Remove kernel Solution: Load the virtual optical drive, place the image in the real machine, mount it to the virtual CD drive, set the virtual machine as the CD-ROM boot, enter the system
Select trouble that option, enter rescue mode Chroot/mnt/sysimage and then DF to see if the Sr0 (Sr0 is a virtual CD-ROM), into the directory hanging in the virtual CD-ROM, into the PACKAGE,RPM-IVH kernel3-Brabulabra exit reboot
Remove Boot method Workaround: Set root= ' HD0,MSDOS1 '
linux16/boot/vmlinuz3-Brabalabra ro root=/dev/vda1
initrd16/boot/initramfs3-Bra No pull
Boot
Execute Grub2-mkconfig >/boot/grub2/grub.cfg after entering the system

DNS Cache
Service side:
Yum Install Bind-y
Systemctl Enable named
Systemctl Start named
Firewall-cmd--list-all
Firewall-cmd--permanent-add-service=dns
Firewall-cmd Reload
Netstat-antulpe | grep named
Vim/etc/named.conf
Change 4 configurations
Systemctl Restart named

Client:
Vim/etc/resolv.conf
Write NameServer 172.25.254.201 in it (that's your server-side IP)

Are configured well after dig www.baidu.com

Configuration of the DNS server (forward parsing)
Service side:
Vim/etc/named.rfc1912.zones
Copy and paste the 19th to 24th line in line 25 to modify
Zone "Lcdqqq.com" in {
Type master;
File "Lcdqqq.com.zone";
allow-update {none;};
29};
Vim/var/named
ll
Copy the Named.localhost to lcdqqq.com
Cp-p named.localhost lcdqqq.com (-P: Full replication, even permissions copied)
Enter the file to modify: (4) (in this configuration file to the. End, otherwise auto-lcdqqq.com)
@ in SOA dns.lcdqqq.com. Root.lcdqqq.com. (
0; Serial
1D; Refresh
1H; Retry
1W; Expire
3H); Minimum
NS dns.lcdqqq.com.
DNS A 172.25.254.201
www A 172.25.254.1
At this point, the client dig www.lcdqqq.com will be connected to 172.25.254.1

Further setup: $TTL 1D
@ in SOA dns.lcdqqq.com. Root.lcdqqq.com. (
0; Serial
1D; Refresh
1H; Retry
1W; Expire
3H); Minimum
NS dns.lcdqqq.com.
DNS A 172.25.254.201
www A 172.25.254.1
www A 172.25.254.2
LCD CNAME www.lcdqqq.com.
lcdqqq.com. MX 1 172.25.254.101.
Add the last three lines, line 79th, show the DNS polling, line 80th

DNS server configuration (reverse resolution)
Service side:
Vim/etc/named.rfc1912.zones
Copy and paste line 37th to 42nd into line 43 and modify it to:
Zone "254.25.172" in {
The type master;
The file "Lcdqqq.loopback";
allow-update {none;};
47};
Vim/var/named
ll
Copy the Named.loopback to Lcdqqq.loopback
Cp-p named.loopback Lcdqqq.loopback (-P: Full replication, even permissions copied)
Enter the file to modify: (In this configuration file to the. End, otherwise auto-lcdqqq.com)
@ in SOA dns.lcdqqq.com. Root.lcdqqq.com. (
0; Serial
1D; Refresh
1H; Retry
1W; Expire
3H); Minimum
NS dns.lcdqqq.com.
A 172.25.254.201
1 PTR www.lcdqqq.com.
101 PTR www.hello.com.
At this point, the client Dig-x 172.25.254.1 will be connected to www.lcdqqq.com
Dig-x 172.25.254.101 will be connected to www.hello.com

DNS for internal and external network sensitivity detection:
Modify the etc/name.conf to:
View LocalNet {
match-clients {172.25.254.101;};
Zone "." in {
Type hint;
File "named.ca";
};
Include "/etc/named.rfc1912.zones";
};

View Internet {
match-clients {any;};
Zone "." in {
Type hint;
File "named.ca";
};
Include "/etc/named.rfc1912.zones.inter";
};
Make a copy of/etc/named.rfc1912.zones, named Name.rfc1912.zones.inter.
Copy the Lcdqqq.com.zone to Lcdqqq.com.inter under/var/named
Change all 254 to 0
Copy the Lcdqqq.loopback to Lcdqqq.loopback.inter under/var/named
Change all 254 to 0
Test with 172.25.254.101 and 172.25.254.1, respectively.

Linux Basic Learning (15) System failure recovery and DNS service configuration