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IT General Knowledge

Newline Character

  • Classic Mac OS: \r
  • Unix-like: \n
  • Windows: \r\n

IP

Public Network vs Private Network

Load Balance

Load Balancer (L4 Load Balancer)

  • L4 meaning transport layer in OSI model.
  • It make route decision based on IP & port.

Reverse Proxy (L7 Load Balancer)

  • L7 meaning application layer in OSI model.
  • It's a public virtual host for internal web servers.

HTTP Request Headers

Non-Standard Headers

  • X-Forwarded-For
  • X-Real-IP
  • X-Forwarded-Host
  • X-Forwarded-Proto

RC7239

  • Forwarded

tmux

Prefix

  • All tmux shortcut started with prefix
  • Default: ^B
    • I prefer: ` or Escape
    • My tmux configuration: .tmux.conf

User Configuration

  • Location: ~/.tmux.conf






























# VI-style control in copy-mode
set-window-option -g mode-keys vi
# emacs-style control in status-line
set-option -g status-keys emacs

# Move around panes with hjkl like VIM
bind -r h select-pane -L
bind -r j select-pane -D
bind -r k select-pane -U
bind -r l select-pane -R

# Resize panes
bind -r M-h resize-pane -L 1
bind -r M-j resize-pane -D 1
bind -r M-k resize-pane -U 1
bind -r M-l resize-pane -R 1

# Change tmux prefix to Esc
unbind C-b
set-option -s escape-time 0
set-option -g prefix Escape
bind Escape send-prefix

# Toggle status bar
bind t set-option status

# Binding some shortcuts
bind-key C-r source-file ~/.tmux.conf\; display "~/.tmux.conf reloaded."
bind-key C-l clear-history\; display "clear history"
bind-key C-c list-commands\; display "list commands"

Session & Client Operations

  • List sessions: tmux ls
  • List clients: tmux lsc, I never used.
  • Attach to last used session: tmux attach or tmux a
  • Attach to specific session: tmux a -t <target session>
  • Detach client: tmux detach or prefix, d
  • Switch to previous client: prefix, (
  • Switch to next client: prefix, )

Window & Pane Operations

  • Split window (to panes): prefix, %/"
  • Move around panes: prefix, arrow keys(↓/←/→/↑)
  • Kill pane: prefix, x
  • New window: prefix, c
  • Select the previous window: prefix, p
  • Select the next window: prefix, n

History Operations

  • Enter copy mode: prefix, [
  • Leave copy mode: q

Other Shortcuts

  • List key bindings: prefix, ?

PowerShell

Create TLS Certificate

New-SelfSignedCertificate -DnsName mydomain.com -FriendlyName mydomainAlias -NotAfter (G
et-Date).AddYears(1)

Disable Weak Cipher

Disable-TlsCipherSuite -Name "TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA"

TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA (ecdh_x25519) - A
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (ecdh_x25519) - A
TLS_DHE_RSA_WITH_AES_256_CBC_SHA (dh 2048) - A
TLS_DHE_RSA_WITH_AES_128_CBC_SHA (dh 2048) - A
TLS_RSA_WITH_AES_256_CBC_SHA (rsa 2048) - A
TLS_RSA_WITH_AES_128_CBC_SHA (rsa 2048) - A
TLS_RSA_WITH_3DES_EDE_CBC_SHA (rsa 2048) - C
TLS_RSA_WITH_RC4_128_SHA (rsa 2048) - C
TLS_RSA_WITH_RC4_128_MD5 (rsa 2048) - C

Nmap

SSL/TLS

  • nmap -sV --script ssl-enum-ciphers -p 443 <host>

SELinux

SSH Port

  • sudo semanage port --list | grep ssh
    • semanage port -l
  • sudo semanage port --add --type ssh_port_t --proto tcp 1234
    • semanage port -at ssh_port_t -p tcp 1234

Database

MySQL

Reset root password

Create & Grant user








CREATE USER 'newuser'@'localhost' IDENTIFIED BY 'user_password';
CREATE USER 'newuser'@'10.8.0.5' IDENTIFIED BY 'user_password';
CREATE USER 'newuser'@'%' IDENTIFIED BY 'user_password';
GRANT permission1, permission2 ON database_name.table_name TO 'database_user'@'localhost';
GRANT CREATE, DROP, DELETE, INSERT, SELECT, UPDATE ON database_name.* TO database_user@'localhost';
GRANT ALL PRIVILEGES ON *.* TO 'adminuser'@'%' WITH GRANT OPTION;
FLUSH PRIVILEGES;

Create Database



CREATE DATABASE mydatabase;
CREATE DATABASE my_unicode_database CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

Oracle 11g

Export / Import

  1. Table Exports/Imports


expdp username/password@ORCL1 tables=AAA,BBB dumpfile=AAA_BBB.dmp
impdp username/password@ORCL2 tables=AAA,BBB dumpfile=AAA_BBB.dmp
  1. Schema Exports/Imports


expdp username/password@ORCL1 schemas=username dumpfile=username.dmp
impdp username/password@ORCL2 schemas=username dumpfile=username.dmp
  1. Database Exports/Imports


expdp \"sys/password@ORCL as sysdba \" full=Y dumpfile=ORCL.dmp
impdp \"sys/password@ORCL as sysdba \" full=Y dumpfile=ORCL.dmp

Oracle 19c

Install on Centos 7

  1. RPM




sudo yum install bind-utils compat-libcap1 compat-libstdc++-33 glibc-devel ksh libaio-devel libstdc++-devel net-tools nfs-utils psmisc smartmontools sysstat unzip xorg-x11-utils xorg-x11-xauth
curl -O https://yum.oracle.com/repo/OracleLinux/OL7/latest/x86_64/getPackage/oracle-database-preinstall-19c-1.0-1.el7.x86_64.rpm
sudo rpm -i oracle-database-preinstall-19c-1.0-1.el7.x86_64.rpm
sudo rpm -i oracle-database-ee-19c-1.0-1.x86_64.rpm
  1. Config /etc/hosts


sudoedit /etc/hosts
<server fixed IP> <server hostname>
  1. Config





sudoedit /etc/init.d/oracledb_ORCLCDB-19c
export ORACLE_SID=ORCL
export CREATE_AS_CDB=false
sudo cp /etc/sysconfig/oracledb_ORCLCDB-19c.conf /etc/sysconfig/oracledb_ORCL-19c.conf
sudo /etc/init.d/oracledb_ORCLCDB-19c configure
  1. Check config log


sudo su - oracle
less /opt/oracle/cfgtoollogs/dbca/ORCLCDB/ORCLCDB.log
  1. Set Environment Variables for Oracle user





sudo su - oracle
vim ~/.bash_profile
export ORACLE_HOME=/opt/oracle/product/19c/dbhome_1
export ORACLE_SID=ORCL
export PATH=$PATH:$ORACLE_HOME/bin
  1. Create a DBA user


sudo su - oracle
sqlplus / as sysdba



CREATE USER mydba IDENTIFIED BY "mypassword";
GRANT DBA TO mydba;
  1. Firewall


sudo firewall-cmd --permanent --add-port=1521/tcp
sudo firewall-cmd --reload

Install on CentOS 8

  1. RPM



sudo dnf install https://yum.oracle.com/repo/OracleLinux/OL8/baseos/latest/x86_64/getPackage/oracle-database-preinstall-19c-1.0-1.el8.x86_64.rpm
sudo echo <ip> <FQDN> <short hostname> >> /etc/hosts
sudo rpm -i oracle-database-ee-19c-1.0-1.x86_64.rpm
  1. Config
sudoedit /etc/init.d/oracledb_ORCLCDB-19c
export ORACLE_SID=ORCL
export CREATE_AS_CDB=false
sudo /etc/init.d/oracledb_ORCLCDB-19c configure
  1. Check config log


sudo su - oracle
less /opt/oracle/cfgtoollogs/dbca/ORCLCDB/ORCLCDB.log
  1. Set Environment Variables for Oracle user





sudo su - oracle
vim ~/.bashrc
export ORACLE_HOME=/opt/oracle/product/19c/dbhome_1
export ORACLE_SID=ORCL
export PATH=$PATH:$ORACLE_HOME/bin
  1. Create a DBA user


sudo su - oracle
sqlplus / as sysdba



CREATE USER mydba IDENTIFIED BY "mypassword";
GRANT DBA TO mydba;
  1. Firewall


sudo firewall-cmd --permanent --add-port=1521/tcp
sudo firewall-cmd --reload

Oracle

Tablespace



CREATE TABLESPACE TTT DATAFILE 'ttt_data.dbf' SIZE 10m AUTOEXTEND ON NEXT 10m;
DROP TABLESPACE TTT INCLUDING CONTENTS AND DATAFILES CASCADE CONSTRAINTS;

Privileges








-- DBA
SELECT * FROM ROLE_TAB_PRIVS;
SELECT * FROM DBA_SYS_PRIVS;
SELECT * FROM DBA_TAB_PRIVS;
SELECT * FROM DBA_ROLE_PRIVS;
-- Normal user
SELECT * FROM USER_ROLE_PRIVS;

Backup Table

create table aaa_bak as select * from aaa;

Log Rotation

logrotate

logrotate.d

Convention is to put logrotate config file in directory logrotate.d









/path/to/log/file {
    compress
    copytruncate
    daily
    dateext
    dateformat .%Y%m%d
    rotate 999
}

logrotate manually

logrotate -s /path/to/logrotate/state/file /path/to/logrotate/config/file
logrotate -fs /dev/null /path/to/logrotate/config/file
logrotate /path/to/config/file &>/dev/null

/usr/sbin/logrotate /etc/logrotate.d/config &>/dev/null

/var/log/tomcat/catalina.out {
    compress
    copytruncate
    rotate 3
    postrotate
        find /var/log/tomcat/ -mtime +3 -exec rm {} \;
    endscript
}

Shell Scripts

Bash

Absolute path of script



file=$(cd "$0"; pwd)
directory=$(cd $(dirname "$0"); pwd)

Container

Docker

Restart Policy

  • docker run --restart always ...
  • docker update --restart=always <container>
    • docker inspect <container>

Windows

Excluded Port ranges

  • Clear a lot of ports that used by The Windows NAT Driver service.




netsh interface ipv4 show excludedportrange protocol=tcp
net stop winnat
net start winnat
netsh interface ipv4 show excludedportrange protocol=tcp

Linux

RPM Download (RedHat, CentOS)

  • Download RPM that already installed.
    • --destdir: destination directory
    • --resolve: download dependencies (that are not installed).


sudo yum install yum-utils
yumdownloader <package-name>

Create User





groupadd --gid 1234 mygroup
useradd --gid mygroup --uid 1234 myuser
passwd mypassword
usermod -aG wheel myuser

Network

Disable IPv6 on a specific interface

  • CentOS 7
    • /etc/sysctl.conf
      • net.ipv6.conf.eth0.disable_ipv6 = 1
  • CentOS 8
    • nmcli con modify eth0 ipv6.method disable

CentOS 8

PPPoE

  1. Install (by DVD)




sudo mount /dev/sr0 /mnt
sudo rpm -i /mnt/BaseOS/Packages/ppp-<package version>.x86_64.prm
sudo rpm -i /mnt/BaseOS/Packages/NetworkManager-ppp-<package version>.x86_64.rpm
sudo umount /mnt
  1. Install (by network)
sudo dnf install NetworkManager-ppp
  1. Configure Network Manager







sudo nmcli connection edit type pppoe
nmcli> set pppoe.username <PPPoE username, e.g. xxxxxxxx@hinet.net>
nmcli> set pppoe.password <PPPoE password>
nmcli> set connection.interface-name eth0
nmcli> set connection.zone external
nmcli> set ipv6.method disabled
nmcli> save & quit

DHCP Server









sudo nmcli con modify Wired\ connection\ 1 connection.id eth1
sudo nmcli con modify eth1 ipv6.method disabled
sudo nmcli con modify eth1 ipv4.method manual ipv4.addresses 192.168.255.254/16
# sudo nmcli con modify eth1 connection.autoconnect yes
sudo nmcli con up eth1
sudo dnf install dhcp-server
sudo cp /usr/share/doc/dhcp-server/dhcpd.conf.example /etc/dhcp/dhcpd.conf
sudoedit /etc/dhcp/dhcpd.conf

NAT




sudo nmcli con modify pppoe connection.zone external
sudo nmcli con modify eth1 connection.zone internal
sudo firewall-cmd --set-default-zone=external
Last changed by 
 
Mark Zhang
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