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系統管理技巧

完整複製 Home 目錄

由於 User 的 Home 目錄內有許多隱藏檔，若要完整複製它們，有兩個方法：

方法一：可以複製成一個新目錄

cd /home
cp -a user1/ user1_new/

方法二：複製到一個現有目錄內

cd /home
cp -a user1/.[^.]* user1_new/

手動建立一個新的 Home 目錄

cp -r /etc/skel /home/user1
chown -R user1.group1 /home/user1
chmod 0700 /home/user1

為什麼 /home 目錄權限有一個點

Answer: 有(點)的權限目錄表示有在 SELinux 監控清單，既使關閉了 SELinux ，權限也不會變更。

# ls -ld /home
drwxr-xr-x. 2 root root 4096 Mar 28  2017 /home

# ls -Zd /home
drwxr-xr-x.  root root system_u:object_r:home_root_t:s0 /home


自訂 PATH

PATH 新增一個客制 bin 目錄

~/.bashrc :

case :$PATH: in
    *:/home/$USER/bin:*) ;;
    *) PATH=/home/$USER/bin:$PATH ;;
esac
清除 Zombie 程序(defunct)

One may deal with zombie processes in any one of the following ways:

  • Fix the parent process to make it execute wait(2) on child process exit
  • Kill the parent process of the zombie
  • Reboot system
  • Ignore it

列出 zombie processes

ps aux |grep "defunct"
ps aux |grep Z

# How many Zombie process running on your server
ps aux | awk {'print $8'}|grep -c Z

# List the PID of Zombie
ps aux | awk '{ print $8 " " $2 }' | grep -w Z

Kill zombie process

# find the parent process list
pstree -paul

kill -9 <PARENT-PID>

RHEL Documents:

  • What_is_a_zombie_(defunct)_process.pdf
  • How_to_kill_Zombie_Defunct_process.pdf
資安 & Auditing 相關
# Parse /var/log/secure
grep "authentication failure" /var/log/secure | awk '{ print $13 }' | cut -b7-  | sort | uniq -c

# Login failed attempts
lastb -F
lastb -F <username>

Check Linux Login History

#!/bin/bash
#Filename: intruder_detect.sh
#Description: Check Linux Login History
AUTHLOG=/var/log/secure

if [[ -n $1 ]];
then
  AUTHLOG=$1
  echo Using Log file : $AUTHLOG
fi

# Collect the failed login attempts
FAILED_LOG=/tmp/failed.$$.log
egrep "Failed pass" $AUTHLOG > $FAILED_LOG 

# Collect the successful login attempts
SUCCESS_LOG=/tmp/success.$$.log
egrep "Accepted password|Accepted publickey|keyboard-interactive" $AUTHLOG > $SUCCESS_LOG

# extract the users who failed
failed_users=$(cat $FAILED_LOG | awk '{ print $(NF-5) }' | sort | uniq)

# extract the users who successfully logged in
success_users=$(cat $SUCCESS_LOG | awk '{ print $(NF-5) }' | sort | uniq)
# extract the IP Addresses of successful and failed login attempts
failed_ip_list="$(egrep -o "[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+" $FAILED_LOG | sort | uniq)"
success_ip_list="$(egrep -o "[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+" $SUCCESS_LOG | sort | uniq)"

# Print the heading
printf "%-10s|%-10s|%-10s|%-15s|%-15s|%s\n" "Status" "User" "Attempts" "IP address" "Host" "Time range"

# Loop through IPs and Users who failed.

for ip in $failed_ip_list;
do
  for user in $failed_users;
    do
    # Count failed login attempts by this user from this IP
    attempts=`grep $ip $FAILED_LOG | grep " $user " | wc -l`

    if [ $attempts -ne 0 ]
    then
      first_time=`grep $ip $FAILED_LOG | grep " $user " | head -1 | cut -c-16`
      time="$first_time"
      if [ $attempts -gt 1 ]
      then
        last_time=`grep $ip $FAILED_LOG | grep " $user " | tail -1 | cut -c-16`
        time="$first_time -> $last_time"
      fi
      HOST=$(host $ip 8.8.8.8 | tail -1 | awk '{ print $NF }' )
      printf "%-10s|%-10s|%-10s|%-15s|%-15s|%-s\n" "Failed" "$user" "$attempts" "$ip"  "$HOST" "$time";
    fi
  done
done

for ip in $success_ip_list;
do
  for user in $success_users;
    do
    # Count successful login attempts by this user from this IP
    attempts=`grep $ip $SUCCESS_LOG | grep " $user " | wc -l`

    if [ $attempts -ne 0 ]
    then
      first_time=`grep $ip $SUCCESS_LOG | grep " $user " | head -1 | cut -c-16`
      time="$first_time"
      if [ $attempts -gt 1 ]
      then
        last_time=`grep $ip $SUCCESS_LOG | grep " $user " | tail -1 | cut -c-16`
        time="$first_time -> $last_time"
      fi
      HOST=$(host $ip 8.8.8.8 | tail -1 | awk '{ print $NF }' )
      printf "%-10s|%-10s|%-10s|%-15s|%-15s|%-s\n" "Success" "$user" "$attempts" "$ip"  "$HOST" "$time";
    fi
  done
done

rm -f $FAILED_LOG
rm -f $SUCCESS_LOG

System Audit

# Install Audit
yum install audit
systemctl start auditd

# Authentication Report
# To get authentication report for all the attempts which was made
aureport -au -i | more
# To get authentication report for all the success attempts which was made
aureport -au -i --success | more
# To get authentication report for all the failed attempts which was made
aureport -au -i --failed | more
# To get success login information
aureport -l --success | more
# To get failed login information
aureport -l --failed | more
# To get success login summary report for all the success attempts which was made
aureport -l --success --summary -i | more

Check if a RHEL system is vulnerable to a specific CVE

# rpm -q --changelog [package-name] | grep [CVE-NUMBER]
rpm -q --changelog openssl | grep CVE-2021-3450
rpm -q --changelog openssl | grep CVE
rpm -q --changelog openssl | grep CVE-2021

# Using yum command
yum install yum-plugin-security
yum update yum
yum updateinfo info --cve CVE-2021-3445

Auditd

Auditing tool for UNIX/Linux like - Lynis

rsh

rsh server

# install on CentOS 6/7
yum install rsh-server

# Startup the service on CentOS 6
chkconfig rsh on
chkconfig rlogin on
service xinetd reload

# Startup the service on CentOS 7
systemctl start rsh.socket
systemctl start rlogin.socket
systemctl start rexec.socket
systemctl enable rsh.socket
systemctl enable rlogin.socket
systemctl enable rexec.socket
strace 程式除錯
# Trace the command
strace df -h

# Trace the process ID
strace -p 33259

# Get Summary of Linux Process
strace -c -p 3569

# Print Instruction Pointer During System Call
strace -i df -h

# Show Time of Day For Each Trace Output Line
strace -t df -h

# Print Command Time Spent in System Calls
strace -T df -h

# Trace Only Specific System Calls
strace -e trace=write df -h
strace -p 3569 -e poll
停用 suspend, hibernation
# disable the following systemd targets
sudo systemctl mask sleep.target suspend.target hibernate.target hybrid-sleep.target

sudo systemctl restart systemd-logind.service

# Then reboot the system and log in again
# Verify if the changes have been effected using the command
sudo systemctl status sleep.target suspend.target hibernate.target hybrid-sleep.target

# To re-enable the suspend and hibernation modes, run the command
sudo systemctl unmask sleep.target suspend.target hibernate.target hybrid-sleep.target

To prevent the system from going into suspend state upon closing the lid, edit the /etc/systemd/logind.conf file.

[Login] 
HandleLidSwitch=ignore 
HandleLidSwitchDocked=ignore
磁碟資訊
# Approach #1
lsblk

nvme0n1     259:0    0 465.8G  0 disk 
├─nvme0n1p1 259:1    0   512M  0 part /boot/efi
└─nvme0n1p2 259:2    0 465.3G  0 part /
nvme1n1     259:3    0 953.9G  0 disk /media/alang/AlangsData

# Approach #2, requires to run as super-user.
sudo lshw -short -class disk,volume

H/W path                 Device           Class          Description
====================================================================
/0/100/14/0/3/4/0.0.0    /dev/sda         disk           Mass-Storage
/0/100/14/0/3/4/0.0.0/0  /dev/sda         disk
last
# To check the last ten login attempts, you can pipe it with "head"
last | head -n 10

# using complete usernames and hostnames
last -w

# find the device used by the user
tty

# To find the last login by date,
last --since <date>
last --until <date>
last --since -2days

# find the last bad login attempts
sudo lastb
tail -f -n 100 /var/log/auth.log | grep -i failed

# find the last SSH logins
tail -f -n 100 /var/log/auth.log | grep -i sshd
sudo journalctl -r -u ssh | grep -i failed

# find last login times for all users
lastlog
lastlog -u <user>
檢測虛擬平台類型
dmidecode -s system-manufacturer
systemd-detec-virt
virt-what
硬體資訊
sudo lshw -short

H/W path                 Device           Class          Description
====================================================================
                                          system         NUC8i7HVK
/0                                        bus            NUC8i7HVB
/0/0                                      memory         64KiB BIOS
/0/2f                                     memory         16GiB System Memory
/0/2f/0                                   memory         8GiB SODIMM DDR4 Synchronous Unbuffered (Unregistered)
/0/2f/1                                   memory         8GiB SODIMM DDR4 Synchronous Unbuffered (Unregistered)
/0/34                                     memory         256KiB L1 cache
/0/35                                     memory         1MiB L2 cache
/0/36                                     memory         8MiB L3 cache
/0/37                                     processor      Intel(R) Core(TM) i7-8809G CPU @ 3.10GHz
/0/100                                    bridge         Xeon E3-1200 v6/7th Gen Core Processor Host Bridge/DRA
/0/100/1                                  bridge         Xeon E3-1200 v5/E3-1500 v5/6th Gen Core Processor PCIe
/0/100/1/0               /dev/fb0         display        Polaris 22 [Radeon RX Vega M GH]
/0/100/1/0.1                              multimedia     Advanced Micro Devices, Inc. [AMD/ATI]
/0/100/1.1                                bridge         Xeon E3-1200 v5/E3-1500 v5/6th Gen Core Processor PCIe
/0/100/1.1/0                              bus            ASMedia Technology Inc.
/0/100/1.1/0/0           usb3             bus            xHCI Host Controller
/0/100/1.1/0/1           usb4             bus            xHCI Host Controller
...
#
sudo lshw -html > HardwareSummary.html

Finding Number of Ram Slots

sudo dmidecode -t memory

sudo lshw -class memory
編譯與開發工具

Install

# RedHat/CentOS 6
yum install make libtool autoconf subversion git cvs wget libogg-devel gcc gcc-c++ pkgconfig

# RedHat/CentOS 7
yum group install "Development Tools"
dd
# 備份 MBR
dd if=/dev/hdx of=/path/to/image count=1 bs=512

# 光碟轉成 iso 檔
dd if=dev/cdrom of=/root/cd.iso

# 銷毀硬碟資料
dd if=/dev/urandom of=/dev/hda1

# 備份整個 USB-Flash
dd if=/dev/sdb | gzip > ./my-usb_flash.img.gz

# 還原 USB-Flash
gzip -dc ./my-usb_flash.img.gz | dd of=/dev/sdb

# 建立一個測試用的大檔案10GB
dd if=/dev/zero of=/path/to/image bs=1G count=10
# NOTE: 新版 Linux 可以改用指令
fallocate -l 1G test.img 

# Test network bandwidth between 2 Linux servers
dd if=/nas-mount-point/samplefile of=/dev/null bs=1M count=1024 iflag=direct
dd if=/dev/zero of=/nas-mount-point/samplefile bs=1M count=1024 oflag=direct 
# NOTE: the samplefile is greater than 1GB and the RAM is preferably more than 2GB.
history

See time stamp in bash history

echo 'export HISTTIMEFORMAT="%F %T "' >> ~/.bash_profile
系統效能管理

使用 Swap 的 processes

for file in /proc/*/status ; do awk '/VmSwap|Name/{printf $2 " " $3}END{ print ""}' $file; done | sort -k 2 -n -r | less
Swap 管理
# 檢查目前的 swap 配置
free
swapon -s

# 開啟/關閉 swap
swapon /dev/sda3
swapoff /dev/sda3

# 製作一個 swap 系統
mkswap /dev/sda3

make-swapfile.sh：1GB

#!/bin/bash

dd if=/dev/zero of=/swapfile bs=1024 count=1024k
chown root:root /swapfile
chmod 0600 /swapfile
mkswap /swapfile
swapon /swapfile
echo "/swapfile          swap            swap    defaults        0 0" >> /etc/fstab
sysctl vm.swappiness=10
echo vm.swappiness=10 >> /etc/sysctl.conf
free -h
cat /proc/sys/vm/swappiness

Extend the existing SWAP partition:

[root@tycitpdb05-a ~]# grep -i swap /etc/fstab
/dev/mapper/rootvg-swap swap                    swap    defaults        0 0

[root@tycitpdb05-a ~]# ls -al /dev/mapper/rootvg-swap
lrwxrwxrwx 1 root root 7 Aug 15 11:49 /dev/mapper/rootvg-swap -> ../dm-1

[root@tycitpdb05-a ~]# swapon -s
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       4194300 0       -2

[root@tycitpdb05-a ~]# swapoff -v /dev/mapper/rootvg-swap
swapoff /dev/mapper/rootvg-swap

[root@tycitpdb05-a ~]# lvextend -L16G /dev/rootvg/swap
  Size of logical volume rootvg/swap changed from 4.00 GiB (1024 extents) to 16.00 GiB (4096 extents).
  Logical volume rootvg/swap successfully resized.
  
[root@tycitpdb05-a ~]# mkswap /dev/rootvg/swap
mkswap: /dev/rootvg/swap: warning: wiping old swap signature.
Setting up swapspace version 1, size = 16777212 KiB
no label, UUID=06aac9ae-9e8c-48bd-9f16-5f3d0c32b31f

[root@tycitpdb05-a ~]# swapon -v /dev/rootvg/swap
swapon /dev/rootvg/swap
swapon: /dev/mapper/rootvg-swap: found swap signature: version 1, page-size 4, same byte order
swapon: /dev/mapper/rootvg-swap: pagesize=4096, swapsize=17179869184, devsize=17179869184
xfs 檔案系統

修復 xfs 檔案系統

sudo mount -a
mount: /data: mount(2) system call failed: Structure needs cleaning.

sudo umount /data

# with '-n' option to perform a dry run
sudo xfs_repair -n /dev/sdb1
# repair the filesystem
sudo xfs_repair /dev/sdb1

xfs 備份與回復

# Install the xfsdump
dnf install xfsdump

# Create a full-backup
# -L: for dump session
# -M: for media in drive
# -f: the backup destination
xfsdump -L session_0 -M datapart -f /data/boot.xfsdump /boot

# Create a incremental backup with the level 1
# -l: the backup level (0-9)
xfsdump -l 1 -L session_1 -M datapart -f /data/boot.xfsdump1 /boot

# Restore a full-backup
xfsrestore -f /data/boot.xfsdump /test

# Restore an incremental backup
xfsrestore -r -f /data/boot.xfsdump /test
xfsrestore -r -f /data/boot.xfsdump1 /test
Linux Module
監視檔案與目錄異動
比對兩個檔案

列出 A.txt 中没有包含 B.txt 内容的行

diff A.txt B.txt | grep "^<" | cut -c3-
比對兩個目錄
# tree
tree dir1
tree dir2

# diff
diff -q /path/to/dir1 /path/to/dir2
diff -q dir1 dir2
diff -qr dir1 dir2
diff -qrs dir1 dir2
複製整個目錄架構，但不要包含檔案
# tree
tree -dfi --noreport dir1
tree -dfi --noreport dir1 | xargs -I{} mkdir -p "$HOME/Downloads/{}"
tree -a $HOME/Downloads/dir1

# find + xargs
find dir1 -type d
find dir1 -type d | xargs -I{} mkdir -p "$HOME/Documents/{}" 
tree -a $HOME/Documents/dir1

# find + exec
find dir1 -type d -exec mkdir -p "$HOME/Desktop/{}" \;
記憶體管理
# Using meminfo
cat /proc/meminfo
grep -E --color 'Mem|Cache|Swap' /proc/meminfo
  • MemTotal, Total usable RAM (i.e., physical RAM minus a few reserved bits and the kernel binary code).
  • MemFree, The sum of LowFree+HighFree.
  • MemAvailable, (since Linux 3.14) An estimate of how much memory is available for starting new applications, without swapping.
  • Buffers, Relatively temporary storage for raw disk blocks that shouldn’t get tremendously large (20MB or so).
  • Cached, In-memory cache for files read from the disk (the page cache). Doesn’t include SwapCached.
  • SwapCached, Memory that once was swapped out, is swapped back in but still also is in the swap file.
# Using free
free -h
# Repeat printing free command output every N seconds.
free -s 5 -c 10
  • total, Total installed memory
  • used, Used memory (calculated as total – free – buffers – cache)
  • free, Unused memory (MemFree and SwapFree in /proc/meminfo)
  • shared, Memory used mostly by tmpfs (Shmem in /proc/meminfo)
  • buffers, Memory used by kernel buffers (Buffers in /proc/meminfo)
  • cache, Memory used by the page cache and slabs (Cached and SReclaimable in /proc/meminfo)
  • buff/cache, Sum of buffers and cache
  • available, Estimation of how much memory is available for starting new applications, without swapping.
# Using vmstat
vmstat -w
  • swapd, the amount of virtual memory used.
  • free, the amount of idle memory.
  • buff, the amount of memory used as buffers.
  • cache, the amount of memory used as cache.
  • inact, the amount of inactive memory. (-a option)
  • active, the amount of active memory. (-a option)
  • si, Amount of memory swapped in from disk (/s).
  • so, Amount of memory swapped to disk (/s).
特殊字元的檔名

Dash

# The filename with -- or -
rm -i -v -- -foo
rm -i -v -- --foo
rm -i -v ./-foo

# The filename with -- and whitespaces
rm -i -v -- '-- My Resume . txt'
rm -i -v -- '/path/to/dir/-- My Resume . txt'
rm -i -v -- "/path/to/dir/-- My Resume . txt"

# Using find
find . -name '--my-FileNameGoes-Here' -delete
find /path/to/directory/ -name '---filename with a white spaces' --delete
cp: 強制覆蓋多檔案
# 因為內建 alias=cp -i，以致於 -f 參數無效
yes | cp -r /source /target


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