ssh

測試驗證服務的設定

# 重啟 sshd 前先驗證設定
# 沒有任何內容輸出，表示設定沒有錯誤
# 指令必須使用絕對路徑
/usr/sbin/sshd -t

# 詳細驗證模式
# 可檢視詳細設定檔參數
/usr/sbin/sshd -T

檢視連線參數的設定

$ ssh -F ~/.ssh/config -G remote-host-name
user root
hostname 173.82.136.138
port 22
addressfamily any
batchmode no
canonicalizefallbacklocal yes
canonicalizehostname false
challengeresponseauthentication yes
checkhostip yes
compression no
...

~/.ssh/config

Host my-k8s
    HostName 192.168.31.81
    User alang
    Port 22
    IdentityFile ~/.ssh/alang@mint_rsa
    LocalForward 11000 localhost:11000

自動套用金鑰檔

• %r : remote username
• %h : remote hostname

Host *
    IdentityFile ~/.ssh/id_%r@%h
    IdentityFile ~/.ssh/id_ed25519
    IdentityFile ~/.ssh/id_rsa

建立金鑰檔

mkdir -p $HOME/.ssh
chmod 0700 $HOME/.ssh
ssh-keygen -t rsa

# Specify 4096 bits (default 2048)
# Specify the filename of the key file 
# (default $HOME/.ssh/id_rsa is private key, $HOME/.ssh/id_rsa.pub is public key)
ssh-keygen -t rsa -b 4096 -f ~/.ssh/my-vps-cloud.key -C "My Comment"

以金鑰方式連線

# By custom key file
ssh -i /path/to/the-key-file root＠hostname

# By default key file ~/.ssh/id_rsa
ssh root＠hostname

複製公鑰檔至遠端主機上

指令一：從主機 A 上執行

ssh-copy-id user＠remote-host-ip 
or
ssh-copy-id -f -i $HOME/.ssh/id_rsa.pub user＠remote-host-ip

指令二：從主機 A 上執行

cat ~/.ssh/id_rsa.pub | ssh user＠remote-host-ip "mkdir -p ~/.ssh && cat >>  ~/.ssh/authorized_keys"

從主機 B 上執行，以手動方式上傳 id_rsa.pub：

cd ~/.ssh
mv id_rsa.pub host-A-hostname.pub
cat host-A-hostname.pub >> authorized_keys
chmod 0700 ~/.ssh
chmod 0640 authorized_keys

NOTE:

如果 .ssh 目錄裡已經有 authorized_keys 檔案，可以另存一個檔名加上 2，例如 authorized_keys2

測試連線: 從主機 A 上執行

ssh <remote-userB>@<remote-hostB-name>

不需要輸入密碼就可以登入。

限制來源IP: 編輯 authorized_keys

# Allow login from 192.168.2.0/24 subnet but not from 192.168.2.25
from="!192.168.2.25,192.168.2.*" ssh-ed25519 my_random_pub_key_here vivek＠nixcraft
# Allow login from *.sweet.home but not from router.sweet.home
from="!router.sweet.home,*.sweet.home" ssh-ed25519 my_random_pub_key_here vivek＠nixcraft

公鑰遺失，重新產生

# Generate a public key from a private key
ssh-keygen -y -f ~/.ssh/id_rsa > ~/.ssh/id_rsa.pub

# View the fingerprint of the key
# NOTE: If the private key and the public key is key pair, the fingerprint of them are the same.
ssh-keygen -l -f ~/.ssh/id_rsa.pub
ssh-keygen -l -f ~/.ssh/id_rsa

指令自動補全

自動搜尋 ~/.ssh/config 的連線資訊

編輯 ~/.bashrc 

complete -o default -o nospace -F _ssh ssh

sshpass

整合 shell 做自動化的指令

Install sshpass

sudo apt-get install sshpass     #[On Debian, Ubuntu and Mint]
sudo yum install sshpass         #[On RHEL/CentOS/Fedora and Rocky Linux/AlmaLinux]
sudo emerge -a sys-apps/sshpass  #[On Gentoo Linux]
sudo pacman -S sshpass           #[On Arch Linux]
sudo zypper install sshpass      #[On OpenSUSE]    

1. ssh 執行指令

# Use the -p (this is considered the least secure choice and shouldn't be used)
sshpass -p !4u2tryhack ssh -o StrictHostKeyChecking=no username＠host.example.com hostname

# Use the -f option (the password should be the first line of the filename)
echo '!4u2tryhack' >pass_file
chmod 0400 pass_file
sshpass -f pass_file ssh -o StrictHostKeyChecking=no username＠host.example.com hostname

# Use the -e option (the password should be the first line of the filename)
SSHPASS='!4u2tryhack' sshpass -e ssh -o StrictHostKeyChecking=no username＠host.example.com hostname

2. 整合 rsync

# Use -e
SSHPASS='!4u2tryhack' rsync --rsh="sshpass -e ssh -l username" /custom/ host.example.com:/opt/custom/

# Use -f
rsync --rsh="sshpass -f pass_file ssh -l username" /custom/ host.example.com:/opt/custom/

3. 整合 scp 

scp -r /var/www/html/example.com --rsh="sshpass -f pass_file ssh -l user" host.example.com:/var/www/html

# copying a file to a remote server
sshpass -p "REMOTE_USER_PASSWORD" scp linuxshelltips_v2.txt ubuntu@18.118.208.79:/home/ubuntu/
# copy a directory
sshpass -p "REMOTE_USER_PASSWORD" scp -r Some_Directory/ ubuntu@18.118.208.79:/home/ubuntu/

4. With a GPG-encrypted file

echo '!4u2tryhack' > .sshpasswd
gpg -c .sshpasswd
rm .sshpasswd
gpg -d -q .sshpassword.gpg > pass_file; sshpass -f pass_file ssh user＠srv1.example.com hostname

整合 tar 做異機備份與檔案傳輸

NOTE:

用 tar 做檔案複製，可以保留 Symbolic links, special devices, sockets, named pipes 等等的特殊類型檔案。

Sudo: Please note that you may get an error that read as follows with ssh command when using with sudo or any other command that needs a pseudo-terminal allocation:

• How To Use tar Command Through Network Over SSH Session

ssh user@box tar czf - /dir1/ > /destination/file.tar.gz
ssh user@box 'cd /dir1/ && tar -cf - file | gzip -9' >file.tar.gz

# backups /wwwdata directory to dumpserver.nixcraft.in host over ssh session
tar zcvf - /wwwdata | ssh user@dumpserver.nixcraft.in "cat > /backup/wwwdata.tar.gz"

# With gpg
tar zcf - /data2/ | gpg -e | ssh vivek@nas03 'cat - > data2-dd-mm-yyyy.tar.gz.gpg'

# With sudo
tar zcvf - /wwwdata | ssh -t vivek@192.168.1.201 "sudo cat > /backup/wwwdata.tar.gz"

# Copying from the remote machine (server1.cyberciti.biz) to local system
cd /path/local/dir/
ssh vivek@server1.cyberciti.biz 'tar zcf - /some/dir' | tar zxf -

# With dd
dd if=/dev/sdvf | ssh backupimg@vpc-aws-mumbai-backup-001 'dd of=prod-disk-hostname-sdvf-dd-mm-yyyy.img'
# To restore a local drive from the image on the server
ssh backupimg@vpc-aws-mumbai-backup-001 'dd if=prod-disk-hostname-sdvf-dd-mm-yyyy.img' | dd of=/dev/sdvf

OTP and Two-Factor Authentication

Google Authenticator

• Setting up multi-factor authentication on Linux systems
• How to Setup Two-Factor Authentication (Google Authenticator) for SSH Logins
• How To Setup Multi-Factor Authentication For SSH In Linux
• Enabling 2FA on RHEL 8 using Google Authenticator
• Configure SSH Two factor (2FA) Authentication on CentOS 8 / RHEL 8
• Use oathtool Linux command line for 2 step verification (2FA)
• How to Set Up SSH to Use Two-Factor Authentication
• How to set up 2FA for Linux desktop logins for added security

USB Thumb Drive / Memory Card

• How To Login With A USB Flash Drive Instead Of A Password On Linux Using pam_usb (Fork)

YubiKey

https://github.com/drduh/YubiKey-Guide 

允許可遠端登入的帳號

AllowUsers joe root＠192.168.1.32 axer＠163.* axer＠120.109.* axer＠2001:288:5400:*

# OR
AllowGroups ssh-users

踢出(Kick Out) 遠端登入帳號

方法一:

root@localhost:~# who -u
abhishek pts/0        2021-04-05 09:25 00:01       31970 (223.180.180.107)
prakash  pts/1        2021-04-05 09:26   .         32004 (223.180.180.107)
root     pts/2        2021-04-05 09:26   .         32039 (223.180.180.107)

root@localhost:~# echo "Your session will end in 2 minutes. Save your work!" | write prakash pts/2

root@localhost:~#  kill -HUP 32004

方法二: 用 loginctl

loginctl terminate-user <user-name>

從遠端一行指令修改密碼會顯示明碼

# add the option -t to have the password to be invisible.
ssh -t <username>＠<remote-host-ip> passwd

從遠端執行指令

ssh user1＠server1 'df -H'
ssh root＠nas01 uname -mrs
ssh root＠nas01 lsb_release -a
ssh sk@192.168.225.22 "uname -r ; lsb_release -a"

# Run sudo or su command
ssh -t user＠hostname sudo command 
ssh -t user＠hostname 'sudo command1 arg1 arg2'
ssh user＠nas01 su -c "/path/to/command1 arg1 arg2"
# RHEL/CentOS specific #
ssh user＠nas01 su --session-command="/path/to/command1 arg1 arg2"
ssh vivek＠nixcraft.home.server su --session-command="/sbin/service httpd restart"

# Running and executing multiple ssh command
cat > commands.txt
date
uptime
df -H

ssh user＠server_name < commands.txt

# Run Local Scripts On Remote Systems
ssh sk@192.168.225.22 'bash -s' < my.sh

Sample: The multi-line command syntax

#!/bin/bash
_remote="ls.backup"
_user="vivek"
 
echo "Local system name: $HOSTNAME"
echo "Local date and time: $(date)"
 
echo
echo "*** Running commands on remote host named $_remote ***"
echo
ssh -T $_remote <<'EOL'
	now="$(date)"
	name="$HOSTNAME"
	up="$(uptime)"
	echo "Server name is $name"
	echo "Server date and time is $now"
	echo "Server uptime: $up"
	echo "Bye"
EOL

防止 Idle 狀態被終止連線

ssh 連線可能因為某些原因，例如防火牆，在一段時間沒有操作後，連線會被切斷，要避免這情形，可以透過 Server 端或者 Client 端，設定連線保持就可以解決。

方法一：在 Server 端編輯 /etc/ssh/sshd_config

ClientAliveInterval	300
ClientAliveCountMax	3

 方法二：在 Client 端，要連線加上參數

ssh -o ServerAliveInterval=300 username@server_ip_address

安全設定

停用不安全的密碼演算法

• https://access.redhat.com/solutions/4278651
  

# Find out current encryption_algorithms supported
# From local
sshd -T | grep "\(ciphers\|macs\|kexalgorithms\)"
# Frome Remote
nmap --script "ssh2*" your.ssh.server.ip

# Verify the settings
sshd -t

# Reload the SSH
systemctl reload sshd

# Test
# From Remote
ssh -vv -oCiphers=aes128-cbc,3des-cbc,blowfish-cbc <server.ip>
ssh -vv -oMACs=hmac-md5 <server.ip>

# Find out what algorithms the ssh supports for
# See the section Ciphers, KexAlgorithms, MACs
man 5 sshd_config

For RHEL 8

/etc/sysconfig/sshd

# uncomment the line with the CRYPTO_POLICY= variable in /etc/sysconfig/sshd.
CRYPTO_POLICY=

/etc/ssh/sshd_config

# Fix for 'SSH Weak Algorithms Supported'
# Fix for 'SSH Weak MAC Algorithms Enabled'
# Fix for 'SSH Weak Key Exchange Algorithms Enabled'
# For RHEL8
Ciphers aes128-ctr,aes256-ctr
KexAlgorithms curve25519-sha256,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512
GSSAPIKexAlgorithms gss-group14-sha256-,gss-group16-sha512-,gss-nistp256-sha256-,gss-curve25519-sha256-

For RHEL 7

# Fix for 'SSH Weak Algorithms Supported'
# Fix for 'SSH Weak MAC Algorithms Enabled'
# Fix for 'SSH Weak Key Exchange Algorithms Enabled'
# For RHEL7
Ciphers aes128-ctr,aes192-ctr,aes256-ctr
KexAlgorithms curve25519-sha256,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512

For RHEL 6

/etc/ssh/sshd_config

# Fix for 'SSH Weak Algorithms Supported'
# Fix for 'SSH Weak MAC Algorithms Enabled'
# Fix for 'SSH Weak Key Exchange Algorithms Enabled'
# For RHEL6
Ciphers aes128-ctr,aes192-ctr,aes256-ctr
KexAlgorithms diffie-hellman-group-exchange-sha256
MACs hmac-sha1,hmac-ripemd160

For RHEL 5

# Fix for 'SSH Weak Algorithms Supported'
# Fix for 'SSH Weak MAC Algorithms Enabled'
# Fix for 'SSH Weak Key Exchange Algorithms Enabled'
# RHEL 5 doesn't support for KexAlgorithms
# For RHEL5
Ciphers aes128-ctr,aes192-ctr,aes256-ctr
MACs hmac-sha1,hmac-ripemd160

For AIX 7.1/7.2

# Fix for CVE-2023-48795
# NOTE: Restarting service is required for applying the changes
Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com
MACs umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512

Port Knocking To Secure SSH

• How to Use Port Knocking To Secure SSH Service in Linux

DenyHosts

• Block SSH Server Attacks (Brute Force Attacks) Using DenyHosts

SSHGuard

• How to Block SSH Brute Force Attacks Using SSHGUARD

目錄權限

chmod 0700 ~/.ssh
chmod 600 ~/.ssh/id_rsa
chmod 600 ~/.ssh/id_rsa.pub
chmod 600 ~/.ssh/authorized_keys
chmod 600 ~/.ssh/known_hosts
chmod 600 ~/.ssh/config

登入root通知信

/root/.bashrc , /root/.profile 

# Send an alert when someone logged on as Root
# Put the function into the .bashrc or .profile of Root home directory
Send_Alert() {
    ADMIN_EMAIL="sysadmin@example.com"
    SUBJECT="[ALERT]Root Access from $(who | cut -d'(' -f2 | cut -d')' -f1 | tail -1) to $(hostname -s)"
    echo "
Hostname: $(hostname -s)
Datetime: $(date +'%Y-%m-%d') $(date +'%T')

[More details - Show who is logged on]
$(who)

    " | mail -s "$SUBJECT" "$ADMIN_EMAIL"
}
Send_Alert

Port Forwarding

• A Visual Guide to SSH Tunnels: Local and Remote Port Forwarding

Port Knocking

一種保護 SSH 外部網路存取的方式，原理是外部網路透過 Knock 機制，敲特定埠號組合，可以控制主機端 SSH 防火牆的開啟與關閉。

• Knock, Knock: How to Shield Your VPS From Port Scanning with Port Knocking - LowEndBox

SFTP

• How to Enable SFTP Without Shell Access
• How to Install SFTPGo on Ubuntu 22.04
  • How To Setup SFTP Server With SFTPGo In Linux

Chroot jails

• How to set up SFTP to chroot only for specific users
• How to configure an sftp server with restricted chroot users with ssh keys
• Linux 建立 SFTP 專用帳號，並設定 Chroot 環境教學
• How to set up Linux chroot jails
• Restrict SSH User Access to Certain Directory Using Chrooted Jail

踢掉已登入 sftp 的 user

> ps -ef | grep i04181
root     2165397     988  0 12:01 ?        00:00:00 sshd: i04181 [priv]
i04181   2165401       1  0 12:01 ?        00:00:00 /usr/lib/systemd/systemd --user
i04181   2165403 2165401  0 12:01 ?        00:00:00 (sd-pam)
i04181   2165410 2165397  0 12:01 ?        00:00:00 sshd: i04181@notty
i04181   2165411 2165410  0 12:01 ?        00:00:00 /usr/libexec/openssh/sftp-server
root     2166995 2165217  0 13:22 pts/0    00:00:00 grep --color=auto i04181

> pstree -p i04181
sshd(2165410)───sftp-server(2165411)

systemd(2165401)───(sd-pam)(2165403)

> kill 2165410

SCP

• How to Securely Transfer Files in Linux Using SCP - Make Tech Easier

PuTTY

• windows - How to export/import PuTTY sessions list? - Stack Overflow

Tools

• EasySSH - Multiple SSH Sessions in a Single Window with EasySSH | TechRepublic
• Shellngn - Online SSH Client with SFTP, VNC, RDP - Shellngn

FAQ

RHEL 6 連線至較新版 RHEL 8 時發生錯誤

no hostkey alg

在 RHEL 6 執行以下指令

ssh-keygen -t ecdsa -f /etc/ssh/ssh_host_ecdsa_key -C '' -N ''
chmod 600 /etc/ssh/ssh_host_ecdsa_key
chmod 640 /etc/ssh/ssh_host_ecdsa_key.pub
restorecon /etc/ssh/ssh_host_ecdsa_key.pub

編輯 /etc/ssh/ssh_config

Host <rhel8-hostname/IP>
  HostKeyAlgorithms ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521

Learning

• How To Set up SSH Keys on a Linux / Unix System
• Top 20 OpenSSH Server Best Security Practices
• How to Set a Custom SSH Warning Banner and MOTD in Linux
• Protect SSH Logins with SSH & MOTD Banner Messages
• SSH ProxyCommand example: Going through one host to reach another server
• How To Reuse SSH Connection To Speed Up Remote Login Process Using Multiplexing
• 10 Actionable SSH Hardening Tips to Secure Your Linux Server
• How to audit SSH server and client config on Linux/Unix
• Ezeelogin (Commercial) - Setup a self hosted Jump server  on your premise to manage SSH access to Linux servers,  Routers, Switches, Cloud instances .

Web SSH

• Shell In A Box – A Web-Based SSH Terminal to Access Remote Linux Servers
• Bastillion is a web-based SSH console that centrally manages administrative access to systems.

SSH Auditing

• Teleport - The open source access platform used by DevSecOps teams for SSH, Kubernetes, databases, internal web applications and Windows.
  

Commands