SSH In Case Of

SSH In Case Of

Category & Tags: Nix, Linux, SSH

Note: Putty officially recommends to use Bitvise SSH Client & Server.
See also Powershell.

Sudo #

sudo without passwd

vim /etc/sudoers
# or
sudoedit /etc/sudoers

modify ALL=(ALL) ALL to ALL=(ALL) NOPASSWD:ALL (i.e. add NOPASSWD: before the last ALL), e.g. %sudo ALL=(ALL:ALL) NOPASSWD:ALL

SSHD Config > Less Time #

Add two commands to avoid long time before asking passwords:

GSSAPIAuthentication no
UseDNS no

GSS API is alternative to SSH-keys

SSHD Config > More Security #

Change to get high security by using key auth only:

PasswordAuthentication no
ChallengeResponseAuthentication no

SSHD Config > Keep Alive #

On server:

ClientAliveInterval 100
ClientAliveCountMax 2

(this can also be done on clients, but usually clients_nr > server_nr.)

Config > Remember to Restart #

sudo service ssh restart

SSHDConfig > All Above Together #

echo ' '                                  >> /etc/ssh/sshd_config;
echo 'GSSAPIAuthentication no'            >> /etc/ssh/sshd_config;
echo 'UseDNS no'                          >> /etc/ssh/sshd_config;
echo 'PasswordAuthentication no'          >> /etc/ssh/sshd_config;
echo 'ChallengeResponseAuthentication no' >> /etc/ssh/sshd_config;
echo "ClientAliveInterval 100" | sudo tee -a /etc/ssh/sshd_config
echo "ClientAliveCountMax 2"   | sudo tee -a /etc/ssh/sshd_config
echo ' '                                  >> /etc/ssh/sshd_config;
service ssh restart && sleep 1 && service ssh status

SSHD To Allow Root Login w/ Password #

Note: dangerous, use for tests only.

sed -i 's/.*PermitRootLogin.*//' /etc/ssh/sshd_config
echo 'PermitRootLogin yes' >> /etc/ssh/sshd_config

sed -i 's/.*PasswordAuthentication.*//' /etc/ssh/sshd_config
echo 'PasswordAuthentication yes' >> /etc/ssh/sshd_config

service ssh restart && sleep 1 && service ssh status

Client Config File Example #

vi ~/.ssh/config

# with the following config, we can `ssh mydb`
Host mydb
    Port 22
    User opc
    ProxyJump [email protected]
    # Jump through multiple hosts:
    # ProxyJump user@host1,user@host2,user@host3

    # VNC 5901 port: // On Windows you can see it referenced as :0 which means 5900, :1 is 5901 etc.
    LocalForward 5901 localhost:5901

    # test web
    LocalForward 80 localhost:8080
    # or
    LocalForward client_local_ip_like_127001:80 remote_server:8080

    # Reversed forwarding
    RemoteForward 90 localhost:9999
    # or
    RemoteForward one_of_sever_ips:90 client_localhost:9999

    # SOCKS
    DynamicForward 1337

ref bak

SSH Tunneling for Clients #

port forwarding - direct static forwarding #

Suppose we want the client to Listen on80 and forward all traffic via SSH-server to localhost:8888 (a jupyter is running on “localhost” on the server).

ssh from nix as client: #

c2s (client-to-server) by -L #

ssh -L 80:localhost:8888 remote_server_username@remote_server_ip
  • -L: localforward, listen on a local client port and forward any connect (to this local client port 80) to the remote server.
  • meaning of this cmd: forward client’s local 80 (from ssh client’s perspective) to a specified server & port (localhost:8888, from ssh server’s perspective).
  • the “localhost” in the cmd is the remote server.

s2c (server-to-client) by -R #

ssh -R 90:localhost:9999 remote_server_username@remote_server_ip
  • -R: remote reverse forward, listen on the remote server’s port and forward any connect (to server port 90) to this local client.
  • meaning of this cmd: forward remote (server) 80 to local end host & port (localhost:9999)
  • the “localhost” in the cmd is this local client.

tips #

ssh -NR      90:localhost:9999 remote_server_username@remote_server_ip
ssh -fnNTqgR 90:localhost:9999 remote_server_username@remote_server_ip

this example shows common parameters used together with forwarding (note -R must be the last to combine with forwarding info).

  • -f : fork the process into the background after you type your password (may combine with sleep x, see man ssh).
  • -n : no STDIN (n or N must be used if using -f).
  • -N : no cmd execution on remote server, thus no remote terminal either (n or N must be used if using -f).
  • -T : no pseudo-terminal allocation.
  • -q : quiet mode. Since this is just a tunnel we can make it quiet.
  • -g : globallly allow remote hosts to connect to local forwarded ports (must be specified on the master process for a multiplexed connection).

See ref zhihu for autossh.

ref: tencent, zhihu

ssh from win as client: #

port forwarding - dynamic port forwarding (socks proxy) #

client on nix: #

ssh -D 9876 -fnN -C -q -N myuser@remote_ssh_server

  • -D 9876 : dynamic destinations, port 9876 behaves as a SOCKS server.
  • -C : compression on.

client on win: #

putty ssh tunnels > source port 9876; dest: Dynamic.
bitvise > services > SOCKS proxy > enable > listen port 9876.

usage in firefox: #


network.proxy.no_proxies_on : localhost,,,
network.proxy.socks :
network.proxy.socks_port : 8080
network.proxy.socks.remote_dns : true
network.proxy.socks_version : 5
network.proxy.type : 1


permanent ssh tunneling as a service #

Create a persistent SSH tunnel between servers with systemd, (bak)

Generate Keys for Openssh-server #

(OBS: putty-gen can convert .ppk to openssh, but not vice-versa. cuz .ppk contains key pair.)

in windows with putty #

Run PUTTYGEN.EXE: (OBS: some servers support 2048 only)


  1. Change “Key comment” !!! (will influence .ppk chucksum, so canNOT be easily changed later).
  2. Give passphrase (recommended).
  3. Copy the “public key for … authorized_keys” content to file “authorized_keys” (OpenSSH, most Nix distribution).
  4. “Save private key” to .ppk (private & public pair, putty).
  5. Menu > Conversions > “Export OpenSSH Key” > file “id_rsa” (private, OpenSSH).

in nix #

Generate and distribute:

ssh-keygen -b 2048
# give fileanme or press enter for default
ssh-copy-id -i $HOME/.ssh/ -p 22 username@host1
ssh-copy-id -i $HOME/.ssh/ -p 22 username@host2


method 1: add fingerpring for all nodes #

(this may insert redundant/duplicate items if any is already exiting, but no problems.)

ssh-keyscan -H ip_domain_1,ip_domain_2,ip_domain_3 >> ~/.ssh/known_hosts

Batch from a file example:

ssh-keyscan -f $HADOOP_HOME/etc/hadoop/slaves -H >> ~/.ssh/known_hosts

ssh config:

Host *
    StrictHostKeyChecking no

method 3: ssh w/ -o to each node to trigger auto-adding fingerprint #

(why not use ssh-keyscan instead?)

ssh -oStrictHostKeyChecking=no


ssh user@host "echo \$HOME" # need to escape variables


Nix as driver:

ssh [email protected] 'bash -s' <

Win as driver, see here.


sudo DEBIAN_FRONTEND=noninteractive apt-get -y -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold" upgrade


Config Putty #


Mosh (Mobile-Shell) #

install on servers #


sudo apt install mosh

install on clients #

Nix or Windows WSL:

sudo apt install mosh

Windows cygwin, if needs other termianl emulators (or simply use chrome):

C:\cygwin64\setup-x86_64.exe -q mobile-shell

run #

mosh host_or_ip
# or
mosh --ssh="ssh -p 80" user@host_or_ip

NOTE: here uses --ssh=... to set ssh port, as mosh -p means the mosh port, not ssh one.

udp ports range #

as mosh -p means the mosh port, not ssh one.