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Better SSH experience

This page contains some suggestions on how to improve your SSH experience.

The ssh config file

You user own configuration file is located in your home folder at the path


In some cases this file does not exist, you can create it for example by issuing

touch ~/.ssh/config

This file is very useful to create ssh shortcuts to servers and add specific options for each server. In what follows I show some examples of how this can be used. You can edit the file with any text editor of your choice, it's a simple text file.

Prevent broken pipe when not using the client

Sometimes you're connected to a server but you are not interacting with the ssh client because you have other things to do. In many cases the server will disconnect you if it doesn't see any activity (in jargon, you're idle)

To prevent such disconnection add to your ~/.ssh/config the following:

Host *
  ServerAliveInterval 30

you may tweak that number to be less aggressive eventually. Typical intervals are 60, 90, 120.

This should be done on the machine where you launch the ssh command, to keep that machine in contact with the server.

Speedup connection using tunneling

One can speedup logging in by configuring her/his own ssh client. This will also help in scp-ing data to the cluster.

The configuration example below shows how you can access a machine through another machine with a single ssh command. The scenario is as follows:

 User  --->  machine  ---->  machine ----> machine

And you want to use one single command to login to B or C.

# access
Host hostA
User <username on>
# this line below is equivalent to ssh -X
ForwardX11 yes  
# directly access
Host hostB
User <Username on>
ForwardX11 yes
# Use this command if A uses Openssh server version < 5.4.
# It requires the program netcat (nc) to be installed on the
# machine.
ProxyCommand ssh -q nc 22
# directly access
Host hostC
User <Username on>
ForwardX11 yes
# Use this command if uses Openssh server version >= 5.4,
# that has native support for tunnelling ssh connections.
ProxyCommand ssh -q -W

Example: My user is florido. In the template above, I would change all the <Username …> to florido.

then to login to I will do:

ssh hostB

And I will have to input 2 passwords: one for, one for

In general you will have to input as many passwords as the number of hops. To overcome this problem, use a private/public ssh keypair technique described below.

Reduce passwords using a private/public ssh key pair

If you plan to use this, read the following:




I will give you a set of quick steps how to setup ssh keys. But it is important that one understands the concepts behind, so please read the sections below.

What is SSH PKI

SSH key pair is a form of PKI, Public Key Infrastructure1). In this technology every user or machine has a pair of keys:

  • A Public key that can be shared with everyone and copied on servers
  • A Private key that should be kept secure at all times and only readable by your user

Host keypairs

The machine or host keys are usually stored in the /etc/ssh/ system folder, and they are generated at installation time by the OpenSSH scripts. They are of different kinds as they support different encryption schemas, their names being like ssh_host_<schema> (private) and ssh_host_<schema>.pub (public). In the example below, note how the private key is readable and writable ONLY by root while all the .pub ones are readable by everyone (but not writable!)

ls -ltrah /etc/ssh/ssh_host*
-rw-r--r-- 1 root root  393 mar 30  2016 /etc/ssh/
-rw------- 1 root root 1,7K mar 30  2016 /etc/ssh/ssh_host_rsa_key
-rw-r--r-- 1 root root  601 mar 30  2016 /etc/ssh/
-rw------- 1 root root  672 mar 30  2016 /etc/ssh/ssh_host_dsa_key
-rw-r--r-- 1 root root  173 mar 30  2016 /etc/ssh/
-rw------- 1 root root  227 mar 30  2016 /etc/ssh/ssh_host_ecdsa_key
-rw-r--r-- 1 root root   93 mar 30  2016 /etc/ssh/
-rw------- 1 root root  399 mar 30  2016 /etc/ssh/ssh_host_ed25519_key
-rw-r--r-- 1 root root  651 maj 10  2016 /etc/ssh/
-rw------- 1 root root  986 maj 10  2016 /etc/ssh/ssh_host_key

Every ssh pki key has a fingerprint, that is, a unique way to identify that key. You can check your machine unique fingerprint by using the ssh-keygen script provided by OpenSSH:

tjatte:~> ssh-keygen -lf /etc/ssh/ssh_host_key
ssh-keygen: /etc/ssh/ssh_host_key: Permission denied
tjatte:~> ssh-keygen -lf /etc/ssh/ 
2048 7a:21:c6:ee:4c:a1:a0:0e:4d:72:71:e9:3d:2f:b6:d7 (RSA1)

What happens when connecting to a server

Every time a user connects to a server, the server presents its key fingerprint and the user is requested to acknowledge he/she/ze is aware of trusting that fingerprint. The sysadmin has a list of trustworthy fingerprints, so if you're unsure, ask me!

When a fingerprint is accepted, it is stored in the user's home folder inside the file ~/.ssh/known_hosts in encrypted form:

 head ~/.ssh/known_hosts
|1|MPQKqI2ylQAvJh/w7R3iXekDlv8=|aGwTSWRlJL4O3olYfRmzato1h3o= ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCm0y1t5RLpgUZ16Et8To6lKkh6AyTaCBQyQQ2Z/ASLpr6qTYWIX9Ebx3ZYr7kgUUsikJzuPe8CFHflpNnIAFhVdih1j9y4RJXRi4vo/3SlmT59jq6I6OqG5qcnhys6gFx21Yd8dLiKCgAyWw296gzTsBJDTIp8Myfz0MCPIEO+qWeSOcWq/6wm6iWnTGnfBkJyGnwqZooqO2XgVF3gYbVonES7MB6nTADVSUIAZ8P1TZ2V8vJbXkogUkHD8kv1wt4N/O7yFEtYySsoXLZlb7zFQyw/REVaW1Jn1RVtg76KMhvxBwlgM0J9bbUjcok5E/kIVvu5oDijmMahKBpiOUrB
|1|/vDgSpeBUKtJVgcul94pqe5nWyE=|1eF4hBt+gSqT1mf+X7psKVdWeWQ= ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCm0y1t5RLpgUZ16Et8To6lKkh6AyTaCBQyQQ2Z/ASLpr6qTYWIX9Ebx3ZYr7kgUUsikJzuPe8CFHflpNnIAFhVdih1j9y4RJXRi4vo/3SlmT59jq6I6OqG5qcnhys6gFx21Yd8dLiKCgAyWw296gzTsBJDTIp8Myfz0MCPIEO+qWeSOcWq/6wm6iWnTGnfBkJyGnwqZooqO2XgVF3gYbVonES7MB6nTADVSUIAZ8P1TZ2V8vJbXkogUkHD8kv1wt4N/O7yFEtYySsoXLZlb7zFQyw/REVaW1Jn1RVtg76KMhvxBwlgM0J9bbUjcok5E/kIVvu5oDijmMahKBpiOUrB

As you can see the above is not human readable. But if you want to know if you've trusted a machine, you can do for example 2):

ssh-keygen -lf ~/.ssh/known_hosts -F watto
# Host watto found: line 274 type RSA
2048 2f:21:6b:19:fc:fc:9d:62:8f:88:c2:2b:c4:d6:0c:70 |1|vyeQU5q0QfKZzq9/helQLGGK9s4=|ZN50r7hlYRTlCeSEXzzz+80XZKw= (RSA)

User keypairs

These user keys are usually stored in the user's ~/.ssh/ folder, the default names are id_rsa (private) and (public). But one can choose any location and any name.

A user key can have a password or not. :!: It is strongly discouraged to use passwordless keys. Should your private key get stolen, this will generate an enormous security breach. :!:

The password is used to “unlock” the key, that is, to allow the ssh client (more precisely, the ssh-agent )to use it to connect on the user's behalf.

Using user ssh key pair to login

In order for the user key to work, the machine where one has to login needs to trust that key.

The overall mechanism is as follows. A user willing to connect to a server has to

  1. Copy his/her/hir public key to that server
  2. Instruct the server that such key is trustworthy by modifying the ~/.ssh/authorized_keys file
  3. run an ssh-agent on his/her/hir local machine that will take care of authenticating the user to the server when logging in. Usually this is automatically started by the ssh client if the user is using PKI.

OpenSSH on linux provides a set of scripts for managing keys:

  • ssh-keygen: does key management
  • ssh-copy-id : copies a key to a server and updates the authorized_keys automatically
  • ssh-agent : takes care of remembering which key as been used for which host, remembers key password

SSH key pair quick setup

We will generate a public/private key pair called myid_rsa and and copy it to a machine called in order to login to it.

  1. Generate a private/public keypair with password and strong encryption3): Command:
     ssh-keygen -b 4096 -f ~/.ssh/myid_rsa


    Generating public/private rsa key pair.
    Enter passphrase (empty for no passphrase): 
    Enter same passphrase again: 
    Your identification has been saved in /nfs/users/floridop/.ssh/myid_rsa.
    Your public key has been saved in /nfs/users/floridop/.ssh/
    The key fingerprint is:
    The key's randomart image is:
    +--[ RSA 4096]----+
    |          .o .o+.|
    |         .+ . +. |
    |          .+ o oo|
    |         o..o =o*|
    |        S o  +.*o|
    |         .  .  E.|
    |                 |
    |                 |
    |                 |
  2. Make sure the permissions are correct: Commands:
    chmod 600 ~/.ssh/myid_rsa; chmod 644  ~/.ssh/; ls -ltrah ~/.ssh/myid_rsa*


    -rw------- 1 pflorido hep 3,3K maj  3 13:59 /nfs/users/floridop/.ssh/myid_rsa
    -rw-r--r-- 1 pflorido hep  751 maj  3 13:59 /nfs/users/floridop/.ssh/
  3. Copy the key to the target server, say watto: Command:
    ssh-copy-id -i ~/.ssh/


    /usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
    /usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
    Number of key(s) added: 1
    Now try logging into the machine, with:   "ssh ''"
    and check to make sure that only the key(s) you wanted were added.
  4. Add the key to the agent keyring:Command:
    ssh-add ~/.ssh/myid_rsa


    Enter passphrase for /nfs/users/floridop/.ssh/myid_rsa: 
    Identity added: /nfs/users/floridop/.ssh/myid_rsa (/nfs/users/floridop/.ssh/myid_rsa)
  5. Try to login to the server using the identity created: Command:
    ssh -i ~/.ssh/

For every entry in your config you will need to run ssh-copy-id if you want to use ssh keys on that entry. This will copy the key to the target machine (for example beast)

You can now create entries in your ~/.ssh/config file to use tunnelling as described in Speedup connection using tunneling, and the agent should automatically forward your keys.

Debugging SSH problems and useful commands

The best to debug is to enable ssh verbose mode:

ssh -vv

See which keys are tried by the agent

ssh-add -l

Delete a key from the agent keyring

ssh-add -d ~/.ssh/myid_rsa

Check if the agent is running

ps aux | grep ssh-agent

Remove an offending key from known_hosts

ssh-keygen -R <hostname>

Remove an offending key from known_hosts

ssh-keygen -R <hostname or IP>


it_tips/ssh.txt · Last modified: 2023/10/09 14:27 by florido

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