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--- it_tips:ssh [2017/05/03 11:41]
florido
+++ it_tips:ssh [2023/10/09 14:27] (current)
florido
@@ Line 12: / Line 12: @@
 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:
+<code bash>
+Host *
+  ServerAliveInterval 30
+</code>
+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 =====
@@ Line 69: / Line 83: @@
 :!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!::!:
-This tutorial is nice:
+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.
-https://wiki.archlinux.org/index.php/SSH_Keys
-:!: the below part of the document is work in progress. :!:
 ==== What is SSH PKI ====
@@ Line 82: / Line 91: @@
   * A **Private** key that should be kept secure at all times and only readable by your user
-These user keys are usually stored in the user's ''~/.ssh/'' folder, the default names are ''id_rsa'' (private) and ''id_rsa.pub'' (public). But one can choose any location and any name.
+=== Host keypairs ===
-The machine 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,
+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!)
@@ Line 108: / Line 117: @@
 </code>
-Every time a user connects to a server, the server presents the 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!
+==== 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:
@@ Line 121: / Line 132: @@
 2f:21:6b:19:fc:fc:9d:62:8f:88:c2:2b:c4:d6:0c:70 |1|vyeQU5q0QfKZzq9/helQLGGK9s4=|ZN50r7hlYRTlCeSEXzzz+80XZKw= (RSA)
 </code>
+==== User keypairs ====
+These user keys are usually stored in the user's ''~/.ssh/'' folder, the default names are ''id_rsa'' (private) and ''id_rsa.pub'' (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 ====
@@ Line 136: / Line 155: @@
   * ''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 //myid_rsa.pub// and copy it to a machine called ''watto.matfys.lth.se'' in order to login to it.
+  - Generate a private/public keypair **with password** and strong encryption((NIST complexity recommendations, http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-57Pt3r1.pdf)): Command: <code:bash> ssh-keygen -b 4096 -f ~/.ssh/myid_rsa</code>Result:<code:bash>
+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/myid_rsa.pub.
+The key fingerprint is:
+d:1d:94:b9:71:35:59:f8:79:26:92:b5:a3:f5:d4:e3 pflorido@tjatte.hep.lu.se
+The key's randomart image is:
++--[ RSA 4096]----+
+|          .o .o+.|
+|         .+ . +. |
+|          .+ o oo|
+|         o..o =o*|
+|        S o  +.*o|
+|         .  .  E.|
+|                 |
+|                 |
+|                 |
++-----------------+
+</code>
+  - Make sure the permissions are correct: Commands:<code:bash>chmod 600 ~/.ssh/myid_rsa; chmod 644  ~/.ssh/myid_rsa.pub; ls -ltrah ~/.ssh/myid_rsa*</code>Result:<code:bash>
+-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/myid_rsa.pub
+</code>
+  - Copy the key to the target server, say watto: Command:<code:bash>ssh-copy-id -i ~/.ssh/myid_rsa.pub pflorido@watto.matfys.lth.se</code>Result:<code:bash>
+/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
+Password:
+Number of key(s) added: 1
+Now try logging into the machine, with:   "ssh 'pflorido@watto.matfys.lth.se'"
+and check to make sure that only the key(s) you wanted were added.
+</code>
+  - Add the key to the agent keyring:Command:<code:bash>ssh-add ~/.ssh/myid_rsa</code>Result:<code:bash>
+Enter passphrase for /nfs/users/floridop/.ssh/myid_rsa:
+Identity added: /nfs/users/floridop/.ssh/myid_rsa (/nfs/users/floridop/.ssh/myid_rsa)
+</code>
+  - Try to login to the server **using the identity created**: Command:<code:bash>ssh -i ~/.ssh/myid_rsa.pub pflorido@watto.matfys.lth.se</code>
+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:
+<code:bash>ssh -vv pflorido@watto.matfys.lth.se</code>
+==== See which keys are tried by the agent ====
+<code:bash>ssh-add -l</code>
+==== Delete a key from the agent keyring ====
+<code:bash>ssh-add -d ~/.ssh/myid_rsa</code>
+==== Check if the agent is running ====
+<code:bash>ps aux | grep ssh-agent</code>
+==== Remove an offending key from known_hosts ====
+<code:bash>ssh-keygen -R <hostname></code>
+==== Remove an offending key from known_hosts ====
+<code:bash>ssh-keygen -R <hostname or IP></code>
 ====== References ======
+  * Arch linux SSH PKI tutorial, https://wiki.archlinux.org/index.php/SSH_Keys

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