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en:linux:ssh [2023/10/18 22:49] – angelegt psycoreen:linux:ssh [2024/12/17 13:08] (current) – created psycore
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-====== Back up the sshd ======+{{tag>deutsch linux debian sshd it-security}} 
 +====== Backing up the sshd ======
  
-The pre-installed SSH daemon (sshd) is insecure in the basic configuration. To ensure greater protection, it is necessary to integrate a [[wpde>Asymmetrisches_Kryptosystem|key authentication]].+The pre-installed SSH daemon (sshd) is insecure in the basic configuration. To ensure greater protection, it is necessary to integrate a [[wp>Public-key_cryptography|to integrate key authentication]].
  
  
-===== Create key pair =====+===== 1.0 Key pair =====
  
-First, we create a key pair under Linux:+==== 1.1 Create key pair ==== 
 + 
 + 
 +Firstly, we create a key pair under Linux: 
 + 
 +<code bash> 
 +$ ssh-keygen -t ed25519 
 +Generating public/private ed25519 key pair. 
 +Enter file in which to save the key (/home/user/.ssh/id_ed25519): /home/user/.ssh/id_ed25519 
 +</code> 
 + 
 +We are asked to enter a password. This process is important, **because only the combination of key file and password** enable server access.
  
 <code bash> <code bash>
-$ ssh-keygen -t rsa -b 4096 
-Generating public/private rsa key pair. 
-Enter file in which to save the key (/home/user/.ssh/id_rsa): /home/user/.ssh/id_rsa 
 Enter passphrase (empty for no passphrase): Enter passphrase (empty for no passphrase):
 Enter same passphrase again: Enter same passphrase again:
-Your identification has been saved in /home/user/.ssh/id_rsa. +Your identification has been saved in /home/user/.ssh/id_ed25519. 
-Your public key has been saved in /home/user/.ssh/id_rsa.pub.+Your public key has been saved in /home/user/.ssh/id_ed25519.pub.
 The key fingerprint is: The key fingerprint is:
 35:9f:6e:c2:46:62:09:2d:dc:dd:1e:79:cc:56:d9:2b root@v05-s42 35:9f:6e:c2:46:62:09:2d:dc:dd:1e:79:cc:56:d9:2b root@v05-s42
 </code> </code>
-**Be sure to enter a password, otherwise you can access the server just by possessing the private key!** 
  
-We rename id_rsa.pub to authorised_keys and download id_rsa locally to our computer. **It is important to delete id_rsa securely afterwards!** (if necessary install wipe with //apt-get install wipe//)+With ''id_ed25519.pub'' we now do the following:
  
-<code bash>$ wipe id_rsa +  - rename it to ''authorised_keys'' 
-Okay to WIPE 1 regular file ? (Yes/No) yes +  - move to the folder ''.ssh'' 
-Operation finished. +  - Set access authorisations
-1 file wiped and 0 special files ignored in 0 directories, 0 symlinks removed but not followed, 0 errors occured.</code> +
- +
-We repeat the process with all users who should have access to the sshd. +
- +
-Set modes:+
  
 <code bash> <code bash>
 +mv id_ed25519.pub .ssh/authorized_keys
 chmod 0700 .ssh chmod 0700 .ssh
 chmod 0600 .ssh/authorized_keys chmod 0600 .ssh/authorized_keys
 </code> </code>
  
-===== Putty Private Key =====+We then load ''id_ed25519'' locally on the computer.
  
-Now we download [[http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html|puttygen.exe]] down. We open puttygen.exe and navigate in the menu to **Conversion / Import Key**. At this point we select the generated private key that we have downloaded to our computer. Now add a suitable comment and we can click on **save private key** button.+==== 1.2 Secure deletion of the private key ====
  
-We need this key with the .ppk extension to log in later for putty. **Never upload this key to the server!**+**It is important, ''id_ed25519'' securely afterwards!** (If necessary, install wipe with //apt-get install wipe//) 
 + 
 +<code bash>$ wipe id_ed25519 
 +Okay to WIPE 1 regular file ? (Yes/No) yes 
 +Operation finished. 
 +1 file wiped and 0 special files ignored in 0 directories, 0 symlinks removed but not followed, 0 errors occured.</code> 
 + 
 +===== 2.0 Putty Private Key ===== 
 + 
 +Now we download [[http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html|puttygen.exe]] down. We open puttygen.exe and navigate in the menu to ''Conversion / Import Key''. At this point, we select the generated private key that we have downloaded to our computer. We mark ''EdDSA'' in the lower area. Now add a suitable comment and we can click on ''Save private key'' button. 
 + 
 +Save this key with the ''.ppk'' extension for later login to putty. **Never upload this key to the server!**
  
  
 ===== Login test ===== ===== Login test =====
  
-Now we test whether the connection is established with the generated key. To do this, we open putty.exe and enter the host name as usual. Before we click on **open** we switch to the left in the tree view to **Connection / SSH / Auth** and enter the following under **private key file for authentication** enter the path to our ppk file. Now click on **open** button. If the login was successful, and without error messages, we can completely disable password authentication in our sshd.+Now we test whether the connection is established with the generated key. To do this, we open putty.exe and enter the host name as usual. Before we now click on ''open'' in the tree view on the left, we switch to ''Connection'' -> ''SSH'' -> ''Auth'' and enter under ''private key file for authentication'' enter the path to our ppk file. Now click on ''open'' button. If the login was successful, and without error message, we can completely deactivate the password authentication in our sshd.
  
-===== sshd configuration =====+===== SSHd configuration =====
  
-Now we edit the sshd configuration file <code text>/etc/ssh/sshd_config</code>+Now we edit the SSHd configuration file <code bash>nano /etc/ssh/sshd_config</code>
  
 <code text> <code text>
-# Hier ist es sinnvoll einen Port oberhalb von 1024 zu nehmen+# choose a port above 1024
 Port 22 Port 22
  
-# Unbedingt Protokoll 2 verwenden!+# use protocol 2 !
 Protocol 2 Protocol 2
  
-# RSAAuthentication deaktivieren+# deactivate RSAAuthentication
 RSAAuthentication no RSAAuthentication no
  
-# PubkeyAuthentication aktivieren+# activate PubkeyAuthentication
 PubkeyAuthentication yes PubkeyAuthentication yes
  
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 IgnoreRhosts yes IgnoreRhosts yes
  
-# Wollen wir nicht+# unwanted options
 RhostsRSAAuthentication no RhostsRSAAuthentication no
 HostbasedAuthentication no HostbasedAuthentication no
 PermitEmptyPasswords no PermitEmptyPasswords no
- +PermitRootLogin no 
-# Wollen wir erst recht nicht +PasswordAuthentication no
-PasswordAuthentication no #UNBEDINGT AUSKOMMENTIEREN UND AUF NO SETZEN!!!!+
 ChallengeResponseAuthentication no ChallengeResponseAuthentication no
 </code> </code>
  
-===== Restart sshd =====+===== SSHd restart =====
  
 <code bash>/etc/init.d/ssh restart</code> <code bash>/etc/init.d/ssh restart</code>
  
-==== Note ==== +==== Hint ====
- +
-The current SSH session is not closed. **To correct configuration errors, the current session should remain open until everything is working properly!** +
- +
-===== Copyright and notes ===== +
- +
-© by [[psycore]] 2020+
  
-The author accepts no liability for any damage resulting from the use of this tutorial. This tutorial is written to the best of my knowledge and belief. The distribution of this tutorial is only allowed if the original text including copyright remains untouched.+The current SSH session is not closed. **To correct configuration errors, the current session should remain open until everything is working correctly!**