265 lines
14 KiB
Markdown
265 lines
14 KiB
Markdown
# Pushing SSH a bit further
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## What is SSH
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### Origins
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SSH is *the* current standard for remote logins but you might want to read up a bit on what was used before SSH existed.
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[This](https://www.jeffgeerling.com/blog/brief-history-ssh-and-remote-access) is a pretty good blog post on the history of SSH.
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You should never use the following the following programs anymore but it's good to be aware of their historic existance.
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* rlogin
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* rsh
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* rcp
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* telnet (still has some legitimate usage such as with munin)
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The main advantage of SSH is it's encryption.
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It works similarly to SSL which you use all the time to do most of your web browsing.
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When using encryption it becomes **very** hard to sniff the data traveling between the client and the server.
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There are two versions of SSH, version 1 and version 2, and you should only use version 2 as the former is not considered [secure]() anymore.
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The recommended encryption used by most SSH servers is [AES](https://en.wikipedia.org/wiki/Advanced_Encryption_Standard).
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If you're interested in understanding the mathematics behind AES, [this](https://www.youtube.com/channel/UC1usFRN4LCMcfIV7UjHNuQg) class is exceptionally good but not for the faint of heart.
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It's not mandatory to fully understand the math behind encryption to use it though.
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The main takeaway would be the number of **bit's used** where **higher** is **better**.
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By default ssh uses a very secure cipher but you can specify which one you want with the `-c` flag to `ssh`.
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Do keep in mind that the server needs to support the cipher you're requesting.
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## SSH keys
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SSH encryption and SSH keys are not the same thing.
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**Keys** are used for **authentication** with a server.
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Once the client is authenticated and granted access to the server, the encryption is set to **encrypt** the **traffic** from client to server and visa versa.
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SSH keys are [asymmetric](https://en.wikipedia.org/wiki/Public-key_cryptography) key pairs where you have two simple text files.
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One with the **private** part, which is used for **decrypting**, and one **public** part which is used for **encrypting**.
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Both parts together form one **key pair**.
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If you're interested in the maths behind key pairs, have a look at this 15min [video](https://www.youtube.com/watch?v=4zahvcJ9glg&t=1s), it's a lot easier than you expect!
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### Generating keys
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SSH comes with `ssh-keygen` to generate keys.
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Keys are by default stored in `~/.ssh/` and named `id_rsa` for the **private** part, and `id_rsa.pud` for the **public** part.
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**Important**, each generated key is **unique** so if you overwrite a key you made previously that old key is **gone!**.
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There is **no way** to recover that old key anymore, gone is gone.
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I highly advise you to keep backup's of your important keys, preferably in multiple locations.
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GNOME [Seahorse](https://wiki.gnome.org/Apps/Seahorse/) or [keepass2](https://keepass.info/) are good password managers you can use to store both passwords and key pairs.
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An ssh key can be generated by running the `ssh-keygen` command.
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You'll be prompted with a dialog that will ask to name and where to save the key pair, plus whether you want a password for that key or not.
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For automated logins you should not set a password as it would defeat the purpose but you **must** realise that everyone that has access to the **private** key will be able to log in to each server the key is deployed to.
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Keeping this file safe becomes very important.
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**Remember the bandit SSH key level?**
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```
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➜ ~ git:(master) ✗ ssh-keygen
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Generating public/private rsa key pair.
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Enter file in which to save the key (/home/waldek/.ssh/id_rsa): my_ssh_key
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Enter passphrase (empty for no passphrase):
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Enter same passphrase again:
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Your identification has been saved in my_ssh_key.
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Your public key has been saved in my_ssh_key.pub.
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The key fingerprint is:
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SHA256:ndWgt3PjUjrdfaBMudQFJM8xCrjPKXlvg9TRYsWz2UQ waldek@helloworld
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The key's randomart image is:
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+---[RSA 2048]----+
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| .. oo=.E|
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| . o B++.|
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| .. +o+*.|
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| .. ++++..|
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| S+o+*o* |
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| o *+.X +.|
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| + oB o +|
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| . +o .|
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| . . |
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+----[SHA256]-----+
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➜ ~ git:(master) ✗
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```
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The current default is to generate an [RSA](https://en.wikipedia.org/wiki/RSA_(cryptosystem)) key of 2048 bits which is currently considered secure.
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You can change the scheme and bit depth with the `-t` and `-b` arguments respectively.
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Have a look at the `man ssh-keygen` pages for more info.
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### Deploying keys
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Just generating a key is not that useful, we need to know how to **use** them.
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In order to understand that we need to have a look at how an ssh connection works.
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When you try to establish an ssh connection you can add a `-v` flag to make the output more verbose.
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You'll see output along the lines of this:
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```
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➜ qualifying git:(master) ✗ ssh student@localhost -v
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OpenSSH_7.9p1 Debian-10+deb10u2, OpenSSL 1.1.1d 10 Sep 2019
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debug1: Reading configuration data /home/waldek/.ssh/config
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debug1: Reading configuration data /etc/ssh/ssh_config
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debug1: /etc/ssh/ssh_config line 19: Applying options for *
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debug1: Connecting to localhost [127.0.0.1] port 22.
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debug1: Connection established.
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debug1: identity file /home/waldek/.ssh/id_rsa type 0
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debug1: identity file /home/waldek/.ssh/id_rsa-cert type -1
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debug1: identity file /home/waldek/.ssh/id_dsa type -1
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debug1: identity file /home/waldek/.ssh/id_dsa-cert type -1
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debug1: identity file /home/waldek/.ssh/id_ecdsa type -1
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debug1: identity file /home/waldek/.ssh/id_ecdsa-cert type -1
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debug1: identity file /home/waldek/.ssh/id_ed25519 type -1
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debug1: identity file /home/waldek/.ssh/id_ed25519-cert type -1
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debug1: identity file /home/waldek/.ssh/id_xmss type -1
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debug1: identity file /home/waldek/.ssh/id_xmss-cert type -1
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debug1: Local version string SSH-2.0-OpenSSH_7.9p1 Debian-10+deb10u2
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debug1: Remote protocol version 2.0, remote software version OpenSSH_7.9p1 Debian-10+deb10u2
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debug1: match: OpenSSH_7.9p1 Debian-10+deb10u2 pat OpenSSH* compat 0x04000000
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debug1: Authenticating to localhost:22 as 'student'
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debug1: SSH2_MSG_KEXINIT sent
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debug1: SSH2_MSG_KEXINIT received
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debug1: kex: algorithm: curve25519-sha256
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debug1: kex: host key algorithm: ecdsa-sha2-nistp256
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debug1: kex: server->client cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
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debug1: kex: client->server cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
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debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
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debug1: Server host key: ecdsa-sha2-nistp256 SHA256:iALTEcfl6AjvOnT0TWBNrp/PsuWem/ZiP+uGRVEeFaE
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debug1: Host 'localhost' is known and matches the ECDSA host key.
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debug1: Found key in /home/waldek/.ssh/known_hosts:1
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debug1: rekey after 134217728 blocks
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debug1: SSH2_MSG_NEWKEYS sent
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debug1: expecting SSH2_MSG_NEWKEYS
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debug1: SSH2_MSG_NEWKEYS received
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debug1: rekey after 134217728 blocks
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debug1: Will attempt key: /home/waldek/.ssh/id_rsa RSA SHA256:tOuvE+Qq1B/eXyGcyIfs0MVXaaSI/GNYjLqO3D+Tz+k
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debug1: Will attempt key: /home/waldek/.ssh/id_dsa
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debug1: Will attempt key: /home/waldek/.ssh/id_ecdsa
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debug1: Will attempt key: /home/waldek/.ssh/id_ed25519
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debug1: Will attempt key: /home/waldek/.ssh/id_xmss
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debug1: SSH2_MSG_EXT_INFO received
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debug1: kex_input_ext_info: server-sig-algs=<ssh-ed25519,ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521>
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debug1: SSH2_MSG_SERVICE_ACCEPT received
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debug1: Authentications that can continue: publickey,password
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debug1: Next authentication method: publickey
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debug1: Offering public key: /home/waldek/.ssh/id_rsa RSA SHA256:tOuvE+Qq1B/eXyGcyIfs0MVXaaSI/GNYjLqO3D+Tz+k
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debug1: Authentications that can continue: publickey,password
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debug1: Trying private key: /home/waldek/.ssh/id_dsa
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debug1: Trying private key: /home/waldek/.ssh/id_ecdsa
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debug1: Trying private key: /home/waldek/.ssh/id_ed25519
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debug1: Trying private key: /home/waldek/.ssh/id_xmss
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debug1: Next authentication method: password
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student@localhost's password:
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```
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The connection will try any form of authentication allowed by the server in order of most secure to least secure.
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Publickeys are considered more secure than passwords so it will try that first.
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You can see this at the following line `debug1: Will attempt key: /home/waldek/.ssh/id_rsa RSA SHA256:tOuvE+Qq1B/eXyGcyIfs0MVXaaSI/GNYjLqO3D+Tz+k`.
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The `student` account does not have the matching public key as an authorized key so the client tries a password login next `debug1: Next authentication method: password`.
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On the server side the sshd process will look for a matching **public** key in the users home directory.
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By default it will try every line in the `~/.ssh/authorized_keys` file for authentication.
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If a match is found, you'll be granted access, if not it will try an other authentication method until every method allowed by the server is exhausted and you'll be denied access.
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The verbose output of a successful login with a key pair can be seen below.
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```
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➜ ~ git:(master) ✗ ssh student@localhost -v
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OpenSSH_7.9p1 Debian-10+deb10u2, OpenSSL 1.1.1d 10 Sep 2019
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debug1: Reading configuration data /home/waldek/.ssh/config
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debug1: Reading configuration data /etc/ssh/ssh_config
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debug1: /etc/ssh/ssh_config line 19: Applying options for *
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debug1: Connecting to localhost [127.0.0.1] port 22.
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debug1: Connection established.
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debug1: identity file /home/waldek/.ssh/id_rsa type 0
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debug1: identity file /home/waldek/.ssh/id_rsa-cert type -1
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debug1: identity file /home/waldek/.ssh/id_dsa type -1
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debug1: identity file /home/waldek/.ssh/id_dsa-cert type -1
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debug1: identity file /home/waldek/.ssh/id_ecdsa type -1
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debug1: identity file /home/waldek/.ssh/id_ecdsa-cert type -1
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debug1: identity file /home/waldek/.ssh/id_ed25519 type -1
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debug1: identity file /home/waldek/.ssh/id_ed25519-cert type -1
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debug1: identity file /home/waldek/.ssh/id_xmss type -1
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debug1: identity file /home/waldek/.ssh/id_xmss-cert type -1
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debug1: Local version string SSH-2.0-OpenSSH_7.9p1 Debian-10+deb10u2
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debug1: Remote protocol version 2.0, remote software version OpenSSH_7.9p1 Debian-10+deb10u2
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debug1: match: OpenSSH_7.9p1 Debian-10+deb10u2 pat OpenSSH* compat 0x04000000
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debug1: Authenticating to localhost:22 as 'student'
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debug1: SSH2_MSG_KEXINIT sent
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debug1: SSH2_MSG_KEXINIT received
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debug1: kex: algorithm: curve25519-sha256
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debug1: kex: host key algorithm: ecdsa-sha2-nistp256
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debug1: kex: server->client cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
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debug1: kex: client->server cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
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debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
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debug1: Server host key: ecdsa-sha2-nistp256 SHA256:iALTEcfl6AjvOnT0TWBNrp/PsuWem/ZiP+uGRVEeFaE
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debug1: Host 'localhost' is known and matches the ECDSA host key.
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debug1: Found key in /home/waldek/.ssh/known_hosts:1
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debug1: rekey after 134217728 blocks
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debug1: SSH2_MSG_NEWKEYS sent
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debug1: expecting SSH2_MSG_NEWKEYS
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debug1: SSH2_MSG_NEWKEYS received
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debug1: rekey after 134217728 blocks
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debug1: Will attempt key: /home/waldek/.ssh/id_rsa RSA SHA256:tOuvE+Qq1B/eXyGcyIfs0MVXaaSI/GNYjLqO3D+Tz+k
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debug1: Will attempt key: /home/waldek/.ssh/id_dsa
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debug1: Will attempt key: /home/waldek/.ssh/id_ecdsa
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debug1: Will attempt key: /home/waldek/.ssh/id_ed25519
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debug1: Will attempt key: /home/waldek/.ssh/id_xmss
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debug1: SSH2_MSG_EXT_INFO received
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debug1: kex_input_ext_info: server-sig-algs=<ssh-ed25519,ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521>
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debug1: SSH2_MSG_SERVICE_ACCEPT received
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debug1: Authentications that can continue: publickey,password
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debug1: Next authentication method: publickey
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debug1: Offering public key: /home/waldek/.ssh/id_rsa RSA SHA256:tOuvE+Qq1B/eXyGcyIfs0MVXaaSI/GNYjLqO3D+Tz+k
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debug1: Server accepts key: /home/waldek/.ssh/id_rsa RSA SHA256:tOuvE+Qq1B/eXyGcyIfs0MVXaaSI/GNYjLqO3D+Tz+k
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debug1: Authentication succeeded (publickey).
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Authenticated to localhost ([127.0.0.1]:22).
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debug1: channel 0: new [client-session]
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debug1: Requesting no-more-sessions@openssh.com
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debug1: Entering interactive session.
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debug1: pledge: network
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debug1: client_input_global_request: rtype hostkeys-00@openssh.com want_reply 0
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debug1: Remote: /home/student/.ssh/authorized_keys:1: key options: agent-forwarding port-forwarding pty user-rc x11-forwarding
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debug1: Remote: /home/student/.ssh/authorized_keys:1: key options: agent-forwarding port-forwarding pty user-rc x11-forwarding
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debug1: Sending environment.
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debug1: Sending env LC_CTYPE = en_GB.UTF-8
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debug1: Sending env LANG = en_GB.UTF-8
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Linux helloworld 4.19.0-17-amd64 #1 SMP Debian 4.19.194-1 (2021-06-10) x86_64
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The programs included with the Debian GNU/Linux system are free software;
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the exact distribution terms for each program are described in the
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individual files in /usr/share/doc/*/copyright.
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Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
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permitted by applicable law.
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student@helloworld:~$
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```
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## Standard usage
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## Tweaking the sshd configuration file
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All server configuration is done in the `/etc/ssh/sshd_config` file.
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Starting version TO_CHECK you can use the modern `/etc/ssh/sshd_config.d/` folder system to override default system configuration.
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This way any changes to the standard configuration made by the package maintainers won't mess with your custom preferences and tweaks.
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### Version
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A modern sshd configuration will only allow version 2 but you can check or specify this in the configuration file.
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You'll probably never have to set this yourself but do keep it in mind when you're confronted with old installations.
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```
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Protocol
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Specifies the protocol versions sshd(8) supports. The possible values are ‘1’ and
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‘2’. Multiple versions must be comma-separated. The default is ‘2’. Protocol 1
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suffers from a number of cryptographic weaknesses and should not be used. It is
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only offered to support legacy devices.
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Note that the order of the protocol list does not indicate preference, because the
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client selects among multiple protocol versions offered by the server. Specifying
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“2,1” is identical to “1,2”.
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```
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## Tunnels
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## SFTP
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## Autossh
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## SSHFS
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## SSHuttle
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