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How to install and configure SSH client and server. Understand the keys and security issues. Common questions and practical ansswers.
SSH uses public-key cryptography to authenticate the remote computer and allow it to authenticate the user, if necessary. There are several ways to use SSH; one is to use automatically generated public-private key pairs to simply encrypt a network connection, and then use password authentication to log on.
Another is to use a manually generated public-private key pair to perform the authentication, allowing users or programs to log in without having to specify a password. In this scenario, anyone can produce a matching pair of different keys (public and private). The public key is placed on all computers that must allow access to the owner of the matching private key (the owner keeps the private key secret).
Secure Shell (SSH) is a cryptographic network protocol for operating network services securely over an unsecured network.
If your SSH service allows password based authentication, then your Internet connected SSH server will be hammered day and night by bot-nets trying to guess user-names and passwords. The bot net needs no information, it can just try popular names and popular passwords. There's an awful lot of people named john with a password of qwerty123. Apart from anything else this clogs your logs.
except that public-key auth will do nothing for your logs getting clogged with bots trying to connect. To stop that, run your SSH server on a high port (i.e. 9876 instead of 22). Then if they want to hit you they have to portscan you first, and bots generally don't waste that much time... there are plenty of SSH servers on 22.
The logic is that there are a lot more combinations of SSH keys than passwords so it is a lot harder to guess. Using SSH keys also allows you to disable password authentication meaning that most of the automated attacks going round the internet will be useless.
This isn't so much a technical question as it is conceptual. I understand the cryptography used in an SSH key is far stronger than a regular password, but I don't understand why it is considered more
IMPORTANT - do not install openssh-server on your local machine (laptop/desktop) unless you wish to permit incoming connections from other remote machines ... you do NOT need this package to ssh to other machines since ubuntu comes with the Client half of this Server
How to restart SSH after Ubuntu 15.04?
sudo systemctl restart ssh
How do I stop/start ssh? I've tried
sudo service ssh restart
sudo restart ssh
I get errors every time.
- Public and Private Keys
- Key-Based SSH Logins
- Generating RSA Keys
- Choosing a good passphrase
- Key Encryption Level
- Password Authentication
- Transfer Client Key to Host
- Where to From Here?
Desserts References and More
The .ssh directory will contains (in the home directory of any user) :
- authorized_keys : list of public keys allowed to be used to connect to this server
- config : optional file with configuration parameters for the SSH client
- the public and private keys you've generated on this host for this user.
- known_hosts : maintain a list of hosts to which you already connected together with a hash to detect if the host key has changed since the last time.
On client, after I do ssh-add to add my private key everything works fine, ssh-add -l lists key and I can connect to hosts that have corresponding public key, but nothing changes in .ssh dir as it
[Host's public key] SSH host keys are stored in /etc/ssh/, which you generally do not need to choose. These keys were generated when the openssh-server package was installed.
You can list the fingerprint of the keys by ssh-keygen -l -f /etc/ssh/ssh_host_key.pub though you will need to repeat this for each public key.
[private key] ... By default, ssh-keygen will create a key for the current user, which, by default, will be stored in ~/.ssh.
I installed openssh-server and created a key with ssh-keygen. I then attempted to test it using local port forwarding by doing ssh -L 8080:www.nytimes.com:80 127.0.0.1. However, the key fingerprint...
View the publc key in different formats: MD5, SHA1, SHA256
$ ssh-keygen -l -E md5 -f dustin.pem
2048 MD5:29:ed:da:d3:5a:8c:78:4f:62:d3:fd:0c:77:5b:6d:d9 dustin.pem.pub (RSA)
$ ssh-keygen -l -E sha1 -f dustin.pem
2048 SHA1:x2ENPL+vzVdlgkIyu0tAhVQ+H4U dustin.pem.pub (RSA)
$ ssh-keygen -l -E sha256 -f dustin.pem
2048 SHA256:agJs/axI8QPzet/eoPMDxLSf37fd1bgsMX4Di0gqMy4 dustin.pem.pub (RSA)
It seems like this should be easy. I run "ssh-keygen -l -f " (the default seems to be SHA1). All of the examples show it printing the hex-formatted digest with a little extra, harmless information....
DSA is faster in signing, but slower in verifying. A DSA key of the same strength as RSA (1024 bits) generates a smaller signature. An RSA 512 bit key has been cracked, but only a 280 DSA key.
Also note that DSA can only be used for signing/verification, whereas RSA can be used for encryption/decrypt as well.
It appears they are both encryption algorithms that require public and private keys. Why would I pick one versus the other to provide encryption in my client server application?
Perhaps you have a new installation, or there is a man-in-the-middle attack.
Solution #1: Remove keys using ssh-keygen
Solution #2: Add correct host key in /home/user/.ssh/known_hosts
Solution 3: Just delete the known_hosts file If you have only one ssh server
Explains how to fix - warning: remote host identification has changed! Openssh server and client error under any Linux / UNIX / BSD like operating systems.
Did you recently reinstall the OS on your server or anything like that? That would cause this.
From what I can tell, all these answers are about suppressing the warning, instead of dealing with it. In short, the warning is telling you that the server doesn't look like it used to look; see https://en.wikipedia.org/wiki/Man_in_the_middle_attack for why this may be a danger.
I got this message today when trying to log into my server. What should I do? Whats going on?
$ ssh 10.10.10.69
@ WARNING: REMOTE HOST
How to know which key belongs to which server, and how do I clean up selectively?
To find out which entry is for a known hostname in known_hosts:
# ssh-keygen -H -F <hostname or IP address>
To delete a single entry from known_hosts:
# ssh-keygen -R <hostname or IP address>
I run an Ubuntu desktop with a bunch of virtual servers in Virtual Box to test stuff out, etc. In the past I have also been connecting to other kinds of remote VPS Linux boxes. Currently my .ssh/
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