The post is mostly to myself, but hopefully useful for others too.
When connecting to an OpenSSH server, one of the first things that the server does is to perform a reverse DNS loopuk on the client’s IP, which is in general a slow operation and causing for an unnecessary delay during authentication. However this reverse DNS lookup can be disabled.
Do the following to disable it:
# In /etc/ssh/sshd_config:
UseDNS no
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This howto demonstrates how to convert an OpenSSL key to a public/private OpenSSH key-pair.
The motivation for converting this is simple — dual use. That is to say, any user or application that has been issued a certificate can now use their SSL-based credentials for both SSL- and SSH-based authentication.
Requirements
You’ll need a valid certificate and private key — actually only the key is required. This recipe assumes that your certificate and key will have the names user-crt.pem and user-key.pem, respectively. If your key is encrypted (which it should be), you’ll also need to have it’s passphrase handy.
You’ll need a shell account on a system that supports OpenSSH logins using public/private key authentication.
Solution
The solution is to extract the public key from the private key using ssh-keygen, copy the new key-pair into place, and test them out.
1. Copy the private SSL key to ~/.ssh/id_ssl.
$ cp user-key.pem ~/.ssh/id_ssl
$ chmod 600 ~/.ssh/id_ssl
2. Extract the public SSH key using ssh-keygen.
$ ssh-keygen -y -f ~/.ssh/id_ssl > ~/.ssh/id_ssl.pub
$ chmod 600 ~/.ssh/id_ssl.pub
3. Add the public key to your authorized_keys
$ cat ~/.ssh/id_ssl.pub >> ~/.ssh/authorized_keys
4. Test the new key by attempting to SSH to localhost.
$ ssh -i ~/.ssh/id_ssl localhost
At this point, you’ll need to enter your passphrase (assuming you had one), and if all goes well, you’ll be sitting at a new shell prompt.
5. Remove the test key from your authorized_keys file.
In theory, a single certificate and key issued to an employee would be sufficient to access all participating SSL- and SSH-based resources in a given environment (or perhaps the entire company).
This post is based on an original recipe by Klayton Monroe.
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Here are the commands used to open and forward the host’s port 2222 to the guest’s port 22 (SSH Server Port).
Type this into a terminal:
VBoxManage setextradata nameofyourguest "VBoxInternal/Devices/pcnet/0/LUN#0/Config/ssh/HostPort" 2222
VBoxManage setextradata nameofyourguest "VBoxInternal/Devices/pcnet/0/LUN#0/Config/ssh/GuestPort" 22
VBoxManage setextradata nameofyourguest "VBoxInternal/Devices/pcnet/0/LUN#0/Config/ssh/Protocol" TCP
Could also be used to forward PostgreSQL and HTTP:
VBoxManage setextradata nameofyourguest "VBoxInternal/Devices/pcnet/0/LUN#0/Config/postgresql/HostPort" 5432
VBoxManage setextradata nameofyourguest "VBoxInternal/Devices/pcnet/0/LUN#0/Config/postgresql/GuestPort" 5432
VBoxManage setextradata nameofyourguest "VBoxInternal/Devices/pcnet/0/LUN#0/Config/postgresql/Protocol" TCP
VBoxManage setextradata nameofyourguest "VBoxInternal/Devices/pcnet/0/LUN#0/Config/http/HostPort" 8080
VBoxManage setextradata nameofyourguest "VBoxInternal/Devices/pcnet/0/LUN#0/Config/http/GuestPort" 80
VBoxManage setextradata nameofyourguest "VBoxInternal/Devices/pcnet/0/LUN#0/Config/http/Protocol" TCP
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From time to time I work on remote servers and some times it would be very useful to be able to mount the remote file system on my local machine. Recently I found sshfs, which is a filesystem client based on the SSH File Transfer Protocol and since most *nix like servers; Linux, FreeBSD, NetBSD, OpenBSD servers with OpenSSH installed already support this protocol it is very easy to set up – Read: on the server side there’s nothing to do, setup or configure. On the client side mounting the filesystem is as easy as logging into the server with SSH.
Technically (more) the sshfs is based on FUSE, which is the best userspace filesystem framework for linux 
Here is how I connect to my remote SSH-based servers and hosts:
$ sshfs -o uid=1000,gid=1000 cc@remote.host.com:/home/cc /media/mountpoint
I use the uid and gid arguments to give my own normal user with those uid and gid to have write access to the mounted filesystem.
This post describes the sshfs in more details and moreover howto use it on Ubuntu and Debian.
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By default, the network connection in VirtualBox is set to NAT (Network Address Translation), that is every packet coming from the Guest machine is modified so that it seems as it has come from the Host machine. In this way it’s easy for the Guest machine to connect to all the rest of the network (the internet included) but nobody can start a connection with the Guest Machine since it’s hidden behind the Host one.
This blog post describes a technique to route incoming connections to VirtualBox guest instances in order to enable for example SSH and sshfs connections
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You can run a single command on an OpenSSH server by adding the command to the end of the ssh command.
ssh -fCT user@server
-f = Allows ssh to close after the connection is established.
-C = Use Compression
-T = No terminal session will be started
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As a regular Linux user I use SSH over and over again during a workday. Normally it is almost the same hosts I connect to every day or just a few different, all in all the amount of hosts I connect to regularly is fairly small.
To avoid tedious and repetitive username and host type-ins, I found a small GNOME applet called SSHMenu, which keeps all regular SSH connections available within a single mouse click.
This screenshot depicts the idea.
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