I have collected some interesting and not such obvious SSH features in the post below. Some are simple, another complex - but all of them were useful in my daily practice. Enjoy!



For adding SSH keys to remote system use command ssh-copy-id which automates a procedure of adding a key to remote .ssh/authroized_keys file.


Regular ssh uses direct TTY access to make sure the password is entered by an interactive keyboard input. Sshpass runs ssh in a dedicated tty, foolling it into thinking it is getting the password in interactive way.

sshpass -pMy.Password ssh bastion

ssh_config and known_hosts

OpenSSH deprecated DSA keys

Some time ago OpenSSH deprecated DSA keys. If you still want to use it, add PubkeyAcceptedKeyTypes +ssh-dss to ssh_config file.

OpenSSH hashes known_hosts names

It looks like despite what the manual says, the default now (OpenSSH 7.2p2) is “yes”. It results in unreadable hosts names in known_hosts file. So usually I expect it to look like it:

myserver4, ssh-dss AAAAB3NzaC1kc3MAAACBAIgmj+TuTQbfvrhNzYLY+4d4DW+xq

But with HashKnownHosts default to yes I have this instead:

|1|SuF6AQaEqk97eWxjfsSDuTTEk8E=|Qc/Ln0XgjYn4osmXz2TYkA6SwNA= ssh-rsa AAAAB3NzaC1

Because of it I needed explicitely say I do not need hashing known hosts:

HashKnownHosts No


The HostKeyAlias also applies to known_hosts. When there is no matching DNS record, I am aliasing hosts using Host directive and HostName to refer to real host name or IP address like below:

Host bastion
   User jurek
   ProxyCommand corkscrew proxy.kedra.com 80 %h %p ~/.ssh/passwords

With such the configuration the server key gets stored in known_hosts as the entry specified with HostName directive, thus it will be To make it more meaningful, lets add HostKeyAlias directive and its argument will be now used to save server keys and searched in known_hosts during the connection phase.

Host bastion
   HostKeyAlias bastion
   User jurek
   ProxyCommand corkscrew proxy.kedra.com 80 %h %p ~/.ssh/passwords

The corkcrew command is actually behind scope of the task but for the note let me say it allows going through authenticated HTTP NTLM proxy. There are also other options like using cntlm transparent proxy for this purpose.

Removing a particular host from known_hosts

ssh-keygen -R hostname

Above solution found here.

Host key gets changed over and over

In a situation when you are connecting a remote host which gets reinstalled and its host key gets regenerated, ssh drops a warning the remote host identification gets changed:

The RSA host key for foo-bar.net has changed,
and the key for the corresponding IP address
is unchanged. This could either mean that
DNS SPOOFING is happening or the IP address for the host
and its host key have changed at the same time.
Offending key for IP in /home/user/.ssh/known_hosts:6

Sometimes it is actually expected. In such the case I can force ssh to avoid adding a volatile key to the ssh_keys, replacing its location by /dev/null: UserKnownHostsFile=/dev/null, and allowing connections to hosts without maching host keys: StrictHostKeyChecking=no.

 Host bastion
   HostKeyAlias bastion
   User jurek
   ProxyCommand corkscrew proxy.kedra.com 80 %h %p ~/.ssh/passwords


Subjects mentioned so far were just small useful tricks. Using SSH with multihops can be a big thing.

Multihop allows to manage limitations of the network. Usually it happens because the only points connecting the external world with our network are HTTP proxy or bastion hosts, so direct SSH connection to a host outside of the internal network is not possible.

However SSH tools are smart enough to overcome restrictions mentioned. Keep reading.

Scenario 1: HTTP proxy + bastion host

+---------+    +------------+    +---------+   +--------+
| laptop  |--->| HTTP proxy |--->| bastion |-->| target |
+---------+    | NTLM auth  |    |  host   |   |  host  |
               +------------+    +---------+   +--------+

Scenario like above occurs when I try to connect from an internal network, through NTLM authenticated HTTP proxy to AWS target host which is behind some bastion host.

Dealing with HTTP proxy first

Going through the HTTP proxy needs a configuration which has been already mentioned:

Host bastion
   HostKeyAlias bastion
   User jurek
   ProxyCommand corkscrew proxy.kedra.com 80 %h %p ~/.ssh/proxypass
   ServerAliveInterval 60

The corkscrew allows going over NTLM authenticated HTTP proxy. Each time the target is bastion, above configuration applies automatically and calls corkscrew, authenticates at HTTP proxy and reaches the bastion host through it.

Easy Approach

It is the most straightforward way, do ssh to first host, then continue connection to another in just one line:

ssh -t bastion ssh -i KEY.pem ec2-user@target

Without a pseudo-TTY allocated (-t option) you will not be able to enter an interactive ssh session. The pseudo-TTY is not required for a single-run, no keyboard input command (like ssh bastion ls) but is a prerequisite for an interactive ssh.

Because the second ssh runs at the bastion, the private key file KEY.pem, required to access target EC2, needs to be copied to the bastion before.

With the changes in ~/.ssh/config I am able to reach the target with one liner ssh.

Sophisticated Way

The Easy Approach is enough for ad-hoc connections but also tiresome for daily operations. For a regular access through multi-hops I would ideally had a simple version of ssh with an alias host.

ProxyCommand Magic

You have seen the ProxyCommand already with the corkscrew command. From the documentation: Specifies the command to use to connect to the server. […] The command can be basically anything, and should read from its standard input and write to its standard output. It should eventually connect an sshd server running on some machine. […] any occurrence of ‘%h’ will be substituted by the host name to connect, ‘%p’ by the port, and ‘%r’ by remote user name

Let’s analyze an example:

(1)                   (2)
ssh  -o 'ProxyCommand ssh -xaq hophost nc %h 22' ec2-user@target

The ssh (1) uses another, inner ssh (2) to connect to hophost host first (ssh -xaq hophost). The inner ssh (2) runs nc target 22 which connects to the target:22 and now all standard input/output are now redirected to port 22 of the target. And it does the trick. The outer ssh (1) performs the SSH protocol over the standard input/output instead of TCP/IP (effectively ignoring the target, using the username only) and proceeds with the authentication. Lets prove it and run it without a hostname, providing the target directly to nc instead:

ssh -o 'ProxyCommand ssh -xaq hophost nc target 22' ec2-user@

You will find it working. What is worthy of note - the authentication key does not need nor can be located at the hophost host. It is the netcat (nc) which opens the connection and it is the outer ssh (1) which does the authentication. The outer ssh runs at its own node, not at the hophost one.

Instead of using the netcat, the latest versions of ssh have its own solution of forwarding the standard i/o to given port. It is -W host:port flag. So here is a counterpart of the netcat version:

ssh -o 'ProxyCommand ssh -W %h:22 hophost' ec2-user@target

Looks simpler. In a similar way the authentication throuth an NTLM proxy works. You have the corkscrew option for that purpose.

Now how to set it up in a nice way? With the first scenario you have already put a magic proxy in the user ~/.ssh/config file. Whenever you ssh to bastion, it runs the proxy corkscrew, authenticates through your HTTP NTLM proxy and setup SSH connection over it. It just matches the hostname (after the Host directive you may use multiple names or even matching patterns). I specified the ProxyCommand above with the -o flag. But the more appropriate place to specify it is the ~/.ssh/config file.

Because you are free to setup another ~/.ssh/config at the bastion host, and the next one, and another… this way you may involve multiple proxies to work your way to the target node automatically. So instead of specifing a chain of ssh, you can ssh1 directly to aws target node, ssh1 based on its config will run a proxy ssh2 which connects to bastion first. The ssh2 to bastion will be handled by another rule which uses its own proxy (corkscrew or nc) to skip over HTTP proxy and get connected to bastion. The ssh1 will work over this connection to the target host.

Complete multihop solution

Contents of ~/config/.ssh:

Host bastion
   HostKeyAlias bastion
   User mybastionuser
   ProxyCommand corkscrew proxy.kedra.com 80 %h %p ~/.ssh/proxyauth
   ServerAliveInterval 60

Host awstarget
   HostKeyAlias awstarget
   User ec2-user
   ProxyCommand ssh -W %h:%p bastion

When I invoke ssh bastion, it automatically connects to it using corkscrew over HTTP proxy. This is straightforward and I had it working already. However when I call ssh awstarget it chaines proxycommand with ssh bastion first (which uses corkscrew and so on). This is great - a simple and clean ssh call.

Final test:

jurek@ub16a|2106(master)$ ssh -i myAWSkey.pem awstarget
Warning: Permanently added 'bastion' (ECDSA) to the list of known hosts.
Last failed login: Thu Mar 22 12:29:05 EDT 2018 from ip-10-110-50-197.ec2.internal on ssh:notty
There were 2 failed login attempts since the last successful login.
Last login: Thu Mar 22 12:25:02 2018 from ip-10-110-50-197.ec2.internal
[ec2-user@ip-10-10-35-17 ~]$

Scenario 2: Connect a Database available from 2nd host

Another real life scenario: Multi-tier networks with a few connecting points. I have Oracle RAC database which I want connect to, but since I cannot connect to it directly, I have to SSH from my home node (home) to a jump host first (jump), then from the jump to an application host (app) and only then I can reach the database which runs at its database server (oradb). It is not an AWS example but it could be similar. No HTTP proxy here. It is all about tunneling a port at localhost to another port at remote node.

+-------+    +---------+    +---------+   |       SCAN1 | VIP1 |
| home  |--->|   jump  |--->|  app    |-->| oradb SCAN2 +------+
|       |    | :22 ssh |    | :22 ssh |   | :1521 SCAN3 | VIP2 |
+-------+    +---------+    +---------+   +------------ +------+

Oracle Oddities

We have two nodes between us and the database we are interested in. It already looks complicated. But it is even more complex because of the RAC’s configuration which is a nightmare. First, the RAC usually exposes SCAN round-robin DNS address. SCAN makes hacker’s life complicated. SCAN is a kind of dispatcher which takes the incoming traffic and redirects it further to another Virtual IP (VIP). And there can be a few of them depending on the cluster size. What a headache! How overcome it?

My solution? Skip the SCAN completly. Use the VIP only. Connect to VIP directly. Test it first. Forward a single VIP, skip the SCAN.

How to find the VIP and the proper VIP? Yes, not all of them might work. Getting the VIP is tricky and boring - it is SCAN role to give you a proper VIP. But in real life of a developer once you pick a proper one it rarely changes. Ask your DBA to help you. Ask him to be smart because some VIPs are not appropriate for singleton services. Use uniform services instead (which usually are exposed on all VIPs) when you can.

Facing the challenge

Port 9999 used in the example here is just an example, you may use another not occupied TCP port or even randomly picking one. The command below runs at home and opens port 9999 at localhost interface as the tunel entry, then it connects to jump host and attaches the other end of the tunel to jump’s localhost interface, port 9999.

It makes no sense until something listens on port 9999 of localhost interface at jump host. Nothing does so far. This is why - once again - another ssh runs at the jump host, attaches a new, second tunel entry to port 9999 of localhost (effectively gluing both tunels together), then the second ssh connects the app host and redirects the end of the second tunel out of the app reaching directly to port 1521 of the remote host oradb’s VIP. An oracle server is expecting to listen there.

The chain of tunels described above is represented by the following command:

ssh -tL 9999:localhost:9999 jump ssh -L 9999:oradb-vip1:1521 app

Now connecting to the target database is a simple as issuing:

sqlplus scott/tiger@//localhost:9999/service_name

Voila! It will work.

  1. SSH throush multiple hosts using ProxyCommand
  2. Transparent MultiHop
  3. ProxyCommand use for multiple hops
  4. ProxyCommand passing through one host
  5. SSH Agent Forwarding
  6. scp files via intermediate host
  7. Generating random numbers in bash
  1. OpenSSH Wiki, particularly a chapter on Proxies and Jump Hosts.
  2. Why OpenSSH deprecated DSA keys.

SSH + Python

Paramiko is Python implementation of SSHv2 protocol for both client and server. Supports parsing of regular OpenSSH config files, in particular ssh_config and known_hosts. It is able to use ProxyCommand ssh feature.


Basic ad-hoc VPN with SSH is easy to acomplish. Read this superuser post and documentation about sshuttle which is a kind of framework in Python to forward selected networks to remote nodes.