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SSH-like Shell via AWS SSM

Project description

smsh

A command line utility enabling SSH-like interactive sessions and continuous output for long running commands via the AWS SSM API. The tool provides continuous output from the SendCommand operation, despite the output not being available via the AWS API.

installing

Currently, smsh is only distributed as source. Clone this repository and then bulid it with setuptools:

python setup.py install

interactive usage

smsh allows you to enter an interactive shell session by just providing the instance-id or IP address:

smsh i-123450asdf12345

Once "connected", the user is then given an input prompt, similar to a shell prompt:

[root@demo-instance root]#

The instance name will get set to the "Name" tag of the instance, or will default to the instance-id if no Name is set. The user can then interact with the session like it was a remote shell:

[root@demo-instance root]# ls -al
total 40
dr-xr-x---  3 root root 4096 Mar 15 21:47 .
dr-xr-xr-x 25 root root 4096 Mar 15 18:44 ..
-rw-r--r--  1 root root   18 Jan 15  2011 .bash_logout
-rw-r--r--  1 root root  176 Jan 15  2011 .bash_profile
-rw-r--r--  1 root root  176 Jan 15  2011 .bashrc
-rw-r--r--  1 root root  100 Jan 15  2011 .cshrc
drwx------  2 root root 4096 Mar 14 21:53 .ssh
-rw-r--r--  1 root root  129 Jan 15  2011 .tcshrc
[root@demo-instance root]#

ephemeral usage

smsh also allows for single remote commands to be executed. This can be run by providing the -c flag:

smsh i-123450asdf12345 -c 'ls -al'

The output will be printed directly to the console.

output

This tool supports unbuffered output mode (still technically buffered), where users can get constant real time output for long-running commands, despite the output not being available via the AWS API. This is a large limitation of SSM. By default, all smsh output is unbuffered. This can lead to performance issues though, as the client will be constantly polling the instance for the latest output.

Clients can run in buffered mode by adding the argument, --buffered-output.

Because there is near real-time output, long running commands become much more usable. Additionally, things like tail -f are natively supported.

[root@demo-instance root]# tail -f my.log
long running process Thu Mar 15 21:51:02 UTC 2018!!!!
long running process Thu Mar 15 21:51:03 UTC 2018!!!!
long running process Thu Mar 15 21:51:04 UTC 2018!!!!
long running process Thu Mar 15 21:51:05 UTC 2018!!!!
long running process Thu Mar 15 21:51:06 UTC 2018!!!!
long running process Thu Mar 15 21:51:07 UTC 2018!!!!
long running process Thu Mar 15 21:51:08 UTC 2018!!!!
long running process Thu Mar 15 21:51:09 UTC 2018!!!!
long running process Thu Mar 15 21:51:10 UTC 2018!!!!
long running process Thu Mar 15 21:51:11 UTC 2018!!!!

state

In interactive mode, the session state is maintained throughout the session. This includes the current working directory, the current environment variables (both local and exported), and the current user.

root@demo-instance root]# pwd
/root
[root@demo-instance root]# cd /var/log
[root@demo-instance log]# pwd
/var/log
[root@demo-instance log]#

canceling a command

In interactive mode, CTRL+C will cancel any command with the SSM API, and as such SIGTERM should be sent to the executing command on the target managed instance. Additionally, sending a SIGTERM to smsh in either interactive or ephemeral mode should likewise result in the SIGTERM being sent to the instance.

editor

This tool also allows for local Vi/Nano editors to be used. Just like when normally using them on a remote machine, they can open up existing or new files. However, because of the nature of the program, any new files that are opened in an editor and saved without any content are discarded.

[root@demo-instance log]# vi somefile.txt

considerations

The following are some considerations for using this tool:

  • This tool polls. If there are many users in an account, or if there are many commands being executed, API calls could be throttled. This tool attempts to do exponential backoff to try to avoid this situation.
  • DescribeCommandInvocation does not support tag based permissions. So, while you can control the SendCommand based on tag, anyone will effectively be able to see the output of every command.
  • SSM limits the script size that can be sent via the SendCommand action. This size limit can impact the Editor usage, when editing very large files.
  • Output buffering. Even the unbuffered mode buffers up to 100 lines at a time. If the output is happening faster than that, the command can get backed up.

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