Simple telnet client functionality on EventMachine
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Provides telnet client functionality.

This class was once derived from the Net::Telnet class in Ruby's standard library. It uses EventMachine, adds some functionality around logging and other useful features that came up useful back when I was using this code productively.

It was developed with simplicity in mind. It tries to hide the complexity of asynchronous programming using Fibers.

Here's the API documentation of the current release.


With the coming up release I plan to modernize this repository a bit. That means porting it to a standard bundle gem-like structure, and improve documentation and make it more YARD compatible, which is used on

I plan to add specs as soon as I've released a related gem called em-telnet_server (see below).


This section has been copied and slightly modified from Net::Telnet's documentation.

The telnet protocol allows a client to login remotely to a user account on a server and execute commands via a shell. The equivalent is done by creating a SimpleTelnet::Connection instance with the :host option set to your host along with a block which defines the task to be done on the host. The established connection (login already performed by #login) is passed to the block. In the block, you'd normally make one or more calls to #cmd. After the block returns, the connection is automatically closed.

This class can also be used to connect to non-telnet services, such as SMTP or HTTP. In this case, you normally want to provide the :port option to specify the port to connect to, and set the :telnet_mode option to false to prevent the client from attempting to interpret telnet command sequences. Generally, #login will not work with other protocols, and you have to handle authentication yourself.

Differences to Net::Telnet

  • based on EventMachine and Fibers
  • uses lowercase Symbols (like :host) for options (instead of `"Host")
  • provides per connection logging for:
    • general activity (see #logger)
    • output log (see the option :output_log, #output_logger, and #output_log)
    • commands sent (see the option :command_log, #command_logger, and #command_log)
    • debug logging (if logger.debug?)
      • prints recently received data in a more human-friendly 0.5s interval, as opposed to single characters (because EventMachine is fast)
  • no hexdump log
  • can handle extremely big outputs by deferring checking for the prompt in the output in case it's getting huge
  • the :connect_timeout which specifies the timeout for establishing new connections
  • the :wait_time option which is useful for commands that result in multiple prompts
    • it specifies the time to wait for more data to arrive after what looks like a prompt
  • #last_command sent
  • #last_prompt last prompt matched
  • #logged_in time when login succeeded
  • #last_data_sent_at time when last data was sent
  • SimpleTelnet::TimeoutError exceptions know the causing command and on which host it happened (#hostname, #command)


If you're starting from scratch and simply want to access a single host via telnet, do something like this:

opts = {
  host: "localhost",
  username: "user",
  password: "secret",
} do |host|
  # At this point, we're already logged in.

  host.cmd("touch /my/file")

  # get some output
  puts host.cmd("ls -la")

  host.timeout(30) do
    # custom timeout for this block
    host.cmd "slow command"

If you already have an EventMachine reactor running, you can use SimpleTelnet inside it, like here: do

  opts = {
    host: "localhost",
    username: "user",
    password: "secret",
    output_log: "output.log", # log output to file
    command_log: "command.log", # log commands to file
  } do |host|
    # already logged in
    puts host.cmd("ls -la")

By the way, SimpleTelnet::Connection and EventMachine::Protocols::SimpleTelnet are the same.

Related Projects

I'm planning to release em-massive_telnet for massively parallel telnet connections (from client to server) and em-telnet_server which can be used to build your own telnet server based on EventMachine.

I've written the code for these two gems years ago, but I'll need to carefully extract it so any sensitive information is stripped off.


There is a large number of RFCs relevant to the Telnet protocol. RFCs 854-861 define the base protocol. For a complete listing of relevant RFCs, see


The gem is available as open source under the terms of the ISC License. See the LICENSE file.