SSH Connection, unable to read any reply from host, others work fine

I am trying to read some data from a Cisco network switch (SG300-28SFP) over ssh.

Initially i couldn’t connect as the switch only support some pretty old ciphers e.t.c, but added those to the connection and i can connect. However i cannot read any replies from the device.

Initially i was trying sshex and sshkit, but am now using erlang :ssh directly to make sure it wasn’t a framework problem.

Here is my (pretty basic, and badly written), code.

  def send_command(command) do
    {:ok, ssh_pid} =
      :ssh.connect('', 22,
        user: 'cisco',
        save_accepted_host: false,
        silently_accept_hosts: true,
        password: 'cisco',
        modify_algorithms: [
          prepend: [kex: [:"diffie-hellman-group1-sha1", :"diffie-hellman-group-exchange-sha1"]],
          prepend: [public_key: [:"ssh-rsa", :"ssh-dss"]]
        # ssh_msg_debug_fun: fn _, _, msg, _ -> IO.inspect(msg) end
        # user_dir: Path.expand("../../ssh", __DIR__)

      :ssh.connection_info(ssh_pid, [

    {:ok, chn} = :ssh_connection.session_channel(ssh_pid, 5000)

    _success = :ssh_connection.exec(ssh_pid, chn, command, 5000)

    for _ <- 0..3 do
      receive do
        {:ssh_cm, ^ssh_pid, value} -> IO.inspect(value)

    :ok = :ssh.close(ssh_pid)

If i run this, it hangs forever at the receive do loop, like this:

iex(1)> FibreTest.Cisco.send_command("up")
  client_version: {{2, 0}, 'SSH-2.0-Erlang/4.12.5'},
  server_version: {{2, 0}, 'SSH-2.0-OpenSSH_5.9p1.RL'},
  user: 'cisco',
  peer: {'', {{192, 168, 1, 254}, 22}},
  sockname: {{192, 168, 1, 100}, 60691},
  options: [
    modify_algorithms: [
      prepend: [
        kex: [:"diffie-hellman-group1-sha1",
        public_key: [],
        cipher: [client2server: [], server2client: []], 
        mac: [client2server: [], server2client: []],
        compression: [client2server: [], server2client: []]
      prepend: [
        kex: [],
        public_key: [:"ssh-rsa", :"ssh-dss"],
        cipher: [client2server: [], server2client: []],
        mac: [client2server: [], server2client: []],
        compression: [client2server: [], server2client: []]
    password: 'cisco',
    pref_public_key_algs: [:"ssh-rsa", :"ssh-dss", :"ecdsa-sha2-nistp384",
     :"ecdsa-sha2-nistp521", :"ecdsa-sha2-nistp256", :"ssh-ed25519",
     :"ssh-ed448", :"rsa-sha2-256", :"rsa-sha2-512"],
    preferred_algorithms: [
      kex: [:"diffie-hellman-group1-sha1",
       :"diffie-hellman-group-exchange-sha1", :"ecdh-sha2-nistp384",
       :"ecdh-sha2-nistp521", :"ecdh-sha2-nistp256",
       :"diffie-hellman-group16-sha512", :"diffie-hellman-group18-sha512",
       :"diffie-hellman-group14-sha256", :"curve25519-sha256",
       :"", :"curve448-sha512"],
      public_key: [:"ssh-rsa", :"ssh-dss", :"ecdsa-sha2-nistp384",
       :"ecdsa-sha2-nistp521", :"ecdsa-sha2-nistp256", :"ssh-ed25519",
       :"ssh-ed448", :"rsa-sha2-256", :"rsa-sha2-512"],
      cipher: [
        client2server: [:"",
         :"", :"aes256-ctr", :"aes192-ctr",
         :"", :"aes128-ctr", :"aes256-cbc", :"aes192-cbc",
         :"aes128-cbc", :"3des-cbc"],
        server2client: [:"",
         :"", :"aes256-ctr", :"aes192-ctr",
         :"", :"aes128-ctr", :"aes256-cbc", :"aes192-cbc",
         :"aes128-cbc", :"3des-cbc"]
      mac: [
        client2server: [:"",
         :"", :"hmac-sha2-256", :"hmac-sha2-512",
         :"", :"hmac-sha1"],
        server2client: [:"",
         :"", :"hmac-sha2-256", :"hmac-sha2-512",
         :"", :"hmac-sha1"]
      compression: [
        client2server: [:none, :"", :zlib],
        server2client: [:none, :"", :zlib]
    save_accepted_host: false,
    silently_accept_hosts: true,
    user: 'cisco'

If i ssh into the switch from the same machine manually, i get the expected login prompt.

Here’s my open_ssh config (which i also had to edit to enable me to connect)

# This is the ssh client system-wide configuration file.  See
# ssh_config(5) for more information.  This file provides defaults for
# users, and the values can be changed in per-user configuration files
# or on the command line.

# Configuration data is parsed as follows:
#  1. command line options
#  2. user-specific file
#  3. system-wide file
# Any configuration value is only changed the first time it is set.
# Thus, host-specific definitions should be at the beginning of the
# configuration file, and defaults at the end.

# This Include directive is not part of the default ssh_config shipped with
# OpenSSH. Options set in the included configuration files generally override
# those that follow.  The defaults only apply to options that have not been
# explicitly set.  Options that appear multiple times keep the first value set,
# unless they are a multivalue option such as IdentityFile.
Include /etc/ssh/ssh_config.d/*

# Site-wide defaults for some commonly used options.  For a comprehensive
# list of available options, their meanings and defaults, please see the
# ssh_config(5) man page.

# Host *
#   ForwardAgent no
#   ForwardX11 no
#   PasswordAuthentication yes
#   HostbasedAuthentication no
#   GSSAPIAuthentication no
#   GSSAPIDelegateCredentials no
#   BatchMode no
#   CheckHostIP yes
#   AddressFamily any
#   ConnectTimeout 0
#   StrictHostKeyChecking ask
#   IdentityFile ~/.ssh/id_rsa
#   IdentityFile ~/.ssh/id_dsa
#   IdentityFile ~/.ssh/id_ecdsa
#   IdentityFile ~/.ssh/id_ed25519
#   Port 22
Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc
MACs hmac-md5,hmac-sha1,hmac-sha2-256,
#   EscapeChar ~
#   Tunnel no
#   TunnelDevice any:any
#   PermitLocalCommand no
#   VisualHostKey no
#   ProxyCommand ssh -q -W %h:%p
#   RekeyLimit 1G 1h
#   UserKnownHostsFile ~/.ssh/known_hosts.d/%k
Host *
    SendEnv LANG LC_*
HostkeyAlgorithms ssh-dss,ssh-rsa
KexAlgorithms +diffie-hellman-group1-sha1,diffie-hellman-group14-sha1

I have various other linux/debian servers as well as some Pi’s that i can issue commands on from elixir/erlang fine.

So, a bit stumped here and wondering if anyone had any suggestions before i resort to the serial port :frowning:


Have you inspected what’s actually being returned here?


Sorry seem to have replied twice, and can’t see a way to delete the first reply . . .

Yes it’s a :success atom.

Ok, I know this probably doesn’t help you much, but I would rewrite the code into:

 :success = :ssh_connection.exec(ssh_pid, chn, command, 5000)

To make sure it stops there if that step doesn’t succeed.

Yes entirely valid, and this is not how I plan to actually do this if I can get it to work :+1: