SSH notes

SSH Config

See SSH config for information on configuring the ssh client and sshd server.

RSA versus DSA keys

Use RSA

1. There are not current patents for either RSA or DSA.
2. RSA is the default for OpenSSH.
3. The default RSA key length of 2048 bits is much longer than the maximum key length of 1024 bits for DSA.

SSH Chaining

SSH Chaining allows you to create a chain SSH logins through an intermediate gateway. You can do it all in one line if you specify the -t option. The following example connects to a server at 192.168.1.100, which is accessable only from example.com.

ssh -t noah@example.com "ssh noah@192.168.1.100"

share an existing open SSH session with other SSH tools

This speeds up additional connections to the same host because the client does not have to create a new socket connection and it does not have authenticate to the remote host.

WARNING! There is a dangerous side effect to using this. If you 'exit' from the first shell (master) it will hang until all the other shells have also exited. If you force the shell to exit by killing it with `kill -9` or by closing its xterm window then it will kill every other shell that was using the shared connection.

Add these lines to your local client ~/.ssh/config to enable connection sharing:

Host *
ControlMaster auto
ControlPath ~/.ssh/master-%r@%h:%p

Share X11 desktops

You can share a single mouse and keyboard seamlessly between two X11 servers. You must have `x2x` installed on the remote machine. This example lets you access a monitor located to the left (west) of your main monitor. When you move the mouse to the left edge of the main screen if will show up on the right edge of the remote X11 server.

ssh -f -Y noah@server.example.org x2x -west -to :0.0

You may need to specify the full path to x2x as /usr/bin/x2x or /usr/bin/X11/x2x.

Using `dd` and `ssh` to copy a disk image over a network

dd if=/dev/sdb0 | ssh noah@server.example.org "dd of=ops-tools.img"

Using `ssh` to copy files through a DMZ gateway to a machine behind a firewall

This example copies a directory, pictures/ to a tarball on the remote server.

tar c pictures/ | gzip -c - | ssh user@192.168.1.100 "dd of=pictures.tar.gz"

This makes use of `ssh` chaining to copy a file. This doesn't require a tunnel, but it requires that keys be setup between the DMZ gateway and the server on the remote LAN.

This example copies a file called pictures.tar.gz and requires a key between 208.77.188.166 and 192.168.1.100:

dd if=pictures.tar.gz | ssh user@208.77.188.166 "ssh user@192.168.1.100 \"dd of=pictures.tar.gz\""

You can't use the '-t' option for chaining `ssh` commands because this will cause one of the `ssh` commands to attempt to read the password from the output of `dd`. As far as I can tell there is no way to handle this situation with password authentication. Only public keys work in this situation. For example, this will not work:

dd if=pictures.tar.gz | ssh -t user@208.77.188.166 "ssh user@192.168.1.100 \"dd of=pictures.tar.gz\""

VPN over SSH

OpenSSH versions starting with v4.3 have a built-in VPN feature.

# cat /proc/sys/net/ipv4/ip_forward
0
# echo 1 > /proc/sys/net/ipv4/ip_forward
# cat /proc/sys/net/ipv4/ip_forward
1

remote server sshd_config

Check the settings on the remote server in /etc/ssh/sshd_config. If you change any settings be sure to reload the config settings by running `/etc/init.d/sshd reload`. The sshd_config file should have these settings:

# Allow either layer 2 or layer 3 (point-to-point ''tun'' or ethernet ''tap'').
PermitTunnel yes

remote server bridge setup

# tunctl -b -t tap_vpn_0
tap_vpn_0
# brctl addbr br_vpn_0
# brctl addif br_vpn_0 eth0
# brctl addif br_vpn_0 tap_vpn_0
# brctl show
bridge name     bridge id               STP enabled     interfaces
br_vpn_0        8000.0016366db84d       no              eth0
                                                        tap_vpn_0
# ifconfig tap_vpn_0 up
# ifconfig br_vpn_0 up

local client bridge setup

# tunctl -b -t tap_vpn_c0
tap_vpn_c0
# brctl addbr br_vpn_c0
# brctl addif br_vpn_c0 eth0
# brctl addif br_vpn_c0 tap_vpn_c0
# brctl show
bridge name     bridge id               STP enabled     interfaces
br_vpn_c0       8000.0016cbaa352a       no              eth0
                                                        tap_vpn_c0
# ifconfig tap_vpn_c0 up
# ifconfig br_vpn_c0 up

unfinished notes

Here are some additional notes and a script I found on the topic. Also mentions setting up tun/tap drivers for Mac OS X.

[1-sshvpn]
#!/bin/bash

# # prereqs:
# # remote host's sshd_config must have "PermitRootLogin=no", "AllowUsers user", and "PermitTunnel=yes"
# # "tunctl", in debians it is found in uml-utils, redhats another (dont remember but "yum provides tunctl" must tell)
# # remote user must be able to sudo-as-root
# # can opt by routing as in this case or soft bridge with brctl and you get full remote ethernet segment membership :D
# # that last i think i'll implement later as an option
# # other stuff to do is error checking, etcetc, this is just as came from the oven
# # juako
#
# http://ubuntuforums.org/showthread.php?t=926435&p=7587049#post7587049
#
# Sorry for the thread bump but the "Disabled Privacy Extensions" is probably because you haven't added
#     Tunnel yes
#     TunnelDevice any:any
# to /etc/ssh/ssh_config
#
# http://ubuntuforums.org/showthread.php?t=926435&page=3&p=7886699#post7886699
#
# Ok, I run across so many of these unfinished threads involving OSX and SSH
# interface tunneling (VPNs) that I decided to start suggesting the solution I
# use. For ease of use, I'll avoid describing port and proxy forwarding. Feedback
# is welcome!
#
# Drivers
#
# First, OSX doesn't come with tun/tap drivers. Download them at SourceForge's
# TUN/TAP for OSX project. You'll also need the drivers loaded on any other OS,
# but most UNIX/Linux flavors have them standard.
#
# Privileges
#
# Both the ssh client and the remote login user need enough privileges to
# configure the tun/tap interfaces. This means running ssh as root and loging in
# remotely as root. I will forever be looking for a means around this (setting
# read/write permissions on the tun/tap device files doesn't cut it), but it's
# really not a security threat. Just don't leave a root terminal open and you'll
# be fine.
#
# Server Configuration
#
# You must be able to log in as root. So far, the only user I can get to
# configure tun/tap interfaces is the root user. As far as SSH is concerned,
# PermitRootLogin should default to yes, but make sure it is indeed set to yes.
#

userhost='user@host'
sshflags='-Ap 2020 -i /path/to/some/authkey'
vpn='10.0.0.0/24'
rnet=192.168.40.0/24

# START VPN
if [ "$1" == "start" ]; then
  echo setting up local tap ...
  ltap=$(tunctl -b)
  ifconfig $ltap ${vpn%%?/*}2/${vpn##*/} up

  echo setting remote configuration and enabling root login ...
  rtap="ssh $sshflags $userhost sudo 'bash -c \"rtap=\\\$(tunctl -b); echo \\\$rtap; ifconfig \\\$rtap ${vpn%%?/*}1/${vpn##*/} up; iptables -A FORWARD -i \\\$rtap -j ACCEPT; iptables -A FORWARD -o \\\$rtap -j ACCEPT; iptables -t nat -A POSTROUTING -o eth0 -s ${vpn%%?/*}2 -j SNAT --to \\\$(ip r | grep $rnet | sed \\\"s/^.*src \\\(.*\\\$\\\)/\1/g\\\"); sed -i -e \\\"s/\\\(PermitRootLogin\\\).*\\\$/\1 without-password/g\\\" -e \\\"s/\\\(AllowUsers.*\\\)\\\$/\1 root/g\\\" /etc/ssh/sshd_config; /usr/sbin/sshd -t\"'"
  rtap=$(sh -c "$rtap")

  echo setting up local routes ...
  # since my ISP sucks with transparent filters (i can't opt for another where i live), i'll just use my work net as gateway
  ip r a $(ip r | grep default | sed "s/default/${userhost##*@}/")
  ip r c default via ${vpn%%?/*}1 dev $ltap

  echo bringing up the tunnel and disabling root login ...
  ssh $sshflags -f -w ${ltap##tap}:${rtap##tap} -o Tunnel=ethernet -o ControlMaster=yes -o ControlPath=/root/.ssh/vpn-$userhost-l$ltap-r$rtap root@${userhost##*@} bash -c "\"sed -i -e 's/\(PermitRootLogin\).*\$/\1 no/g' -e 's/\(AllowUsers.*\) root\$/\1/g' /etc/ssh/sshd_config; /usr/sbin/sshd -t\""

  echo connected.
# STOP VPN
elif [ "$1" == "stop" ]; then
  echo searching control socket and determining configuration ...
  controlpath=$(echo /root/.ssh/vpn-$userhost*)
  ltap=${controlpath%%-rtap*} && ltap=tap${ltap##*-ltap}
  rtap=${controlpath##*rtap} && rtap=tap${rtap%%-*}

  echo bringing the tunnel down ...
  ssh $sshflags -o ControlPath=$controlpath -O exit $userhost

  echo restoring local routes ...
  ip r c default $(ip r | grep ${userhost##*@} | sed "s/${userhost##*@}\(.*$\)/\1/g")
  ip r d ${userhost##*@}

  echo restoring remote configuration ...
  sh -c "ssh $sshflags $userhost sudo 'bash -c \"tunctl -d $rtap; iptables -D FORWARD -i $rtap -j ACCEPT; iptables -D FORWARD -o $rtap -j ACCEPT; iptables -t nat -D POSTROUTING -o eth0 -s ${vpn%%?/*}2 -j SNAT --to \$(ip r | grep $rnet | sed \"s/^.*src \(.*\$\)/\1/g\")\"'"

  echo deleting local tap ...
  tunctl -d $ltap

  echo disconnected.
fi

Remote Server Security Enhancement with SSH Keys

You can make port forwarding even more secure by limiting what a privileged account can do. When you add a key to authorized_key you may pass parameters to fine tune the connection. This can be used to restrict what the client is allowed to do. On the remote server, add the following to ~username/.ssh/authorized_keys:

from="192.168.1.69",command="/bin/false",no-pty,no-X11-forwarding,no-agent-forwarding,no-port-forwarding,permitopen="localhost:143"
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEA8XIr8LEXdvc4VZEvNenWkJrerTzNhqTT7QvCD+Y2EjCUPQwfBcSnvhY3oasNigNonghQFqm7/HqWBLpcN+4mqDUrXrEdj6HQmHvCV6WozNUVb5jjiyQ/JF4hqcQd6oelCkVw8wD32I2jlYqydpqOGY4xqakWDAfm3SOx5il3Kl49mKCg5B3GQPexhTujaTT3y/Q1eeT3zGpHE9Mp7k20X8rMxSjp5ncLAmdf42fRh05HY5f1GrupQIEdi0/TDcPNWL1ml89zttrDOLgDnwny7P0x2jmcX41cSxL/8svER7BAk2sroyQe6L21pJ7o2MYz1IwnsQgji/GjJoaA7hTNCQ== username@client.example.com

• from="192.168.1.69": accept connection only from the given IP address
• command="/bin/false": forces this command to be run no matter what is passed via ssh from the client
• no-pty: never allocate a PTY for interactivity
• no-X11-forwarding: No X11
• no-agent-forwarding: we don't want or need ssh-agent
• no-port-forwarding: prevent ssh -R ...
• permitopen="localhost:143": allow only localhost connections to port 143 for `ssh -L` requests

Turn off remote host key identity checks (ignore known_hosts)

The short answer to this question:

ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o LogLevel=ERROR -o BatchMode=yes noah@example.org "uptime"

WARNING! This totally defeats detection of Trojan or man-in-the-middle attacks.

You want to not be bothered by messages like the following:

The authenticity of host 'www.example.org (192.168.0.2)' can't be established.
RSA key fingerprint is 7c:05:58:a2:fb:ca:b5:2b:e3:37:3c:c4:ff:55:48:b1.
Are you sure you want to continue connecting (yes/no)?

or

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!

or

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@       WARNING: POSSIBLE DNS SPOOFING DETECTED!          @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

Sometimes you want to connect to a remote host where you do not care about the host identification. This note shows how to automatically ignore the identity checks that the SSH client normally enforces.

One situation where identity checks will make your life miserable is when connecting to a host that is running multiple SSH services on different ports. SSH only stores the remote host's name and identity key in ~/.ssh/known_hosts. It does not store which port number an identity key is associated with. SSH will complain about a remote host's identification changing even though there might be multiple SSH services running on ports other than 22. Another situation where identity checks become tiresome is when working with a virtual machine management system that needs to destroy and rebuild lots of guests. Usually such a system will have a mechanism to install host identity keys, but sometimes that mechanism is not available.

There are two options that control host key checks, StrictHostKeyChecking and UserKnownHostsFile. The StrictHostKeyChecking option controls whether ssh will ask if you want to continue connecting in the case where the authenticity of host can't be established (the host key is not found in known_hosts). It also controls whether ssh will allow connections to a known host where the identity key does not match. By setting this option to no you tell ssh to always connect. This creates a small problem because ssh will still store the key in your known_hosts, which can pollute this file with lots of useless keys. The UserKnownHostsFile option tells ssh which known_hosts file to use. By setting this option to /dev/null you tell ssh read and write the host keys in /dev/null, which is as good as throwing it away. This will cause an annoying message every time a connection is made, which reads like, Warning: Permanently added 'www.example.org,192.168.0.2' (RSA) to the list of known hosts.. This happens because ssh still thinks it's storing the host key in a known hosts file even though that file is /dev/null. You can get rid of this message by setting the LogLevel option to ERROR. The default LogLevel is INFO, which reports warning messages like this.

The following examples shows how to connect to a remote host while disabling host identity key checks:

ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o LogLevel=ERROR noah@example.org
ssh -p 2222 -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o LogLevel=ERROR noah@example.org

If you intend to use these settings to script SSH to run remote commands, then you may also want to add the BatchMode=yes option. This will disable requests for a password so that even if something is misconfigured or if your remote certificate is missing or broken then the SSH command won't just sit there waiting for someone to type a password. Instead, the command will fail right away so that your script can more quickly detect that something is wrong (if only it were this easy to script apt-get to just fail if there is ever a situation where it wants to ask the user for input.) For example, this checks the load of a remote server:

ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o LogLevel=ERROR -o BatchMode=yes noah@example.org "uptime"

If you want to live dangerously you can put these options in your SSH client config ~/.ssh/config. You may want to put these options under a Host section so that these options are only modified for a specific host or network of hosts. In the following example I assume that the 192.168.100.0/24 network is where I build and test virtual machine guests. I never care about the host identity keys for these guests.

Host 192.168.100.*
StrictHostKeyChecking no
UserKnownHostsFile /dev/null
LogLevel ERROR

Finally, I didn't tell you to actually do any of this, and don't say I didn't warn you.

scp warning: "stty: standard input: Invalid argument"

While using scp to copy a file to or from a remote server you may see the following warning message:

stty: standard input: Invalid argument

The problem is in the remote host's .bashrc or /etc/bashrc (and other places). When scp connects to the remote host the server on the remote end will source the .bashrc file. Most .bashrc file have some logic at the top to prevent running if the connection does not have a terminal. There are two common variants:

[ ! -t 0 ] && return

The other, more common style;

# If not running interactively, don't do anything
[ -z "$PS1" ] && return

If that logic is not there and the .bashrc attempts any interaction with stdin then you are likely to get a warning message. It seems counterintuitive that scp would invoke the remote's .bashrc.

To fix this you need to add one of those two lines to the remote's .bashrc file. This will get rid of the warnings.

regain terminal control from a locked SSH session

Press Enter then type '~.'. In other words:

<ENTER>~.

SSH for Windows

Putty is the best free SSH client for Windows. You can also use Cygwin, but the ANSI terminal emulation isn't as good unless you start the X11 server and run xterm.

MindTerm SSH client Java Applet

MindTerm_2.1 (non-commercial). This was the last free version of MindTerm.

   mindterm.jar

Put this applet on a web page and point the <applet> "archive" attribute to the URL of the JAR file:

 <applet archive="mindterm.jar" code="com.mindbright.application.MindTerm.class" width="620" height="440"> 
     <param name="te" value="xterm-color"> <!-- "vt102" -->
     <param name="fs" value="18">
     <param name="gm" value="80x32+0+0">
     <param name="port" value="22">
     <param name="cipher" value="blowfish"> <!-- "des" -->
     <param name="usrname" value="">
     <param name="sepframe" value="false">
     <param name="quiet" value="false">
     <param name="cmdsh" value="false">
     <param name="verbose" value="true">
     <pa ram name="autoprops" value="none">
     <param name="idhost" value="false">
     <param name="alive" value="10">
     <param name="appletbg" value="white">
 </applet>

<include iframe src="http://www.noah.org/engineering/mindterm/mindterm_iframe.html" width="660" height="480px" />