C[omp]ute

From: andrew cooke <andrew@...>

Date: Fri, 22 Apr 2011 18:07:26 -0300

My latest cable connection does everything in one small box that also works as
a router and WiFi.  That makes it very simple to use, but it has custom
firmware with minimal configuration, which means that I cannot accept any
incoming connections.

This is a nuisance if I am away from home, as I have no way to connect to my
computer(s) to do things like read email.

The obvious solution is to use ssh to open a remote tunnel on another machine
and then connect through there.  But my web site is on a virtual host, which
means that I cannot open ports "at random".  And work's computers are behind a
mess of firewalls that I don't fully understand (and also, I don't really want
to rely on work for non-work activities).

So today I configured an Amazon micro instance to do the work for me.  It
wasn't so complicated, and is free for a year.  Here's what I did:

 1 - Sign up with Amazon.  I didn't really understand what I was doing but it
 turns out that you don't need to - it's pretty much idiot proof.  You can use
 your existing Amazon account, but you need to supply credit card details and
 then reply to an automated phone call verification.  If it helps, what you
 want is "EC2" (you don't need to specify any details at this stage).

 2 - Start a "micro" EC2 instance.  Once you sign up you can get access to a
 web console.  There you can choose from a variety of different machine
 configurations.  I went to the "community" selection and chose a recent
 (11.4) OpenSuse (the "Suse" option on the initial list are not free).  For
 free, you want a yellow star next to the machine.  At some point in this
 process you get to download a key file, which you should save to your ~/.ssh
 directory (I'm assuming you're running Linux at home and at Amazon).

 3 - Once you have an instance started, fiddle with the "security group" in
 the web console.  This is basically a firewall.  Enable ports 22 and 2222
 (the latter will be the remote port for tunnelling through to home).  You
 don't need to restart the instance when you change the security group
 settings (but do check "Apply Rule Changes").

 4 - Connect to your instance using ssh.  The easiest way to do this is to
 modify ~/.ssh/config so it contains:

host *.amazonaws.com ssh.example.com
     user root
     StrictHostKeyChecking no
     UserKnownHostsFile /dev/null
     CheckHostIP no
     IdentityFile ~/.ssh/amazon-key.pem

  where amazon-key.pem is the file you downloaded at some point (also, make
  sure that has 600 access permissions).  Once you have that in config you can
  just use "ssh blah-blah.amazonaws.com" where blah-blah is the "public DNS"
  of your instance (see the web console).  At this point you already the proud
  owner of a little virtual computer....

  5 - Next, you need to associate your virtual computer with a real, permanent
  address.  Amazon has a service for this, called "Elastic IPs" (on the left
  of the web console).  Click that and after a few more clicks you'll have a
  permanent numeric address (which is quite something given that IPv4 is
  exhausted).

  6 - Optional, I guess, you can associate a DNS name with that IP address.
  Amazon doesn't do this, but your local DNS register or web host will.  In my
  case I created a new subdomain at webfaction.com and then pointed that at
  the EC2 instance.  This name should then be the "ssh.example.com" in the
  config file above.

  7 - On the instance, change the config to allow the SSH tunnel to bind
  correctly.  Go to /etc/ssh and edit sshd_config (IMPORTANT - this is not
  ssh_config, it's sshd_config, with a "d" - I wasted an hour here...).
  Uncomment the GatewayPorts line and set it to "yes" or "clientspecified"
  then restart sshd.

  8 - Back on your home machine, you can now create the tunnel:

ssh -fN2R 0.0.0.0:2222:localhost:22 ssh.example.com

  where "0.0.0.0" is the binding on EC2 (all NICs), 2222 is the port on EC2
  (what you will connect to), localhost is where that will be tunneled to
  locally, 22 is the local (ssh) port.  So connecting to port 2222 on your EC2
  example (now configured as ssh.example.com, or using the numeric "public IP"
  address) will log you in to your own computer.  Sweet.

  9 - To make this permanent, install autossh locally (eg with Yast).  Then
  configure root with the key file and config in root's .ssh directory.  As
  root you can then run:

autossh -f -M 2223 -N -R 0.0.0.0:2222:localhost:22 ssh.example.com

  (after killing the one you started as yourself earlier).  Even better, you
  can make a little script in /etc/init.d (or add the line above to rc.local
  if you're using anything other than OpenSuse) and this will start when your
  local machine starts.

And that's it.  "ssh -p 2222 ssh.example.com" will connect you to your home
computer, for free, from outside, via Amazon.

Andrew