Remote Login to the Command Line

The other day I was away from home when my wife called up and asked me where I'd put the papers I'd promised to print out.

I let out a very soft damn, because of course I'd forgotten to print out anything.

But all was not lost. I simply logged onto my home computer, found the appropriate file, and printed it, on my home printer. She was suitably impressed.

This, of course, is the whole point of a long-ago post, where we figured out how to determine a router's external IP address. What we want to do today is to use that information to log onto one of your home computers from afar. We'll assume you have figured out your current dynamic IP address, perhaps by looking at the bottom of your gmail page, or using the software I assembled in the previous post.

So let's say that you've found that your router currently has the dynamic IP address aaa.bbb.ccc.ddd . Then, in principle, all you have to do to log onto your account at home would be to open up a terminal and type

ssh rcjhawk@aaa.bbb.ccc.ddd

where you'd replace rcjhawk by your own account, of course. Then, if you have ssh set up properly, you'll be logged to a CLI on your home machine.

In the usual home setup, with many computers connected to a router with only one external IP address, this will never work out of the box. First, the router doesn't (usually) do ssh on its own, and if it did there wouldn't be any useful information for you there, unless you wanted to trash fix your own network. Second, the router doesn't know which machine you want to talk to. And third, even if it did, that machine might not be accepting ssh requests (and won't, if you've followed my previous recommendation).

So what do we need to do? Let's see: First, we need to tell the router which computer gets the ssh session. Then, we need to set that computer up to receive ssh commands. Finally, if we want to be able to contact multiple computers, all behind the same router, we have to figure out how to do that, as well.

Setting up the router

First things first: how does the router pick a computer for ssh? Well, though Wikipedia tries to explain it in technical detail, basically your computer connections to the outside world are controlled by a set of 65536 (or 2¹⁶) ports. If you like, you can consider each port as a connection in an old-style telephone switchboard. When you call up a specific service on a computer, you ask to be hooked up to a specific phone number.

Some of these phone numbers, or ports, are customarily assigned to specific services. For example, port 80 is usually reserved for a web server. It doesn't have to be that way, though, you could start your web server running on port 31526, if you liked. The problem is that everyone's browser would be looking for your server on port 80 of your computer, and you wouldn't get many hits. (If that's what you desire, go for it.)

ssh, it turns out, is typically assigned port 22. So when I run

ssh rcjhawk@aaa.bbb.ccc.ddd

the computer I'm on asks the operator (my home router), to connect it to port 22 so that it can talk to whoever is there. If that contact speaks ssh, then we can communicate. If, however, the contact is speaking telnet, or ftp, we're going to have problems.

The router, as we said above, knows nothing about ssh, or at least doesn't want to talk to the unwashed masses of the Internet. It has to pass the call on to a computer that I've designated to receive ssh requests. This works by what is known as Port Forwarding. This tells that router that whenever someone comes calling at port 22 it is to pass that signal along to a specific computer.

How do you do port forwarding? Well, that depends on your router, your firewall, if any, and maybe on your modem. I can't give general directions, though PortForward.com might be able to help. There is also a YouTube instructional video, and you ought to be able to find specific instructions for your own router/firewall.

Since I'm on Verizon FIOS here at the house, I have a Verizon MI424WR modem/router. There is an old set of instructions for port forwarding online, but the software seems to have changed. That did provide a starting point, however, and I've figured how how to set up forwarding on my current setup:

1. Point your Browser at http://192.168.1.1/
2. Click Firewall at the top of the page.
3. Click Port Forwarding on the left.
4. You see a line that says Create new port forwarding rule: and below that a box that says IP Address forward to or select from menu. Click on that, and then click on the machine you want to pipe your ssh requests to.
5. Click on the box labeled Application to forward … and select SSH. When things settle down, click apply.

Setting up your home machine

This is fairly straightforward. Let's assume that we've instructed the router to forward port 22 to hal, here. On hal, we've followed all the instructions to get the ssh daemon running, so that we know what to do with incoming ssh requests.

Currently, however, we have the line
ALL: ALL
in our /etc/hosts.deny file, and
ALL: 192.168.1.0/255.255.255.0
in /etc/hosts.allow, which means that only computers on the local network can ssh to hal. One way to fix this would be to add
sshd: ALL
to /etc/hosts.allow. This allows hal to receive ssh requests from any machine, anywhere. Another way to do it would be to specify those external machines that can access hal by host. For example, if I worked for Google, the line might read:
sshd: 72.14.0.0/255.255.0.0
or some-such, letting me log in from any computer with an ip number 72.14.xxx.yyy.

Once we have your home computer's /etc/hosts.allow file properly set up, then running
ssh youraccount@aaa.bbb.ccc.ddd
from an allowed machine, where youraccount is the account name on your home machine, and aaa.bbb.ccc.ddd is your router's current IP address, should get you in, at least after you enter the appropriate password.

Other machines on your network

The problem with all of that is that I can only access hal by this method. Suppose I want to access another computer, fred, instead. I could, of course, get into my router and change port 22 forwarding from hal to fred, but that's difficult to do from the road, unless I allowed my router to be administered remotely.

However, the port/service assignment is more of what you'd call a guideline. We can, in fact, use any port in a storm for ssh services. In the example above the Verizon router had presets for ssh using port 22, but we could have used any other port instead.

Again, the exact way of doing this will vary from router to router, but to set up forwarding of an arbitrary port to a machine with the Verizon router,

1. Point your Browser at http://192.168.1.1/
2. Click Firewall at the top of the page.
3. Click Port Forwarding on the left.
4. Under IP Address … click on your target machine.
5. Under Application click Custom Ports. When things settle down you'll have a bunch of boxes to click.
6. Under Protocol select TCP.
7. Under Source Ports select Any.
8. Under Destination Ports select Specify, and then put in your selected port. In this case we'll use 1022. We don't have to use 1022, but it's simple to remember, once we know that ssh ordinarily uses port 22.
9. Click on Apply and you're done here.

Now go back to your target machine. Open up a terminal window. Then,

1. Run
   sudo vi /etc/hosts.allow
   and edit the file to allow sshd access from your favorite machines, just as you did with the first machine.
2. Run
   sudo vi /etc/ssh/sshd_config
   Look for a line that says Port 22
3. Below that line add the line
   Port 1022
   or whatever other number you've chosen.
4. Stop and restart the ssh daemon by running the command:
   sudo /etc/init.d/ssh restart
5. Now go over to your remote machine. Run
   ssh youraccount@aaa.bbb.ccc.ddd -p 1022
   if 1022 is the port you've chosen. You should be logged on to the second machine in your network.

That concludes the tutorial on accessing your home machines' command lines. Ubuntu also has a remote desktop feature, which I've yet to use. When I get around to trying it out we'll put that online here as well.

Finding Your External IP Address

For various reasons you might want to find your public IP address, that is, the 32-bit number, expressed in pseudo-decimal form as something like 72.14.204.99, that tells the world how to reach your computer.

For most businesses and government offices this is trivial: your computer is assigned a static IP address, one that never changes with time. For home users, though, it's a little more complex.

Start with the IP address itself. The largest possible IP address is 255.255.255.255. That means that you can have no more than 4,294,967,296 unique IP addresses, and some of those are reserved for local use, as we'll see. We're coming up on 7 billion people in the world. Not everyone has a computer (yet, and remember iPhones, iPads, Kindle, and all 'Droids count), but many people have more than one (see iPhones, iPads, …). So there aren't enough IP addresses to go around. Eventually, everyone will switch over to the IPv6 address scheme, which will give us 340,282,366,920,938,463,463,374,607,431,768,211,456 (2¹²⁸, or 3.4×10³⁸) possible addresses, which is probably enough for every non-hydrogen nucleus in the observable universe to have its own address.

But we're not there yet, so something has to be done to shrink the address space. Your local ISP solves this problem by 1) charging you extra to get a static IP address; 2) giving you only one IP address per gateway (your cable, FIOS, DSL, or dial-up modem, or some other router or hub that connects you to the internet); and 3) taking your IP address away from you when you're not using it.

That last one is called dynamic IP addressing, and it's part of what we're going to talk about today. If you have a dynamic IP address, then when you turn your gateway connection (usually some type of modem or router) on, your ISP assigns you an IP address from its bank of addresses. As long as your connection is turned on, you'll have an IP address, but your ISP can change it at any time. It will almost certainly change it if you turn off your connection and turn it on again. So if you know your IP address right now, there's no reason to expect it will be the same tomorrow, or even five minutes from now — though to be fair, Verizon FIOS has kept our house on the same IP address for weeks.

And there is worse to come: Suppose your ISP has given you the address 244.117.16.25. The computer you're reading this on doesn't have a clue as to what that address is. Go back to 2), above. Your house has only one IP address, and it belongs to your gateway. Every other computer in the house belongs to your home network. When you want to download your email, your computer politely requests the gateway to go fetch it:

Please, sir, may I have my email?

Oh, all right. Just this once. But don't tell any of the other computers what we're doing.

Thank you, sir! I'll be eternally grateful

You'd better be, or no Internet for you.

Let's see how this works. ping is a command that will poke a stick at a computer and get information about it. Let's try it with a well known site:

ping google.com
PING google.com (72.14.204.99) 56(84) bytes of data.
64 bytes from iad04s01-in-f99.1e100.net (72.14.204.99): icmp_seq=1 ttl=53 time=21.8 ms
64 bytes from iad04s01-in-f99.1e100.net (72.14.204.99): icmp_seq=2 ttl=53 time=20.2 ms
64 bytes from iad04s01-in-f99.1e100.net (72.14.204.99): icmp_seq=3 ttl=53 time=23.4 ms
^C

(you'll need to hit ctrl-C to stop this)

So we see that google.com has an IP address of 72.14.204.99 — that's actually one of many, but don't worry about that. Now let's try it with a local machine:

ping george
PING george.home (192.168.1.2) 56(84) bytes of data.
64 bytes from george.home (192.168.1.2): icmp_seq=1 ttl=64 time=9.77 ms
64 bytes from george.home (192.168.1.2): icmp_seq=2 ttl=64 time=2.70 ms
64 bytes from george.home (192.168.1.2): icmp_seq=3 ttl=64 time=0.961 ms
^C

So far has the home network is concerned, george lives at 192.168.1.2, which is one of those reserved addresses we mentioned above. The range 192.168.0.0 to 192.168.255.255 is reserved for internal networks. As we read this, there are no doubt hundreds of thousands of computers with the local IP address of 192.168.1.2. Every one of them, however, has a different public IP address.

Great. But sometimes I need to know my external IP address, and it's apparent that Hal, here, hasn't got a clue as to what it is. How can I find it?

The easiest way is to go to another website. Every website you connect with gets your public IP address automatically, unless you use some kind of anonymous service. Some websites are set up to tell you what your IP address is. One example is What Is My IP?. They even provide a way to get your IP address with a simple one-line terminal command:

wget www.whatismyip.com/automation/n09230945.asp -O - -q ; echo

If you read your Gmail via Thunderbird, Evolution, Outlook, etc., and keep your web client on all day, gmail will tell you the address of your computer. Just log on to your gmail account and look for the line that says Last account activity: …. The IP address there is the last computer to download email. If you're using more than one computer from different locations, say you left your office machine on over the weekend and now you're using a home computer, click on the Details link and you'll find all of the activity for this account.

There's a company DynDNS, which will not only find your IP address, but give you a domain name that constantly updates to your current numerical address. Of course, they do this by having your computer constantly tell them the public IP address.

And, of course, your gateway knows its public IP address. Just log onto it through your browser (something like http://192.168.0.1, http://192.168.1.1, or http://192.168.100.1, depending on your system), and your IP address will be up there someplace.

But all of these systems have flaws: Say you need your IP address in a script. You can get it via your browser from your gmail or your gateway machine, but it's hard to parse a browser page inside a script. The What's My IP? one-liner works in a script, but what if their site goes down, or, worse, they go out of business? Same for DynDNS. We want a solution that works any time your gateway is on and your computer has an internet connect, and we want to get it from the command line.

Any other way just ain't elegant.

If you search the Internet you find surprisingly few ways (i.e., none) to do this. Partly I think it's because there isn't a consistent way to find your gateway from your current machine. For example, in the days when we used Comcast, we had a cable modem as the gateway. On the house side of the modem we connected a Linksys router, which was the hub of our home network. All the computers in the house were connected through the router to the modem — a two step-process. Now we have Verizon FIOS, with a Actiontec MI424WR router that serves as gateway/modem and router — a one-step process.

By stumbling around, and with a little help from nixCraft, I was able to come up with a solution that works for me. It's a bash script. Let me put it here (finally, I hear you say), and we'll dissect it below:

#! /bin/bash

# Determine the public IP address for your DHCP system by bootstrapping

#  calls to your gateway/router

# Requires "route," available in Ubuntu package net-tools, and
# nslookup, available in Ubuntu package dnstools

# Disclaimers:
# 1) This works with Verizon FIOS using the Actiontec MI424WR Router
#    If you have another system YMMV.
#
# 2) And, as you'll no doubt note, this is set up for a Debian system,
#    specifically Ubuntu 10.04 .  Package names and installation
#    procedures for other systems WILL vary.
#

###
# Start Here:
###

#
# Get Router internal Gateway IP
# Information found on
# http://www.cyberciti.biz/faq/how-to-find-gateway-ip-address/
# If route is not installed, run
# sudo apt-get install net-tools
# and make sure /sbin is in your $PATH
# Note "U" = up, and "G" = gateway

internalip=`route -n | grep UG | awk '{print $2}'`

echo Your Gateway\'s Internal IP Address is: $internalip

# Now we need to get the name of the router.  If you don't
#  have nslookup installed, run
# sudo apt-get install dnsutils

echo -n "Your Gateways's name is: "

routername=`nslookup $internalip | grep "name =" | awk '{print $4}'`

# On the Actiontec, at least, the name has an annoying "." at the end.
# Remove it:

routername=${routername%\.}

echo $routername

echo -n "Your Public IP Address is: "

nslookup $routername | grep Address | tail -1 | awk '{print $2}'

OK, what's going on? First of all script only works if you have the route and nslookup commands installed on your system. If they aren't there, in Ubuntu, and presumably and Debian system, you can install them using

sudo apt-get install net-tools dnsutils

The first call, route -n, will look at the Kernel's IP routing table:

$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     0.0.0.0         255.255.255.0   U     1      0        0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 eth0
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 eth0

which on this machine tells us that we have several local connections. The U under Flags means that the connection is up, and G means it's the gateway. From this we see that the numerical address of the gateway is 192.168.1.1. Let's see what nslookup can tell us about that. Note that the script has to parse all of the output to get this far.

$ nslookup 192.168.1.1
Server:  192.168.1.1
Address: 192.168.1.1#53

1.1.168.192.in-addr.arpa name = Wireless_Broadband_Router.home.

Here's something new, our gateway's got a name. Wireless_Broadband_Router.home. Again, the script has to do a little work to tease that name out of nslookup's output, but we'll just do it by hand here:

$ nslookup Wireless_Broadband_Router.home
Server:  192.168.1.1
Address: 192.168.1.1#53

Name: Wireless_Broadband_Router.home
Address: 192.168.1.1
Name: Wireless_Broadband_Router.home
Address: xxx.yyy.zzz.aaa

The first address, 192.168.1.1, is just the router's internal IP address. The last address (Of course I'm not giving you my IP address. I'm not daft, you know.) is the one we want. xxx.yyy.zzz.aaa is the external address of this machine.

Of course there are many, many, problems with this script.

• So far it only works with the Verzion/Actiontec setup. A friend, running Ubuntu, wasn't able to pick out his gateway machine with the route -n command.
• Some machines can have multiple gateways.
• The Mac's route command doesn't work in the same way as under Linux, so you can't parse out the gateway, even if you have none.
• You might be able to tease the gateway IP address out of the /etc/resolv.conf file. I don't know what happens if you have multiple gateways here. Under Ubuntu, the one numerical gateway is 192.168.1.1, on the Apple it gives me 192.168.1.1 and xxx.yyy.zzz.aaa, my public IP address.

If you try the script, leave a comment below. Let me know your internet setup (gateway/router), if it works, and any tweaks you did to make it work. Maybe we can come up with a universal script.

Now, by all that's geekly, why did we do all of this? That's the subject of an upcoming post, boys and girls …

Smart ssh tricks

I want my computers to talk to one another.

More specifically, I want to be able to remotely log in to one of my computers from another one, and I want to be able to easily copy files from one computer to another. The easiest, most secure, way to do that these days is to use the Secure Shell protocol, aka ssh.

Examples: I have an account on a computer called grumpy (sleepy and doc were already taken). To log onto grumpy from hal, I open up a terminal window and type:

$ ssh rcjhawk@grumpy

The program then logs me onto grumpy, where I can access the system just as I can for any desktop machine.

Or, say, I have a file at /home/rcjhawk/Documents/strasburg_pictures.ppt on grumpy. To copy it over to Hal, I use the secure copy program, scp:

$ scp rcjhawk@grumpy:Documents/strasburg_pictures.ppt .

which, obviously, works pretty much like the regular copy program except that I have to specify the machine and account name right up front.

And, finally, I can send files the other way:

$ scp The_Hard_Times_of_Zach_Greinke.txt rcjhawk@grumpy:Documents

which sends that regrettable file into the Documents folder on grumpy.

Except:

You will probably find that this doesn't work out of the box in your Ubuntu setup. You'll get a message that looks something like this:

ssh: connect to host grumpy port 22: Connection refused

That's because Ubuntu ships ssh in two parts:

• openssh-client: this lets you use ssh and scp from your machine to log onto another machine. This is installed with the standard Ubuntu installation, and is all you need if, for example, you want to log onto your work computer from home.
• openssh-server, on the other hand, allows other machines to log onto your personal desktop machine. This is not installed by default in Ubuntu because it's an obvious security hole unless you know what you're doing. If you do know, or assume that you will learn, install the package with
  
  $ sudo apt-get install openssh-client
  
  Once you do that, sshd, the daemon that controls remote access, will automatically start up, and will start up every time you reboot the machine. Then you're good to go.

In the example above, grumpy needs to have openssh-server installed and running, but hal does not.

Note for Mac Users: You get the equivalent of openssh-server by going to System Preferences => Sharing and clicking Remote Login.

Security

ssh encrypts the transmissions between machines, but it also has other security features. The first time you try to log onto a machine, for example, you'll get a message that looks something like this:

$ ssh rcjhawk@grumpy
The authenticity of host 'grumpy (192.168.54.42)' can't be established.
RSA key fingerprint is xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx.
Are you sure you want to continue connecting (yes/no)?

If you answer yes, then you'll get the message:

Warning: Permanently added 'grumpy,192.168.54.42' (RSA) to the list of known hosts.

After that you'll be able to log onto grumpy without the warning message. If you want to start over, i.e. not recognize any computer as being previously visited, erase the file $HOME/.ssh/known_hosts

You can also limit access to the machine by editing your /etc/hosts.deny and /etc/hosts.allow files. As I've noted before, if you don't want ever Tom, Dick, and Susie to have a chance to access your machine, your /etc/hosts.deny file should read:

ALL: ALL

after which only addresses listed in your /etc/hosts.allow file will have access to the machine. In my /etc/hosts.allow file I currently have

ALL: 192.168.1.0/255.255.255.0 ,

which allows all machines on our local network to access grumpy.

OK, everything is good, right? Well, not quite. As things are set up right now, every time you ssh or scp to grumpy you must enter a password. This isn't a big annoyance with ssh, because presumably you will keep the remote login running for a relatively long time. However, it is an annoyance with scp, if you want to copy a large number of files from different places, requiring a large number of scp commands.

The solution is to use ssh-keygen to generate a a key. Here's how it works:

Start the program:

$ssh-keygen -t dsa Generating public/private dsa key pair.
Enter file in which to save the key (/home/rcjhawk/.ssh/id_dsa):

We'll use the default directory location, so just hit return here.

Enter passphrase (empty for no passphrase):

Frankly, I don't know anyone who does use a passphrase. It does protect your key against someone who's gotten access to your account, but at that point you're probably hosed anyway. So for now we'll just hit return.

Enter same passphrase again:
Your identification has been saved in /home/rcjhawk/.ssh/id_dsa.
Your public key has been saved in /home/rcjhawk/.ssh/id_dsa.pub.

Now let's take a look at the .ssh directory:

$ ls -la $HOME.ssh
total 16
drwx------  2 rcjhawk rcjhawk  4096 2010-06-19 13:04 .
drwxr-xr-x 34 rcjhawk rcjhawk  4096 2010-06-19 13:01 ..
-rw-------  1 rcjhawk rcjhawk   672 2010-06-19 13:04 id_dsa
-rw-r--r--  1 rcjhawk rcjhawk   599 2010-06-19 13:04 id_dsa.pub

Here you'll notice two files. One, id_dsa, is your private key. The second, id_dsa.pub, is your public key. Key the private key private. Don't let anyone have access to it. Don't copy it over into the .ssh folder of another account, or to another computer. You'll note that both the .ssh directory and id_dsa are set so you are the only person who can read those files. Keep it that way.

The public key, now, that's another matter. If you place id_dsa.pub into the file $HOME/.ssh/authorized_keys on another computer, you'll be able to access that computer from your home computer. Here's one way to to it (I'm leaving out all the password prompts):

$ ssh rcjhawk@grumpy
Welcome to grumpy!
rcjhawk:~ $ cd .ssh
rcjhawk:~/.ssh $ scp rcjhawk@hal:.ssh/id_dsa.pub tempid
rcjhawk:~/.ssh $ cat tempid >> authorized_keys
rcjhawk:~/.ssh $ rm tempid
rcjhawk:~/.ssh $ exit

What did we do? We logged into grumpy, copied hal's public key over to grumpy's .ssh directory, and added it to the end of the authorized_keys file. I renamed the file during the copy so as not to write over grumpy's own id_dsa.pub file, if one was there. Notice that scp was able to get that file even though Hal's .ssh directory is copy protected, because you gave the system the right password.

The next time you try to ssh or scp from hal to grumpy, ssh will find the public key on grumpy, compare it to the private key on hal, and conclude that you are, indeed, allowed to log in or transfer files.

The discerning reader will note that I'm only applying this to my local network, which all sits behind a router and firewall. How, you may ask, can I use ssh to log onto my local machine from outside my home?

Well, I wish I could tell you, but at the moment I can't. It's something to be researched in the future. If you're reading this, and you know how to do it, post a link to the procedure in the comments.