Uwe Hermann | A slightly paranoid Debian developer

Here's a quick HOWTO for setting up your own secure pseudo-dynamic DNS (DDNS) server.

It's not a "real" DDNS service, i.e. you won't be able to use standard DNS tools or protocols to talk to the server, but it covers 98% of all functionality I expect from a service such as DynDNS or similar ones: It tells me the IP address of a certain box which doesn't have a static IP address (e.g. my home-server).

Requirements

You'll need:

• A Linux box with dynamic IP address (dial-up modem/DSL), I'll call it homeserver from now on. This is the box whose public IP address I want to be able to find out.
• A public Linux box with static IP address (or known DNS name) where you have a user account and ssh access. I'll call this box publicserver.

Setup

On the homeserver:

• Add a non-root user account (e.g. user) just for the purpose of this mechanism: adduser user. The user doesn't need any special permissions.
• Create an ssh key with an empty passphrase for the user: ssh-keygen -t rsa -b 4096. This is required as you'll want to run ssh commands via cronjob later.
• Add a cronjob which runs a random command such as ls regularly (as user), e.g. once per 10 minutes:
  
5,15,25,35,45,55 * * * * user ssh -x user@publicserver ls

  The command to run (e.g. ls) doesn't really matter at all, more on that later.

On the publicserver:

• Add a non-root user account (e.g. also named user) just for the purpose of this mechanism: adduser user. The user doesn't need any special permissions.
• Add the public ssh key (/home/user/.ssh/id_rsa.pub) of user@homeserver to the publicserver's /home/user/.ssh/authorized_keys, so that the homeserver user can login on the remote publicserver without password (i.e. non-interactively). We'll also limit which ssh commands this user can run using the command keyword in /home/user/.ssh/authorized_keys file:
  
command="echo $SSH_CLIENT | cut -d \" \" -f 1 > /home/user/homeserverip.txt && chmod 644 /home/user/homeserverip.txt",no-port-forwarding,no-X11-forwarding,no-agent-forwarding ssh-rsa AAAAAAAAAA...AAAAAAA user@homeserver

  In the above example AAA...AAA is the public key, command specifies which command should be run if this user "logs in" via ssh, and we use some other options such as no-port-forwarding,no-X11-forwarding,no-agent-forwarding to minimize what this user can do via ssh.

So to summarize: the homeserver's user simply executes the above commands on the remote publicserver, which in turn abuses the $SSH_CLIENT environment variable which contains the public IP the ssh connection was coming from (which is exactly what we're looking for). We store that IP in the homeserverip.txt file, which will always contain the latest-known IP address of the homeserver (because of the cronjob).

Getting the current homeserver IP address

You can now retrieve the current IP address of your homeserver easily from anywhere (e.g. from your laptop when you're in another, possibly hostile network) in order to connect to your homeserver:

  $ ssh -x otheruser@publicserver cat /home/user/homeserverip.txt

To make this a bit more convenient you can add a shell alias (e.g. into ~/.bashrc):

  alias homeserverip='ssh -x otheruser@publicserver cat /home/user/homeserverip.txt'

Or, to conveniently login to your homeserver as johndoe:

  alias homeserverlogin='ssh -x johndoe@`ssh -x otheruser@publicserver cat /home/user/homeserverip.txt`'

Conclusion, advantages

This may not be the most elegant solution, and it has a number of drawbacks when compared to services such as DynDNS, but it's sufficient for me and it also has some advantages:

• You're not dependent on the DDNS service provider. For instance DynDNS recently changed their policy to only allow one update per 28 days, which totally sucks. They then disabled the service completely until I updated my ddclient config and contacted them, i.e. I wasn't able to connect to my homeserver for quite a while, which also sucks.
• The ssh-based solution is secure and encrypted, in contrast to some other DDNS services, which only allow unencrypted HTTP-based connections (yes, some do allow https/SSL connections).
• This solution doesn't require in-depth DNS server config knowledge, neither does it require a DNS server you control. You only need a (non-root) ssh account on a public server (or virtual server).

Personally I'm currently using this mechanism for two things, more might follow:

• Connect to my homeserver via ssh.
• Get the homeserver's IP address so I can update my OpenVPN client config file on my laptop (I use my homeserver as OpenVPN server).

So far it works pretty nicely.

Update 2008-06-24: Various fixes and simplifications. SSH key must be password-less. Don't run cronjob once per minute, that's overkill.
Update 2008-07-02: Simplify setup by removing the need for extra scripts. Limit the commands the user can perform via ssh in the authorized_keys file. Make the RSA keys 4096 bits strong.

OK, so I've been hacking on and testing my shiny new One A110 mini-laptop during the last few days and I must say I'm very happy with it. I'll write up some more details later (check the wiki if you're impatient), but today I want to highlight a very nice feature of this laptop (compared to, for instance, the Eee PC): The VIA C7-M ULV CPU in the laptop has VIA Padlock support.

VIA Padlock is a hardware feature in recent VIA CPUs which provides hardware-accelerated AES and SHA-1/SHA-256 support, among other things. This can be used in Linux (with the proper drivers and patches) to improve performance of dm-crypt, OpenSSL (and all programs using it), scp, sha1sum, OpenVPN, etc. etc.

I have written a quite extensive VIA Padlock HOWTO and benchmarks in the A110 wiki (but all of this will work on other systems which have VIA Padlock, too). To summarize, here are the most important benchmarks:

dm-crypt (256bit AES, cbc-essiv:sha256)

Without VIA Padlock support:

$ hdparm -tT /dev/mapper/hdc2_crypt
/dev/mapper/hdc2_crypt:
 Timing cached reads:   448 MB in  2.00 seconds = 223.47 MB/sec
 Timing buffered disk reads:   22 MB in  3.07 seconds =   7.17 MB/sec

With VIA Padlock support:

$ hdparm -tT /dev/mapper/hdc2_crypt
/dev/mapper/hdc2_crypt:
 Timing cached reads:   502 MB in  2.00 seconds = 250.41 MB/sec
 Timing buffered disk reads:   90 MB in  3.07 seconds =  29.36 MB/sec

The native speed of the SSD in the laptop is 31.01 MB/sec, so there is almost no performance penalty when using VIA Padlock.

OpenSSL

OpenSSL speed benchmark, first line without Padlock, second line with Padlock enabled:

$ openssl speed -evp aes-256-cbc [-engine padlock]
type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes
aes-256-cbc       9187.18k    10572.28k    11054.32k    11179.36k    11218.02k
aes-256-cbc      47955.92k   150619.73k   325730.73k   458320.11k   520520.79k

ssh/scp

Without VIA Padlock support:

$ scp -c aes256-cbc bigfile.dat localhost:/dev/null
bigfile.dat                100%  159MB   5.9MB/s   00:27

With VIA Padlock support:

$ scp -c aes256-cbc bigfile.dat localhost:/dev/null
bigfile.dat                100%  159MB  14.5MB/s   00:11

OpenVPN

A real speed benchmark is pending (not measurable easily on 100MBit LAN, will try on a slower link), but as OpenVPN uses OpenSSL it should have roughly the same speedup iff you tell OpenVPN to use AES (it uses Blowfish per default).

Also, there's a measurable difference in CPU load while tranferring large files over OpenVPN: 8% CPU load with VIA Padlock (vs. 20% CPU load without VIA Padlock).

sha1sum / phe_sum

phe_sum is a small C program which can be used as drop-in replacement for sha1sum (which doesn't support VIA Padlock yet). Quick benchmark:

sha1sum, without VIA Padlock:

$ time sha1sum bigfile.dat
real    0m6.511s
user    0m5.864s
sys     0m0.412s

phe_sum (with VIA Padlock support):

$ time ./phe_sum bigfile.dat
real    0m1.149s
user    0m0.704s
sys     0m0.424s

All in all VIA Padlock gives you a pretty impressive speedup for many crypto-using applications on Linux, which is especially useful on the A110 mini-laptop (think OpenVPN or scp for mobile usage, and dm-crypt for an encrypted SSD, of course).

Just in case you haven't yet watched it: Big Buck Bunny.

Great animated video created mainly using Blender, released under the Creative Commons Attribution 3.0 license.

The soundtrack/score is now also available under a CC license (as is lots of other "raw" material for the movie).

OK, so I've spent my last money on the One Mini A110 subnotebook recently. Yep, yet another ASUS Eee PC clone, but this one has the great benefit of costing only 199.- Euros and has similar specs as the Eee PC 2G Surf (700), I think.

This is really a great little machine as far as I can tell. It's a VIA C7-M ULV 1GHz with 512MB DDR2 RAM and a 2 GB Solid-State-Disk (SSD), 7" screen at supposedly 800x480, VGA out, card reader slot for SD/MMC/MS, 2x USB, wireless, modem, audio. No webcam, no bluetooth.

Yesterday I created a wiki at a110wiki.de (for the A110, but also the A120 from the same vendor, which has a 4 GB SSD), where A110 users can collect information, HOWTOs, photos, etc. There's already quite some content there, especially some early tutorials and photos on the inner workings of the A110.

Today I've installed a stock Debian unstable distro on the SSD with 2.6.25 kernel, and I'm currently checking which parts of the hardware work out of the box, and which need further fixing. There's a a bunch of source code tarballs and patches on the vendor website, but most of it seems to be meant for 2.6.22, we'll see if and/or how much work it'll take to merge all this upstream (if it's not already done)...

My Debian Installation HOWTO is also available from the wiki, of course; I'll add more info and photos during the day.

Now for all interested parties: The vendor of the A110 has (again) announced a special weekend offer (valid until Sunday, June 1, 2008, i.e. tomorrow) where they'll sell the A110 for 199,- Euros again, the regular price will be 229,- Euros after that. So if you're thinking about buying one, now is probably the right time.

Check the wiki for issues which are important to you, some quirks remain at this point (but will probably mostly be figured out sooner or later), e.g. the wifi seems to have issues (the vendor said they'll send a driver update to all affected customers), the RAM is builtin and can't be upgraded, and some other, more or less important issues, depending on what you expect from the laptop.

For real-time communication there's also the #a110 IRC channel on Freenode.

FYI, if you're not using xdm/kdm/gdm but are instead starting the X11 server manually with startx (which is what I usually do) you might have experienced brokenness in Debian unstable recently:

Fatal server error:
Unrecognized option: /etc/X11/xinit/xserverrc

This is already reported as bug #482425 and #482527 and should hopefully be fixed soon, but in the meantime this patch against /usr/bin/startx should work around the issue:

--- /usr/bin/startx.orig 2008-05-26 18:21:26.000000000 +0200
+++ /usr/bin/startx     2008-05-26 18:21:36.000000000 +0200
@@ -107,9 +107,7 @@
 if [ x"$server" = x ]; then
     # if no server arguments or display either, use rc file instead
     if [ x"$serverargs" = x -a x"$display" = x ]; then
-       server=$defaultserver
        serverargs=$defaultserverargs
-       display=$defaultdisplay
     else
        server=$defaultserver
     fi

Hope that saves some people out there lengthy investigations and hassle.