trivia

I have just built myself a new intel core 2 duo based machine to replace one of my older machines which was beginning to struggle under the load of video transcoding I was placing upon it. The new machine is based on an E8400 and is nice and shiny and fast. Because it is a new build, I decided to install the OS and all my preferred applications, tools and utilities from scratch. Yes, I could have just copied my old setup, or at the least, my home directory and system configuration from my older machine, but I chose to do a completely new clean build on top of a clean install of ubuntu 8.04. I did this largely because my older system has been upgraded and “tweaked” so often I am no longer sure exactly what is on there or why. I am sure that it contains a lot of unnecessary cruft and I felt it was time for a clear out. A new build should ensure that I only installed what I actually needed. Of course I copied over my mail, bookmarks and other personal data, but the applications themselves I simply installed from new and then configured to my preferred standard.

Like most modern linux distros, Ubuntu is pretty secure straight out of the box. Gone are the (good old, bad old) days when umpteen unnecessary services were fired up by init or run out of inetd by default. But old habits die hard and I still like to check things over and stop/remove stuff I don’t want, or don’t trust. I also like to check outbound connections because a lot of programs these days have a habit of “calling home” - a habit I dislike. I noticed and cleared up one or two oddities I’d forgotten about (Ubuntu uses ntpdate to call a canonical server if ntpd is not configured for example. Since I use my own internal ntp server, this was easy to sort). However, after clearing, or identifying all other connections I was left with one outbound http connection I didn’t recognise, and worse, it was to a network I know to be untrustworthy. The connection was to 66.235.133.2. This machine is on the omniture network. Omniture is notorious for running the deeply suspicious 2o7.net. Omniture market webanalytics services and are used by a whole range of (perfectly respectable) companies who pay them for web usage statistics. But omniture have never successfully explained why they choose to use a domain name which looks like, but isn’t, a local RFC 1918 address from the 16 bit block (e.g. 192.168.112.207). I don’t trust them, and I didn’t like the fact that my shiny new machine was connecting to them. So what was responsible? And what to do?

Well, the “what to do” bit is easy - just blackhole the whole 66.235.128.0 - 66.235.159.255 network at my firewall. But that feels a bit OTT, even for me. A bit of thought, and a bit of digging gave me a better solution, and one which incidentally solves a range of related problems. What I actually needed was a way of preventing oubound connections to any hosts I don’t like or don’t trust. So long as the IP addresses of the hosts are not hard coded in the application (as sometimes happens in trojans) the classic way to do this is to simply map the hostname to the local loopback address in your hosts file. But this can become tedious. Fortunately, it turns out that a guy called Dan Pollock maintains a pretty comprehensive hosts file on-line at someonewhocares.org. Result.

Because I run my own local DNS server (DNSmasq on one of the slugs) it was easy for me to add Dan’s host file to my central hosts file. So now all my machines will routinely bin any attempted outbound connection to adservers, porn sites, or whatever in the list. The downside, of course, is that this is a bit of blunt instrument and may cause some difficulty with some sites (ebay for example). But I’m prepared to put up with that whilst I fine tune the list. I can also pull the list regularly and automatically via cron so that I stay up to date (but of course I won’t just blindly update my DNS, I’ll pull the file in for inspection and manual substitution…..).

So what was making the connection? Well it looks to me as if adobe is the culprit. I had installed the acroreader plugin for firefox.

Silly me. Must remember to avoid proprietary software.

(Oh, and you just have to love omniture’s guidance on how to opt-out of their aggregation and analysis. You have to install an opt-out cookie. Oh yes, indeedy, I’ll do that.)

Have you ever wondered what was eating your network? Would you like to be able to check exactly which application was responsible for that sudden spike in outbound traffic? NetHogs might help. This neat little utility calls itself a “small ‘net top’ tool”, and that is exactly what it is. NetHogs groups bandwidth usage by PID so you can immediately see which application is responsible and take whatever action you deem appropriate.

Recommended.

(Oh, and if you want a nice graphical representation of the connections your PC is making whilst you are using it, I recommend you install etherape. It can be a highly educational (not to say scary) experience to leave etherape running whilst you fire up your browser. You will find that your PC is making HTTP connections all over the place. Now try leaving it running whilst you are not doing anything and watch what happens.)

I recently needed to set up a mailing list for a group of friends (my bike club). I had become tired of mail bounces and failures because we were all relying on an out of date list of addresses originally cobbled together by one member. That list of addresses was routinely used in “reply all” messages to others about forthcoming social events. An obvious improvement would be a mail list - ideally one which members could manage themselves. I originally looked at using a quick and dirty system using a mail forwarding mechanism which would simply explode mail sent to one address to the complete list of aliases (I can be lazy). However I discovered that neither my mail/web provider, nor my ISP really offered this facility in quite the way I wanted it. So, an obvious way forward would be do it myself using a slug.

I’ve used mailman in the past and knew it offered everything I wanted (including a web interface for membership management and access to archived messages), but I don’t (or rather didn’t) run a mail server on my home network. So that had to be fixed first. The necessary ingredients for the list management were: mailman itself; an MTA (I chose postfix because I know it, like it and find the default debian exim unnecessarily complicated); and a webserver (I was already running lighttpd on both slugs because it performs better than apache on low memory machines). I also wanted to use SSL encryption on the webserver to preserve password integrity (but not to authenticate the webserver itself).

There were a number of steps required to get this all working to my satisfaction. These were:

Step 1 - upstream SMTP authentication using TLS with postfix;
Step 2 - getting a mailman listserver running with postfix;
Step 3 - configuring lighttpd with SSL for mailman;
Step 4 - putting it all together and letting the world in.

It all worked, but the main drawback turned out to be the performance of the slug when running mailman. The combination of SSL encryption and mailman python scripts is too big a hit for a device with only 32 Mb of RAM. It would be perfectly feasible to run mailman on the slug if we limited ourselves to management by email alone (i.e. ignore the web management interface). But doing this would severely limit its functionality and in such case we might as well look at alternative list managers such as Majordomo or Listproc. In the end, the attractiveness of mailman’s web interface meant that I moved it all off the slug and onto a more powerful platform (also running debian). Nevertheless, the documentation here may be of use to anyone considering a mailman install with postfix and lighttpd on any linux distro. The notes on SSL usage at step 3 can, of course, also be applied (with suitable modification) to apache or any other webserver supporting SSL certificates.

Any half decent sysadmin will routinely test the security of his or her own systems. A good, and sensible, sysadmin will follow up those tests with an independent security audit by a professional company - preferably one which is a member of a recognised industry body (such as CREST). Finding the holes in your security mechanisms (and there will be some - probably more than you will be happy about) before the bad guys do is essential if you want to sleep at night (and keep your job).

There are a huge number of security testing tools available for free if you know where to look. Most sysadmins keep a toolbox of their favourites (nmap, nessus, ettercap, dsniff et al.) to hand ready for testing any new build. But it can sometimes be difficult to know just which tool to use, and where to get it. Enter backtack. I first came across this collection of tools as recently as february 2006 and found it an excellent resource. Essentially backtrack is a collection of all the security testing tools you are likely to need packaged into one linux distribution. Think of it as a knoppix for security testing. A complete list of all the tools in the collection can be seen here.

Bactktrack Version 3 has just hit the streets. Get it here.

(Oh, and don’t think that using a toolset like this makes you a pen-tester. It doesn’t. What it might do is make you more security aware, and a better sysadmin.)

On a mail list I subscribe to I have recently been involved in a discussion about the restrictions sometimes placed on users of WiFi hotspots or hotel networks (to say nothing of the restrictions placed on corporate networks). Some of the suggested solutions involve tunnelling ssh connections over http(s). Other solutions assume that the network is simply restricting access with packet filters so that you may just need to connect to a non-standard port (such as 80 or 443). If this is the case, then you simply have to configure your target ssh daemon to listen on that port. However, some networks force you through a proxy, in which case you need a utility like corkscrew. I had not previously heard of this neat little utility - but it turns out to merit some exploration if you find yourself needing such a tool.

Corkscrew is relatively simple to set up, but if you have problems, take a look at Andrew Savory’s blog entry of 27 February 2008.

As I mentioned in an earlier post, I found several references to successful reflashes of the BT hub to a genuine Thomson 7G image on a variety of sites. None of those sites gave instructions as to how to do this if you run a linux PC.

So I have documented how I did it here.

BT home hub