Jim Moy

This stuff is great. I tell my wife how cool I think it is, and she says “What’s the big deal? It’s just tape…” Not just electrical tape. It’s self-amalgamating tape! Just the thing for us folks with antennas subject to wind, rain, and snow. Wrap it around those N connectors between my antenna and the LMR400 to my wireless network card, and feel safe that it’s a weather-tight seal.

I guess it’s also called Tommy Tape, or self-fusing tape. My neighbor turned me onto this stuff as he was investigating our 802.11b neighborhood WLAN. I haven’t found any really good pictures on the web of its use yet, but here’s one. And the first link has some instructions on how it’s used. Better yet, just Google it. Looks pretty ordinary, so I guess I see what she means. I just got my connectors wrapped up again after moving my antenna. All ready for the Spring rains.

Now that Yahoo has decided to charge for POP3 access to email, I’m becoming more serious about using my public server as an email server. Up until now I’ve only beeen using it as a forwarder for my domain, to get mail to my “real” address, which has typically been a Yahoo address or some other place easily abandoned for spam control. I could just switch to another free service, but that’s not the point. Having my own domain should really mean that I have more control over my email. So now I’m using Postfix as my SMTP server. Not that it’s particularly better than Sendmail or Qmail, but it’s what came with my Mandrake distribution! Not that I’ll push the envelope on any of these programs with my email needs. It sure is nice to have unlimited aliases though.

Here are a couple of FCC links regarding RF safety, and the Federal rule that preempts local restriction on access to wireless signals, including Internet access. Basically, if your antenna is less than a meter in diameter and on a mast less than 12 feet above your roofline, the preemption applies. But there are plenty of exceptions (ham radio, common areas, etc.) answered by the Q/A in the article.

Of course, our 802.11b connections are too much of a good thing, you knew someone had to start. (From one of my favorite 802.11b resources.)

The new antenna location is working great. It appears to solve the weird behavior I was seeing before, and even stand up to some accumulation of snow and ice with headroom to spare. Over the past few days we’ve had a snowstorm that caused a half-inch of snow and ice to accumulate on the antenna overnight. You can see the signal strength effect on the MRTG graph shown here.

The accumulation begins a little before midnight, and by the time I got started at work the next morning the signal strength had dropped a good ten points. The new location has greater fade margin, so I never noticed the difference. I would typically see the full “11Mbps” 802.11b speed if my signal strength was greater than 30. These are Linux Aironet driver numbers, which have been discussed, but I’m still not exactly sure what relation, if any, these are to dBm, although the 2nd technique described here gives numbers that are consistent with my experiences with an OmniSky CDPD modem, to compare apples and oranges.

I’ve moved my antenna. The new location has clearer line of sight to the access point on the barn, my first wireless hop to the Internet, and is farther away from the picture window and my house. The downside is that it’s closer to the ground and more likely to be blocked occasionally. But, it’s a good tradeoff visually so I’ll have to see how it fares this Spring and Summer when everyone’s out in the yard.

It took a longer length of LMR400 than I had before, 80 feet, very promptly delivered by the Davis RF folks. But I’m showing better signal strength compared to the old location, and it was behaving well even when we still had snow on the ground.

The default shell in Dachstein is ash, a lightweight shell that handles most of the system scripting chores. I prefer bash as my login shell to get features like command line history, and whatever other behaviors I’ve become used to over the years.

A bash LRP package is available for Dachstein, and what this does via a link from /bin/sh is make bash the default shell for the entire system. There is an incompatibility between bash and some LRP packages in Dachstein, dnscache and tinydns are a couple of examples. One way to work around this problem is to install bash and fix up the incompatible scripts to use ash instead.

However, replacing a shell wholesale like that doesn’t make me feel confident that I’m getting the tested behavior, so I’ve created a modified version of the bash LRP package that can be downloaded here which simply omits the link from /bin/sh. Then I edit /etc/password for those users I want bash to be their login shells, and ash continues to be the shell for the rest of the system scripts.

My domain is hosted on a machine that is twice removed from a wired connection to the Internet. That is, traffic to my site goes through two 802.11b connections, one provided by the local neighborhood WLAN, and another provided by the local co-op, largely because I live in a somewhat rural area not serviced by DSL or Cable Internet. The first hop from my site is a half-mile, and the second hop is four miles.

Over the past few months, I’ve noticed some odd behavior. My signal appears to drop off dramatically for periods of time, most pronounced when there is snow on the ground, which I haven’t figured out. Most of the time the connection is perfect, but the occasional behavior is to lose signal strength during the day, and it comes back up at night. I have clear line of sight over that first hop, so it’s not something simple like snowplows parking in the way. I watch signal strength on the /proc entry for the driver that I use for my Aironet 340.

Today when I noticed the signal strength was down pretty low, I tore down my firewall with my wireless NIC and took it out on my deck. I didn’t use my fixed antenna, I used the little rubber-ducky that came with the NIC, and got better signal strength than the fixed antenna was providing. The fixed antenna has a little bit of obscuring by my neighbors trees, but the main difference may be that the fixed location is under the eaves of my house, and only a few inches from a picture window. But it doesn’t explain that it works fine most of the time, and I’ve only been able to correlate it with the snowfall. I’m going to move the antenna.

One of the warm-fuzzies you get from running a LEAF based firewall like Dachstein is that it’s not a full-blown Linux distribution. You start with just the stuff you need for a firewall, and add other things you want.

But… there can be gotchas. I bumped into the /proc/cmdline limit on kernel params recently, while adding some more LRP packages to my 1.0.2 Dachstein system. You can tell if you’ve got the problem by comparing the LRP= list in your syslinux.cfg file, and what you see when you type

cat /proc/cmdline

If it’s getting truncated, then you’ve got a problem. I fixed the problem by moving the LRP package list out of syslinux.cfg, and into a separate file. Look for the line in /linuxrc that says
ROOTMAP="`sed 's/.*LRP=/1/; s/ .*//1' /proc/cmdline`"
and change it to two lines like this:
pkglist=`cat /boot/etc/lrppkgs.cfg`
ROOTMAP=`echo $pkglist | sed ’s/ /,/g’`
then put your list of LRP packages in /boot/etc/lrppkgs.cfg in a simple list, one per line, no punctuation. Backup root.lrp and boot, and you should see all your packages loading.