Nov 08
Categories: sysadmin Tags: , , ,

I recently noticed that the downstream bit-rate on my ADSL modem was fluctuating. This can happen because the noise on the copper pair between the house and the exchange can vary over time. There isn’t much you can do about that, but it is possible to somewhat influence the performance at the house end by reducing the length of the wire from the master socket to the modem, by using better quality wire and even by disconnecting the bell terminal at the master socket.

But before making any changes, I needed to be able to monitor the effects, so I thought this Muinin plug-in was in order. This plugin monitors the upstream and downstream bit-rates of my ZyXEL Prestige P-660HWP-D1 ADSL modem. I’m sure it’ll work with any Prestige modem. Here is an example plot.

Munin graph of modem bitrates

Munin graph of modem bitrates from zyxel_prestige_adsl_chandata plugin

You can get the script here: See the documentation therein for set-up instructions.

You can see the bitrate drop twice during Friday evening. On Saturday morning I moved the modem from an extension socket to the master socket. Initial indications were that I had improved the downstream bitrate significantly. However by the evening you can see that it subsequently dropped back to a rate slightly lower than at the same time 24 hours earlier.

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Sep 17
Categories: sysadmin Tags: , ,

I’ve been having intermittent problems with my internet access for a while (who hasn’t?) where everything would be really slow for a day or so and then it would mysteriously recover. I’d kick the modem, the squid cache, the local caching DNS server, etc but I was never sure where the problem lay.

Recently it got so bad that I investigated a bit harder. When it’s all gone bad (which is most of the time now, is seems) It looks like my DNS queries are transmitted as expected but the replies are never forthcoming. All the while I have solid connectivity. I can even flood ping the DNS servers that are not responding!

Before I go hassle my ISP, I thought I’d collect some evidence. Hopefully this will help me bypass the stupid “have you tried rebooting?” questions. So I’ve written a Munin plugin which I will configure to poll my ISP’s DNS servers. I expect that this will show that I frequently don’t get a response, in the evenings at least; I wonder if their traffic shaping is broken…

You can find the plugin at See the documentation therein.

Example daily plot from Munin plugin dnsresponsetime

Example daily plot from Munin plugin dnsresponsetime

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