Archive for the 'Network' Category

Dec 13 2007

Managing known_hosts file

Published by under Linux,Mac,Network

Recent Debian installations of openSSH default to a hashed host name in the known_hosts file used by openSSH. This makes it more difficult to manual maintain and edit this file. A good start on the commands that can (should?) be used to manage know_hosts can be found on this blog entry.

Two useful commands:

search for an entry: $ ssh-keygen -H -F hostname
delete an entry: $ ssh-keygen -R hostname

An online version of the ssh-keygen man page provides more detail on the options it provides.

Since a one way hash is used, there isn’t any way to list all the entries in known_hosts with machine names “de-hashed”.

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Oct 04 2006

Bandwidth between two laptops on a wireless router (part 2)

Published by under Network

Following up from my previous post on very slow file transfers between two laptops, I ran some tests in which I changed the router configuration with the following results.

TiBook to iBook – kbps
Router Configuration
mixed ‘b only’
wireless to wireless 400 1600
wired to wireless 20800 5000

The TiBook(11b) to iBook(11g) using a mixed mode router configurations remains in the same range as prior tests, but the switch to “b only” configuration shows a significant improvement. But, the performance is still mediocre: 1.6Mbps vs 6Mbps which should be reached. The TiBooks poor wireless performance is a factor in that reduction. (Note: wireless tests were done at my normal usage distance of about 30 ft). A price paid for the somewhat improved wireless to wireless performance is worse wired to iBook performance.

Lastly, I did some up/download tests to a remote server out on the internet and the relatively low bandwidth of my dsl connection (384up/1.5down) dominates all other factors and performance on the two machines is the same when up or downloading. The greater efficiency of using a common mode on the router is a significant factor but only on potentially high speed internal transfers.

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Oct 03 2006

Bandwidth between two laptops on a wireless router

Published by under Mac,Network

I have two OS X based laptops and a linux based server connected to a linksys WRT54GL wireless router which in turn connects them all to the internet. Internet performs is okay and pretty much consistent with the stated speed of the dsl connection to the internet. Connection speed among the local machines is less than expected: laptop to server 200-400 kB/s but only 30-40 kB/s between the laptops. Both of those bandwidths are low, but laptop to laptop is ridiculous! Traceroute says the two laptops are communicating with no intermediate stops.

In an attempt to pin down the bottleneck, I ran various comparisons which are summarized in the following table. All the tests used scp and there were no other machines connected to the router when the tests were run. The iBook is two years old and the TiBook is four years old.

File transfer via scp – avg bandwidth in kbits/sec
to—-> 802.11g 802.11b 100baseT 100baseT 100baseT
from: iBook TiBook iBook TiBook Linux
iBook-802.11g 2240 8800 8800
TiBook-802.11b 200-440 800-3200 2728
100baseT 2672 8800

So where’s the bottleneck?

  1. Looking at the first row, the wireless iBook(802.11g) is capable of maxing out 10baseT on a linux box, but can only push about a fourth of that rate to the wireles TiBook. So it seems neither the router nor iBook are the source of the problem.
  2. In the second row, I’ve put a couple of ranges which reflect varying the distance of TiBook from the wireless base station. Distance (and correspondingly poorer reception) is a factor, but does not account for much of the speed difference. Even when the TiBook is within 10 feet of the base station, it can only reach about 30% of the 100baseT, though it does double the throughput to the iBook. But that is still terribly low. From this second line of data, it seems that the TiBook wireless can only get up to about 2.7Mbits/sec. Well below the theoretical max (11Mb/s) and about 50% of the practical max of around 6Mb/s. The difference between the wireless to wireless and wired to wired speeds must reflect some sort of interference/inefficiency between the three devices; possibly due to mixing g and b on the same router.
  3. And finally, the last line seems to indicate that the TiBooks wireless reception capacity is about the same 2.7Mb/s that it could send to a wired machine. i.e. the max is symetric.

Seemingly, the mixing of b & g is the most likely culprit. Next test will set the router to b only and I’ll see if the wireless to wireless transfer rate improves.

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