Michael Wheeler

IMG_3339 – TheSkorm

Just getting a quick post in before the end of the month. As you may notice by the picture, a hard drive in my server failed. Luckily this was a backup drive in a LVM which actually wasn’t being used, so there was no data loss. As you can see the damage to the drive is quite large (the most I’ve ever seen in pulling a drive apart). LVM wasn’t too nice to me though. I was quite disappointed that LVM failed to notify me of the drive failure, and/or recognise that there was a problem and mount things in read only, instead LVM continued on it’s merry way, until I found the source of the “clicking” noise in the computer room. Once I worked out which drive it was I had to remove the inode and get LVM to remove it and carry on.

From this, I think there needs to be some improvements for LVM. LVM should have some sort of error detection on drives and send out alert emails. LVM should also do data redundancy. I know LVM isn’t designed to do either, but it seems logical. I want something like a Drobo, but for free running on Linux or FreeBSD. Something where the hard drives can be what ever size they want with redundancy with error reporting. This is just a dream….

I’m also getting into my Uni work, starting two courses, Foundations of Business Computing and Data Communications. Both seem fairly boring so far but I’ll see how it goes.

Cisco – queued

Cisco messages can be annoying when you are working a switch that logs ACL rules on a Cisco switch, so something that I’ve found quite useful is editing the line configuration to make the command you are typing in on a Cisco switch be re-printed below the log message.

It’s pretty simple to use, and all you need to do is this, in “configure terminal”.


line con 0
logging synchronous
line vty 0 4
logging synchronous
line vty 5 15
logging synchronous


Another handy tip is enabling SSH. SSH is secure unlike telnet, meaning passwords or important information can’t be seen if an attacker was able to log packets.

SSH is fairly easy to enable, first off make sure you have a username configured or radius, enable the ssh transport under the lines, set a domain-name and then generate some keys.


line vty 0 15
login local
transport input telnet ssh
!
username admin password god
ip domain-name internaluse.net
! out of config mode run
crypto key generate rsa


Make sure you put down 1024 for the RSA keys, other most SSH clients will turn you down.

TCP-Header – flickr_xtaci

When setting up servers and services I used to wonder why people would use UDP. It’s annoying to setup and test with firewalls, however lately while doing my CCNA study I have learned the important uses of UDP and TCP.

TCP has lots of features, like error recovery and acknowledgment which UDP doesn’t support. Both use “ports” and both have error detection. So why use UDP?

I’ve linked to a flickr scan of TCP header. As you can see a TCP header is 20 Bytes long, without it’s payload, where as if you look at a UDP header it’s only 8 Bits long. UDP already has the advantage of being smaller in size, but what really makes UDP useful is it’s lack of error recovery.

In services like VoIP , Video Conferencing and Gaming, by the time TCP has detected an error, sent a request for the missing packet, and continued on it’s way it has interrupted the call, making TCP a bad choice. Since UDP doesn’t have error handling, it doesn’t matter if a packet was lost or damaged, because UDP won’t request (or check) for the missing packet and continue on it’s merry way. Thus keeping the call at low latency.

Ps. Now that I have gotten half way through the first Cisco CCNA book I might share a few other network tips, tricks or info.

Old Jeans are the Best – digicla

I’ve made many mistakes in my life (too many in my opinion) but one mistake I havn’t made is buying jeans. Jeans form the base for nearly everything I wear. I wear them around the home and work. The best thing I have done with my life is buying Targets 92 Relaxed Bootcut jeans. Alot more loose, more pocket space, and are great with big boots.

These jeans have turned my life around.

Cadbury Cream Eggs – Craig Rodway

Not really a fan of them, but I found this on youtube and it’s a wonderful set of advertisement.

Link

Have a look, I loved it.