Aleksandar's computer workshop

Let's see what Aleksandar was fixing today.
My findings, tips & tricks related to computers, internet, programming and other stuff I was working with.

SMTP tar pit feature for Microsoft Windows Server 2003

Tarpitting is the practice of deliberately inserting a delay into certain SMTP communications that are associated with spam or with other unwanted traffic. To be effective, these kinds of communications typically rely on generating a high volume of traffic. By slowing an SMTP conversation, you can dramatically reduce the rate at which automated spam can be sent or at which a dictionary attack can be conducted. Legitimate traffic may also be slowed by tar pitting.

The tar pit feature is available in Microsoft Windows Server 2003 and in several third-party SMTP servers. The tar pit feature in Windows Server 2003 works by slowing all responses that contain SMTP protocol 5.x.x error codes. An administrator can configure the delay that is introduced by the tar pit feature.

Tar pitting affects only anonymous SMTP connections. Authenticated sessions are exempt. Therefore, if you regularly exchange lots of SMTP mail with another organization,
and you find that tar pitting is affecting that traffic, you can bypass tar pitting for that organization by authenticating SMTP communications.

More details you can find here:
http://support.microsoft.com/kb/842851

How to change Name Server (DNS Client) Settings under Linux

In order to be able to use conical names you have to use name servers (also called DNS servers) which will convert some name into IP address. I will here show you where are configuration files under Linux

Edit /etc/resolv.conf which can like this:
nameserver 192.168.1.1
nameserver 10.0.0.138

Also you might hardcode some IP's in hosts file. It looks like this:
127.0.0.1 localhost.localdomain localhost
194.109.6.67 ns.xs4all.nl

To have your new settings valid you have to restart the computer or do this in command line which will restart all your network services and devices:

/etc/init.d/network restart

If you are connected with SSH be patient connection will be restored.

How to wire Ethernet Cables also known as UTP CAT5 patch cables

Every so often I come across a cable only running in half duplex with severe packet loss, only to find out it was incorrectly wired. In 1991 EIA/TIA released their joint 568 Commercial Building Wiring Standard. These standards help reduce electro-magnetic interference to ensure rated cable speeds.

About the Cable

You can find bulk supplies of the cable at many computer stores or most electrical or home centers. You want UTP (Unshielded Twisted Pair) Category 5 cable for basic 10/100 functionality. You need CAT 5e for gigabit (1000BaseT) operation and CAT 6 gives you a measure of future proofing. Bulk cable comes in many types, there are 2 basic categories, solid and braided cable. Braided cable tends to work better in "patch" applications for desktop use. It is more flexible and resiliant than solid cable and easier to work with, but really meant for shorter lengths. Solid cable is meant for longer runs in a fixed position. Plenum rated cable should/must be used whenever the cable travels through an air circulation space. For example, above a false celing or below a raised floor. 


CAT5 Cable internal structure

Inside the cable, there are 8 color coded wires. These wires are twisted into 4 pairs of wires, each pair has a common color theme. One wire in the pair being a solid or primarily solid colored wire and the other being a primarily white wire with a colored stripe (Sometimes cable doesn't have any color on the striped cable, the only way to tell is to check which other wire it is twisted around). The twists are extremely important. They are there to counteract noise and interference. It is important to wire according to a standard to get proper performance from the cable. 

About the RJ45 Ends

The RJ45 end is a 8-position modular connector that looks like a large phone plug. There are a couple variations available. The primary variation you need to pay attention to is whether the connector is intended for braided or solid wire. For braided/stranded wires, the connector has contacts that actually pierce the wire. For solid wires, the connector has fingers which pierce the insulation and make contact with the wire by grasping it from both sides. The connector is the weak point in an ethernet cable, choosing the wrong one will often cause grief later. If you just walk into a computer store, it's nearly impossible to tell what type of connector it is.

 
RJ45 Plugs


About the cable types

There are 3 main types of RJ-45 cables that you will run across in networking, both LAN and WAN. Some interfaces can cross and un-cross a cable automatically as needed. The TIA/EIA-568-A specifies two wiring standards for a 8-position modular connector such as RJ45. The two wiring standards, T568A and T568B vary only in the arrangement of the colored pairs. Your choice might be determined by the need to match existing wiring, jacks or personal preference, but you should maintain consistancy. 
  • Straight-through or standard patch cable, the colored wires are in the same sequence at both ends of the cable, which is used to connect to a hub or switch.
  • Crossover or cross-connect cable, the first (far left) colored wire at one end of the cable is the third colored wire at the other end of the cable, used to operate in a peer-to-peer fashion without a hub/switch.
  • Rolled, Rollover or Console cable, the colored wires at one end of the cable are in the reverse sequence of the colored wires at the other end of the cable. Mostly used to configure routers via console.

T568B CAT5 Specification and RJ45 Plug pin numbering

 

Different EIA/TIA-568 wiring specifications

 

Sample of prepared plugs as standard patch cable 

 

RJ45 Crimping tool

 

 

 

 

Video: How to make a Cat5e Network Cable