Cisco DHCP Configuration

Posted on March 12, 2010 by admin

Dynamic Host Configuration Protocol (DHCP) is a protocol used by networked devices to obtain information required to operate on an Internet Protocol network.

When a DHCP client is first switched on, it sends a broadcast packet on the network with a DHCP request. If there is a DHCP server exists in the network, this request will be picked up by the DHCP server. The DHCP server will allocate an IP address to the PC, from one of the pools of addresses it managed.

DHCP clients use a broadcast packet to find a DHCP server, so if a client is on one side of a router, and the server on the other, normally these broadcast packets will be filtered out. Many routers today include support for DHCP- forwarding, but it is not enabled by default.

Many system administrators do not realize that DHCP service can be enabled in Cisco devices. In this article, we will show the method on configuring DHCP on a Cisco router. Continue reading →

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Windows Clustering

Posted on March 11, 2010 by admin

Clustering is the connection of multiple computers together in such a way that to the outside world, the cluster appears to be a single system. Each computer in a cluster is referred to as a node.

Business suffers when work stops because of critical service is offline. A properly installed and configured cluster provides continued operation during hardware failure, software failure, and/or planned maintenance. Ability to recover from failed server hardware and failed software is the main benefit of clustering.

Windows Server 2003 clustering consists of two different technologies: Server Cluster and Network Load Balancing (NLB). Each of these technologies can be used to provide high availability for different types of services. Continue reading →

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The Fewer Services Your System Runs, The Better

Posted on March 10, 2010 by admin

It is only a matter of time before an unsecured system becomes contaminated and losing valuable data. In addition, poorly secured system that is compromised can be used for a number of illegal activities. It was reported that several major Internet sites were disabled by attacks that were launch through the use of thousands of these devices.

Whether it is a Windows PC, router, switches, Linux machine or network enable device, it is recommended to stop unnecessary services that are run by default. Every running, but unused, service on your system is an unnecessary security vulnerability. Users may not even be aware that many of these services are running. Each additional service running on it may be a possible avenue for a network cracker to penetrate the system.

It is important to know the security implication of any network service before enabling it in. For example, if the telnet service is activated, a telnet client running on another device on the network can easily establish a telnet session to the system; it needs only the address of the system and the port or socket number of the network service running on the system. The rule of thumb for TCP and UDP ports is: Turn off any services or listeners that you do not need for your application to function. One way to check the list of ports running on your system is to go to one of the many web sites that can run a remote scan of your system and tell you how secure the system. Continue reading →

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Using nslookup to verify DNS registration for domain controllers

Posted on November 16, 2009 by admin

The nslookup command is a standard command-line tool provided in most DNS service implementations. It offers the ability to perform query testing of DNS servers and obtain detailed responses as the command output. This information is useful in troubleshooting name resolution problems, verifying that resource records (RRs) are added or updated correctly in a zone, and debugging other server-related problems.

When trying to join a Windows 2003 DC using NetBIOS name, you may receive this message: “A domain controller for the domain could not be contacted”. Common issue with this error may due to the DNS SRV records required to locate a domain controller for the domain are not registered in DNS. These records are registered with a DNS server automatically when a domain controller is added to a domain.

To verify DNS registration for domain controllers, we may use the nslookup command

On your Command Prompt.

Type:

nslookup
Continue reading →

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Disable unnecessary network services for security

Posted on August 14, 2009 by admin

Whether it is a Windows PC, router, switches, Linux machine or network enable device, it is recommended to stop unnecessary services that are run by default. Every running, but unused, service on your system is an unnecessary security vulnerability. Users may not even be aware that many of these services are running. Each additional service running on it may be a possible avenue for a network cracker to penetrate the system.

It is important to know the security implication of any network service before enabling it in. For example, if the telnet service is activated, a telnet client running on another device on the network can easily establish a telnet session to the system; it needs only the address of the system and the port or socket number of the network service running on the system. The rule of thumb for TCP and UDP ports is: Turn off any services or listeners that you do not need for your application to function. One way to check the list of ports running on your system is to go to one of the many web sites that can run a remote scan of your system and tell you how secure the system.

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DNS – the basic TCP/IP system

Posted on August 14, 2009 by admin

DNS or Domain Name System is one of the base system service in TCP/IP. It is used by email system, windows active directory, any web access and so on. Without DNS, a user will need to know the IP addresses of all the services s/he uses.

Understanding of DNS is a must in managing network system. As mentioned, almost all network service require the service of a DNS. A trouble happened in DNS will make everything not working properly. Using a DNS tools like nslookup may be useful in troubleshooting this problems.

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Print using IPP via HTTP or HTTPS

Posted on May 14, 2009 by admin

The Internet Printing Protocol (IPP) is a standard network protocol for remote printing as well as managing print jobs, media size, resolution, and so forth in distributed environments.

The idea behind the IPP is to define a protocol for end users to print over the Internet, provide users with the same printing controls and concepts that they use to print locally or to LAN-attached printers. There are two main components to IPP:

- Web-based printer management which offers the ability to administer, to connect to, and to view printers by using a Web browser.
- Internet printing which allows you to use the printer’s URL to connect to a printer.

In IPP, print jobs are sent using Hypertext Transfer Protocol (HTTP), the same protocol used to connect to web pages. Unlike other printing protocols, IPP also supports access control, authentication, and encryption, making it a much more capable and secure printing solution than older ones.

As IPP transfers are made using HTTP v. 1.1, once a connection is made, multiple commands (files) can be sent over a single TCP connection. However, by using HTTP/1.1, IPP faces criticism for its protocol overloading. This allegedly makes for a more complex and bloated protocol and implementation than necessary — for example the venerable lp protocol was extended to cover the same functionality.

Anyway, the advantage of implementing IPP on top of HTTP is in the fact that the latter protocol is already well-tested on the Internet as a method to transfer files, which enables reuse of proven, well-tested and debugged client and server code.

It is a design goal of IPP to not invent new security features when existing protocols can be used. For example, the original RFC suggested authorization be done for example via HTTP’s Digest access authentication mechanism or via SSL3. Encryption is not done by IPP itself either, but it may be handled by SSL/TLS protocol layer.

Because the service is using HTTP or HTTPS, this is typically port 80 or port 443. As IPP supports HTTPS traffic, communication can be encrypted, depending on the user’s Internet browser settings. IPP printing via HTTPS is offered in Windows XP and Windows Server 2003. Windows Vista and Windows Server 2008 also support IPP printing over RPC in Medium-Low security zone. IPP uses the client/server model, where the client resides in a PC or workstation and the server is embedded in a printer device or resides in a separate print server machine. The process is as follows:
Continue reading →

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Using POP3 not SMTP to retrieve email

Posted on April 25, 2009 by admin

The Post Office Protocol version 3 (POP3) as defined in RFC1939 is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. The protocol defines how the server responds to requests sent from an e-mail client.

A POP3 mail server receives e-mails and filters them into the appropriate user folders. The POP3 service makes e-mail messages available for download from a server, enabling a server to host e-mail accounts and provide basic e-mail access. When a user connects to the mail server to retrieve his mail, the messages are downloaded from mail server to the user’s hard disk. Virtually all modern e-mail clients and servers support this protocol.

POP3 works over a TCP/IP connection using TCP on network port 110. POP3 listens on this TCP port 110 for connections from e-mail clients, authenticates the client, and manages the connection with the client. The design of POP3 and its procedures supports end-users with intermittent connections (such as dial-up connections), allowing these users to retrieve e-mail when connected and then to view and manipulate the retrieved messages without needing to stay connected. Although most clients have an option to leave mail on server, e-mail clients using POP3 generally connect, retrieve all messages, store them on the user’s PC as new messages, delete them from the server, and then disconnect.
Continue reading →

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Trouble shoot domain name issue using nslookup

Posted on March 29, 2009 by admin

nslookup is a computer program used in Windows and Unix to query Domain Name System (DNS) servers to find DNS details, including IP addresses of a particular computer, MX records for a domain and the NS servers of a domain. The name nslookup means “name server lookup”.

nslookup is also use as a command-line administrative tool for testing and troubleshooting DNS servers. In the very simple usage, if you enter a domain name, you get back the IP address to which it corresponds, and if you enter an IP number, then you get back the domain name to which it corresponds.

There are two modes available in nslookup: interactive and non-interactive. Interactive mode allows the user to query name servers for information about various hosts and domains or to print a list of hosts in a domain. Non-interactive mode is used to print just the name and requested information for a host or domain. Following is an example in using interactive mode of nslookup.

C:\>nslookup
Default Server: dns3.maxonline.com.sg
Address: 202.156.1.58

> ifreetuition.com
Server: dns3.maxonline.com.sg
Address: 202.156.1.58

Non-authoritative answer:
Name: ifreetuition.com
Address: 74.220.215.82

Continue reading →

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Study of ARP using Wireshark

Posted on March 28, 2009 by admin

Wireshark is a very useful tool for the study of network protocol!

In my previous post on ARP, we have learned some basic about the ARP protocol. Let’s look more detail into the ARP request packet in this post.

When the MAC address for a next hop device is not found in the local ARP cache, a broadcast is sent out on the network, with a special format called the ARP request. The following diagram shows an example of an ARP request packet captured using Wireshark.

As shown in the diagram, the ARP packet is contained directly in an Ethernet packet and does not have an IP header. On Ethernet networks, these packets use an EtherType of 0×0806 as indicated in the Packet Details Pane of Wireshark.

As Ethernet protocol specification only assigns a single type value to ARP, an Ethernet frame that contains an ARP request or ARP response message will have the same type indicated. As such, the receiver must examine the Opcode field in the ARP packet to determine whether it is a ARP request or ARP response message.
Continue reading →

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