Some Notes on Firewall Implementations

For anyone considering implementing a new firewall onto a network here are a few notes to help you through the process.  However before you get started there’s a very important first step that you should always take when implementing on medium to large networks.  This step is to establish a Firewall change control board, which consists of user, system administrators and technical managers from throughout your organisation.  Failing to establish proper change control and implementation processes can be very dangerous on a firewall.  A badly thought out rule could create huge security issues or operational problems – that ‘deny all’ rule might look safe but if it ends up blocking mission critical applications you won’t be popular.

Hardware firewalls are amazingly secure and not too expensive. The very first reported type of network firewall is referred to as a packet filter. Establishing a firewall for your infrastructure is an excellent method to present some simple security for your expert services.


Firewalls frequently have such functionality to hide the real address of computer that is linked to the network. You can install most firewall products on a customized network and have it’s protection almost immediately. The host-based firewall might be a daemon or service as part of the operating system or an agent application like endpoint security or protection. These firewalls often arrive in conjunction with antivirus program. Otherwise, a software firewall can be set up on the computer in your house that has an online connection. Or, you may add an extra software component to your firewall.  If you are primarily responsible for your company’s firewall it’s best to have some secure remote access in case of emergencies.  Be careful with rules which allow your access though, you don’t want to let through users’ streaming through UK TV through a VPN service.

In case the connection is controlled by NetworkManager, you may also utilize nm-connection-editor to modify the zone. The secure connection is currently established and now is the time to launch vncviewer so that it employs the secure tunnel. The SSH connection is currently established. Especially in case you allow connection from anywhere online and on the normal SSH port (22).
After you own a server to try from and the targets you want to evaluate, you may continue with this guide. As stated in the past edition you also may want to locate a repository closer to your server. By applying the forwarder you may override the DNS servers supplied by your ISP and utilize fast, higher performance servers instead. Repeat this for each domain that you would like the server to manage. It is necessary for a standard server. Also many servers block dynamic dns hosts, so you could discover your server becomes rejected. At this point you have a simple mail server!

The application shouldn’t be confused with malware behavior. Some Antivirus software applications may ask that you switch off the firewall and disable the Antivirus to be able to install it. Before you put in a software, the very first important step is to look at the configuration of your computer, and the system prerequisites of the program. Update the neighborhood package index and install the software if it’s not already offered.

The configuration of your computer must match the demands of the software to be set up. If you are pleased with your present configuration and have tested that it’s functional once you restart the service, it is possible to safely permit the service. The only configuration you should make that actually impacts the functionality of the service will probably be the port definition in which you determine the port number and protocol you desire to open. If all your interfaces can best be managed by a single zone, it’s probably simpler to just pick out the best default zone and use that for your configuration. You may then modify your network interfaces to automatically choose the right zones. Whenever you are transitioning an interface to a different zone, be conscious that you are most likely modifying the services which are going to be operational. Opening up an entire interface to incoming packets might not be restrictive enough and you may want to have more control concerning what to allow and what to reject.

James Hellings

Author of IP Cloaker



ATM – Routing IP

There are of course many different network architectures many of which have been around for many years. One of them is known as ATM (Asynchronous Transfer Mode) and was considered in the 1990’s to be the ultimate network architecture design. The belief was that in the future every computer or device would be fitted with an ATM network adapter rather than the alternatives which at the time were token-ring or ethernet.

The reality has turned out somewhat different of course, and it’s unlikely that we will ever see the extensive use of ATM based networks. However many corporations installed ATM backbone switches for an important reason because they have the ability to handle network traffic at extremely high speeds.

There is a difficulty though for using these switches, that is ATM is a virtual circuit based, cell based networking scheme which is primarily connection orientated. Compare this with Ethernet which powers the majority of commercial networks which is actually a connection less frame based networking scheme. In fact to integrate the two systems, you need to use one of the available overlays which have been developed in order to allow Ethernet to be connected to the ATM backbones and switches.

These normally work by using layer 3 routing algorithms which can discover the initial routes through the network, Then layer 2 virtual circuits can be established through the ATM fabric delivering data without actually going directly through the routers. This technique is normally known as ‘shortcut routing’ although you will often here it described by other terms as it’s a useful technique. If you need more detailed information check your normal networking references or search online using search terms like ‘IP routing over ATM’.

There are difficulties with these improvised techniques one of the most common is knowing when to route and when to switch the traffic at layer 2. Long data transmissions such as Netflix video streams should be switched as the more efficient method of transport. However for shorter transmissions then the router is normally the best option.

Layer 3 traffic will not under normal circumstances identify the length of the transmission so it may or may not be suitable to be switched. There are ways of identifying the length of the transmission normally by inspecting the content of the datagrams itself. There are many different methods of identifying the flow mostly developed by different networking companies, some are no longer commonly used but you will find others being developed or utilized extensively in various environments. See the references below for some examples that can be researched for more information.

3 Com Fast IP
Ipsilon IP Switching
Switch IP Address – Watch UK TV in USA

No Comments Protocols, Proxies, VPN

Creating a Proxy Hierarchy

Although most networks and organisations would benefit from implementing proxy servers into their environment it can be a difficult task to decide the location and hierarchy of these servers.  It is very important and there are some questions which can aid the decision making process.

Flat or Hierarchical Proxy Structure?

This decision will largely depend on the both the size and the geographical dispersion of the network.  The two main options are firstly whether a standard single flat level of proxies will be sufficient, or whether something larger is required.  This would be a larger hierarchy based on  tree structure much like an Active Directory forest structure used in complexed windows environments.

Indeed in such environments it may be suitable to mirror the Active Directory design with a proxy server structure.   Many technical staff would use the following rule of thumb – each branch office would require an individual proxy server.  Again this may map onto an AD design where each office exists with it’s own Organisational Unit (OU) . This has other benefits because you can apply custom security and configurations options based on that OU, for example allowing  the sales OU more access through the proxy than administrative teams,

This of course needs to be carefully planned in line with whatever physical infrastructure is in place.   You cannot install heavy duty proxy hardware at the end of a small ISDN line for example.  The proxy servers should be installed in line with both the organisation configuration and network infrastructure.    Larger organisations can base these along larger geographical areas for example a separate hierarchy in each country.  So you would have a top level UK proxy server above regional proxies further down in the organisation.

If the organisation is fairly centralized you’ll certainly find a single level of proxies a better solution.  It is much easier to manage and the latency is minimised without tunnelling through multiple layers of servers and networks.

Single or Proxy Arrays

A standard rule of thumb for proxy servers is usually something like one proxy for every 3000 potential users.   This is of course only an estimate and can vary widely depending on the users and their geographic spread.  This doesn’t mean that the proxies need to be automatically independant, but can indeed be installed in a chain together.

For example you can set up four proxies in parallel to support 12000 users using the Cache Array Protocol (CARP).  These could be set up across different boundaries even across a flat proxy structure.   Remember that the servers will have different IP address ranges if across national borders.   Make sure that your proxy with the Irish IP address can speak to all the other European sites, most proxies should ideally be multihomed to help with routing.

Using the caching array will allow multiple physical proxies to be combined into a single logical device.    This is normally a good idea as it will increase things like the cache size and eliminates redundancy between individual proxy caches.

It’s normally best to run proxies in parallel whenever the opportunity exists. However sometimes this will not be possible and specific network configurations may stop this method meaning you’ll have to run proxies individually in a flat mode.   Even if you have to split up proxy resources in to individual machines be careful about creating network bottlenecks.  Individual proxies should not be pointing to single gateways or machines, even an overworked firewall can cause significant impact on a network’s performance and latency.

Digital Interface Testing – Cisco

If you need to check the physical layer status and the quality of digital circuits then there are two tools which you are likely to need.   The first is a breakout box which can be used to determine the connection integrity between the DTE and the DCE. This box (also know as ‘BOB’) has two external connections which can be extended on the DTE and DCE.

The box supplies status information on the digital circuit and will also display any data being transmitted at the time.   The device will normally display real-time status information about data, clocking, space and activity.  On most of the breakout boxes, this information is displayed using status LEDs.  It is normally quite a compact device powered by batteries in order to increase it’s portability.   The box contains buffered electrical circuitry which does not interfere with the actual line signal during testing,  Most are also capable of verifying the electrical resistance and line voltage too.

These are focused on physical problems on a network primarily, although errors can occur for other reasons.  If you’re looking at other issues perhaps conflicts on a proxy IP address or an application error then you should look at other tools.

The second piece of equipment you’ll need has a variety of names but is most commonly known as BERT.  This stands for bit-error-rate tester and is actually a lot more sophisticated piece of kit.   This can effectively measure the error rate in a digital signal.  This error rate can be measured both from end to end circuit or on a portion of a circuit for isolating individual faults.  The bit error rate is often measured during installation and commissioning so that it can be used as a baseline.

The BERT also is used to measure error rates on the variety of different bit patterns that it can generate. You can use this information for timing or noise issues on the circuit.  It does take time but allows a line to be monitored accurately and a traffic and error analysis can be performed

John Williams

UK VPN Free trial

Internet Control Message Protocol – ICMP

The Internet control message protocol has a wide variety of different message types many of which are extremely useful for managing and troubleshooting an IP Network.   Most of us are familiar with the command ‘ping’ which uses at it’s core both ICMP echo and echo reply.   Another well used ICMP tool is that of traceroute which is useful for monitoring TTLs (time to live) and hop counts.

There are however quite a number of these ICMP messages, beyond the ones used by these tools and most are extremely useful for anyone managing a complex IP based network.   Here’s some of the most useful ones:

ICMP unreachable – an IP host will produce an ICMP unreachable message if there is no valid path to the requested host, network, protocol or port.  There are several of these messages which are often grouped together for convenience.  They are often generated from routers and switches, for example if local access lists are restricting access to the requested resource.   You should be careful about allowing these messages to be propagated as they contain source addresses.  Particularly if the connection is being used externally perhaps through an external connection like a BBC VPN for instance.    The messages can be blocked by using the no ip unreachables command on Cisco hardware.

ICMP redirects – a router will produce a redirect message if it receives a packet on a given interface and the route is on  the same device.   These can be used to help update local routing tables with the correct information.   There is an interesting protocol from Cisco which can be configured to help with these situations it’s called the Hot Standby Routing Protocol (HSRP).

ICMP  mask request and reply – some hosts do not have their subnet masks statically defined and have no way of learning it.  Here they can use an ICMP mask request which can be responded to by the router with an ICMP mask reply.

ICMP source quench – these messages provide an important function within ICMP that of congestion control on the network.   If a network device such as a router detects network congestion perhaps because of dropped packets or overflows in buffers and on it’s interfaces then it will send an ICMP source quench message to the source of these packets.

ICMP Fragmentation – this type of message is sent when an IP packet is received which is larger than the MTU specified within the LAN or WAN environment yet it also has the flag DF set (do not fragment). Here the packet cannot be forwarded however the ICMP message can be used to at least pass back some information on the issue.  There are actually quite a few scenarios where the DF bit is set automatically by devices as the packet is distributed.

Further Reading:

John Summer, Proxy for Netflix – video, Harvard Press, 2017

HTTP (Hypertext Transfer Protocol)

For many of us a network is either our little home setup consisting of perhaps a modem and wireless access point and a few connected devices, or perhaps that huge global wide network – the internet.  Whatever the size all networks need to allow communication between the various devices connected to them.  Just like human beings need languages to communicate so do networks only in this context we call them ‘protocols’.

The internet is built primarily using TCP/IP protocols to communicate, this is used to transport information between ‘web clients’ and ‘web servers’.   It’s not enough though to enable the media rich content delivered to our web browsers and a host of secondary protocols site above the main transport protocol – the most important one which enables the world wide web is called HTTP.

This provides a method for web browsers to access content stored on web servers, which is created using HTML (Hypertext Markup Language).  HTML documents contain text, graphics and video but also hyperlinks to other locations on the world wide web.   HTTP is responsible for processing these links and enabling the client/server communication which results.

Without HTTP the world wide web simply wouldn’t exist and if you want to see it’s origins search for RFC 1945 where you’ll find HTTP defined as an application level protocol designed with the lightness and speed necessary for distributed, collaborative, hypermedia information systems.   It is a stateless, generic and object orientated protocol which can be used for a huge variety of tasks – crucially it can be used on a variety of platforms which means it doesn’t matter whether you’re platform your computer is on (linux, Windows or Mac for instance) – you can still access the web content via HTTP.

So what happens? When someone types a web name or address into the address field of their web browser, the browser attempts to locate the address on the network it is connected to.  This can either be a local address or more commonly it will look out on to the internet looking for the designated web server.   HTTP is the command and control protocol which enables communication between the client and the web server allowing commands to be passed between the two of them.   HTML is the formatting language of the web pages which are transferred when you access a web site.

The HTTP connection between the client and server can be secured in two specific ways – using secure HTTP (SHTTP) or Secure Sockets Layer (SSL) which both allow the information transmitted to be encrypted and thus protected.  It should be noted though that the vast majority of communication is standard HTTP and is transmitted in clear text insecurely which is why so many people use proxies and VPNs like this to protect their connections.