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From Wikipedia

In computer security, a DMZ (sometimes referred to as a perimeter networking) is a physical or logical subnetwork that contains and exposes an organization's external services to a larger untrusted network, usually the Internet.

Why does it say …

larger untrusted network, usually the Internet.

I often see that the Internet is said to be an untrusted network. Are there any reasons for it?

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Because trusting it would be like leaving your car, full of valuable stuff, unlocked in a seedier neighborhood of New York. –  Daniel R Hicks Jul 11 '12 at 11:44
You should probably think in terms of all networks being "Untrusted" meaning that any network can contain a compromized computer that might attack yours. A truly "Trusted" network would be rare, but "Trusted" is really just shorthand for a "More Trusted" network which you can consider just about anything behind a firewall without public access. –  Bill K Jul 11 '12 at 16:12
(I forgot to mention leaving the keys in the car.) –  Daniel R Hicks Jul 11 '12 at 16:54
Have you seen the Internet? ;) –  Steven Noto Jul 11 '12 at 18:20
'untrusted' means 'not trusted'. It's different from 'distrusted', which means you specifically suspect it. Example: "you shouldn't trust everyone you meet" is good advice, whereas "you should distrust everyone you meet" has a different meaning, or "I don't know either way whether Bob's statement is accurate" vs "I think Bob is lying". –  barrycarter Jul 12 '12 at 16:32
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9 Answers

up vote 79 down vote accepted

I like analogies. You should too.

Don't trust the internet. It's scary.

I haven't slept in 35 hours Imagine the internet is the ocean. It is pretty big and imposing and full of weird and wonderful creatures that may or may not want to eat you alive. Fortunately for you, you've been taught from a young age that wherever you go there's going to be a few creatures out there that can't wait to nibble on your innards, but that they're like 30 feet long and such a rare sight you will probably win the lottery 3 times in a row before you get bit by one, and that you shouldn't worry too much about them. What they didn't teach you in school is that these nibblers are literally everywhere and come in various sizes.

Its harder than you think to come up with 7 fish off the top of your head.

Your innards are pretty important, and you don't want anything nibbling on them. Being the persistent person determined to go for a swim, you strive to find a way to swim without a care in the world, knowing that it can't be you whose luck is so short to get nibble'd.

Luckily for you, your parents are veterans of the Nibbler War of the 70's, and partially solved the problem by surrounding themselves in a Faraday cages. So, they put you in a cage (despite your protests) and drop you in the ocean with some scuba gear. In your cage, you are safe from the innard-eating sea creatures, and you can swim happily within its confines without fear of the sea creatures. Maybe the cage isn't as tight as your parents thought it could be, and you manage poke your appendages out (which the innard-eating fishes will jump on in a flash if they smell you); but that's your parents fault for not putting enough bars in the cage.

It's lonely out there on the internet.

Ok that's a pretty terrible analogy, but the point is this; big companies don't want their data compromised so they put things into private networks where they know that hackers won't be able to touch them without going through a great deal of effort first (or some amazing social engineering). But since you can still access the internet, there is a chance that your own computer will be compromised, which would expose the greater network.

Since the company controls what information can go through, they can mitigate the damage of public facing sites, and be happy that none of their internal network stuff were exposed

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+1 Morbid analogy, but works. –  kinokijuf Jul 11 '12 at 11:31
Or how about a private swimming pool? In a swimming pool, you can control what goes in and out; in the ocean, you can't. –  BlueRaja Jul 11 '12 at 15:43
@BlueRaja But you still get those pesky kids who somehow manage to drive an '87 chevy through your back yard and just ruin it for you and everyone else for a few weeks. And, you also have to deal with the people who insist that there should be a slip-n-slide from your private pool to the public pool 10 blocks away; and lets not forget the people who bring their own pool toys when it was outlined pretty clearly in 24 point bold comic sans notice that you DONT BRING YOUR OWN POOL TOYS TO THE PRIVATE POOL. –  qweet Jul 11 '12 at 15:51
+1 for the drawings. –  Michael0x2a Jul 11 '12 at 17:46
+1 for MagiKarp –  Mechaflash Jul 11 '12 at 20:28
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To answer "why Internet is unsecure?", we actually need to understand "How Internet works?". And taking it one step further, let's ask "What is Internet?".

What is Internet?

A junior grade text book will define Internet as network of networks, and that remains true to a CTO level. In practical terms, start thinking from the fact how are you reading this text. You are reading this from either your personal computer/laptop or from an office desktop. If it is a personal computer, you are connected by dialing to the ISP, or if you are on LAN, someone else has done that step for you. A LAN itself is a network, though smaller. A LAN will have computers and routers (may be servers).

When LAN gets connected to ISP, to which more PCs, Servers, routers and LANs are connected, it becomes part of a larger network. When these larger network gets further connected, we end in having a huge network, called Internet.

How does the Internet work?

Again let's go back to the basics. How can any two computers talk? They send packets of information to each other, which are provided in a well defined protocol, which both systems understand. Think of it as one person sending a letter to another, letter is the packet, and protocol is some simple rules that will make sure that information is conveyed properly.

For example, I am writing in English and you understand what it means. Now if the second person is far away such that person one cannot deliver the letter himself, he will need to trust mediators. You may use the post office or a courier service. Now if the place is far away, one post office will send the letter to second, which will pass it further till it reaches the destination.

The same analogy works for Internet. When you are sending or fetching information on the Internet, it has to pass through many routers and servers.

Why is Internet unsafe?

Is the information in your letter safe when you post it? Yes, but only till a point when a post office worker, or someone on the way opens it. Same is true for Internet.

As the information is passing through so many routers and servers, or data is actually residing on some server, anyone who can gain access can fetch that information. Of course there are safety measures, protocols (SSH/ https) and encryption are commonly used. But any algorithm that can secure the information, will also have a counter-algorithm, that will enable to gain access.

So simply putting it, your data is hundred percent safe till you are on an isolated system, the moment you get connected to a network, someone can access the data (exaggerated? Yes). It will come down to the smartness to person who is trying to save the information vs the person who is trying to access the information

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This answer hits it on the head. What you send through the Internet goes through routers. Another name for "routing" is "forwarding" - and "forwarding" (oversimplified a little) is essentially taking a copy of the packets you're sending and resending them elsewhere. So such routers (that you don't know, have control over, or trust) can read everything you are sending, indeed, that EVERYONE is sending. That's the crux of where the "untrusted" part comes in. –  ultrasawblade Sep 4 '12 at 2:31
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Information that you get from the internet is coming from a specific computer that's... well... it's out there somewhere. You don't know who owns or operates that computer. You don't know who put the information on it, either.

To get from that computer to yours, the information has to travel through several routers along the way. Each router has the opportunity to modify the data passing through it and you don't know who owns or operates the routers.

This is why you can't trust the internet, at least not in the sense of "trust" as used in discussions of security: You could be getting data from a malicious creator, or the data could be getting sent by a malicious server, or the data could have been modified in transit by a malicious router.

Unless you have taken some measures to verify both the identity of the originator (e.g., having the source provide a signed digital certificate) and the integrity of the communication channel (e.g., using an encrypted protocol), you can't really do much more than cross your fingers and hope that what you get will be the same as what you requested.

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Untrusted means the data that travels through it layers is not secured. You never knows what is happening to you data. Anybody can manipulate it.Data can be lost or corrupted during transmission. It may lost its integrity and confidentiality. A man with lot of skills can hack into your data. Usually there are lot of technique through you can secure yourself but still it is prone to be hacked by the hackers.

Internet is also called unsecured because it uses IPv4 protocol which is unreliable and connectionless datagram protocol. It provides no error control and flow control. For the reliablity it is paired with reliable protocol TCP for the transmission of data in transport layer.

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IPv6 is also a unreliable connectionless datagram protocol and TCP doesn't provide any real security –  ratchet freak Jul 11 '12 at 13:39
Yes,TCP does not provide any real security but it provides some sort of reliablity to IP, as tcp is a connection oriented service. and if you talk about IPv6 yes it is also connectionless but it has different header format then IPv4, much longer than IPv4 and some of its fields accommodate encryption and authentication of data for some application that IPv4 does not provide. –  Deb Jul 11 '12 at 16:43
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Because you can't trust everyone

The internet is "everyone in the world with a network connection".

Do you trust everyone in the world with a network connection? Do you want all of them to be able to connect to your company's payroll database?

If not, that's why the internet is "untrusted."

If so, please let us know the IP address so that we can start getting paychecks. ;)

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You can't trust anyone. –  Trufa Jul 11 '12 at 19:49
Except StackExchange ;) –  Gaʀʀʏ Jul 11 '12 at 21:48
Even StackExchange. –  Polynomial Jul 13 '12 at 14:32
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Imagine yourself at home, there is your mom your dad your sister. If one of them would ask you to borrow 100$ from you what would you do? Now image being in a stadium full of people you don't know, and someone asked you 100$ would you react differently?

On the internet there are people who have allot to gain from your identity. They can take control of your computer and clean your bank account. They can use your computer to attack other computers. There are people who use social engineering to scam you. There are people who use trojans and viruses to spread their reach.

Minute you are connected to internet you are making yourself vulnerable to these people.

Default behavior of internet savy users is distrust to everybody and everything. That's why it is considered an untrusted network, cause you never know who is on the other end of the line and what he wants from you.

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Briefly, "trust" in computer security is not quite the same as "trust" in the ordinary sense. It has to be extended to include the concept of identity.

Let's start with the ordinary definition of the word. The OED defines "trust" as "Confidence in or reliance on some quality or attribute of a person or thing, or the truth of a statement". In the pre-Internet era, you might want to send a confidential message to your friend Sally. You could give it to a third party, Bob, if you were confident that he would deliver the message to Sally and to nobody else. In that case you are relying on a particular quality of Bob — his ability to deliver your message discretely. In other words, you trust Bob.

Online, it's possible to fake identity. So "trust" has to be extended beyond reliance on a third party's qualities. It has to include reliance on the the third party's identity. Suppose your trusted third party is the message board at bob.com. In that case, you're relying not just on the discretion of a particular online system, but the assumption that the address bob.com actually points to the system you think it does. But that last assumption is a bad one — there are many ways to and hijack a domain name. If the privacy of your message to Sally is really, really important, you have to make bob.com prove its identity. And that's one of the functions of SSL.

So, we say that the internet in general is untrusted not because we think everybody's out to get us, but because that "trust" online means proving that entities are what they say they are. Trust mechanisms are not built into the internet, because of its origins as an imformal research-based network based on mutual trust — in the plain English sense of the word. Trust — in the computer security sense — has to be layered on.

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+1 for the Alice-Bob-like explanation. For clarity, I'd use Bob for the recipient, and Tim for Third party. (what does the 's' of Sally stand for?) –  ignis Jul 12 '12 at 3:18
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Note that there are basically three things to be concerned about:

  1. Whether the guy at the other end can be trusted
  2. Whether you can trust the connection between you and the other end
  3. Whether someone you don't even know about can connect to your system without your permission

In a fully-trusted network (eg, a network consisting of only the computers in your own household) you have none of these concerns. But get outside of such a limited environment and you need to be concerned.

With an un-firewalled computer connected to an untrusted network, you're exposed on all three counts. The guy at the other end could use the data you send him inappropriately, or he could send malicious data to your system. Even if the guy at the other end is trustworthy, someone with access to the "pipe" could read/manipulate the bits and bytes to extract your private data or send malicious content to either you or the other end. And if someone can connect to your computer without your consent and manipulate it's innards, everything is exposed.

Trusting the other end is of course a matter of judgment on your part. You (hopefully) use some care and don't transact business on sites which you do not have good reason to trust (and never use a debit card to make internet purchases). And you use an anti-virus/firewall setup that will prevent a malicious (or simply hacked) site from installing nasties on your box.

Assuring a good, uncompromised connection to the other end is mainly a matter of using an encrypted protocol. For HTTP, this is generally HTTPS -- a version of the HTTP protocol that adds the SSL "Secure Socket Layer" encryption layer. All reputable sites that deal in private/financial matters should be using the HTTPS protocol (which you will know because the URL prefix is "https:" and because your browser displays a "padlock" icon or wording like "Verified by: VeriSign, Inc." if you hover the mouse cursor over the icon ahead of the URL in the address line). There are other approaches, such as using a VPN (Virtual Private Network), but they're more for business/commercial stuff.

In terms of keeping the bad guys from connecting directly your box, this comes down to having a good firewall. This can either be firewall software in your computer (as a part of the antivirus package, generally) or as a separate hardware box. (This function is often included in wireless routers, for instance.)

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Use of "HTTPS" is optional and not always used for sites that do use ssl. Also, some pages leave most of the page unencrypted, but still use ssl for the data, but I find it very confusing even though it works. –  Joe Jul 18 '12 at 18:23
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Why trust any network, large or small? Trust is elusive, and perhaps it's the worst word to use in this context. When you traverse any network boundary, you need to consider the risk and invoke the necessary mitigation.

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