How insecure is web?

I hav开发者_开发问答e just started writing socket programs. Came to know that single UDP packet has source port destination port and some MAC address representing router..etc. I wonder why anybody cannot create custom packets with a fake information in and send it over internet. I would like to know how safe are our PCs. What should be done to secure it ?

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There are a couple of different aspects to the answer.

One is that the web relies on TCP, not UDP. Which means that it is connection-oriented. Your package will be rejected, unless it appears to be part of an existing connection (which means, among other things, that it has to have the right source IP and port as well. And it has to have the right sequence number to fit into the receive window). This can still be faked without too much trouble, of course. But it does require you to know a bit about the packets being sent on the original connection.

Another part is that whenever we need to be sure that the sender of a packet is who they claim to be, we use encryption. :)

Most packets don't really need this. It's not a huge deal if someone sends a request to Google which appears to come from my IP. But when making credit card transactions, it becomes a bit more important.

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Most of the TCP/IP stack "leaks trust", as I once put it -- and there isn't much that you, as a software developer (assuming you're looking for a programming solution, otherwise, stackoverflow's the wrong forum, go to serverfault or superuser;-) can do about it -- beyond choosing and carefully implemented protocols that are reasonable in terms of security expectation.

HTTPS (with strong checks of certificates, etc) is one reasonably strong approach; for stronger security, look into SSH and VPN-based approaches. Of course, nobody should assume privacy or strong authentication is in place unless they've taken specific steps towards it (if they HAVE taken such steps, they may be still subject to successful attacks, which is why using existing, more or less "proven" solutions such as HTTPS, SSH, VPNs, is advisable;-).

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Yes, anyone can create packets with whatever data they want and send them out over the internet. Especially with UDP, you can pretend to be anyone you want (unless your ISP does egress filtering). Source addresses for UDP cannot be trusted. Source addresses for TCP can to an extent (you know the data has to be coming from the IP address in question, or someone along the route).

Welcome to the internet :)

Edit: just to clarify egress filtering is something the sending ISP would have to do. As a reciever, there's not really anything you can do to verify the address on a UDP packet without communicating back to the sender. The only reason you can at least partially trust an incoming TCP connection is that TCP requires certain control data flow back to the sender (and hence needs a valid IP address/port to set the connection up and maintain it).

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Well, many many people create invalid packets and send them over Internet; for instance, read Ping of death.

A [completly] secure computer is a computer turned off. To make your running PC more secure from this thread kind, you should rely on firewall softwares/hardwares, which can detect that malformed packets.

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Custom packets with fake information can easily be created. Therefore you have to make sure you're not vulnerable to them.