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I want to understand http protocol in depth. Does anyone know where can I find relevant material for HTTP. A few questions that I have in particular are:
Why does when press enter after typing for eg. "google.com" it gets translated to "http://www.google.com" and not "ftp://www.google.com" or something else.
How does the http request gets transferred to a server, what routing algorithm it uses to decide which server to used, and inside that server where to locate the uniform resource".
Any pointers in this direction are most welcome!!
Because HTTP is simply the web standard for requests. We all agreed on using that when the web started, and now we are stuck with it (whether it is good or not).
HTTP stands for 'Hypertext Transfer Protocol' and is in fact the way how to exchange that information you see on your display when browsing.
FTP, and numerous other protocols are their standards (or in general methods) for their purpose. HTTP serves the purpose of communicating between client and server for the web.
And about how HTTP works internally, well, they wrote a few books on it. The standard is described by the IETF: Hypertext Transfer Protocol -- HTTP/1.1.
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I am confused as to what the statelessness of HTTP means in terms of the client.
I get that stateless-ness means the server does not store any state information about the client but does the client still remember the identities of the servers it has interacted with, assuming cookies are not used?
The “remembering” logic you are talking about would have to be added to either the server, or the client, or both. HTTP is just a protocol, not an implementation, and that protocol is stateless because it doesn’t define any remembering logic as part of its specification
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I am studying networking and operating systems and I stumbled upon this statement:
If there is no default gateway in the routing table we can't transfer packets to all the addresses.
I am asking whether the above statement is correct?
Yes the statement is true. Here is the definition from wikipedia
A default gateway in computer networking is the node that is assumed
to know how to forward packets on to other networks. Typically, in a
TCP/IP network, nodes such as servers, workstations and network
devices each have a defined default route setting, (pointing to the
default gateway), defining where to send packets for IP addresses for
which they can determine no specific route.
A routing table can not store information of all the possible routes to which the computer may connect. So it needs a default mechanism to send traffic to computers for which no known path exists. The way is to route all such packets to default gateway.
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Suppose my Skype integration wants to filter notifications for a user to messages that contain the string "server down". Is this possible?
Per default Skype for Business (=SfB) can only block URL or attachments as written here. Maybe there exist an 3rd party application which can do that, but I do not think so. Reason: The problem here is that SfB can perform peer2peer connections (see more here). That means if you and your workmade are in the same network and no firewall restricts you both and you both perform a chat together, the chat is done between both of you (without a real server connection; for more infos see the official documentation inside the 2nd link). So a 3rd party software (which might sitting on the server) will not capture that kind of information. So what you might need is a client software on every PC. But what about the mobile clients then? So I do not think that such a software exists or would be possible to build.
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EDIT: i would really appreciate a source or an example that talks about this being done.
Some companies have a distributed program written in C++, it runs on different machines, it uses TCP/IP for inter communication between different machines (windows and linux). Is it possible to write a program from the ground up using other languages (python, java) that will be able to join their network?
Yes. Of course, TCP/IP is language agnostic. The key issue is to get the protocol correct.
If you have the specification for what is being communicated (or can reverse-engineer it), then yes. Without knowing what to send, and what to expect back it'll be a losing battle.
Of course! That's why it is a protocol: you don't have to know who is on the other side, as long as it follows the protocol specification
Based on edit:
This info is anywhere, based on the language you want to use. Here is a basic python example for tcp communication
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I've noticed that my employer blocks outgoing traffic on a few problematic ports, such as SSH. It looks like a timeout to every application being blocked. Why isn't this implemented by refusing the connection? Is this simply that the SYN doesn't make it to the destination? I'm trying to make a list of ports that I am sure are blocked and I'm thinking perhaps I can just port scan a known host outside of the network, such as my VPS.
Are these statements true for most implementations?
If the connection is refused or accepted, then the port is open
through the firewall.
If the connection times out but the host
certainly exists and doesn't have any kernel-level features turned on
to make it look like it doesn't exist, then the firewall is blocking
it.
Your firewall may block/allow depending on IP address as well, so the port alone doesn't say anything.
If the connection is accepted, it doesn't mean the firewall isn't blocking, it might just mean the firewall redirects it elsewhere. For example, redirect all traffic to port 80 outside your organization to some "you can't get there from here" webpage.
If the connection times out, it may have lots of reasons, one of them being the firewall, but it might also be your DSL line is down, or routing is misconfigured somewhere, or just about anything that can go wrong on the network.
Even if the connection is accepted AND connects the correct target (your VPS), it might have been redirected to a transparent proxy.
Think twice about the port scan. If the network people of your company manage their network well, they will detect the port scan, and you'll have to answer some embarrasing questions to them.
It's a security consideration. Sending a reset (refusing the connection) indicates that the resource exists, which is itself an information leak. Sending nothing reveals nothing, leaks nothing: it is indistinguishable from the case where the entire host doesn't exist. There was one firewall product in the 1990s that sent resets, which was considered poor practice.
I'm away from my library at the moment but I'll provide more details on Monday.
You can profile what outbound ports are blocked with Firebind.
Check out scanme.firebind.com
It uses a custom client/server solution to specifically enumerate firewall rules.
Dave