I'm trying to send messages between multiple computers on different networks via HTTP.
From my understanding, a computer has an internal IP for their home network, and an external IP for their router.
To send my HTTP packet from one computer to another, I assume it makes at least three hops:
From my device to my router
From my router to another router with a given external IP
From that router to the device with a given internal IP
And that's given there aren't any other subnetworks in the way.
My assumption, therefore, is that to contact such a device, you need a list of at least one IP address. But from what I've seen from tools like cURL or ping, that's not how it works.
What am I misunderstanding, and how do I go about sending a packet to such a device with cURL?
Related
I just started learning about networking and IP/MAC addresses. I understand if you want to send data to another device within the same network, you would sent out an ARP broadcast to the local network and the target device will respond with its mac address so they can start a connection.
My question is that does public IP also work the same way? Like if my home router wants to connect to the google.com IP, does it send out a ARP broadcast to the entire internet and wait for google to respond with its mac address?
Usually your gateway router will arp for the next hop router along the path to the destination, this path is often the default route (cisco calls this "the gateway of last resort"; the "default route" in cisco-ese is the default route only if routing is disable in the router which typically only happens if an image is not loaded so it is boot strapping). But sometimes your gateway will only have an out going interface, not a next hop ip address. This is fine for serial links since there can only be one next hop. But for ethernet, this causes your gateway and the next hop router to use proxy arp. There are other way to do this as well.
I'm writting a simple network sniffer that should be able to reconstruct network structure.
When an interface has set up a DHCP, I can easily read interface settings such as client IP address, subnet mask, DNS server etc. by catching a DHCP packet and analysing it.
When an interface has a static IP, I'm catching ARP Announcement packet to get static IP address and then ARP request from the gateway, to get geteway IP address. I'm also saving MAC addresses.
My problem is: how to get subnet mask from one or more static IPs in the network and the gateway address. Or by caching some packets. I didn't see packets that could have such informations.
I also need DNS address, but it's less important.
The program should work in OpenWRT (C++).
My problem is: how to get subnet mask from one or more static IPs in the network and the gateway address.
Possibly, you can't.
If the sniffed network uses DHCP then you can monitor the DHCP requests (which should be broadcast) for their subnet mask and router fields which mirror the server's offer.
Without DHCP, all you can do is take an educated guess. If your passive sniffer registers broadcasts from addresses 192.168.1.1 through 192.168.1.29, you know that the prefix length is at most /27. It could also be anything shorter, down to /16, with potential addresses being (currently) absent or silent. The prefix could be even short than /16 if the network admin is ignoring RFC 1918. With public addresses you're mostly on your own.
If you can scan actively you could send ARP requests and see which ones get answered - you'd also see nodes that don't originate any traffic/broadcasts.
The gateway is also just a guess. In a network with mostly Internet-bound traffic, the default gateway is most likely the one being ARPed most often. If the network traffic is mostly server-centric, ARP requests for their addresses outnumber the ones for any gateway.
Your sniffer is severely limited when it is just attached to a switch and listening to broadcast packets only. If the sniffer manages to listen to all traffic on the network (via a monitoring/mirroring switch port) then you can easily identify the gateway by its MAC address that packets for arbitrary IP addresses is sent to and vice versa.
As above, if you can actively send probe packets you could test the gateway(s) with packets that they accept (and hopefully forward) and which ones they reject.
That's essentially my question. Isn't the network portion in a computer's IP address so that, when it is sent, other computers can look at that network portion and know where to send it back to? So why do routers have their own IP address?
The router needs to be a node on the same network as the computer using it. When your PC tries to communicate with a system on a different network, it consults it's routing table to figure out which router (there can be several) has the route to the destination. Without an IP on the router, there would be no way to send packets to the router, and thus no way to get out of your network.
I suppose the IP protocol could have been designed to use broadcasts to find the route out, but that would have caused issues with traffic congestion. Thankfully it wasn't designed like that.
I recently found that packets are encapsulated within ethernet frames. Packets use IP addresses, frames use MAC addresses.
Why aren't IP addresses used in ethernet frames for routing? I understand that when trying to access a basic website, the computer goes to a DNS to find the IP address relevant to the user-entered domain name. How do computers find the correct MAC address?
Really, how are MAC addresses used in routing internet traffic?
Thanks
IP packets aren't always encapsulated in Ethernet frames. There are other physical media such as ISDN, etc. When packets are routed, IP addresses are used to determine the next hop and the physical address is used to physically identify the interface serving as the next hop. Only the former (determining next-hop) is usually called routing.
To answer your second part, MAC addresses are discovered through ARP (Address Resolution Protocol) in IPv4 & ND6 (Neighbor Discovery) in IPv6.
Update:
The destination IP address in the IP header is the final destination. In the process of routing (at each hop), you get the next hop's IP address to (eventually) reach the final destination from the routing table (this could be a default gateway's IP address). To send the packet to the next hop, you need its MAC address. While hopping through intermediate links, the IP address in the IP header don't change - only the MAC addresses change.
Bit late but still here is my answer :) ...
To send data you need two address, the MAC address and the IP address.
Basically the sending host will ARP for a MAC address, this occurs when the local host doesn't know the MAC address of the host it has an IP address for or it will ARP for the default gateway MAC address (if it doesn't already know it) if the IP address in on a different subnet/ network. Once it obtains a MAC address the IP packet is encapsulated in a L2 frame and sent across the media. If the IP packet is meant for a host on a different subnet/ network, it will be sent to the default gateway, this router will de-encapsulate the L2 frame (remove and discard it) check the IP address and will forward it. For the router to do this it needs a MAC address to send it over the media, It will look up the next hop in it's routing table, encapsulate the IP packet with the same source and destination IP address that was sent from the original host into a new L2 frame. This time the MAC address for the source address will be that of the forwarding interface of the router, and the receiving interface of the next hop will be the destination MAC address. This will continue from hop to hop until it reaches the final host, each time the MAC addresses will change, but the original IP address will remain the same.
Here's the key point -- there can be more types of packets than INTERNET traffic. You could be using IPX, which is non-routable. How do clients identify each other? By the MAC address.
Routing != Addressing, which is really where the MAC comes into play.
In order to be routed, the OSI model adds a layer to allow for path discovery to the next gateway. This layer is responsible for routing, but knows nothing about the MAC address.
As a side note, at the hardware level, MAC addresses ARE used by switches, but not for routing. From How Stuff Works:
The switch gets the first packet of data from Node A. It reads the MAC
address and saves it to the lookup table for Segment A. The switch now
knows where to find Node A anytime a packet is addressed to it. This
process is called learning.
In this way, a switch can make sure that traffic is only outputted to the correct port. This isn't accomplishing routing so much as reducing network congestion. Only broadcasts and traffic destined specifically for that MAC address should be sent out the port.
Recently I have been thinking about the same and came upon this question. Here is my answer to this question. Actually MAC address is needed for correctly sending the packet to right destination. This is specially true when packet is needed to sent over a VLAN. There can be multiple switches/routes connected on that VLAN over multiple physical interfaces. However IP Routing is unaware of these physical interface. It only knows about the logical connectivity. For example, route 10.10.10.0/24 is reachable via VE/VIF0.10(logical VLAN interface) and/or nexthop neighbor is 20.20.20.1. There could be multiple interfaces under VLAN 10. Then to which interface packet is sent out? This is where ARP comes in the picture. ARP helps to discover the MAC address associated with the next-hop IP address. When switch/router learns the nexthop MAC. along with that it learns the physical interface also via which that MAC is reachable. Hence while routing packet, firstly MAC corresponding to the destination IP is searched and then the physical interface associated with that MAC is searched. Finally packet is sent out via that physical interface. The MAC corresponding to that destination IP is used as destination MAC. In absence of this, routed packets will always be flooded in the outgoing VLAN.
Hope this helps.
Thanks.
Answer: MAC addresses are not used in the process of routing of a packet.
segment -> transport layer (TCP ports)
packets -> network layer (IP addresses)
frame -> data link layer (MAC addresses)
bits -> physical layer (electric/optical signals)
Create your own packet/segment visit http://wirefloss.com/wireit/
There are 2 models (TCP/IP and ISO/OSI)
In detail:
Your app has some data. This is encapsulated by mentioned layers. Encapsulation means that a header with fields is added at each layer. If your data never leave the local network the MAC address will be the same. Once your data needs to be delivered outside your network the frame header is stripped by router and is replaced by router fields.
UPDATE 2021: Some people seems never heard of ISO OSI model and put this answer as incorrect.
I'm trying to learn the basics of ARP/TCP/HTTP (in sort of a scatter-shot way).
As an example, what happens when I go to google.com and do a search?
My understanding so far:
For my machine to communicate with others (the gateway in this case),
it may need to do an ARP Broadcast (if it doesn't already have the
MAC address in the ARP cache)
It then needs to resolve google.com's IP address. It does this by
contacting the DNS server. (I'm not completely sure how it knows
where the DNS server is? Or is it the gateway that knows?)
This involves communication through the TCP protocol since HTTP is
built on it (TCP handshake: SYN, SYN/ACK, ACK, then requests for
content, then RST, RST/ACK, ACK)
To actually load a webpage, the browser gets the index.html, parses
it, then sends more requests based on what it needs? (images,etc)
And finally, to do the actual google search, I don't understand how
the browser knows to communicate "I typed something in the search box
and hit Enter".
Does this seem about right? / Did I get anything wrong or leave out anything crucial?
Firstly try to understand that your home router is two devices: a switch and a router.
Focus on these facts:
The switch connects all the devices in your LAN together(including the router).
The router merely connects your switch(LAN) with the ISP(WAN).
Your LAN is essentially an Ethernet network which works with MAC addresses.
For my machine to communicate with others (the gateway in this case),
it may need to do an ARP Broadcast (if it doesn't already have the MAC
address in the ARP cache)
Correct.
When you want to send a file from your dekstop to your laptop, you do not want to go through the router. You want to go through the switch, as that is faster(lower layer). However you only know the IP of the laptop in your network. For that reason you need to get its MAC address. That's where ARP kicks in.
In this case you would broadcast the ARP request in the LAN until someone responds to you. This could be the router or any other device connected to the switch.
It then needs to resolve google.com's IP address. It does this by
contacting the DNS server. (I'm not completely sure how it knows where
the DNS server is? Or is it the gateway that knows?)
If you use DHCP, then that has already provided you with the IP of the DNS server. If not, then it means that you manually provided the IP of the DNS. So the IP of the DNS server is stored locally on your computer.
Making a DNS request is just about putting its IP in the packet with the request and forwarding the packet to the network.
Sidenote: DHCP also provides the IP address of the router.
This involves communication through the TCP protocol since HTTP is
built on it (TCP handshake: SYN, SYN/ACK, ACK, then requests for
content, then RST, RST/ACK, ACK)
Yes. To clarify things: When your computer sends the request
FRAME[IP[TCP[GET www.google.com]]]
The frame is being sent to your LAN's switch which forwards it to the MAC of the router. Your router will open the frame to check the destination IP and route it accordingly(in this case to the WAN). Finally when the frame arrives at the server, the server will open the TCP segment and read the payload, which is the HTTP message. The ACK/SYN etc. messages are being processed just by your computer and the server and not any router or switch.
To actually load a webpage, the browser gets the index.html, parses
it, then sends more requests based on what it needs? (images,etc)
Yes. An HTML file is essentially a tree structure which can have embedded resources like images, javafiles, CSS etc. For each such resource a new request has to be sent.
Once your browser gets all these recourses, it will render the webpage.
And finally, to do the actual google search, I don't understand how
the browser knows to communicate "I typed something in the search box
and hit Enter".
When you type a single character, it is being sent to the server. The server then responds with its suggestions. Easy as that.
References(good reads):
http://www.tcpipguide.com/free/t_TheNeedForAddressResolution.htm
http://www.howtogeek.com/99001/htg-explains-routers-and-switches/
http://www.eventhelix.com/realtimemantra/networking/ip_routing.htm#.UsrYAvim3yO
http://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol