I want to trace packets as they move from my host to the destination - something like 'traceroute'. However, I need to know what to what port the packets are accepted at each of the network nodes while 'traceroute' gives me only the node.
Is this possible ? What tools should I use ?
By network nodes, I assume you mean router in the path. The question makes no sense since ports are transport layer addresses, but routing happens with network layer addresses.
When network traffic is routed, a router will strip off the layer-2 frame and inspect the layer-3 destination address (IP, IPX, etc.) in the layer-3 packet. The router never sees the layer-4 segment, so it knows nothing about the layer-4 address (TCP, UPD, SPX, etc. port). The router will switch the layer-3 packet to the next interface and encapsulate it in a new layer-2 frame for that interface, before sending it on its way.
Related
Actually i have three Questions in this regard
IP ADDRESS:- Does router and default gateway same? if same , then why router (internet IP) address is public and Gateway IP address is local (i mean why both are different)
MAC ADDRESS:- what is the difference between WAN MAC address and ROUTER MAC address and Default Gateway MAC address , why are they different?
Difference between Data packets and IP packets? are data packets travel from a host to a remote server present in some other country through submarine cables or through router to router by hop-by-hop transport?
I am a new learner , please don't get angry if i asked something silly
Router and gateway are essentially the same - a gateway provides access to remote subnets. The hardware for that is called router. Routers route between networks.
A default gateway is a gateway to provide access to all subnets that are not specifically configured. At a minimum, a host requires a default gateway to connect to the Internet.
IP addresses are configured on (logical) interfaces. A router usually requires at least two interfaces, each configured with an IP address from the connected subnet.
Likewise, a MAC address is required for a physical interface - specifically for the data link layer to work (OSI layer 2). It uniquely identifies interfaces within a network segment and is used by network switches for forwarding.
A data packet can be many things. An IP packet is a packet (also protocol data unit PDU) on the network layer (OSI layer 3). IP packets travel between hosts around the globe and transport user data (payload) between them.
User data is actually transported by an application-layer protocol (e.g. HTTP) that in turn uses a transport-layer protocol (e.g. TCP) between processes/applications. The transport layer uses the network layer (IPv4 or IPv6) which in turn uses the data link layer (e.g. Ethernet) which in turn uses the physical layer (physical interfaces and cables).
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.
I am trying to understand the functioning of the different layers in the TCP/IP stack, and I just wanted some clarification on how the link layer derives the MAC address of NICs to receive packets.
This isn't a function of TCP/IP, per se. Instead, the ARP (Address Resolution Protocol) is used in IPv4 to translate the destination IP address to the correct MAC address.
This is accomplished by the stack by first determining if the delivery is local (within the subnet) by comparing the destination to the configured network mask. If it is local, ARP will be used to generate broadcast frames at the link layer, attempting to resolve the known IP address to the known MAC address.
On the other hand, if the destination IP address is determined not to be on the local subnet, the ARP protocol will be used to send a broadcast ARP at the link layer to discover the MAC address of the router that should be used based on the configured routing table.
Using IPv6, ARP is eliminated and replaced with multicast (more specifically, solicited node multicast) using the Neighbor Discovery Protocol over ICMP6.
When we establish a TCP connection from PC1 to Server and send data through this connection, how does the Router know to which of the two PCs (PC1 and PC2) should it communicate on the way back from Server to PC1?
And
How does ping (ICMP) know to which internal node it should send the answer?
NAT (Network Address Translation) is stateful. It creates a translation table that has the layer-3 and layer-4 protocols and addresses. By looking up the return traffic addresses in the translation table, the NAT process can determine which inside addresses should be placed in the packet.
Edit:
Per the edit to your question, asking about ICMP (it is very bad form to change the question in order to ask a different question because it can invalidate the perfectly acceptable answers already given):
It is all the same as TCP or UDP. NAT creates a state table that is dynamically built as traffic passes from inside to outside. NAPT will allow you to overload a single IP address with traffic from multiple inside addresses, and it will translate the return traffic by looking up where to send it in its state table.
With NAPT, besides looking at and translating the IP address, NAPT looks at the layer-4 protocol (TCP, UDP, ICMP) and translates the layer-4 addresses (TCP or UDP port numbers or ICMP identifies), too, storing the translations in its translation table. When return traffic is destined to a particular layer-3 and layer-4 address combination, from a particular layer-3 and layer-4 address combination, the NAPT process finds that in its translation table, and it can see where to send the traffic on the inside.
NAT is very resource intensive, and it breaks the IP paradigm of end-to-end connectivity, where every host is uniquely identified by its own IP address, which is why it is called a kludge (or worse). NAT was developed to extend the life of IPv4 until IPv6, with its nearly unlimited addressing, can become ubiquitous.
RFC 2663, IP Network Address Translator (NAT) Terminology and Considerations:
4.1.2. Network Address Port Translation (NAPT)
NAPT extends the notion of translation one step further by also
translating transport identifier (e.g., TCP and UDP port numbers, ICMP
query identifiers). This allows the transport identifiers of a number
of private hosts to be multiplexed into the transport identifiers of a
single external address. NAPT allows a set of hosts to share a single
external address. Note that NAPT can be combined with Basic NAT so
that a pool of external addresses are used in conjunction with port
translation.
For packets outbound from the private network, NAPT would translate
the source IP address, source transport identifier and related fields
such as IP, TCP, UDP and ICMP header checksums. Transport identifier
can be one of TCP/UDP port or ICMP query ID. For inbound packets, the
destination IP address, destination transport identifier and the IP
and transport header checksums are translated.
A NAPT router in figure 2 may be configured to translate sessions
originated from N-Pri into a single external address, say Addr-i.
Very often, the external interface address Addr-Nx of NAPT router is
used as the address to map N-Pri to.
There is a large pool of resources describing NAT (Network Address Translation), which is available if you search "nat explained". A great resource is What is NAT and how it work tutorial.
The most important detail is that commonly we use NAPT (commonly used as PAT - Port Address Translation) (Network Address and Port Translation), alongside NAT.
When a device needs to use the Internet, it must open a local (source port) and send the IP request to the other end. For example, a notepad with source IP address 192.168.1.2 needs to communicate with a web server at 216.58.212.35.
It fires up random source port 1234 and requests information from target port 80 (HTTP - Web page).
This goes through the networks NAT device, which stores the information 192.168.1.2/1234 with the next information that it computes, and sends the request as 46.103.93.105 (its own IP public IP) and a new source port, for example 2345.
The web server responds to the NAT device, which in turn finds the correlated information (source port 2345 targets 192.168.1.2/1234). The notepad receives the information and displays it to the user.
The router adds information to the request header sent to the server that allows it to look up the sender when the reply is received. This is usually accomplished by using a table stored in the router's memory that maps the PC's address to the token added to the header.
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.