sherubthakur / rustyroute

a traceroute clone

RustyRoute

traceroute and tracert are computer network diagnostic commands for displaying possible routes (paths) and measuring transit delays of packets across an Internet Protocol (IP) network. The history of the route is recorded as the round-trip times of the packets received from each successive host (remote node) in the route (path); the sum of the mean times in each hop is a measure of the total time spent to establish the connection. This a toy-clone of that application.

Requirements

• Understanding of IP
• Understanding of ICMP

Theory

Let's start by diving into some netorking theory.

IP header has a field ttl (Time to live). It helps to prevent a packet from looping forever. How?

From the RFC -

Time to Live Time to live in seconds; as this field is decremented at each machine in which the datagram is processed, the value in this field should be at least as great as the number of gateways which this datagram will traverse.

The key point to note is that this field is decremented at each machine in which the datagram is processed. i.e. every router, etc along the path will decrement this field by 1.

What happens to a packet whose ttl reaches 0?

When a Router sees a packet whose ttl is expired (i.e. the ttl count has reached 0)

• the router discards the packet
• sends back an ICMP message with TTL_EXPIRED back to the host
  • Note: we can extract the details of the router that rejects the packet from TTL_EXPIRED message that we receive

This gives us the necessary insight required to make something like traceroute. What if we start with a ttl value of 1 and increase it till we reach a stopping condition (discussed in the next section). Each packet will be rejected by the next router in path (compared to the previous one). Thus helping us trace the whole path.

How to stop?

There are various ways one can decide to stop this process.

Check source address of ICMP response is same as destionation

We check if the source address of the ICMP response that we receive is same as the destination that we want to reach. If that is indeed the case then we can stop.

Using UDP packets

This approach relies what happens when you send a UDP packet to an incorrect port number. When this case arises, the destination host -

• discards the packet
• sends back a PORT_UNREACHABLE ICMP packet

High level flow

1. Sets the value of the TTL=1
2. Create an IP packet with the ttl value
   • dosen't matter what the contents of this packet are
3. Start a timer
4. Send the packet out
   • The first router sees this packet decrements the TTL field sees that it is zero, consequently discards the packet and sends the TTL_EXPIRED message with it's address in the SOURCE_ADDRESS field.
5. Upon receiving the packet validates it
6. Read the SOURCE_ADDRESS
7. stop the timer
8. Calculate the elpased time
9. Disply this info to the user
10. Increment the TTL
11. Repeat steps 2 to 9 again and again untile stopping condition is triggered

Execution

./run.sh --url=<ipv4 address of the destination>

Note: In order to keep the code simple

• we don't do dns resolution so it expects an ipv4 address and not a url
• no reverse dns resolution is done for the ip addresses that we recevieve from each hop
• only one packet is sent out and not 3 like the original traceroute does