Tag Archives: how does tracert work

Fundamentals

Protocol fundamentals – Traceroute differences between Windows and Linux

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My last post about Traceroute over here: http://mellowd.co.uk/ccie/?p=609 – got some interesting conversation going on in the comments.

Basically there is quite a big difference in the way in which Windows and Linux handle traceroute. I tested on both Windows 7 and Ubuntu 10.04, but my guess is that all Windows follow the same format as do all *nix’s (please let me know if otherwise though!)

I would recommend reading the above post again quickly to get all the basics out the way before we delve into the differences.

Step-wise, this is what happens on Windows:

  1. The OS send a DNS PTR request to 1.2.2.4.in-addr.arpa to get the hostname for 4.2.2.1
  2. I get a DNS PTR response giving me a hostname
  3. The OS send an ICMP ECHO request with a TTL of 1
  4. I get an ICMP TTL Exceeded packet back from my local router
  5. 3 & 4 above happens twice more
  6. The OS send a DNS PTR request to my local router
  7. My local router responds with it’s hostname
  8. The cycle above (3-7) is then repeated with a TTL of 2, then 3 and so on
  9. We finally get to 4.2.2.1 – which sends back an ICMP ECHO reply – Once I get 3 the job is complete.

Ubuntu does this completely differently though. Step-wise this is what’s going on:

  1. The OS immidiately sent 3 UDP packets with a high port number straight to 4.2.2.1 with a TTL of 1
  2. The local router responded with 3X ICMP TTL Exceeded message
  3. The above (1 & 2) is then repeated until we get to 4.2.2.1
  4. 4.2.2.1 does not generate a ICMP ECHO reply as an ECHO request was not sent. Rather we get 3 ICMP Code 3 (Port unreachable) replies
  5. The OS now throws out 7 DNS PTR request specifically to each IP it determined in the path from above (Including 4.2.2.1 iself!)
  6. As soon as all the replies come, the job is complete.

The main differences are that Windows will send a DNS PTR request from the start, then send ICMP ECHO requests. At each hop it’ll send a DNS PTR request and then move onto the next hop.
Linux starts with sending UDP packets to a high port number straight away. When it finally gets to the last hop it’ll then send out a mass DNS PTR request to every hop in the path that it has determined.

Fundamentals

Protocol fundamentals – Traceroute

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Traceroute is a powerful tool. Extremely useful when checking the path of a packet through the network. But how does it ACTUALLY work? What is REALLY going on?

Layer3 packets all have a TTL. A Time To Live. If a router receives a packet with a TTL of 1 (and the packet is addresses to a host not directly connected to this router) it will drop the packet. It will also then create an ICMP error packet and send it back to the original source of the packet to let it know that the address was unreachable this time.

If you ping another machine, the OS will generally create a TTL of 255 for sent packets, though it doesn’t HAVE to be 255.

traceroute1

Traceroute will force an ICMP error message so it can get more information from each hop in the path.

As an example, let’s run a quick traceroute to 4.2.2.1 and see what it gives us:

C:\Users\Darren>tracert 4.2.2.1

Tracing route to vnsc-pri.sys.gtei.net [4.2.2.1]
over a maximum of 30 hops:

  1   1 ms  <1 ms  <1 ms  DD-WRT [10.50.80.1]
  2   8 ms   9 ms   8 ms  10.3.280.1
  3   8 ms   9 ms  26 ms  walt-cam-1a-ge96.network.virginmedia.net [80.1.170.69]
  4  18 ms   7 ms   8 ms  popl-core-1a-ae2-0.network.virginmedia.net [195.182.175.229]
  5   8 ms   6 ms  16 ms  popl-bb-1a-as2-0.network.virginmedia.net [213.105.174.234]
  6  31 ms  15 ms  15 ms  195.50.91.69
  7  12 ms  13 ms  31 ms  ae-11-51.car1.London1.Level3.net [4.69.139.66]
  8  14 ms  16 ms  22 ms  vnsc-pri.sys.gtei.net [4.2.2.1]

Trace complete.

I’ve loaded up Wireshark to tell me exactly what’s going on. The very first packet sent from my PC was sent with a destination IP of 4.2.2.1 – but with a TTL of only 1.

traceroute2When my router (10.50.80.1) received that packet, it noticed that the TTL was 1, but also that 4.2.2.1 was not directly connected to it. It responded to my PC with an ICMP code 11 – Time-to-live exceeded. It also responded with it’s own source address.

traceroute3

Traceroute now knows that the very first hop to 4.2.2.1 happens to be my local router. It also knows that the routers IP is 10.50.80.1 – It send 3 ECHO reply requests with a TTL of 1. This is the reason you see the 3 values in the traceroute output

It doesn’t stop there though. As soon as traceroute knows that 10.50.80.1 is the first hop, it’ll ask 10.50.80.1 what it’s hostname is via a DNS PTR request. The router will respond to my PC with a DNS PTR response letting it know the hostname. Now traceroute knows what the IP address and hostname is for the first hop. Note that not ALL devices on the internet will respond with a PTR record and so you won’t ALWAYS get a hostname.

Traceroute will now start again and send 3 more packets with a TTL of 2. Exactly the same will happen as above, but for the second hop in the path. This will continue to happen until we eventually get to the host, or we run into the maximum hop count.

traceroute4

When we finally get ICMP echo replies from 4.2.2.1 – traceroute knows it’s job is complete.

traceroute5

btw, traceroute attempts to get PTR records from the devices in the path only when it gets a TTL time exceeded reply. The actual host you are trying to get to is different though. As soon as I ran traceroute 4.2.2.1 it immediately tried to get the PTR record of 4.2.2.1 first. Once it had that it started with all of the above.