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Douxx

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23 Strangers Standing Between You and This Article

#linux #beginners #networking #tutorial

You clicked this link. Quite simple, right? But before these words appeared in your browser, they went on a little journey, hopping through routers, data centers, and cables you'll never see, operated by people you'll never meet.

And you know what? You can see every one of those stops, and even trace the full path from your machine all the way to mine, or any server, really.

The tool that lets you observe your route through the constellation of routers called the internet is traceroute (or tracert on Windows, I guess they wanted to be original).

Let's run a traceroute:

› tracert 152.53.236.228

Tracing route to theserver.life [152.53.236.228]
over a maximum of 30 hops:

  1     3 ms     1 ms     1 ms  192.168.1.1
  2    20 ms    18 ms    11 ms  77-56-216-1.dclient.hispeed.ch [77.56.216.1]
  3    15 ms    14 ms    13 ms  217-168-61-145.static.cablecom.ch [217.168.61.145]
  4    48 ms    41 ms    27 ms  carbsm101-be-2.aorta.net [84.116.211.21]
  5    15 ms    15 ms    13 ms  ch-otf01b-rc2-ae-54-0.aorta.net [84.116.202.225]
  6    25 ms    13 ms    18 ms  zur01lsr01.ae1.bb.sunrise.net [212.161.150.164]
  7     *        *        *     Request timed out.
  8    39 ms    54 ms    14 ms  213.46.171.182
  9    23 ms    20 ms    20 ms  ae2-2015.nbg60.core-backbone.com [80.255.15.250]
 10    23 ms    22 ms    24 ms  ae12-500.nbg40.core-backbone.com [80.255.9.21]
 11    36 ms    24 ms    45 ms  theserver.life [152.53.236.228]

Trace complete.
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Looks like gibberish, right?

But actually, it's easy to decipher. Let's analyze our data's journey:

Each line is a stop. Let's walk through the journey.

  • The first one, 192.168.1.1, is my own router, still in my living room. The first stranger is actually myself.
  • Hops 2 and 3 are my ISP: the entrance to the highway. You can even see it in the hostnames: cablecom.ch, a Swiss internet provider, handing my data off to the wider world.
  • Hops 4 through 6 are that highway. aorta.net, sunrise.net are transit backbones you've probably never heard of, but your data uses constantly, every single day.
  • Hop 7 is a ghost. Three * instead of a response. Looks like someone doesn't want to be seen. We'll come back to that.
  • Hop 8 is another silent one, no hostname, just a raw IP. Not hiding, but not introducing itself either.
  • And then 9 and 10 are another backbone, core-backbone.com, and if you squint at the hostname you can see nbg: Nuremberg, Germany. My data just crossed a border.
  • 11 is home, well, my home. The server.

Your output will look different: different ISP, different city, maybe even different countries in between. But the story is the same: a chain of strangers, passing your data along.

So in 11 hops, my request crossed my living room, my ISP, some of the thousand of internet backbones, crossed Germany, and landed on my server. All this in about 40 milliseconds.

But how does traceroute even know all this? Well, it shouldn't. It's exploiting a small feature built into every router on the internet, originally designed to prevent flooding the network.

Internet's Structure

The internet works a lot like the postal system. When you send a letter abroad, your local postman doesn't know the way to Germany, he just drops it at the sorting center. The sorting center sends it to the national hub. The national hub hands it to an international carrier. Nobody has the full map. Everyone just knows their next step.

Your request leaves your home, climbs through your ISP, then through bigger and bigger backbone networks, like aorta.net or core-backbone.com from our traceroute, until it reaches the destination network, and works its way down to the target server. Each of those networks is owned by a different company, and they all just agreed to hand traffic to each other.

All of this is coordinated by Border Gateway Protocol, a fascinating rabbit hole for another day. (Finish this article first.)

How Traceroute Exploits This Network

What traceroute actually does is map every step your packet takes through the internet, and it does it by exploiting a feature that was never meant for this.

To do that, it diverts the original purpose of TTL (Time To Live): as defined in the first IP spec (RFC 791), its intent was to "kill stale packets before they clog the network forever".

Every IP packet has a TTL field that gets decremented each time it crosses a new router (transit point). When it reaches 0, the packet gets destroyed and the router that killed it warns the sender about it.

You might already see the trick coming: by setting the TTL to 1, we can get the first router to drop the packet, and tell us it did. That gives us the first router's IP. Then we just repeat, incrementing the TTL each time to peel back one more hop.

We keep going until the destination itself replies with either ICMP_ECHOREPLY or ICMP_DEST_UNREACH depending on the implementation, and that's our signal to stop.

Here's the core loop in C, if you're curious:

for (int ttl = 1; ttl <= MAX_HOPS; ttl++)
    {
        // Set the TTL to the current iter ttl
        setsockopt(send_sock, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));

        char buf[BUF_SIZE];
        struct sockaddr_in from;
        socklen_t from_len = sizeof(from);

        // Ping the target router 3 times to get the 3 delays
        for (int i = 0; i < PROBE_COUNT; i++)
            ms[i] = ping(send_sock, recv_sock, buf, &dest, &from, &from_len);

        /* print stats
           Skipped it for this example
        */

        if (icmp_hdr->type == ICMP_ECHOREPLY && from.sin_addr.s_addr == dest.sin_addr.s_addr)
            break;
    }
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The full C code can be found on this gist.

The Ghosts, and Other Lies

Remember hop 7, the one that went silent? And hop 8, the no-name one?

They're not broken, they just don't want to be seen.

Some routers are configured to drop ICMP packets (the ones traceroute uses to probe the network). They still forward the traffic just fine, but they don't want to play traceroute's little spy game. Others, like hop 8, simply don't have a reverse DNS record.

And it gets worse. The route we just saw? It might not exist anymore. Run two traceroutes with a one-hour difference and you might see completely different routes, maybe even different countries. The internet reroutes itself constantly, reacting to routing metrics, failures, and countless other factors. There's no fixed path, but at least there will always be a path.

Even the timings can lie. See how hop 4 shows 48 ms while hop 9 shows only 23 ms? A further hop that is faster than a closer one?! That's because routers prioritize "real" traffic over responding to ICMP probes. The latency numbers tell you something, but never everything.

Remember that traceroute is a window into the internet, but not a clear one.

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