Writing Stories in Traceroute

2020-10-11 Update

Now a better way exists that doesn't involve a bunch of Docker containers. Please refer to Writing Stories in Traceroute, Elegantly.

Intro

Traceroute is one of the popular tools for network inspection. It shows the IP addresses of routers on the route from your computer to a destination server, similar to:

Domains are shown on the last 2 hops, which is the IP's reverse DNS record. Reverse DNS records exist as PTR records in the format of 4.3.2.1.in-addr.arpa. For more information, you may refer to Setting IP Reverse Records in DN42 (Chinese Only).

However, PTR records need not be real domains. They can be any string that "looks like" a domain. With this, we can write one sentence on each hop of a Traceroute path to make a story:

This story is set up in DN42. If you have already joined DN42, you may ping or traceroute IPs in this post. However, this configuration is not limited to DN42. If you have a public IP block with control on reverse DNS, you may use the same method.

Prepare Routes

The first step is setting up a bunch of routers that forward packets for a specific IP one by one, to create a route long enough to show an essay.

The primitive method is chaining a bunch of physical routers with network cables. But first, I need the bunch of routers; and then I need to connect them to DN42.

On a second thought, Linux has routing capabilities. I can set up some Linux VMs with their own IPs, and execute this command on each of them:

ip route add [Target IP]/32 via [Next Hop IP]

Packets to the destination IP will be forwarded through each VM to create a path.

Next, I need to think of a way to create the VMs. I have ESXi on Kimsufi, and it shouldn't be that resource-intensive to start a bunch of Alpine Linux VMs. But I'd have to do manual configuration for each of them, which is too much of a hassle!

From another perspective, Docker is a management tool for LXC containers, and LXC containers have their independent network namespaces with their own IPs and routes. For my purpose, they are perfect replacements for full Linux VMs.

So I went to create Docker images. All I need is an Alpine image that runs the following script on startup:

#!/bin/sh
echo Target IP is $TARGET_IP
THIS_IP=$(ip addr show dev eth0 | grep inet | cut -d' ' -f6 | cut -d'/' -f1)
echo My IP is $THIS_IP
NEXT_IP=$(echo $THIS_IP | awk -F. '{print $1 "." $2 "." $3 "." $4 + 1}')
if [ $THIS_IP == $NEXT_IP ]; then
    echo I\'m the target, listening
else
    echo Routing $TARGET_IP to $NEXT_IP
    ip route add $TARGET_IP/32 via $NEXT_IP
fi
ping 127.0.0.1 -q

The last ping keeps the container running, or it will exit. The full Dockerfile is available at https://github.com/xddxdd/dockerfiles/tree/25625c20fd1b47b3057cf59b6b84b8401d1b3e1e/dockerfiles/route-next.

Then I need to generate a docker-compose.yml for simpler management. An example is available on the repo above. Or, you can use the mk-compose.py tool to quickly generate a docker-compose.yml, but you would still have to change the network information manually in the file.

Then copy it onto the server, and run docker-compose up -d to start the containers.

Finally, run this command on the Docker host:

ip route add 172.22.76.102/32 via 172.22.76.98

So packets will flow into the first Docker container. Now a Traceroute will show the packet path:

Prepare Essay

Since PTR records are limited to English, I'll need a short English essay. Since it's almost Aug 17, I chose this one:

One should uphold his country』s interest with his life, he should not do things just to pursue his personal gains and he should not evade responsibilities for fear of personal loss.

The path I created has 5 hops, so I split the essay to 5 sentences, and removed punctuations PTR records aren't allowed to have:

• one should uphold his country s interest with his life
• he should not do things
• just to pursue his personal gains
• and he should not evade responsibilities
• for fear of personal loss

The replace spaces with periods:

• one.should.uphold.his.country.s.interest.with.his.life
• he.should.not.do.things
• just.to.pursue.his.personal.gains
• and.he.should.not.evade.responsibilities
• for.fear.of.personal.loss

The fill it to PTR records for the IPs on the path:

Save and wait for DNS updates to propagate:

You will see your small essay in Traceroute.