Present Perfect

After last week's Linode incident I was getting a bit more worried about security than usual. That coincided with the fact that I found I couldn't run puppet on one of my linodes, and some digging turned up that it was because /tmp was owned by uid:gid 1000:1000. Since I didn't know the details of the breakin (and I hadn't slept more than 4 hours for two nights, one of which involving a Flumotion DVB problem), I had no choice but to be paranoid about it. And it took me a good half hour to realize that I had inflicted this problem on myself - a botched rsync command (rsync arv . root@somehost:/tmp).

So I wasn't hacked, but I still felt I needed to tighten security a bit. So I thought I'd go with something simple to deploy using puppet - port knocking.

Now, that would be pretty easy to do if I just deployed firewall rules in a single set. But I started deploying firewall rules using the puppetlabs firewall module, which allows me to group rules per service. So that's the direction I wanted to head off into.

On saturday, I worked on remembering enough iptables to actually understand how port knocking works in a firewall. Among other things, I realized that our current port knocking is not ideal - it uses only two ports. They're in descending order, so usually they would not be triggered by a normal port scan, but they would be triggered by one in reverse order. That is probably why most sources recommend using three ports, where the third port is between the first two, so they're out of order.

So I wanted to start by getting the rules right, and understanding them. I started with this post, and found a few problems in it that I managed to work out. The fixed version is this:

UPLINK="p21p1"
#
# Comma seperated list of ports to protect with no spaces.
SERVICES="22,3306"
#
# Location of iptables command
IPTABLES='/sbin/iptables'

# close everything else
${IPTABLES} -A INPUT -j REJECT --reject-with icmp-port-unreachable


And it gives me this iptables state:
71242

So the next step was to reproduce these rules using puppet firewall rules.

Immediately I ran into the first problem - we need to add new chains, and there doesn't seem to be a way to do that in the firewall resource. At the same time, it uses the recent iptables module, and none of that is implemented either. I spent a bunch of hours trying to add this, but since I don't really know Ruby and I've only started using Puppet for real in the last two weeks, that wasn't working out well. So then I thought, why not look in the bug tracker and see if anyone else tried to do this ? I ask my chains question on IRC, while I find a ticket about recent support. A minute later danblack replies on IRC with a link to a branch that supports creating chains - the same person that made the recent branch.

This must be a sign - the same person helping me with my problem in two different ways, with two branches? Today will be a git-merging to-the-death hacking session, fueled by the leftovers of yesterday's mexicaganza leftovers.

I start with the branch that lets you create chains, which works well enough, bar some documentation issues. I create a new branch and merge this one on, ending up in a clean rebase.

Next is the recent branch. I merge that one on. I choose to merge in this case, because I hope it will be easier to make the fixes needed in both branches, but still pull everything together on my portknock branch, and merge in updates every time.

This branch has more issues - rake test doesn't even pass. So I start digging through the failing testcases, adding print debugs and learning just enough ruby to be dangerous.

I slowly get better at fixing bugs. I create minimal .pp files in my /etc/puppet/manifests so I can test just one rule with e.g. puppet apply manifests/recent.pp

The firewall module hinges around being able to convert a rule to a hash as expressed in puppet, and back again, so that puppet can know that a rule is already present and does not need to be executed. I add a conversion unit test for each of the features that tests these basic operations, but I end up actually fixing the bugs by sprinkling print's and testing with a single apply.

I learn to do service iptables restart; service iptables stop to reset my firewall and start cleanly. It takes me a while to realize when I botched the firewall so that I can't even google (in my case, forgetting to have -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
) - not helped by the fact that for the last two weeks the network on my home desktop is really flaky, and simply stops working after some activity, forcing me to restart NetworkManager and reload network modules.

I start getting an intuition for how puppet's basic resource model works. For example, if a second puppet run produces output, something's wrong. I end up fixing lots of parsing bugs because of that - once I notice that a run tells me something like

notice: /Firewall[999 drop all other requests]/chain: chain changed '-p' to 'INPUT'
notice: Firewall[999 drop all other requests](provider=iptables): Properties changed - updating rule

I know that, even though the result seems to work, I have some parsing bug, and I can attack that bug by adding another unit test and adding more prints for a simple rule.

I learn that, even though the run may seem clean, if the module didn't figure out that it already had a rule (again, because of bogus parsing), it just adds the same rule again - another thing we don't want. That gets fixed on a few branches too.

And then I get to the point where my puppet apply brings all the rules together - except it still does not work. And I notice one little missing rule: ${IPTABLES} -A INPUT -p tcp --syn -j door

And I learn about --syn, and --tcp-flags, and to my dismay, there is no support for tcp-flags anywhere. There is a ticket for TCP flags matching support, but nobody worked on it.

So I think, how hard can it be, with everything I've learned today? And I get onto it. It turns out it's harder than expected. Before today, all firewall resource properties swallowed exactly one argument - for example, -p (proto). In the recent module, some properties are flags, and don't have an argument, so I had to support that with some hacks.

The rule_to_hash function works by taking an iptables rule line, and stripping off the parameters from the back in reverse order one by one, but leaving the arguments there. At the end, it has a list of keys it saw, and hopefully, a string of arguments that match the keys, but in reverse order. (I would have done this by stripping the line of both parameter and argument(s) and putting those on a list, but that's just me)

But the --tcp-flags parameter takes two arguments - a mask of flags, and a list of flags that needs to be set. So I hack it in by adding double quotes around it, so it looks the same way a --comment does (except --comment is always quoted in iptables --list-rules output), and handle it specially. But after some fidgeting, that works too!

And my final screenshot for the day:
71245

So, today's result:

• fixes for the recent module
• an implementation of --tcp-flags
• some small fixes for firewallchain
• A feature integration branch pulling all of these together

Now, I have a working node that implements port knocking:

node 'ana' {

firewall { "106 connection initiation to door":
# FIXME: specifying chain explicitly breaks insert_order !
chain => "INPUT",
proto => "tcp",
tcp_flags => "FIN,SYN,RST,ACK SYN",
jump => "door",
require => [
Firewallchain[':door:'],
]
}
}

and I can log in with

nc -w 1 ana 1234; nc -w 1 ana 3456; nc -w 1 ana 2345; ssh -A ana


Lessons learned today:

• watch iptables -nvL is an absolutely excellent way of learning more about your firewall - you see your rules and the traffic on them in real time. It made it really easy to see for example the first nc command triggering the knock.
• Puppet is reasonably hackable - I was learning quickly as I progressed through test and bug after test and bug.
• I still don't like ruby, and we may never be friends, but at least it's something I'm capable of learning. Puppet might just end up being the trigger.

Tomorrow, I need to clean up the firewall rules into something reusable, and deploy it on the platform.

After last week's Linode incident I was getting a bit more worried about security than usual. That coincided with the fact that I found I couldn't run puppet on one...