Earlier, I posted about upstart and how it illustrated how Ubuntu is not a good operating system for running servers. Today I wanted to provide another example: UFW, the “Uncomplicated FireWall” that is installed by default on Ubuntu servers.
Linux firewalling and UFW
Firewalls in Linux are implemented with iptables. In a nutshell, iptables evaluates packets against lists of rules to determine whether they should be accepted or rejected. iptables is typically used on servers through scripts that set up all these rules the way you want them – these days this process is generally managed through whatever configuration management system you’re using (e.g. chef or puppet). UFW is a tool distributed with Ubuntu that provides a different way of setting up these iptables rules, using command line tools.
How UFW works
UFW keeps its own set of iptables rules in /etc/ufw. When ufw is turned on, it flushes out all the existing iptables rules and replaces them with the rules from /etc/ufw. You can then use the command line tools to add ports to allow or deny, which will update both the running iptables rules as well as the copies stored in /etc/ufw.
Note that all UFW works with is what’s in /etc/ufw – it doesn’t know or care what the running iptables rules are.
A Totally Theoretical Example
Let’s pretend you have a big distributed system where the nodes are all running Ubuntu. It’s a big system, and it’s part of an even larger IT environment, so there’s a large IT support organization that’s staffed with people who were hired because they have fair bit of experience with Linux systems – but some of those folks have more at-home, desktop Linux experience vs datacenter, server Linux experience. Also these folks don’t know the ins and outs of the design of this particular distributed system because they are responsible for all of the varied IT environments at this large organization. The hero in our story is one of these selfless and courageous sysadmins. A report comes in from the users of this distributed system that they’re having trouble reaching some of the resources it provides. Initial troubleshooting leads our hero to posit that the problem is a misconfigured firewall. His experience is with Ubuntu and he’s always used ufw, so his first step is to disable the firewall and see if that helps:
ufw disable
There’s no change, so he figures that this is unrelated, so he turns the firewall back on:
ufw enable
Now he moves on to further troubleshooting.
Here’s the problem: The ‘ufw disable’ command above didn’t actually do anything, as ufw wasn’t enabled to begin with. No harm, no foul. However, running ‘ufw enable’ turns on ufw, and configures it in default mode – which denies all incoming connections. As the server in question provides many key network services for this distributed system, suddenly all of the other nodes in this system cannot reach these central services, and the entire distributed system starts to fall over!
So what went wrong here?
UFW is not a front-end for iptables
It may seem like UFW is a front-end for iptables – because you run ufw commands and it makes changes to iptables rules. But what it’s really doing is throwing away whatever is currently in iptables, and replacing it with what’s been configured in ufw. A real front-end would allow you to inspect and modify iptables stuff that already exists.
How this plays into this situation is that lots of carefully-crafted iptables rules for access, masquerading, etc got silently nuked when the ‘ufw enable’ command was run. It does not take into account the starting state of the system before it does stuff. That’s bad on a server.
UFW’s defaults are appropriate for desktops, not servers
I love ‘deny all’ as a default for firewalls. I could even concede that ‘deny all’ is the best default for servers, although I think that could be debated. However, let’s look at some of the specifics that ufw’s defaults do allow:
- ICMP (this is OK)
- multicast DNS (hmmm….)
- UPnP (!!)
OK, this should go without saying, but does it seem to you like your server operating system should enable UPnP discovery by default? Makes sense for a desktop (maybe) – but not for a server. No way, no how.
UFW is an Ubuntu-specific thing (and makes iptables complicated)
If you have a ‘firewall administrator’ on staff, and you ask them to look at a server, how likely is it that they’re going to know anything about UFW? IMO, very unlikely. They’re probably familiar with iptables, and the first thing they’ll do is look at iptables directly, and they’re going to see dozens of rules that point at other rules and they’re going to be very confused. At best, they’ll refuse to touch it and give you a big lecture about how if packets get to your server’s TCP/IP stack before hitting your firewall, you’re doing it wrong. At worst, they’ll start making changes which due to the complexity of these rules will likely result in more problems, not less.
Make your Ubuntu servers better
If you are going to run servers on Ubuntu, here’s one simple suggestion for how to make them better:
apt-get remove ufw
After this, at least your beleaguered sysadmins will have one less possible way to screw up your systems while doing their jobs.
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