OpenNMS and buggy switches

July 19, 2008 – 12:40 pm

One of my evening projects has been setting up OpenNMS to monitor a network primarily comprised of VENDORNAME switches. OpenNMS is being put in to replace a bundle of Nagios, Cacti, Smokeping, and Groundwork Fruity for Nagios configuration management. The existing system worked well enough, but the lack of autodiscovery of services/nodes along with the poor integration between cacti and nagios was getting a bit annoying.

After setting up and trialling OpenNMS for a bit, we deployed it on this network. And then the switches started failing. They’d still switch packets, and I believe still responded to SNMP, but you couldn’t connect to them via any of the management interfaces.

So, we started looking at the differences between OpenNMS and Nagios/Cacti/Smokeping. Both do SNMP and ICMP queries, and some TCP port availability checks. The combined stack actually does more SNMP traffic because both Cacti and Nagios ended up querying the same OIDs. I’ve often noticed that Cacti sends individual requests for OIDs however, rather than grouping them, whereas OpenNMS defaults to requesting 10 OIDs per PDU. I changed this in the configuration (and later on changed it for real, as it was being set in a different config file as well), and let OpenNMS run against some test switches… and they locked up.

Perry suggested that it could be a memory leak due to the service polling, and set up a test where he polled the SSH server once a minute forever. This test got cancelled after 4 days or so, but the machines hadn’t died at that point, so we decided it wasn’t anything fundamental about the service checks.

I set up a range of services that were being monitored on 10 switches, and let them go for a bit. Due to power outages and equipment moves this step ended up taking longer than it needed to, but the end result was that no matter which services were being monitored, all the switches all locked up at around the same point.

And then I noticed that the switches had a growing number of stale “telnet-d” connections. These switches have capacity for up to 4 concurrent administrative logins – once all 4 slots are full, you can no longer log in. So, the theory is these stale connections were blocking real connections, and never timing out, thus causing the lockout of the management stack. They don’t time out, and you can’t kill them from the switch console short of rebooting the switch. Most of the switches weren’t being actively monitered for telnet, but OpenNMS does do service discovery periodically (I think once a day, and perhaps under other situations too), and this would probe each service. So I firewalled telnet out, and had the switches restarted, thinking this would solve it.

The switches still locked up.

The switches still had stale telnet connections appearing in them.

I turned off the telnet service on each switch, thinking that perhaps there was something else on the network that was talking to them, and restarted them.

Within 5 minutes of rebooting each switch, there was a stale telnet connection listed. Awesome.

So, we’re down to a service that is being misreported as a telnet service. I go through all of them, and discover that none of the other services – FTP, HTTP, HTTPS – even show up as an active session. Which leaves telnet – firewalled out – and SSH.

The OpenNMS plugin which handles discovery of SSH servers is a bit smarter than a basic “is a service listening on port 22″ sort of discovery – it waits for the SSH banner from the server, then sends it’s own SSH banner back, and verifies that it gets a response back. This means it’s getting part way through the SSH establishment, and then canning the connection.

As a quick test, I telnetted to port 22 on a switch and checked the login listing. With the banner is being displayed, nothing even shows up. When I pasted a valid looking SSH banner back, I got a bunch of binary data echoed into my telnet session, and ssh session to the switch locks up. On reconnecting and checking the login listing, sure enough – a stale telnet session was there.

Further tests reveal that if you ssh to one of these switches, but don’t type your password in, the session gets reported as a telnet session. Furthermore, if you kill your ssh process or shell window while the ssh session is waiting for your password, the session never disappears.

So, we have a very live DOS exploit against VENDORNAME switches here, assuming anyone is unwise enough to allow SSH access from random networks and VLANs to their switches that is. I have to point out that while it’s a particular “feature” of OpenNMS that triggered this problem for us, this isn’t a bug in OpenNMS at all, given that it’s trivial to trigger the same problems with the switches directly.

In regards to the actual problem at hand, OpenNMS is quite configurable, so at least I can change the way it does SSH service discovery to revert to a simple “is the port up” check. I’ve left this running for nearly two weeks now, and the switches on my test bed are all still behaving properly.

I held back from posting this until I could get a response from the vendor. They’ve acknowledged the bug, and a fix will be out in the next firmware release apparently. I might update once they have released a new firmware; I’ve edited out the vendor name from this post because I don’t believe it’s responsible to publish denial-of-service vulnerabilities without giving the vendor a chance to fix them.

I also noticed this post on the OpenNMS blog. The author there had similar problems with monitoring a firewall device, and while the scenario seems different, VENDORNAME makes firewalls as well as switches; I wonder if it’s the same vendor in his case.

  1. One Response to “OpenNMS and buggy switches”

  2. It might still be argued that this is a bug in OpenNMS. The point is, though, that this makes no difference whatsoever, since a remote machine should not be able to coax the switch into tripping.

    By Aristotle Pagaltzis on Jul 20, 2008

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