Stop Using the OSI Model

We all use the OSI model to describe the way networks work. I have, in fact, included it in just about every presentation, and every book I have written, someplace in the fundamentals of networking. But if you have every looked at the OSI model and had to scratch your head trying to figure out how it really fits with the networks we operate today, or what the OSI model is telling you in terms of troubleshooting, design, or operation—you are not alone. Lots of people have scratched their heads about the OSI model, trying to understand how it fits with modern networking. There is a reason this is so difficult to figure out.

The OSI Model does not accurately describe networks.

What set me off in this particular direction this week is an article over at Errata Security:

The OSI Model was created by international standards organization for an alternative internet that was too complicated to ever work, and which never worked, and which never came to pass. Sure, when they created the OSI Model, the Internet layered model already existed, so they made sure to include today’s Internet as part of their model. But the focus and intent of the OSI’s efforts was on dumb networking concepts that worked differently from the Internet.

This is partly true, and yet a bit … over the top. 🙂 OTOH, the point is well taken: the OSI model is not an ideal model for understanding networks. Maybe a bit of analysis would be helpful in understanding why.

First, while the OSI model was developed with packet switching networks in mind, the general idea was to come as close as possible to emulating the circuit-switched networks widely deployed at the time. A lot of thought had gone into making those circuit-switched networks work, and applications had been built around the way they worked. Applications and circuit-switched networks formed a sort of symbiotic relationship, just as applications form with packet-switched networks today; it was unimaginable, at the time, that “everything would change.”

So while the designers of the OSI model understood the basic value of the packet-switched network, they also understood the value of the circuit-switched network, and tried to find a way to solve both sets of problems in the same network. Experience has shown it is possible to build a somewhat close-to-circuit switched network on top of packet switched networks, but not quite in the way, nor as close to perfect emulation, as those original designers thought. So the OSI model is a bit complex and perhaps overspecified, making it less-than-useful today.

Second, the OSI model largely ignored the role of middleboxes, focusing instead on the stacks implemented and deployed in hosts. This, again, makes sense, as there was no such thing as a device specialized in the switching of packets at the time. Hosts took packets in and processed them. Some packets were sent along to other hosts, other packets were consumed locally. Think PDP-11 with some rough code, rather than even an early Cisco CGS.

Third, the OSI model focuses on what each layer does from the perspective of an application, rather than focusing on what is being done to the data in order to transmit it. The OSI model is built “top down,” rather than “bottom up,” in other words. While this might be really useful if you are an application developer, it is not so useful if you are a network engineer.

So—what should we say about the OSI model?

It was much more useful at some point in the past, when networking was really just “something a host did,” rather than its own sort of sub-field, with specialized protocols, techniques, and designs. It was a very good attempt at sorting out what a network needed to do to move traffic, from the perspective of an application.

What it is not, however, is really all that useful for network engineers working within an engineering specialty to understand how to design protocols, and how to design networks on which those protocols will run. What should we replace it with? I would begin by pointing you to the RINA model, which I think is a better place to start. I’ve written a bit about the RINA model, and used the RINA model as one of the foundational pieces of Computer Networking Problems and Solutions.

Since writing that, however, I have been thinking further about this problem. Over the next six months or so, I plan to build a course around this question. For the moment, I don’t want to spoil the fun, or put any half-backed thoughts out there in the wild.

Posted in STANDARDS, TECH, WRITTEN

DNS Query Minimization and Data Leaks

When a recursive resolver receives a query from a host, it will first consult any local cache to discover if it has the information required to resolve the query. If it does not, it will begin with the rightmost section of the domain name, the Top Level Domain (TLD), moving left through each section of the Fully Qualified Domain Name (FQDN), in order to find an IP address to return to the host, as shown in the diagram below.

This is pretty simple at its most basic level, of course—virtually every network engineer in the world understands this process (and if you don’t, you should enroll in my How the Internet Really Works webinar the next time it is offered!). The question almost no-one ever asks, however, is: what, precisely, is the recursive server sending to the root, TLD, and authoritative servers?

Begin with the perspective of a coder who is developing the code for that recursive server. You receive a query from a host, you have the code check the local cache, and you find there is no matching information available locally. This means you need to send a query out to some other server to determine the correct IP address to return to the host. You could keep a copy of the query from the host in your local cache and build a new query to send to the root server.

Remember, however, that local server resources may be scarce; recursive servers must be optimized to process very high query rates very quickly. Much of the user’s perception of network performance is actually tied to DNS performance. A second option is you could save local memory and processing power by sending the entire query, as you have received it, on to the root server. This way, you do not need to build a new query packet to send to the root server.

Consider this process, however, in the case of a query for a local, internal resource you would rather not let the world know exists. The recursive server, by sending the entire query to the root server, is also sending information about the internal DNS structure and potential internal server names to the external root server. As the FQDN is resolved (or not), this same information is sent to the TLD and authoritative servers, as well.

There is something else contained here, however, that is not so obvious—the IP address of the requestor is contained in that original query, as well. Not only is your internal namespace leaking, your internal IP addresses are leaking, as well.

This is not only a massive security hole for your organization, it also exposes information from individual users on the global ‘net.

There are several things that can be done to resolve this problem. Organizationally, running a private DNS server, hard coding resolving servers for internal domains, and using internal domains that are not part of the existing TLD infrastructure, can go a long way towards preventing information leaking of this kind through DNS. Operating a DNS server internally might not be ideal, of course, although DNS services are integrated into a lot of other directory services used in operational networks. If you are using a local DNS server, it is important to remember to configure DHCP and/or IPv6 ND to send the correct, internal, DNS server address, rather than an external address. It is also important to either block or redirect DNS queries sent to public servers by hosts using hard-coded DNS server configurations.

A second line of defense is through DNS query minimization. Described in RFC7816, query minimization argues recursive servers should use QNAME queries to only ask about the one relevant part of the FQDN. For instance, if the recursive server receives a query for www.banana.example, the server should request information about .example from the root server, banana.example from the TLD, and send the full requested domain name only to the authoritative server. This way, the full search is not exposed to the intermediate servers, protecting user information.

Some recursive server implementations already support QNAME queries. If you are running a server for internal use, you should ensure the server you are using supports DNS query minimization. If you are directing your personal computer or device to publicly reachable recursive servers, you should investigate whether these servers support DNS query minimization.

Even with DNS query minimization, your recursive server still knows a lot about what you ask for—the topic of discussion on a forthcoming episode of the Hedge, where our guest will be Geoff Huston.

Posted in SECURITY, STANDARDS, TECH, WRITTEN

There is Always a Back Door

A long time ago, I worked in a secure facility. I won’t disclose the facility; I’m certain it no longer exists, and the people who designed the system I’m about to describe are probably long retired. Soon after being transferred into this organization, someone noted I needed to be trained on how to change the cipher door locks. We gathered up a ladder, placed the ladder just outside the door to the secure facility, popped open one of the tiles on the drop ceiling, and opened a small metal box with a standard, low security key. Inside this box was a jumper board that set the combination for the secure door.
First lesson of security: there is (almost) always a back door.

I was reminded of this while reading a paper recently published about a backdoor attack on certificate authorities. There are, according to the paper, around 130 commercial Certificate Authorities (CAs). Each of these CAs issue widely trusted certificates used for everything from TLS to secure web browsing sessions to RPKI certificates used to validate route origination information. When you encounter these certificates, you assume at least two things: the private key in the public/private key pair has not been compromised, and the person who claims to own the key is really the person you are talking to. The first of these two can come under attack through data breaches. The second is the topic of the paper in question.

How do CAs validate the person asking for a certificate actually is who they claim to be? Do they work for the organization they are obtaining a certificate for? Are they the “right person” within that organization to ask for a certificate? Shy of having a personal relationship with the person who initiates the certificate request, how can the CA validate who this person is and if they are authorized to make this request?

They could do research on the person—check their social media profiles, verify their employment history, etc. They can also send them something that, in theory, only that person can receive, such as a physical letter, or an email sent to their work email address. To be more creative, the CA can ask the requestor to create a small file on their corporate web site with information supplied by the CA. In theory, these electronic forms of authentication should be solid. After all, if you have administrative access to a corporate web site, you are probably working in information technology at that company. If you have a work email address at a company, you probably work for that company.

These electronic forms of authentication, however, can turn out to be much like the small metal box which holds the jumper board that sets the combination just outside the secure door. They can be more security theater than real security.
In fact, the authors of this paper found that some 70% of the CAs could be tricked into issuing a certificate for just about any organization—by hijacking a route. Suppose the CA asks the requestor to place a small file containing some supplied information on the corporate web site. The attacker creates a web server, inserts the file, hijacks the route to the corporate web site so it points at the fake web site, waits for the authentication to finish, and then removes the hijacked route.

The solution recommended in this paper is for the CAs to use multiple overlapping factors when authenticating a certificate requestor—which is always a good security practice. Another solution recommended by the authors is to monitor your BGP tables from multiple “views” on the Internet to discover when someone has hijacked your routes, and take active measures to either remove the hijack, or at least to detect the attack.

These are all good measures—ones your organization should already be taking.

But the larger point should be this: putting a firewall in front of your network is not enough. Trusting that others will “do their job correctly,” and hence that you can trust the claims of certificates or CAs, is not enough. The Internet is a low trust environment. You need to think about the possible back doors and think about how to close them (or at least know when they have been opened).

Having personal relationships with people you do business with is a good start. Being creative in what you monitor and how is another. Firewalls are not enough. Two-factor authentication is not enough. Security is systemic and needs to be thought about holistically.
There are always back doors.

Posted in SECURITY, TECH, WRITTEN