SECURITY
The Hedge 10: Pavel Odintsov and Fastnetmon
Fastnetmon began life as an open source DDoS detection tool, but has grown in scope over time. By connecting Fastnetmon to open source BGP implementations, operators can take action when a denial of service event is detected, triggering black holes and changing route preferences. Pavel Odintsov joins us to talk about this interesting and useful open source project.
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IPv6 Backscatter and Address Space Scanning
Backscatter is often used to detect various kinds of attacks, but how does it work? The paper under review today, Who Knocks at the IPv6 Door, explains backscatter usage in IPv4, and examines how effectively this technique might be used to detect scanning of IPv6 addresses, as well. The best place to begin is with an explanation of backscatter itself; the following network diagram will be helpful—
Assume A is scanning the IPv4 address space for some reason—for instance, to find some open port on a host, or as part of a DDoS attack. When A sends an unsolicited packet to C, a firewall (or some similar edge filtering device), C will attempt to discover the source of this packet. It could be there is some local policy set up allowing packets from A, or perhaps A is part of some domain none of the devices from C should be connecting to. IN order to discover more, the firewall will perform a reverse lookup. To do this, C takes advantage of the PTR DNS record, looking up the IP address to see if there is an associated domain name (this is explained in more detail in my How the Internet Really Works webinar, which I give every six months or so). This reverse lookup generates what is called a backscatter—these backscatter events can be used to find hosts scanning the IP address space. Sometimes these scans are innocent, such as a web spider searching for HTML servers; other times, they could be a prelude to some sort of attack.
Kensuke Fukuda and John Heidemann. 2018. Who Knocks at the IPv6 Door?: Detecting IPv6 Scanning. In Proceedings of the Internet Measurement Conference 2018 (IMC ’18). ACM, New York, NY, USA, 231-237. DOI: https://doi.org/10.1145/3278532.3278553
Scanning the IPv6 address space is much more difficult because there are 2128 addresses rather than 232. The paper under review here is one of the first attempts to understand backscatter in the IPv6 address space, which can lead to a better understanding of the ways in which IPv6 scanners are optimizing their search through the larger address space, and also to begin understanding how backscatter can be used in IPv6 for many of the same purposes as it is in IPv4.
The researchers begin by setting up a backscatter testbed across a subset of hosts for which IPv4 backscatter information is well-known. They developed a set of heuristics for identifying the kind of service or host performing the reverse DNS lookup, classifying them into major services, content delivery networks, mail servers, etc. They then examined the number of reverse DNS lookups requested versus the number of IP packets each received.
It turns out that about ten times as many backscatter incidents are reported for IPv4 than IPv6, which either indicates that IPv6 hosts perform reverse lookup requests about ten times less often than IPv4 hosts, or IPv6 hosts are ten times less likely to be monitored for backscatter events. Either way, this result is not promising—it appears, on the surface, that IPv6 hosts will be less likely to cause backscatter events, or IPv6 backscatter events are ten times less likely to be reported. This could indicate that widespread deployment of IPv6 will make it harder to detect various kinds of attacks on the DFZ. A second result from this research is that using backscatter, the researchers determined IPv6 scanning is increasing over time; while the IPv6 space is not currently a prime target for attacks, it might become more so over time, if the scanning rate is any indicator.
The bottom line is—IPv6 hosts need to be monitored as closely, or more closely than IPv6 hosts, for scanning events. The techniques used for scanning the IPv6 address space are not well understood at this time, either.
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.
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.
What’s in your DNS query?
Privacy problems are an area of wide concern for individual users of the Internet—but what about network operators? In this issue of The Internet Protocol Journal, Geoff Huston has an article up about privacy in DNS, and the various attempts to make DNS private on the part of the IETF—the result can be summarized with this long, but entertaining, quote:
Before diving into a full-blown look at the many problems with DNS security, it is worth considering what kinds of information can leak through the DNS system. Let’s ignore the recent discovery that DNS queries can be used to exfiltrate data; instead, let’s look at more mundane data leakage from DNS queries.
For instance, say you work in a marketing department for a company that is just about to release a new product. In order to build the marketing and competitive materials your sales critters will need to stand in front of customers, you do a lot of research around competitor products. In the process, you examine, in detail, each of the competing product’s pages. Or perhaps you work in a company that is determining whether or another purchasing or merging with another company might be a good idea. Or you are working on a new externally facing application, or component in an existing application, that relies on a new connection point into your network.
All of these processes can lead to a lot of DNS queries. For someone who knows what they are looking for, the pattern of queries may be enough to examine strings queried from search engines and other information, ultimately leading to someone being able to guess a lot about that new product, what company your company is thinking about buying or merging with, what your new application is going to do, etc. DNS is a treasure trove of information at a personal and organizational level.
Operators and protocol designers have been working for years to resolve these problems, making DNS queries “more private;” Geoff Huston’s article provides a good overview of many of these attempts. DNS over HTTPS (DoH), a recent (and ongoing) attempt bears a closer look.
DNS is normally sent “in plain text” over the network; anyone who can capture the packets can read not only the query, but also the responses. The simplest way to solve this problem is to encrypt the DNS data in flight using something like TLS—hence DoT, or DNS over TLS. One problem with DoT is it is carried over a unique port number, which means it is probably blocked by default by most packet filters, and can easily be blocked by administrators who either do not know what this traffic is, or do not want it on their network. To solve this, DoH carries TLS encrypted traffic in a way that makes it look just like an HTTPS session. If you block DoH traffic, you will also block access to web servers running HTTPS. This is the logical “end” of carrying everything else over HTTPS to avoid the impact of stateful and stateless packet filters and the impact of middle boxes on Internet traffic.
The good result is, in fact, that DNS traffic can no longer be “spied on” by anyone outside servers in the DNS system itself. Whether or not this is “enough” privacy is a matter of conjecture, however. Servers within the DNS system can still collect information about what queries you are making; if the server has access to other information about you or your organization, combining this data into a profile, or using it to determine some deeper investigation is warranted by looking at other sources of data, is pretty simple. Ultimately, DoH is only really useful if you trust your DNS provider.
Do you? Perhaps more importantly—should you?
DNS providers are like any other business; they must buy hardware, connectivity, and the time of smart people who can make the system work, troubleshoot the system when it fails, and think about ways of improving the system. If the service is free…
DoH, however, has another problem Geoff outlines in his article—DNS is moved up the stack so it no longer runs over TCP and UDP directly, but rather it runs over HTTPS. This means local applications, like browsers, can run DNS queries independently of the operating system. In fact, because these queries are TLS encrypted, the operating system itself cannot even “see” the contents of these DNS queries. This might be a good thing—or might be a bad thing. If nothing else, it means the browser, or any other application, can choose to use a resolver not configured by the local operating system. A browser maker, for instance, can direct their browser to send all DNS queries made within the browser to their DNS server, exposing another source of information about users (and the organizations they work for).
Remember that time you typed an internal hostname incorrectly in your browser? Thankfully, you had a local DNS server configured, so the query did not go out to a resolver on the Internet. With DoH, the query can go out to an open resolver on the Internet regardless of how your local systems are configured. Something to ponder.
The bottom line is this—the nature of DNS makes it extremely difficult to secure. Somehow you have to have someone operate, and pay for, an open database of names which translate to addresses. Somehow you have to have a protocol that allows this database to be queried. All of these “somehows” expose information, and there is no clear way to hide that information. You can solve parts of the problem, but not the whole problem. Solving one part of the problem seems to make another part of the problem worse.
If you haven’t found the tradeoff, you haven’t looked hard enough.
In the end, though, the privacy of DNS queries at a personal and organizational level is something you need to think about.
CAA Records and Site Security
The little green lock—now being deprecated by some browsers—provides some level of comfort for many users when entering personal information on a web site. You probably know the little green lock means the traffic between the host and the site is encrypted, but you might not stop to ask the fundamental question of all cryptography: using what key? The quality of an encrypted connection is no better than the quality and source of the keys used to encrypt the data carried across the connection. If the key is compromised, then entire encrypted session is useless.
So where does the key pair come from to encrypt the session between a host and a server? The session key used for symmetric cryptography on each session is obtained using the public key of the server (thus through asymmetric cryptography). How is the public key of the server obtained by the host? Here is where things get interesting.
The older way of doing things was for a list of domains who were trusted to provide a public key for a particular server was carried in HTTP. The host would open a session with a server, which would then provide a list of domains where its public key could be found in the opening HTTP packets. The host would then find one of those hosts, and hence the server’s public key. From there, the host could create the correct nonce and other information to form a session key with the server. If you are quick on the security side, you might note a problem with this solution: if the HTTP session itself is somehow hijacked early in the setup process, a man-in-the-middle could substitute its own host list for the one the server provides. Once this substitution is done, the MITM could set up perfectly valid encrypted sessions with both the host and the server, funneling traffic between them. The MITM now has full access to the unencrypted data flowing through the session, even though the traffic is encrypted as it flows over the rest of the ‘net.
To solve this problem, a new method for finding the server’s public key was designed around 2010. In this method, the host requests the Certificate Authority Authorization (CAA) record from the server’s DNS server. This record lists the domains who are authorized to provide a public key, or certificate, for the servers within a domain. Thus, if you purchase your certificates from BigCertProvider, you would list BigCertProvider’s domain in your CAA. The host can then find the correct DNS record, and retrieve the correct certificate from the DNS system. This cuts out the possibility of a MITM attacking the HTTP session during the initial setup phases. If DNSSEC is deployed, the DNS records should also be secured, preventing MITM attacks from that angle, as well.
The paper under review today examines the deployment of CAA records in the wild, to determine how widely CAAs are deployed and used.
In this paper, a group of researchers put the CAA system to the test to see just how reliable the information is. In their first test, they attempted to request certificates that would cause the issuer to issue invalid certificates in some way; they found that many certificate providers will, in fact, issue such invalid certificates for various reasons. For instance, in one case, they discovered a defect in the provider’s software that allowed their automated system to issue invalid certificates.
In their second test, they examined the results of DNS queries to determine if DNS operators were supporting and returning CAA certificates. They discovered that very few certificate authorities deploy security controls on CAA lookups, leaving open the possibility of the lookups themselves being hijacked. Finally, they examine the deployment of CAA in the wild by web site operators. They found CAA is not widely deployed, with CAA records covering around 40,000 domains. DNSSEC and CAA deployment generally overlap, pointing to a small section of the global ‘net that is concerned about the security of their web sites.
Overall, the results of this study were not heartening for the overall security of the ‘net. While the HTTP based mechanism of discovering a server’s certificate is being deprecated, not many domains have started deploying the CAA infrastructure to replace it—in fact, only a small number of DNS providers support users entering their CAA certificate into their domain records.
BGP Hijacks: Two more papers consider the problem
The security of the global Default Free Zone DFZ) has been a topic of much debate and concern for the last twenty years (or more). Two recent papers have brought this issue to the surface once again—it is worth looking at what these two papers add to the mix of what is known, and what solutions might be available. The first of these—
Demchak, Chris, and Yuval Shavitt. 2018. “China’s Maxim – Leave No Access Point Unexploited: The Hidden Story of China Telecom’s BGP Hijacking.” Military Cyber Affairs 3 (1). https://doi.org/10.5038/2378-0789.3.1.1050.
—traces the impact of Chinese “state actor” effects on BGP routing in recent years.
Whether these are actual attacks, or mistakes from human error for various reasons generally cannot be known, but the potential, at least, for serious damage to companies and institutions relying on the DFZ is hard to overestimate. This paper lays out the basic problem, and the works through a number of BGP hijacks in recent years, showing how they misdirected traffic in ways that could have facilitated attacks, whether by mistake or intentionally. For instance, quoting from the paper—
- Starting from February 2016 and for about 6 months, routes from Canada to Korean government sites were hijacked by China Telecom and routed through China.
- On October 2016, traffic from several locations in the USA to a large Anglo-American bank
- headquarters in Milan, Italy was hijacked by China Telecom to China.
- Traffic from Sweden and Norway to the Japanese network of a large American news organization was hijacked to China for about 6 weeks in April/May 2017.
What impact could such a traffic redirection have? If you can control the path of traffic while a TLS or SSL session is being set up, you can place your server in the middle as an observer. This can, in many situations, be avoided if DNSSEC is deployed to ensure the certificates used in setting up the TLS session is valid, but DNSSEC is not widely deployed, either. Another option is to simply gather encrypted traffic and either attempt to break the key, or use data analytics to understand what the flow is doing (a side channel attack).
What can be done about these kinds of problems? The “simplest”—and most naïve—answer is “let’s just secure BGP.” There are many, many problems with this solution. Some of them are highlighted in the second paper under review—
Bonaventure, Olivier. n.d. “A Survey among Network Operators on BGP Prefix Hijacking – Computer Communication Review.” Accessed November 3, 2018. https://ccronline.sigcomm.org/2018/ccr-january-2018/a-survey-among-network-operators-on-bgp-prefix-hijacking/.
—which illustrates the objections providers have to the many forms of BGP security that have been proposed to this point. The first is, of course, that it is expensive. The ROI of the systems proposed thus far are very low; the cost is high, and the benefit to the individual provider is rather low. There is both a race to perfection problem here, as well as a tragedy of the commons problem. The race to perfection problem is this: we will not design, nor push for the deployment of, any system which does not “solve the problem entirely.” This has been the mantra behind BGPSEC, for instance. But not only is BGPSEC expensive—I would say to the point of being impossible to deploy—it is also not perfect.
The second problem in the ROI space is the tragedy of the commons. I cannot do much to prevent other people from misusing my routes. All I can really do is stop myself and my neighbors from misusing other people’s routes. What incentive do I have to try to make the routing in my neighborhood better? The hope that everyone else will do the same. Thus, the only way to maintain the commons of the DFZ is for everyone to work together for the common good. This is difficult. Worse than herding cats.
A second point—not well understood in the security world—is this: a core point of DFZ routing is that when you hand your reachability information to someone else, you lose control over that reachability information. There have been a number of proposals to “solve” this problem, but it is a basic fact that if you cannot control the path traffic takes through your network, then you have no control over the profitability of your network. This tension can be seen in the results of the survey above. People want security, but they do not want to release the information needed to make security happen. Both realities are perfectly rational!
Part of the problem with the “more strict,” and hence (considered) “more perfect” security mechanisms proposed is simply this: they are not quiet enough. They expose far too much information. Even systems designed to prevent information leakage ultimately leak too much.
So… what do real solutions on the ground look like?
One option is for everyone to encrypt all traffic, all the time. This is a point of debate, however, as it also damages the ability of providers to optimize their networks. One point where the plumbing allegory for networking breaks down is this: all bits of water are the same. Not all bits on the wire are the same.
Another option is to rely less on the DFZ. We already seem to be heading in this direction, if Geoff Huston and other researchers are right. Is this a good thing, or a bad one? It is hard to tell from this angle, but a lot of people think it is a bad thing.
Perhaps we should revisit some of the proposed BGP security solutions, reshaping some of them into something that is more realistic and deployable? Perhaps—but the community is going to let go of the “but it’s not perfect” line of thinking, and start developing some practical, deployable solutions that don’t leak so much information.
Finally, there is a solution Leslie Daigle and I have been tilting at for a couple of years now. Finding a way to build a set of open source tools that will allow any operator or provider to quickly and cheaply build an internal system to check the routing information available in their neighborhood on the ‘net, and mix local policy with that information to do some bare bones work to make their neighborhood a little cleaner. This is a lot harder than “just build some software” for various reasons; the work is often difficult—as Leslie says, it is largely a matter of herding cats, rather than inventing new things.