DNS Security: The Final Chapter, For Now

August 12, 2018August 22, 2018 Lorin Olsen

As a man of faith, I am often confronted with one sorry truth: my desires often exceed my patience. So it was with my extended DNS security project. I have written three out of four articles about DNS security. But I have taken a detour from my original plan.

The first article that I wrote outlined the merits of using the Trusted Recursive Resolver that showed up in Firefox 61. I concluded that the merits of encrypting DNS payloads were obvious and the investment was a necessary one – if you want to ensure privacy. The second article outlined the merits (and methods) of using DNS -Over-HTTPS (DoH) to secure references to external referrers/resolvers. In the third article, I outlined how I altered my DNS/DHCP infrastructure to exploit DNSMasqd.

That left me with the final installment. And the original plan was to outline how I had implemented Unbound as a final means of meeting all of my DNS security requirements. Basically, I had to outline why I would want something other than a simple DNS referral agent. That is a great question. But to answer that question, I need to provide a little background.

DNS Background

The basic DNS infrastructure is a hierarchical data structure that is traversed from top to bottom (or right to left when reading a domain name). When a customer wants to know the IP address of a particular device, the top-level domain (TLD) is queried first. So if looking for www.lobostrategies.com, one must first search the registry for all ‘.com’ domains. The authoritative server for ‘.com’ domains contains a reference to the authoritative DNS server for lobostrategies.com (i.e., GoDaddy).

The next step is to search the authoritative domain server for the address of the specific server. In my case, GoDaddy would be queried to determine the address for www.lobostrategies.com. GoDaddy would either answer the question or send it to a DNS server supporting the next lower level of the domain hierarchy. Since there are no subdomains (for lobostrategies.com), GoDaddy returns the IP address.

The ISP Advantage

The process of searching from the root to the proper branch (that answers the query) is called recursive searching. And it is the heart of how DNS works. But this burden is not carried by every user. Can you imagine if every user queried the top-level domain servers? It would be an incredible volume of queries. Instead, the results of most queries are stored (i.e., cached) at lower levels of the tree. For example, companies like your cable ISP (or Google, or Cloudflare, or OpenDNS) will be your ‘proxy’ for all requests between you and the host name that you want to resolve into an IP address.

Your ISP has almost every result of top-level domain queries already stored in its cache. So your answer would be delivered with at least one fewer step than it would have required for you to ask the question yourself. And since most public DNS resolvers have massive results already cached, you would never have to go to GoDaddy to get the IP address for my website. So rather than issuing a query to the root and a query to GoDaddy, your ISP can just provide the address directly to you – reducing your name search activity in half. Therefore, most users consult a DNS service that does the searching for them.

Hidden Costs

But think about what it is costing you to do the search yourself. The first time you query a domain, it takes time (and cache memory). But after the one-time ‘charges’ are paid, isn’t running my own recursive DNS a low-cost investment? Yes it is, and no it isn’t. The real cost of DNS is the hidden cost of privacy. If you run your own recursive DNS server, then you have to pay the entry costs (in hardware and in slower initial resolve times).

If you ‘trust’ someone else to do this for you, then they have access to all of your DNS queries. They know who you are and what you are trying to see. They won’t know what you saw in your query to a specific site. But they will know that you wanted to know where a particular site could be found.

Bottom line: To use a DNS resolver/referrer, you are necessarily letting that service provider know about your probably website destinations.

By using a recursive DNS, you are only letting the domain owner for a site know that you are looking for their address. Google would only get query data when you were intending to connect to Google services. So Google would only see a subset of your DNS queries – thereby improving DNS security.

On the flip side, you really do want a service that will encrypt the DNS payload. Recursive DNS tools (like the Unbound tool in DD-WRT and Pi-hole) do not yet support robust encryption for their recursive queries. Indeed, only two DNS providers currently support DOH (i.e., Google and Cloudflare). By selecting to use a recursive DNS that you manage yourself, you are limiting the ability to mask DNS search requests as they move across the Internet. In practice, this means that you will have a higher risk of being exploited by a man-in-the-middle (MIM) DNS attack. And this includes things like DNS spoofing.

The Choice

So I was faced with a simple choice: 1) I could implement a solution with encryption to a trusted recursive DNS provider, or 2) I could pay the upfront price of running my own recursive DNS. When I started to write this series of articles, I was feeling very distrustful of all DNS providers. So I was leaning towards running my own recursive DNS and limiting the search data that my selected provider could exploit. But the more that I thought about it, the more that I questioned that decision. Yes, I don’t trust companies to place me above their bottom line. And I don’t want the ‘gubmint’ to have a choke point that they could exploit. After all, didn’t the 2016 presidential campaign demonstrate that both parties want to weaponize the information technology?

But the fear of all companies and all politicians is a paranoid conceit. And I don’t want to be the paranoid old man who is always watching over his shoulder. More importantly, the real challenge / threat is the proven risk that script-kiddies, hackers, and criminals might target my data while it is in transit. So as I compared a paranoid fear versus a real fear, I started moving towards desiring encrypted DNS queries more than limiting third-party knowledge of my queries.

The Choice Deferred

Just as I was about to implement changes based upon a re-assessment, I inadvertently shot myself in the foot. I was listening to a podcast about information security (i.e., Security Now by Steve Gibson) and I heard about a resurgence of router-based password exploits. I had long ago switched to a password manager. So I wanted more entropy in my randomized password. I checked online to see if there were any password standards for DD-WRT. I found nothing. So I figured that if the software didn’t like a password, then it would stop me before implementing the change.

I plunged ahead and created a 64-character randomized password. The software took the change – even validating the password against a second-entry of the password. But when I went to log back in to the router, my shiny new password was rejected.

Wash, Rinse, Repeat

I was getting frustrated. I looked to see if there was any way to revert back to an older password. But there was no such capability. And the only way to log back into my router would be to factory-reset the device – which I did. But it took a very long-time to recover (~2.5 hours). So after a few hours, I was back to where I started.

Before I tried again, I backed up the non-volatile memory (nvram). Then I decided to try a shorter password. After failing with the 64-character password, I tried a 32-character password. Unfortunately, it resulted in an inaccessible router. So I restored my router and then I was back to the starting line, again.

After researching the issue for forty-five minutes, I found someone that had run into the same problem. They had solved it by using a twenty-two (22) character password. So I earnestly tried to change the password to an eighteen (18) character password. I was hopeful; my hopes were crushed. But I was getting good at doing the factory reset. So after three attempts and almost five (5) hours of effort, I went back to the old password format that I had used before. Lo and behold, it worked.

Overall DNS Security Improvements

After spending the bulk of an evening on this effort, I was glad to be back on track. But I had a fresh appreciation for doing as little as possible to my router and my DNS infrastructure. I already has a working DNS that used DOH to communicate with Cloudflare. And I had done enough research to be less skeptical of the Cloudflare DNS (when compared to ISP DNS and Google DNS).

I now have a DNS service separated from my router. And the DNS and DHCP systems are running on a unified system – thus making reverse-lookups and local queries far more effective. Finally, I have a DNS request facility that should be more secure against man-in-the-middle attacks. So without much more fanfare, I will call this DNS security battle a victory – for now. And I will keep my eyes wide open – both against corporate/government exploitation and against self-inflicted computing wounds!

DNS Security At The Edge

August 9, 2018August 22, 2018 Lorin Olsen

DNS-Security-From-The-Edge — DNS Security From The Edge

In my third installment of this series about DNS security, I am focusing on how we can audit all DNS requests made in our network. In the second installment, I focused on secure communications between DNS servers. And I highlighted the value of DNSSEC and the potential of DNS-Over-HTTPS (DOH). I outlined some of the changes that we’ve made – including the deployment of DNSMasqd on our router and on our Pi-hole. Today, I will focus upon ensuring that end-to-end auditing is possible – if desired.

As noted previously, we upgraded our router (i.e., a Netgear R8000 X6) from stock firmware to DD-WRT. We did this for numerous reasons. Chief among these reasons was the ability to set up an OpenVPN tunnel from our network out to our VPN providers’ VPN endpoints. But a close second was a desire to run DNSMasqd on the router. Since we haven’t chosen to move DHCP functions to the Pi, we wanted a DHCP service that would have better capabilities. More importantly, we wanted to update DHCP option #6 so that we could control what name servers our clients would use. In our case, we knew that all clients would use our Pi-hole as their name server.

After we realized that DNS options on the admin panel didn’t apply to DHCP clients, we figured out how to set name servers for all of our DHCP clients. Once done, all clients began direct interaction with the Pi-hole. [They had previously used the router as a DNS proxy to our Pi-hole system.]

But as is often the case, there were unforeseen consequences. Specifically, reverse lookups (for static address) failed. This meant that DNS security would suffer because we couldn’t correlate elements across the entire request chain. We could have moved dhcpd to the Pi-hole. But we wanted to have a DNS fall-back – just in case the Pi-hole failed. So we changed our processes for assigning static addresses. Now, we will add them to the router as well as to the /etc/hosts file on the Pi-hole. Once implemented, we had clear visibility between request origination and request fulfillment. [Note: We will probably turn this off as it defeats the very anonymity that we are trying to establish. But that is for another day.]

So what’s left? In the first two articles, we set up DNS authentication wherever possible. We also established DNS payload encryption – using DNS-Over-HTTPS. Now we have a means of authenticating DNS server communications. And we have an encrypted payload. But there is one last need: we have to limit the ‘data at rest’ stored by each DNS resolver.

Consider this: If we validate our connection to Google, and we encrypt the traffic between us, then Google can still look at all of the DNS queries that it receives from us. That is fine if you trust your principal DNS resolver. But what if you don’t? A more secure process would be to ask for name resolution directly, rather than through a trusted (or un-trusted) intermediary. In my next article, I will discuss how we implemented a recursive DNS resolver alongside our Pi-hole. Specifically, I’ll talk about using Unbound to limit the amount of ‘data at rest’ that we leave with any single DNS service.

DNS Security Is A Necessary Key To Privacy

August 7, 2018August 22, 2018 Lorin Olsen

DNS Security Is Not Too Expensive — Comparison of DNS Services

Yesterday, I wrote about how Mozilla is updating Firefox to improve DNS security. But my conclusion was that Mozilla may have designed a system with a single point of failure. In fairness, this assessment was far too simplistic. Today, I want to amplify on my thoughts.

The Firefox implementation assumes something that is probably false. It assumes that most people are using Firefox. It also assumes that all Firefox users can choose an appropriate resolver/referrer to meet their specific needs. I would argue that the first assumption is patently wrong while the second assumption is altogether naive. As noted yesterday, I would have much preferred that Mozilla be more transparent about their proposed changes. Also, rather than assume that the code is open and thus reviewed, Mozilla could have asked for more extensive input. [Note: I suspect that Mozilla was transparent with a somewhat limited community. I just wish that their design had been explicitly shared.]

The real problem that I have with their approach is that it is a Firefox-only solution. No, I don’t expect Mozilla to solve this problem for their competitors. But most organizations need a broader solution that will meet everyone’s needs. Enter dnsmasq. In my case, I have implemented DNS over HTTPS (DoH) as my mechanism to improve DNS security.

I am quite fortunate: I am using a DNS server that was just updated. The Pi-hole team has just released Pi-Hole 4.0. And a DNS service provider (Cloudflare) has released an AMD and ARM DNS-over-HTTPS implementation. [Note: Their solution is in binary form. So I will add my voice to the chorus asking for the software to be published under a free/open license. Until that happens, I’m testing the system to see how it performs.]

So what am I seeing thus far?

After switching to cloudflared on Pi-hole 4.0, I ran some benchmarks. And as expected, there was quite a bit more activity on my DNS server. But the overall DNS response time (i.e., the server at 10.42.222.22) was quite acceptable. I want to get about a week’s worth of data. But at this moment, I am very hopeful that the software will maintain acceptable performance levels.

So what should you do? If you’re bold, then use a test system and try it out for yourself. Or keep close tabs on folks who are testing this technology. At the current time, I am thrilled to close down yet another vector of surveillance.

The intersection of vision and action

Tag: #DoH