I’ve had this on my mind a while, but never fully published this until now. I’ve been called out a number of times to various customers who have internet connection issues… and found bad DNS setups the culprit, oftentimes DNS Forwarding. I’ve also argued over more than a few beers about using them.
I also want to mention: I know IT people like to pontificate. I’m talking about companies with 10-2,000 users in a typical Active Directory environment. I’m not talking about companies with 2,500,000 users and external-facing infrastructures or service providers who offer DNS as a service. Those companies are likely to use advanced technologies way outside the scope of your typical Microsoft DNS Server setup and the people who implement those technologies aren’t likely to be reading my blog to understand far simpler setups.
Why You Should Not Use DNS Forwarders:
- DNS Servers Change
… and they change well after you’ve forgotten about your customer. If you’re even aware of the changes, will you remember every customer you’ve set forwarding up for using that IP?
- DNS Servers Go Down
Here in Rochester NY, we’ve had a few circumstances where our area’s primary DNS servers have been down. Granted, it’s 15-20 minutes at most, but that’s enough to anger people. Meanwhile, your internal DNS Server needs to be up using Root Hints or Forwarding anyway, so why have the added dependency?
- You Marry Your ISP
If I had a dollar for each time someone has changed ISPs and then “went offline” I’d … well, I’d be worth about $15. Regardless of how you value your money, you will have a server you forgot about lose DNS when you change your ISP and your forwarders no longer work.
- WAN Failover Breaks
A half-dozen times or more, I’ve assisted engineers with WAN Failover setups that didn’t work when the primary connection went down. Why? Because the DNS Forwarding was configured using the Primary ISP’s DNS server and when they failed over, those DNS Servers stopped answering queries. Worse, people then just slap Google’s 188.8.131.52 in there to “fix” it.
- Problems Are Twice As Complex
I had a school district who couldn’t load specific websites. After some packet-level analysis, I figured out why: the Windows DNS Server and their ISP weren’t playing nicely when using DNSSEC, which the specific sites had configured. The fix was either disable DNSSEC on the Windows DNS Server or use Root Hints. Luckily, they’d recently set up WAN Failover so I could give them my lecture on why it was a bad idea.
- It’s More Secure
ISP DNS Servers are going to be the first targets of hackers trying to redirect your banking traffic, vendor logins or other DNS queries to their malicious servers. Your DNS Server is secured nicely behind a firewall and hackers are much more likely to go after the machines in your ISP’s DNS pool.
- You’ll Never Get Answers from Servers That Aren’t There
I had a customer continuously receive certificate errors whenever they opened Outlook. After investigation, I discovered that the autodiscover.domain.com record was receiving replies – when it shouldn’t have been. Why? Because the ISP’s DNS server was redirecting traffic from the non-existent domain to their
helpfulpain-in-the-ass search website, which was responding on HTTPS with an invalid certificate. Sure, there was some fault in the Exchange setup but this side-effect shouldn’t have happened at all. It’s also happened when someone loads http://domain.com and the record doesn’t exist, and in other situations. Using Root Hints means you’re going to get a real, honest DNS lookup.
- You Get Live Data
When you query against the Root DNS Servers, you get the most recent information possible. I’ve had nameserver records change for domains that required 72 hours for an end-user to properly resolve because their ISP has (likely inappropriate) levels of caching and returned the wrong values when forwarded.
- It’s Idiot Proof
Seriously, direct lookups have been working flawlessly since … well, the mid-1980s. There’s no compelling reason to go the more complicated route with forwarders. This is pretty simple and very reliable technology, there’s no reason to rely on your ISP to get it right when you can.
- You Probably Set It Up Wrong
I’d like to ask Microsoft why, in their wisdom, they left the default timeout for DNS Forwards at 3 seconds. Most DNS resolvers time out after 2 seconds, so the second or third DNS server you’re forwarding to is never going to really service the end user on a lookup unless you drop this to 1 second. Also, have you looked at caching and other considerations? While not really complex, there are some considerations I never see accounted for when forwarding is set up. Again, let the DNS Server do its job.
DNS Forwarding Myths:
- It Reduces Traffic
This is silly – there’s very little difference in traffic between lookups. Recursive or not, the results are cached for the next lookup – the majority of DNS queries come from the cache after an initial loading period.
- It Reduces Server Load
Unless you’re familiar with your DNS Server’s source code, this argument is silly and uninformed. Who is to say that a recursive lookup through an external DNS server is any more or less computationally intensive on a machine than a non-recursive lookup? And unless you’re processing tens of thousands of DNS queries a second, it won’t matter.
- It Prevents DNS Exposure
… just wrong, wrong, wrong. Your DNS Server is going to resolve any query for a name it doesn’t know about – whether forwarding or using root hints. If your DNS server is attempting to resolve internal hostnames through your WAN port you have it configured wrong.
- It’s More Secure
Some people claim that by going to your ISP’s DNS Server, you’re reducing exposure. Firstly, DNS is a pretty simple (and pretty darned secure) protocol, particularly when it’s behind a firewall and not answering public queries. If a potential hacker has the ability to modify or inject malicious packets into lookups done through a DNS root server, why could they not do the same with your ISP’s DNS server addresses?
- It Prevents Cache Poisoning
Since Server 2003, Microsoft’s DNS server has had protections against this by default. But what do you trust more to provide a reliable DNS query/response – your ISP’s DNS servers, or the internet’s heavily fortified root servers? And if the root servers are compromised, would your ISP’s results not also be in question?
I’ll state again – there are plenty of times when you want to use DNS Forwarding. If I had a campus with dozens of buildings or many floors of users, I might have my various internal servers forward external lookups to a DNS setup somewhere else to minimize the WAN traffic of 40 DNS servers (where caching at another level would really benefit things), but those purpose-built servers would be querying the root servers for answers…