DNS Crash Course
DNS is an integral part of the Internet and a very important application layer protocol. Still, developers building Web applications usually don’t think about DNS and expect it to “just work”, or don’t take it into account when designing their systems.
Well, not anymore. Read on for an explanation of what makes DNS tick, how to debug DNS issues and ultimately understand how your Web applications are resolved in order to build better software.
Skip to Tying it all together for a hand on
demonstration of how to use
dig to understand DNS, but it won’t make
much sense without understanding the process first.
Knowing how to control and manipulate DNS will make your life easier, especially as you move towards design and architecture roles.
When developing a new feature, API or product, it’s critical that you know, understand and are able to manipulate the DNS record of the zone that you will be using for it. Features like vanity URLs, custom domains, redirects, SSL, load balancing, all of that depends on the proper configuration of DNS.
It can also significantly speed your local Internet connection, block malware and advertising, and more.
What is DNS?
When someone talks about DNS, they might be referring to multiple things. DNS is a hierarchical, decentralized, heavily cached and recursive naming system. At its core is a database with specific data structures to store various types of records. DNS is also a protocol for how to query that database. It is used mainly to map domain names to IP addresses.
DNS the protocol can use many transport protocols, but most commonly uses UDP at port 53. Unlike HTTP, FTP, POP and other application layer protocols, DNS is a binary protocol.
The DNS system is made up of several parts. It’s important to understand what they are so when you are working with DNS or debugging issues you know what the documentation or tool is referring to.
Parts of the DNS system
- Database system. Used to store records. It’s not a general purpose database, though it has been used to store other types of information, for example to fight email spam.
- Records. Mappings created in the DNS database. There are various types of records. Think of this as rows in a relational table. See Records below.
- Query. A lookup sent to the DNS system asking for a specific domain information.
- Resolver. What you typically use to query the DNS system. You don’t connect directly to a DNS server, instead you connect to a resolver. There are many. Operating systems, standard libraries and applications sometimes use their own, making things more complicated.
- Name servers. When you think about DNS you probably are thinking about name servers. These are the servers that respond to queries and there are many types.
- TTL. Time To Live. This setting tells the resolver how long it should keep a record in cache. Set it too low and it will generate too much traffic to your authoritative server (don’t worry, I explain what these are below). Set it too high and you won’t be able to do quick changes.
Types of DNS records
DNS name servers store many types of records. It’s important to know what they all mean.
- A record. Address Mapping. Also known as host record. Stores a
hostname (for example
lms) and a corresponding IP address like
- AAAA record. Same as the A record, but for IPv6.
- CNAME record. Canonical name. Aliases one hostname to another hostname. When the response to a query is a CNAME record, the DNS system will repeat the query with the new hostname.
The list above would be sufficient for most developers working with Web applications, but there are other records that are also very important and you should know:
- MX records. Mail exchange. Specifies the SMTP servers to use for the zone (read, domain).
- NS records. Name server. Specifies the authoritative name servers that will be used to resolve this zone. See below for an explanation on authoritative name servers. This is how you can, for example, take your domain registered with GoDaddy and administer it with Route53 (the authoritative DNS system by AWS).
- SOA records. Start of Authority. It’s the first record on a zone file. Indicates the authoritative name server for this zone and other zone related information.
How a DNS query is resolved
At the top of it all are 13 servers. These 13 servers are called root servers, and are maintained by the Internet Assigned Numbers Authority or IANA. Every resolver knows the IP addresses of these 13 servers.
|b.root-servers.net||126.96.36.199||2001:500:200::b||University of Southern California|
|d.root-servers.net||188.8.131.52||2001:500:2d::d||University of Maryland|
|f.root-servers.net||184.108.40.206||2001:500:2f::f||Internet Systems Consortium|
|g.root-servers.net||220.127.116.11||2001:500:12::d0d||US Department of Defense|
The official root name servers file can always be found at https://www.internic.net/domain/named.root which I like to think as the bootstrap of the Internet.
When your resolver issues a query, the query will traverse down the server hierarchy until it reaches what is know as its authoritative name server, which is a fancy name of referring to the name server that’s got the answer.
The first stop is one of the root servers.
Assuming that your query is not cached, the root servers are the first stop in the quest for DNS records. It’s job is to direct the query to a top level domain, or TLD. Since your resolver knows about these root servers, it is able to proceed.
IANA manages the TLD servers. The TLD servers are split into two groups:
- Generic top level domains suco as .com, .org, .net, .edu and more. There’s a ton of them.
- Country code top level domains. These are the .mx, .co.uk, .br, and so on.
The root name server will give you an answer about the TLD server that
should follow, based on the top level domain of your query. That is,
.com then go here, if it’s
.net go there, etc.
The job of the TLD is to direct you to an authoritative name server.
Authoritative name server
This is the server that has information about your specific domain. It’s the last step of the resolver. NS records point to authoritative name servers.
The result will be a CNAME (remember canonical records). Now your resolver will recursively take this CNAME record and issue a new query to resolve the DNS record of the name server, until it can get the A record of the query that you originally submitted.
Tying it all together
To better understand it all, let’s say we want to resolve this super
unknown domain name
www.cesarolea.com. We will use the Domain
Information Groper tool otherwise known as
Disclaimer: I will use the
@nsoption in the following examples to force a query done using a specific resolver, however your results might vary as it is highly dependant on your network settings
dig @18.104.22.168 www.cesarolea.com +trace
The result will be pretty verbose, but we can analyze it step by step.
The first step is contacting our resolver in IP
22.214.171.124 in this
case (since we forced
dig use it for name resolution) and the
resolver will contact the root servers to know which TLD to use
; <<>> DiG 9.10.6 <<>> @126.96.36.199 www.cesarolea.com +trace ; (1 server found) ;; global options: +cmd . 510166 IN NS a.root-servers.net. . 510166 IN NS b.root-servers.net. . 510166 IN NS c.root-servers.net. . 510166 IN NS d.root-servers.net. . 510166 IN NS e.root-servers.net. . 510166 IN NS f.root-servers.net. . 510166 IN NS g.root-servers.net. . 510166 IN NS h.root-servers.net. . 510166 IN NS i.root-servers.net. . 510166 IN NS j.root-servers.net. . 510166 IN NS k.root-servers.net. . 510166 IN NS l.root-servers.net. . 510166 IN NS m.root-servers.net. . 510166 IN RRSIG NS 8 0 518400 20201015200000 20201002190000 26116 . xEhSOlZxO8hyn9RB9oSt/4DwHte1eutylIPM6iprEVcFEjOFQ4qJQkXw xN2ogWz12athGKq4JF9WxZB4o+qhd/v2ihGkiqjtxFcQH27F+5dvU1Jd irzrXPo6GO5cndjYK3nkOCtJGjxgGFIIX+0TUhOU59TDkfBY6UBD7lAS tZoGkQFqAcCrnQ3ENucpN7Gp9marRTaYRFNlj45SH+i8X2GYFY14xhlL K6YKUaULNNqsRaQp1w7MlhpXq+KHYLIqFe/+eSEqa04558L/Gfz6RMNP Y/hfqwHnRiapnWC3qebD42hDhbC/Fdr90iO2ZW10EL3qewwAb+fPKv2/ 5UtiDg== ;; Received 525 bytes from 188.8.131.52#53(184.108.40.206) in 31 ms
From the root servers we got the proper TLD servers for our
com. 172800 IN NS i.gtld-servers.net. com. 172800 IN NS g.gtld-servers.net. com. 172800 IN NS m.gtld-servers.net. com. 172800 IN NS e.gtld-servers.net. com. 172800 IN NS a.gtld-servers.net. com. 172800 IN NS b.gtld-servers.net. com. 172800 IN NS l.gtld-servers.net. com. 172800 IN NS d.gtld-servers.net. com. 172800 IN NS j.gtld-servers.net. com. 172800 IN NS c.gtld-servers.net. com. 172800 IN NS k.gtld-servers.net. com. 172800 IN NS f.gtld-servers.net. com. 172800 IN NS h.gtld-servers.net. com. 86400 IN DS 30909 8 2 E2D3C916F6DEEAC73294E8268FB5885044A833FC5459588F4A9184CF C41A5766 com. 86400 IN RRSIG DS 8 1 86400 20201015200000 20201002190000 26116 . mo5DAGGRFxGHjs+H3xi1ev/fSKCxRsT6DOwNbnY4ZEii4EAyc4tk/lCa aXEO1iwKjl5WfZSwRZ8WOuZvLvjYoWbPPDhy05j/wqv4kMW7m5Jvi6sN kdi09u/vEfXGrYltfzInx7/HiDoucp3S1HDlg0zTpUSaryBWmCoCh7De T+BiZgyo2ZnnWe0pwtqWQsnzcv8PwH18+VqwPvtwzfPaNySQiBH9tpJ9 ysRxDOI/W4yJlGA3yTSAxcdgd5Z36N44zlVWLT0/BG+zeKuC/lYlatQP 8MaQ39hkW+LHgvAQ99/PNAvK9PQf+1hEwyzcKHQGoYLgSpUvMp1JiRaW KjS/Fg== ;; Received 1177 bytes from 2001:dc3::35#53(m.root-servers.net) in 70 ms
Authoritative name servers
From the TLD servers we got the authoritative name servers. These
are the servers that ultimately know about our specific domain. In
this case it seems
cesarolea.com is using Route53 (AWS DNS
cesarolea.com. 172800 IN NS ns-624.awsdns-14.net. cesarolea.com. 172800 IN NS ns-435.awsdns-54.com. cesarolea.com. 172800 IN NS ns-1933.awsdns-49.co.uk. cesarolea.com. 172800 IN NS ns-1427.awsdns-50.org. CK0POJMG874LJREF7EFN8430QVIT8BSM.com. 86400 IN NSEC3 1 1 0 - CK0Q1GIN43N1ARRC9OSM6QPQR81H5M9A NS SOA RRSIG DNSKEY NSEC3PARAM CK0POJMG874LJREF7EFN8430QVIT8BSM.com. 86400 IN RRSIG NSEC3 8 2 86400 20201008044114 20201001033114 24966 com. l1OVIJsccUokGhWAyWFevZEQfxZPhdQ75DRjtOTosLNklP/JgVcZ88EK zXkMrVZDRQworiQjLVQIajUlPNE+ZE6J+GxtO54iD75CM8bJ3V4LiOLZ WegKaGX3a+4IMW0B9xXIUSyt+WYQFGBpqbGuzrspSo4in64OIXbkt+7E 1H6qvEcTSOgIbq/Bs1k6Rhbv3NS68fSscvCFR3nB7NBtlQ== S4EV7AUHR1O2EF4EE68320R9LCBN7KEP.com. 86400 IN NSEC3 1 1 0 - S4EVG1T0BSONCRQJ0K9HNUQQ0H2R0HHC NS DS RRSIG S4EV7AUHR1O2EF4EE68320R9LCBN7KEP.com. 86400 IN RRSIG NSEC3 8 2 86400 20201006055010 20200929044010 24966 com. rT28/eZdIgIXIk1i3uBZlghqtAyEBP7usqeJsCiqw/ptM8ZvF0Xj5X/m 5e5mfpnpfRgLOYFWOPrAMJyYWJg6rEsZrEWI6jPnUvQeQT6QGlvY89yV lPFD6CIvRm9DROegRzf8tHqj078y10MPZ2EqX9rogDWDrBDhi63Tu9uQ 1PAFFY7ySsqmXnXySoyaEcgDGF6L0EhRfiVkaL4on5h79A== ;; Received 748 bytes from 2001:502:8cc::30#53(h.gtld-servers.net) in 74 ms
Ultimately what we need is an IP address to go with the domain name. This is precisely what we get from the authoritative name servers.
www.cesarolea.com. 5 IN A 220.127.116.11 cesarolea.com. 172800 IN NS ns-1427.awsdns-50.org. cesarolea.com. 172800 IN NS ns-1933.awsdns-49.co.uk. cesarolea.com. 172800 IN NS ns-435.awsdns-54.com. cesarolea.com. 172800 IN NS ns-624.awsdns-14.net. ;; Received 199 bytes from 2600:9000:5301:b300::1#53(ns-435.awsdns-54.com) in 31 ms
www.cesarolea.com was a CNAME instead of an A record, the name
server would have made another lookup with the CNAME result, and
then finally the A record with the IP address. Here’s an example
mail.google.com. 604800 IN CNAME googlemail.l.google.com. googlemail.l.google.com. 300 IN A 18.104.22.168 ;; Received 87 bytes from 22.214.171.124#53(ns2.google.com) in 34 ms
mail.google.com is a CNAME pointing to
and that, in turn, is an A record pointing to its final IP
As always, there’s more to DNS than meets the eye. I’ve tried to present here very practical information on DNS that should get you started on better understanding how DNS works, and debugging any issues you may find in your own applications.