Why is Amazon Route 53 named that way?

CGP Grey’s recent video about how American interstate roads are numbered made me wonder about Route 53, Amazon’s managed DNS service. If we follow the logic in his video, Route 53 would be a north-south interstate medium. There are quite a few such roads, but that’s probably not how Amazon named it – so if not a road, what is it named after?

The Route 53 marketing pages confirms a popular theory: it’s based on the port used by DNS servers.

The name for our service comes from the fact that DNS servers respond to queries on port 53 and provide answers that route end users to your applications on the Internet.

And then I wondered: why do DNS servers respond to queries on port 53?

DNS is defined by RFC 1035 Domain Names – Implementation and Specification, which was published in November 1987. There’s a section “Transport” which tells us what port to use:

The Internet supports name server access using TCP [RFC-793] on server port 53 (decimal) as well as datagram access using UDP [RFC-768] on UDP port 53 (decimal).

But this isn’t the earliest description of DNS (or its port number). At the top of RFC 1035, we’re told that it obsoletes three earlier RFCs: 882, 883, and 973. If you read all of those, you find that RFC 883 also refers to port 53 (in an appendix, with identical wording to RFC 1035) – and that was published in November 1983.

So why does RFC 883 use port 53?

The answer lies in yet another RFC, referred to in the references of 883 – RFC 870 Assigned Numbers. This explains that in the early days of the Internet, port numbers were allocated by a single person:

If you are developing a protocol or application that will require the use of a link, socket, port, protocol, or network number please contact Joyce [Reynolds] to receive a number assignment.

Joyce Reynolds
USC - Information Sciences Institute
4676 Admiralty Way
Marina del Rey, California 90292

phone: (213) 822-1511


And if we look on page 12 under “Assigned Port Numbers”, there’s a table which tells us that port 53 is assigned for DNS:

Port Assignments:

   Decimal   Octal     Description                         References
   -------   -----     -----------                         ----------
   0         0         Reserved                                 [JBP]
   1-4       1-4       Unassigned                               [JBP]
   5         5         Remote Job Entry                    [8,17,JBP]
   47        57        NI FTP                                 [59,SK]
   49        61        Login Host Protocol                      [PXD]
   51        63        IMP Logical Address Maintenance       [30,AGM]
   53        65        Domain Name Server                       [PM1]

Here [PM1] means Paul Mockapetris, the computer scientist who invented DNS and is the author on RFC 883 and RFC 1035 (among others).

(I was also wondering why only odd numbered ports were being assigned. Apparently this stems from a precursor to TCP/UDP called NCP, in which port numbers were assigned as even-odd pairs. The even port was used for “outgoing” data, and the odd port for “incoming” data – so the port numbers for inbound services were always odd.)

If we look even further back, RFC 870 replaced RFC 820 Assigned Numbers, which was published nine months earlier. (January 1983 rather than October 1983.) That includes a similar table of port assignments, but with a few more gaps:

Port Assignments:

   Decimal   Octal     Description                         References
   -------   -----     -----------                         ----------
   1         1         Old Telnet                            [40,JBP]
   3         3         Old File Transfer               [27,11,24,JBP]
   5         5         Remote Job Entry                    [6,17,JBP]
   47        57        NI FTP                                 [50,SK]
   49-53     61-65     Unassigned                               [JBP]

So presumably at some point in 1983, Paul Mockapetris asked Joyce Reynolds for a port number, she picked the next unassigned number, and 53 became the number used by DNS – and 27 years later, the name of Amazon’s managed DNS service.