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Shortest URLs on the Internet

August 29, 2022

What's the shortest URL for which content is served on the internet?
There are quite a few single-letter second-level domains that are
used in very short URLs, such as Google's http://g.co or Facebook's
https://m.me.

But we can go shorter than this: there's nothing stopping TLD
registry operators serving A records at the apexes of their TLD
zones. For example, if Verisign (the operator of the com TLD
registry) wished, they could add an A record at the apex of the com
TLD zone - com would then resolve to that IP, and your browser would
connect to that IP when you visited https://com.

Does any registry operator actually do this though? Surprisingly, the
answer is yes.

---------------------------------------------------------------------

First, let's find a list of TLDs which have A records.

$ wget https://data.iana.org/TLD/tlds-alpha-by-domain.txt
$ <tlds-alpha-by-domain.txt tail -n+2 | xargs -I{} ./get_a_record_tsv.sh {} > tld-a-records.tsv

where get_a_record_tsv.sh is:

#!/usr/bin/env bash
set -euo pipefail

name="$1"
records=$(dig +short @8.8.8.8 "$name" | head -n1)

echo -e "${name}\t${records}"

Now, let's get the TLDs that have A records:

$ <tld-a-records.tsv grep -Ev $'\t$'
AI      209.59.119.34
ARAB    127.0.53.53
CM      195.24.205.60
IN      e.root-servers.net.
KIDS    127.0.53.53
MD      e.root-servers.net.
MG      a.root-servers.net.
MR      m.root-servers.net.
MUSIC   127.0.53.53
MX      a.root-servers.net.
PN      139.162.17.173
TK      217.119.57.22
UZ      91.212.89.8
WS      64.70.19.33
XN--L1ACC       218.100.84.27
XN--MXTQ1M      127.0.53.53
XN--NGBRX       127.0.53.53

Some of these aren't relevant to us:

  * There's a bunch that have A records pointing at 127.0.53.53
    (which is an internal IP): arab, `rb (translates to "arab"), kids
    , music, and Zheng Fu  (translates to "government"). It turns out this
    is a special IP address designed to alert network adminstrators
    of name collisions: cases where networks have privately been
    using names underneath TLDs which used to be free, but have now
    been allocated as gTLDs. The 53.53 is a reference to port 53, the
    port on which DNS nameservers listen.
  * Some have A records pointing to the root servers. I'm not sure
    why this is - I think it might be because no DNS has been
    configured in the TLD zone for these TLDs. Edit: matttpt on HN
    pointed out the reason for this.

Filtering out the TLDs mentioned above, we get:

AI      209.59.119.34
CM      195.24.205.60
PN      139.162.17.173
TK      217.119.57.22
UZ      91.212.89.8
WS      64.70.19.33
XN--L1ACC       218.100.84.27

These are, in turn, the ccTLDs for Anguilla, Cameroon, the Pitcairn
Islands, Tokelau, Uzbekistan, Samoa and Mongolia (the TLD is an
internationalised ccTLD in this case).

---------------------------------------------------------------------

Let's open up the TLDs we found in a browser and see what happens:

$ <tld-a-records.tsv cut -f1 | xargs -I{} xdg-open 'http://{}'
$ <tld-a-records.tsv cut -f1 | xargs -I{} xdg-open 'https://{}'

Some of them do indeed load:

  * http://ai is a nice retro landing page for "Offshore Information
    Services", who seem to maintain the .ai TLD
  * http://pn serves an "it works" page; https://pn serves "hello
    world" (with an invalid certificate)
  * http://uz serves a HTTP 500; https://uz serves the Uzbekistan
    ccTLD's homepage (with an invalid certificate)
  * http://mon eventually serves a blank page, but does also serve a
    favicon; https://mon eventually serves a default Apache landing
    page (with an invalid certificate)

Screenshot of http://ai

Screenshot of http://ai

The others either time out or refuse the connection.

---------------------------------------------------------------------

This practice - publishing A records at TLD apexes - is discouraged
by ICANN:

    Recommendation: Dotless domains will not be universally reachable
    and the SSAC recommends strongly against their use. As a result,
    the SSAC also recommends that the use of DNS resource records
    such as A, AAAA, and MX in the apex of a TopLevel Domain (TLD) be
    contractually prohibited where appropriate and strongly
    discouraged in all cases.

And, furthermore, the use of dotless domains is prohibited for new
gTLDs.

---------------------------------------------------------------------

Bonus fact: there's also nothing stopping ICANN adding an A record to
the apex of the root zone, which would theoretically make the empty
hostname resolvable. I imagine most browsers etc. would consider a
URL with an empty hostname invalid - Chrome considers both http://
and http://. invalid, at least.

                        James Williams * 2020