[190736] in North American Network Operators' Group
Re: number of characters in a domain?
daemon@ATHENA.MIT.EDU (William Herrin)
Sat Jul 23 12:36:40 2016
X-Original-To: nanog@nanog.org
X-Really-To: <nanog@nanog.org>
In-Reply-To: <CAAAwwbVzY3Qam8q9ERVvKJtM8RaksHmVainLwbxQHs6cNUaugQ@mail.gmail.com>
From: William Herrin <bill@herrin.us>
Date: Sat, 23 Jul 2016 12:36:10 -0400
To: Jimmy Hess <mysidia@gmail.com>
Cc: "nanog@nanog.org" <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
On Sat, Jul 23, 2016 at 11:07 AM, Jimmy Hess <mysidia@gmail.com> wrote:
> In addition, the string component of each DNS label is limited to 63 octets.
This is a hard limit in the DNS packet format. In the packet, the dots
are replaced by either:
1 byte whose high two bits are 0 and whose low six bits are the length
of the next label, 0 to 63. 0 means done, end of the name.
2 bytes, whose high two bits are 1 and whose low 14 bits are the byte
offset within the DNS packet where the name continues
So what happens is: if there are three names in the DNS packet:
www.example.com, ns1.example.com, and ns2.example.com, then the packet
will store www.example.com in full (with the dots replaced with the
next label length) and then it'll store ns1 followed by a pointer to
where "example.com" began in www.example.com and finally it'll store
ns2 followed by a pointer to where "example.com" began in
www.example.com.
Regards,
Bill Herrin
--
William Herrin ................ herrin@dirtside.com bill@herrin.us
Owner, Dirtside Systems ......... Web: <http://www.dirtside.com/>
