reasonable length of FQDN
Mark_Andrews at isc.org
Wed Feb 1 12:49:16 UTC 2006
> Mark, isn't that the maximum length of the wire representation of a
> domain name?
Yes. All other lengths are derived from that.
> arpa/nameser.h in libbind defines NS_MAXDNAME to be 1025,
> with the comment 'maximum domain name', while NS_MAXCDNAME is defined
> as 255 with the comment 'maximum compressed domain name'. I've always
> wondered where the 1025 came from.
#define DNS_NAME_MAXTEXT 1023
* The maximum length of the text representation of a domain
* name as generated by dns_name_totext(). This does not
* include space for a terminating NULL.
* This definition is conservative - the actual maximum
* is 1004, derived as follows:
* A backslash-decimal escaped character takes 4 bytes.
* A wire-encoded name can be up to 255 bytes and each
* label is one length byte + at most 63 bytes of data.
* Maximizing the label lengths gives us a name of
* three 63-octet labels, one 61-octet label, and the
* root label:
* 1 + 63 + 1 + 63 + 1 + 63 + 1 + 61 + 1 = 255
* When printed, this is (3 * 63 + 61) * 4
* bytes for the escaped label data + 4 bytes for the
* dot terminating each label = 1004 bytes total.
#define DNS_NAME_FORMATSIZE (DNS_NAME_MAXTEXT + 1)
* Suggested size of buffer passed to dns_name_format().
* Includes space for the terminating NULL.
> Also, there's an older post on this list that mentions 1004 as the
> limit, quoting RFC1035 as the source (however, the number is not
> explicitly mentioned in that document):
> On 2/1/06, Mark Andrews <Mark_Andrews at isc.org> wrote:
> > From RFC 1035:
> > To simplify implementations, the total number of octets that represent a
> > domain name (i.e., the sum of all label octets and label lengths) is
> > limited to 255.
> > > TIA~
> > >
> > > --
> > > Roman
> > >
> > >
> > --
> > Mark Andrews, ISC
> > 1 Seymour St., Dundas Valley, NSW 2117, Australia
> > PHONE: +61 2 9871 4742 INTERNET: Mark_Andrews at isc.org
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742 INTERNET: Mark_Andrews at isc.org
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