Re: [idn] Length limits on a domain name

[Patrik Fältström <paf@swip.net>]

At 18.11 +0200 00-05-20, Dan wrote:
>A domain names is composed of labels. The current DNS specification
>limits a label to 63 bytes and a domain name to 255.
>The 63 byte limit is because the current protocol does not allow
>longer labels, and the total 255 is just a selected value.

Please read this part of RFC 1035. It sets a number of restrictions 
on the label due to the compression algorithm used.

>4.1.4. Message compression
>
>In order to reduce the size of messages, the domain system utilizes a
>compression scheme which eliminates the repetition of domain names in a
>message.  In this scheme, an entire domain name or a list of labels at
>the end of a domain name is replaced with a pointer to a prior occurance
>of the same name.
>
>The pointer takes the form of a two octet sequence:
>
>     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>     | 1  1|                OFFSET                   |
>     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>
>The first two bits are ones.  This allows a pointer to be distinguished
>from a label, since the label must begin with two zero bits because
>labels are restricted to 63 octets or less.  (The 10 and 01 combinations
>are reserved for future use.)  The OFFSET field specifies an offset from
>the start of the message (i.e., the first octet of the ID field in the
>domain header).  A zero offset specifies the first byte of the ID field,
>etc.
>
>The compression scheme allows a domain name in a message to be
>represented as either:
>
>    - a sequence of labels ending in a zero octet
>
>    - a pointer
>
>    - a sequence of labels ending with a pointer
>
>Pointers can only be used for occurances of a domain name where the
>format is not class specific.  If this were not the case, a name server
>or resolver would be required to know the format of all RRs it handled.
>As yet, there are no such cases, but they may occur in future RDATA
>formats.
>
>If a domain name is contained in a part of the message subject to a
>length field (such as the RDATA section of an RR), and compression is
>used, the length of the compressed name is used in the length
>calculation, rather than the length of the expanded name.
>
>Programs are free to avoid using pointers in messages they generate,
>although this will reduce datagram capacity, and may cause truncation.
>However all programs are required to understand arriving messages that
>contain pointers.
>
>For example, a datagram might need to use the domain names F.ISI.ARPA,
>FOO.F.ISI.ARPA, ARPA, and the root.  Ignoring the other fields of the
>message, these domain names might be represented as:
>
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>     20 |           1           |           F           |
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>     22 |           3           |           I           |
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>     24 |           S           |           I           |
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>     26 |           4           |           A           |
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>     28 |           R           |           P           |
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>     30 |           A           |           0           |
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>     40 |           3           |           F           |
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>     42 |           O           |           O           |
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>     44 | 1  1|                20                       |
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>     64 | 1  1|                26                       |
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>     92 |           0           |                       |
>        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
>
>The domain name for F.ISI.ARPA is shown at offset 20.  The domain name
>FOO.F.ISI.ARPA is shown at offset 40; this definition uses a pointer to
>concatenate a label for FOO to the previously defined F.ISI.ARPA.  The
>domain name ARPA is defined at offset 64 using a pointer to the ARPA
>component of the name F.ISI.ARPA at 20; note that this pointer relies on
>ARPA being the last label in the string at 20.  The root domain name is
>defined by a single octet of zeros at 92; the root domain name has no
>labels.

     paf