Subject: About name syntax

From: [email-masked]
Date: Mon, 28 Feb 2000 16:54:44 +0100

Hello,

In RFC 1035 parragraph 2.3.1. ("Preferred name syntax")  recomended characters
for domain names are a-z, A-Z, 0-9 and "-". What kind of problems could i find
if i used the "/" char inside a host name (for example the/host.my.domain).?

Thanks.

David Fernandez.

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From: Randy Bush <[email-masked]>
Date: Mon, 28 Feb 2000 09:00:00 -0800

> In RFC 1035 parragraph 2.3.1. ("Preferred name syntax") recomended
> characters for domain names are a-z, A-Z, 0-9 and "-". What kind of
> problems could i find if i used the "/" char inside a host name (for
> example the/host.my.domain).?

rfc 2181

11. Name syntax

   Occasionally it is assumed that the Domain Name System serves only
   the purpose of mapping Internet host names to data, and mapping
   Internet addresses to host names.  This is not correct, the DNS is a
   general (if somewhat limited) hierarchical database, and can store
   almost any kind of data, for almost any purpose.

   The DNS itself places only one restriction on the particular labels
   that can be used to identify resource records.  That one restriction
   relates to the length of the label and the full name.  The length of
   any one label is limited to between 1 and 63 octets.  A full domain
   name is limited to 255 octets (including the separators).  The zero
   length full name is defined as representing the root of the DNS tree,
   and is typically written and displayed as ".".  Those restrictions
   aside, any binary string whatever can be used as the label of any
   resource record.  Similarly, any binary string can serve as the value
   of any record that includes a domain name as some or all of its value
   (SOA, NS, MX, PTR, CNAME, and any others that may be added).
   Implementations of the DNS protocols must not place any restrictions
   on the labels that can be used.  In particular, DNS servers must not
   refuse to serve a zone because it contains labels that might not be
   acceptable to some DNS client programs.  A DNS server may be
   configurable to issue warnings when loading, or even to refuse to
   load, a primary zone containing labels that might be considered
   questionable, however this should not happen by default.

   Note however, that the various applications that make use of DNS data
   can have restrictions imposed on what particular values are
   acceptable in their environment.  For example, that any binary label
   can have an MX record does not imply that any binary name can be used
   as the host part of an e-mail address.  Clients of the DNS can impose
   whatever restrictions are appropriate to their circumstances on the
   values they use as keys for DNS lookup requests, and on the values
   returned by the DNS.  If the client has such restrictions, it is
   solely responsible for validating the data from the DNS to ensure
   that it conforms before it makes any use of that data.

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From: "Eric A. Hall" <[email-masked]>
Date: Mon, 28 Feb 2000 10:48:05 -0800

> What kind of problems could i find if i used the "/" char inside a
> host name (for example the/host.my.domain).?

DNS itself doesn't care; you can use any data inside of any label for
any purpose, as long as it meets the length requirements.

But if you plan to use this as a hostname, you will have a wide degree
of problems, depending upon the intended usage. Applications that use
host names will tend to barf when the hostname doesn't confor to RFC
972/1122 rules for host naming (not DNS labels, but host NAMES). You can
expect problems such as web servers rejecting the connection (later
versions of Apache do this I believe, after doing the PTR lookup),
BOOTP/DHCP servers refusing to assign a lease if that host name is
included in the string, X clients/servers not working right, .rhosts and
the like barfing, and so forth.

Lots of stuff still depends on rules for host naming. You'll find them
when you try to do it.

There are also some tricks you'll have to play to put this kind of data
into some DNS servers, who normally see / as an escape character, if you
do end up trying it.

-- 
Eric A. Hall                                            [email-masked]
+1-650-685-0557                                    http://www.ehsco.com

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From: "Eric A. Hall" <[email-masked]>
Date: Mon, 28 Feb 2000 12:31:28 -0800

> Applications that use host names will tend to barf when the hostname
> doesn't confor to RFC 972/1122 rules for host naming (not DNS labels,
> but host NAMES).

I meant RFC 952 (as supplemented by RFC 1122 which changed the max
length to 63 chars, although "man hosts" on many systems will show a 24
character limit which proves my point about backwards compatibility with
952 being important). RFC 972 does not apply to this discussion at all.
Sorry for any confusion.