[unisog] DNS Hostname and SMTP Helo

Paul Russell prussell at nd.edu
Tue Jan 30 20:42:31 GMT 2007

On 1/30/2007 14:58, Bill Martin wrote:
> Some time back, we had saw an influx of mail originating at hosts
> providing a "helo" (hostname), where (hostname) usually started with
> some numerical value.
> Either I've lost my mind or there use to be an RFC that qualified
> "acceptable" host names and if I recall, this ties in with DNS, in that
> a hostname (especially in this case) should never start with a number,
> only a letter.  
> Is this still an acceptable method to reject?  That is, if a host
> announces itself as ([0-9]{x}hostname)??

from RFC 1123 Requirements for Internet Hosts -- Application and Support

 > 2.1  Host Names and Numbers
 >      The syntax of a legal Internet host name was specified in RFC-952
 >      [DNS:4].  One aspect of host name syntax is hereby changed: the
 >      restriction on the first character is relaxed to allow either a
 >      letter or a digit.  Host software MUST support this more liberal
 >      syntax.

This would seem to indicate that rejection based solely on the fact that
the hostname starts with a digit (0-9) is probably not valid.

Paul Russell, Senior Systems Administrator
OIT Messaging Services Team
University of Notre Dame
prussell at nd.edu

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