[unisog] DNS Hostname and SMTP Helo

scott hollatz shollatz at d.umn.edu
Tue Jan 30 20:27:18 GMT 2007

Hash: SHA1

> 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)??
> Any help would be greatly appreciated

RFC 1034 gives this syntax:

<domain> ::= <subdomain> | " "

<subdomain> ::= <label> | <subdomain> "." <label>

<label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]

<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>

<let-dig-hyp> ::= <let-dig> | "-"

<let-dig> ::= <letter> | <digit>

<letter> ::= any one of the 52 alphabetic characters A through Z in
upper case and a through z in lower case

<digit> ::= any one of the ten digits 0 through 9

- --
scott hollatz                                        net shollatz at d.UMn.eDu
information technology systems and services          tel +1 218 726 8851
university of minnesota duluth mn usa                fax +1 218 726 7674
                                               "Asn aD ta zlAp em uT zt33rg"
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