[erlang-questions] is inet:gethostbyname( IP ) correct?

Roberto Aloi <>
Wed Oct 14 11:34:53 CEST 2009


This is due to the dot notation used in inet_addr().

Reading from the doc:
Values specified using the dot notation take one of the following forms:

*/a/*.*/b/*.*/c/*.*/d/*
*/a/*.*/b/*.*/c/*
*/a/*.*/b/*
*/a/*

Each of the four notation types are described below.

    * */a/*.*/b/*.*/c/*.*/d/* notation

      When four parts are specified, each is interpreted as a byte of
      data and assigned, from left to right, to the four bytes of an
      Internet address.

    * */a/*.*/b/*.*/c/* notation

      When a three-part address is specified, the last part is
      interpreted as a 16-bit quantity and placed in the right most two
      bytes of the network address. This makes the three-part address
      format convenient for specifying Class B network addresses as
      128.*/net/*.*/host/*.

    * */a/*.*/b/* notation

      When a two-part address is supplied, the last part is interpreted
      as a 24-bit quantity and placed in the right most three bytes of
      the network address. This makes the two-part address format
      convenient for specifying Class A network addresses as
      */net/*.*/host/*.

    * */a/* notation

      When only one part is given, the value is stored directly in the
      network address without any byte rearrangement.

All numbers supplied as parts in dot notation may be decimal, octal, or
hexadecimal, as specified in the C language (that is, a leading 0x or 0X
implies hexadecimal; otherwise, a leading 0 implies octal; otherwise,
the number is interpreted as decimal).

Sources:
http://www.ietf.org/rfc/rfc3493.txt
http://uw714doc.sco.com/en/man/html.3N/inet.3N.html

Best regards,

Roberto Aloi
Erlang Training and Consulting Ltd.
http://www.erlang-consulting.com
http://aloiroberto.wordpress.com

Garry Hodgson wrote:
> recently, a bug in my code caused us to pass a string
> representing a floating point number to inet:gethostbyname().
> i would have expected it to return an error, but instead it
> returned an ip address, but one that made no sense to me:
>
> 1> inet:gethostbyname( '12.27' ).
> {ok,{hostent,"12.27",[],inet,4,[{12,0,0,27}]}}
>
> so my question is, is this behavior correct, and if so, what
> exactly does it mean that a lookup of '12.27' maps to '12.0.0.27'?
>
> thanks
>



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