[ntp:questions] ntp.h ISBADADR(srcadr) macro

Frank Kardel kardel at ntp.org
Sun Apr 27 13:03:10 UTC 2008


Hi Andrew !

Andrew Stone wrote:
> Hi all,
> 
> I currently need to use ntp where the server for several internal clients is a 127.x.x.x address, not 127.127.x.x.
This is a pretty unusual setup.

> I noticed it is fairly easy for me to modify the ISBADADR macro in ntp.h to allow these types of addresses. Is this the only change required?
ISBADADR() is only used to prohibit exactly that configuration. Thus it 
would be the only change required. But attempting to use 127.0.0.0/8 for 
real network communication will most likely bring you into all sorts of 
trouble. ISBADADR() would be the least of your problems. See below:

> Also, what reservations do you guys have about changing this?

First reservation would be RFC 3330 (http://tools.ietf.org/html/rfc3330).

To quote RFC3330:

    ...
    127.0.0.0/8 - This block is assigned for use as the Internet host
    loopback address.  A datagram sent by a higher level protocol to an
    address anywhere within this block should loop back inside the host.
    This is ordinarily implemented using only 127.0.0.1/32 for loopback,
    but no addresses within this block should ever appear on any network
    anywhere [RFC1700, page 5].
    ...

This wisdom (even older than RFC3330) is put into many configurations
(e. g. interface/routing setups on hosts). Attempting to ignore age old 
practice might burn you.
As I said - it looks like a pretty unusual setup.

There may be more reservations... :-)

You may be better off with RFC1918 if you are trying to find a private
address space.

> 
> Thanks,
> Andrew

Frank




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