While XEP 298 does say:
The namespace of the forwarded stanza MUST be preserved (this is typically
'jabber:client'). If no 'xmlns' is set for the stanza then as per XML
namespacing rules it would inherit the 'urn:xmpp:forward:0' namespace, which is
wrong.
This, by itself, is insufficient to ensure all relevant namespaces of the
original stanza are preserved. Any namespaces of the stream which the original
message was received on, as well those of the original stanza need to preserved
if used in the original stanza, need to preserved. Also, lang tags should
preserved.
One way to do this is for the forwarded to copy all the name space declarations
of the stream the original message was received into the forward element, use a
prefix for the 'urn:xmpp:forward:0' which doesn't clash, and copy all prefixes
of the original message with the original message, using the same prefixes as
on the stream it was received on.
So, for instance, if one received:
<stream:stream
from='[email protected]'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<message>
<body>foo</body>
</message>
</stream:stream>
The element to be forwarded (inside a suitable message element) would be:
<forwarded:forwarded xmlns:forwarded='urn:xmpp:forward:0
xml:lang='en'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'>
<message>
<body>foo</body>
</message>
</forwarded:forwarded>
Or if one received:
<stream
from='[email protected]'
to='im.example.com'
version='1.0'
xml:lang='en'
xmlns='http://etherx.jabber.org/streams'>
<message xmlns='jabber:client'>
<body>foo</body>
</message>
</stream>
<forwarded:forwarded xmlns:forwarded='urn:xmpp:forward:0'xml:lang='en'
xml:lang='en'
xmlns='http://etherx.jabber.org/streams'>
<message xmlns='jabber:client'>
<body>foo</body>
</message>
</forwarded:forwarded>
