I need to know the structure of a server key exchange message when using a psk identity hint. I know the general structure but I can't find an example of where the identity hint fits in. Thanks.
What is the structure of a Server Key Exchange Message structure when using a psk identity hint?
RFC 4279, section 5.2, says this about identity hints:
In the absence of an application profile specification specifying otherwise, servers SHOULD NOT provide an identity hint and clients MUST ignore the identity hint field. Applications that do use this field MUST specify its contents, how the value is chosen by the TLS server, and what the TLS client is expected to do with the value.
So what this means is that it is up to you, the application designer, to define the hint structure. From the TLS point of view, the hint is an opaque sequence of bytes (no longer than 65535 bytes, but otherwise arbitrary).
In this answer, it is claimed that the "hint" was used directly in the key derivation system in the case of a protocol called "NETCONF", but this was dropped during the definition of the RFC (i.e. draft 4 has it, not draft 5 and subsequent). Even if the PSK support was not ultimately used in NETCONF, this shows how such a definition is to be done: each protocol that leverages TLS-PSK is supposed to specify the hint format and processing.