If you provide some more details, somebody will be able to help.
1. How does your header/trailer records look like?- Are they fixed or can vary based on the input?
2. Is there something in your header/trailer records which can be used to dstinguish them from the data records. for e.g. 'HEADER' at pos 1-6 for header record.
3. Input and output RECFM/LRECL
4. position,length and format of the key field.