/usr/share/gromacs/top/amber96.ff/dna.r2b is in gromacs-data 2018.1-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | ; rtp residue to rtp building block table
;GMX Force-field
DA DA DA5 DA3 DAN
DG DG DG5 DG3 DGN
DC DC DC5 DC3 DCN
DT DT DT5 DT3 DTN
|