From RFC7296
Note that IKE_AUTH messages do not contain KEi/KEr or Ni/Nr payloads.
Thus, the SA payloads in the IKE_AUTH exchange cannot contain
Transform Type 4 (Diffie-Hellman group) with any value other than
NONE. Implementations SHOULD omit the whole transform substructure
instead of sending value NONE.
The inclusion of the DH group in the initial proposal list causes IKE_AUTH to fail with libreswan.