[strongSwan] When is DH re-negotiated ?

[Martin Willi]

Hi Graham,

> Is DH re-negotiated everytime we rekey the IKE_SA ?

With strongSwan, yes. We always require a DH exchange. Some other
RFC4306 implementations might support non-DH IKE rekeyings, but in
RFC5996 it is a MUST.

> Here, for our initiators, we give this choice to the SeGW by
> specifying that we accept either (e.g. "aes-sha1-modp1024,aes-sha1!").

We have discussed this once before [1], a fix for strongSwan was
required to support DH and non-DH CHILD_SA proposals.

> My understanding is that by specifying the above, we are leaving it up
> to the SeGW to choose whether to always re-negotiate the DH when the
> CHILD_SA is rekeyed (by sending back to us "aes-sha1-modp1024") or to
> never re-negotiate the DH when the CHILD_SA is rekeyed (by sending
> back to us "aes-sha1").

As long as the rekeying is always initiated by the SeGW, your
understanding is correct.

> So, if the former, does this mean that a new DH is re-negotiated
> everytime we rekey the CHILD_SA AND a new DH is re-negotiated
> everytime we rekey the IKE_SA ?

> If the latter, does this mean that a new DH is re-negotiated ONLY when
> we rekey the IKE_SA ?

> Finally, if the latter, can the SeGW vary when the DH is re-negotiated
> (i.e. re-negotiate the DH every THIRD time the CHILD_SA is rekeyed) ?

Yes, yes. And yes ;-).

Regards
Martin

[1]https://lists.strongswan.org/pipermail/users/2010-April/004789.html