[strongSwan] Same config for strongSwan, different outcome between Android and iOS

[Tobias Brunner]

Hi Laurens,

> I've set up a strongSwan server for IKEv2. Connections with the Android 
> strongSwan app fail, while using the iOS built-in IKEv2 client works 
> without issues. Any ideas on what might be going on?

Looks like it could be an IP fragmentation issue.

> Android strongSwan client server logs:
> 
> Jun 29 01:33:15 irkalla charon: 04[NET] received packet: from 
> 1.1.1.1[40108] to 2.2.2.2[500] (732 bytes)
> Jun 29 01:33:15 irkalla charon: 04[ENC] parsed IKE_SA_INIT request 0 [ 
> SA KE No N(NATD_S_IP) N(NATD_D_IP) N(FRAG_SUP) N((16431)) N(REDIR_SUP) ]
> Jun 29 01:33:15 irkalla charon: 04[IKE] 1.1.1.1 is initiating an IKE_SA
> Jun 29 01:33:15 irkalla charon: 04[IKE] remote host is behind NAT
> Jun 29 01:33:15 irkalla charon: 04[IKE] DH group ECP_256 inacceptable, 
> requesting MODP_2048
> Jun 29 01:33:15 irkalla charon: 04[ENC] generating IKE_SA_INIT response 
> 0 [ N(INVAL_KE) ]

Since you don't have the openssl plugin loaded ECP-256 is not supported
by the server so it requests a different DH group.

> Jun 29 01:33:15 irkalla charon: 04[NET] sending packet: from 
> 2.2.2.2[500] to 1.1.1.1[40108] (38 bytes)
> Jun 29 01:33:17 irkalla charon: 08[NET] received packet: from 
> 1.1.1.1[40108] to 2.2.2.2[500] (732 bytes)
> Jun 29 01:33:17 irkalla charon: 08[ENC] parsed IKE_SA_INIT request 0 [ 
> SA KE No N(NATD_S_IP) N(NATD_D_IP) N(FRAG_SUP) N((16431)) N(REDIR_SUP) ]
> Jun 29 01:33:17 irkalla charon: 06[NET] received packet: from 

This is a retransmit of the original IKE_SA_INIT request (handling this
fails again).

> 1.1.1.1[40108] to 2.2.2.2[500] (924 bytes)
> Jun 29 01:33:17 irkalla charon: 06[ENC] parsed IKE_SA_INIT request 0 [ 
> SA KE No N(NATD_S_IP) N(NATD_D_IP) N(FRAG_SUP) N((16431)) N(REDIR_SUP) ]

This is the request with the new DH group.

> Jun 29 01:33:18 irkalla charon: 06[IKE] 1.1.1.1 is initiating an IKE_SA
> Jun 29 01:33:18 irkalla charon: 06[IKE] remote host is behind NAT
> Jun 29 01:33:18 irkalla charon: 06[ENC] generating IKE_SA_INIT response 
> 0 [ SA KE No N(NATD_S_IP) N(NATD_D_IP) N(MULT_AUTH) ]
> Jun 29 01:33:18 irkalla charon: 06[NET] sending packet: from 
> 2.2.2.2[500] to 1.1.1.1[40108] (440 bytes)

After sending the IKE_SA_INIT response the client is expected to send an
IKE_AUTH message.  If it is too big it gets fragmented into several IP
messages and some firewalls/routers might drop these.  Since the server
does not receive any IKE_AUTH messages it's likely that this happened.

Try configuring `fragmentation=yes` on the server or select the correct
CA certificate in the VPN profile on the client to avoid sending lots of
certificate requests.

Regards,
Tobias