[strongSwan] S2S vpn between strongswan and openbsd with NAT-T doesn't work

[Tobias Brunner]

Hi Flavius,

> I changed the log level enc to 3 and attached the strongswanlog. I also attached a tcpdump from isakmpd which gives detailed information
> about the proposals. (Isakmpd makes packet captures for debugging purposes).

Thanks.  The encap mode attribute (type 4) in the first and only Quick
Mode proposal looks like this:

> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC] parsing TRANSFORM_ATTRIBUTE_V1 payload, 68 bytes left
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC] parsing payload from => 68 bytes @ 0x7f41c4000c0c
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC]    0: 80 04 00 01 80 05 00 05 80 06 01 00 05 00 00 14  ................
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC]   16: 4E 5F EA 41 8A 0F 00 F1 FF 39 9F C4 17 7E 44 F6  N_.A.....9...~D.
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC]   32: 05 00 00 10 04 00 00 00 C0 A8 DE 00 FF FF FF 00  ................
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC]   48: 00 00 00 10 04 00 00 00 C0 A8 01 00 FF FF FF 00  ................
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC]   64: 00 00 00 00                                      ....
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC]   parsing rule 0 ATTRIBUTE_FORMAT
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC]    => 1
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC]   parsing rule 1 ATTRIBUTE_TYPE
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC]    => 4
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC]   parsing rule 2 ATTRIBUTE_LENGTH_OR_VALUE
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC]    => 1
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC]   parsing rule 3 ATTRIBUTE_VALUE
> Nov  9 22:31:45 ubuntu-vpn-gw charon: 11[ENC] parsing TRANSFORM_ATTRIBUTE_V1 payload finished

That is, it's set to tunnel mode (1) and not tunnel mode with UDP
encapsulation (3).  This can also be seen in the tcpdump capture:

> 	payload: SA len: 52 DOI: 1(IPSEC) situation: IDENTITY_ONLY 
> 	    payload: PROPOSAL len: 40 proposal: 1 proto: IPSEC_ESP spisz: 4 xforms: 1 SPI: 0xe79b6410
> 	        payload: TRANSFORM len: 28
> 	            transform: 1 ID: AES
> 	                attribute LIFE_TYPE = SECONDS
> 	                attribute LIFE_DURATION = 1200
> 	                attribute ENCAPSULATION_MODE = TUNNEL
> 	                attribute AUTHENTICATION_ALGORITHM = HMAC_SHA2_256
> 	                attribute KEY_LENGTH = 256

Since there is only one proposal in the SA payload this doesn't seem
correct for an implementation claiming to support RFC 3947

> Nov  9 22:31:45 ubuntu-vpn-gw charon: 03[IKE] received NAT-T (RFC 3947) vendor ID

and wanting to use UDP encapsulation (which it apparently does use
later).  Refer to section 5.1 of RFC 3947 for details on the encap mode
negotiation [1]).

Anyway, this seems to be a known problem since 2011, see e.g. [2] for a
patch and a link to the original report.

> When I configure the tunnel in IKEv2 mode Openbsd sends no NAT-Keepalives to the ubuntuserver and the session times out on the nat-gateway. Don't know why.
> Proably also a proposal negotiation. 

No, in IKEv2 the encap mode is not negotiated.  NAT-T is an integral
part of the protocol.  If both peers support it and a NAT is detected
UDP encapsulation will be used and the hosts behind a NAT SHOULD send
keepalives (see [3]).  Maybe you can use DPD for this purpose (with a
lot more overhead, though).  Also see [4].

Regards,
Tobias

[1] https://tools.ietf.org/html/rfc3947#section-5.1
[2]
http://openbsd-archive.7691.n7.nabble.com/isakmpd-nat-t-patch-td242348.html
[3] https://tools.ietf.org/html/rfc7296#section-2.23
[4]
http://openbsd-archive.7691.n7.nabble.com/OpenIKED-Keepalive-Broken-td301428.html