[strongSwan] Broken CHILD_SA following IKE_SA re-auth with FortiGate remote

Kris KRI2183876 at maricopa.edu
Mon Aug 22 10:16:56 CEST 2016

Why not file issue at https://wiki.strongswan.org/projects/strongswan/issues/new


On Fri, Aug 5, 2016 at 5:56 PM, Tore Anderson <tore at fud.no> wrote:
> Hi,
> We recently experienced that an IKEv2-negotiated ESP site-to-site
> tunnel between strongSwan 5.3.5 running on Ubuntu 16.04 and a Fortinet
> FortiGate router broke following the re-auth of the IKE_SA. Just one
> out of six ESP CHILD_SAs broke.
> I've uploaded config files, charon logs, and other debugging info to:
> https://gist.github.com/toreanderson/effeaae2432abc965398f17c3d2161ef
> Basically what seems to have happened is the following:
> 1) strongSwan decides it's time to re-auth the IKE_SA, and sends an
> IKEv2 INFORMATIONAL with DELETE to the FortiGate. From the logs:
> 15[IKE] sending DELETE for IKE_SA tun1[6447]
> 2) Immediately afterwards strongSwan and the FortiGate both sends and
> IKE_SA_INIT to establish a new IKE SA.
> 09[IKE] initiating IKE_SA tun1[6471] to
> 12[IKE] is initiating an IKE_SA
> 3) Both of these are successfully established along with a CHILD_SA:
> 10[IKE] IKE_SA tun1[6471] established between[]...[]
> 10[IKE] CHILD_SA tun1{14549} established with SPIs cc122363_i 104b86c4_o and TS ===
> 06[IKE] IKE_SA tun1[6472] established between[]...[]
> 06[IKE] CHILD_SA tun1{14551} established with SPIs c1f9cea7_i 104b86c3_o and TS ===
> So now we have a redundant IKE_SA and a redundant ESP_SA.
> 4) The FortiGate detects the mid-air collision and requests to delete
> one of the two IKE_SAs:
> 14[IKE] received DELETE for IKE_SA tun1[6472]
> 5) strongSwan acts IKE_SA DELETE on this by deleting not only the
> IKE_SA, but also the c1f9cea7_i 104b86c3_o CHILD_SA - at least it does
> not occur in the output from "ipsec statusall". The FortiGate does
> however NOT delete that CHILD_SA, indeed, it keeps on actively using
> it. This can be seen from the ESP traffic resulting from a ping packet
> sent from to
> 09:27:19.639105 IP > ESP(spi=0x104b86c4,seq=0xd0f), length 136
> 09:27:19.647375 IP > ESP(spi=0xc1f9cea7,seq=0x121d), length 136
> SPI 0x104b86c4 still obviously exists on both the strongSwan (
> and the FortiGate ( However, strongSwan has already deleted
> SPI 0xc1f9cea7 so it has no way of decrypting those packets. The result
> is that all traffic from to was being
> blackholed until the tunnel was manually brought down and up again by
> NOC staff.
> That concludes my analysis of the incident. I am however not sure if is
> the strongSwan or FortiGate that did someting wrong here (or both), and
> hoping this list might have some insight. The way I see it, the key
> questions are the following:
> - Is the strongSwan behaving correctly when it is also deleting the ESP
>   CHILD_SA when receiving the DELETE IKE_SA from the FortiGate, instead
>   of "moving" it to the other active IKE_SA as it appears the FortiGate
>   has done? RFC4306, section 2.4 says the following:
>   «Closing the IKE_SA implicitly closes all associated CHILD_SAs.
>   ...but this doesn't mention the corner case where there are two
>   parallel CHILD_SAs, as was the case for us.
> - Why does the strongSwan rekey by first deleting the existing IKE_SA
>   and then initiating a new one, instead of the other way around? This
>   seems to me to be a violation of RFC4306, section 2.8, paragraph 4:
>    «SAs SHOULD be rekeyed proactively, i.e., the new SA should be
>    established before the old one expires and becomes unusable.  Enough
>    time should elapse between the time the new SA is established and the
>    old one becomes unusable so that traffic can be switched over to the
>    new SA.»
> It would appear to me that if this SHOULD had been followed, the
> FortiGate would likely not have initiated an IKE_SA of its own
> following the strongSwan's deletion of the old one, and the blackholing
> of traffic would have been avoided.
> Tore
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