[strongSwan] Ping does not come back / but its visible in tcpdump

Valeri Geiser valeri.geiser at km-logistik-service.de
Mon Nov 25 13:56:10 CET 2019

Hi Tobias,

thank you for the explanation. I was not sure what the issue was, but I 
figured that the port 4500 was incorrect. NAT-Traversal was already 
disabled on the Lancom, however, with also disabling MOBIKE now I only 
see port 500 and by your explanation I understand now a little better 
what the issue was.

Best regards,

Am 25.11.19 um 13:31 schrieb Tobias Brunner:
> Hi Valeri,
>> Here is tcpdump from what I think is the ping and its response (pinging
>> which is assigned to Lancom on ethernet port 1):
>> 22:03:20.304824 IP (tos 0x0, ttl 64, id 1894, offset 0, flags [DF],
>> proto ESP (50), length 140)
>>      A.A.A.A > B.B.B.B: ESP(spi=0xbf3e0bb5,seq=0x224), length 120
>> 22:03:20.320540 IP (tos 0x0, ttl 57, id 34530, offset 0, flags [none],
>> proto UDP (17), length 148)
>>      B.B.B.B.ipsec-nat-t > A.A.A.A.ipsec-nat-t: [no cksum] UDP-encap:
>> ESP(spi=0xc9012da8,seq=0x223), length 120
>> I am just clueless now and any help is appreciated. Let me know if any
>> further information is required.
> As you can see, the other peer somehow decides to use UDP-encapsulation
> for ESP, even though there apparently is no NAT between the two.  Since
> the Linux kernel can't process UDP-encapsulated packets for SAs that
> aren't configured for it (a known limitation) the inbound packets will
> be dropped.  I guess by disabling MOBIKE you prevent the other
> implementation from enabling UDP-ecapsulation.
> Regards,
> Tobias

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MFG, Valeri Geiser

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