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First, capture the traffic over the IPsec tunnel of the FortiGate.
Note that there is outbound traffic but no inbound traffic.
diag sniffer packet any 'host 52.138.0.244' 4 0 l
interfaces=[any]
filters=[host 52.138.0.244]
2021-11-03 19:20:31.343034 port7 out 209.15.95.34 -> 52.138.0.244: ESP(spi=0xac6e0cbe,seq=0x4e)
2021-11-03 19:20:31.454182 port7 out 209.15.95.34 -> 52.138.0.244: ESP(spi=0xac6e0cbe,seq=0x4f)
2021-11-03 19:20:32.794925 port7 out 209.15.95.34 -> 52.138.0.244: ESP(spi=0xac6e0cbe,seq=0x50)
2021-11-03 19:20:33.151516 port7 out 209.15.95.34 -> 52.138.0.244: ESP(spi=0xac6e0cbe,seq=0x51)
2021-11-03 19:20:33.151591 port7 out 209.15.95.34 -> 52.138.0.244: ESP(spi=0xac6e0cbe,seq=0x52)
2021-11-03 19:20:33.151669 port7 out 209.15.95.34 -> 52.138.0.244: ESP(spi=0xac6e0cbe,seq=0x53)
2021-11-03 19:20:33.151707 port7 out 209.15.95.34 -> 52.138.0.244: ESP(spi=0xac6e0cbe,seq=0x54)
2021-11-03 19:20:34.791762 port7 out 209.15.95.34 -> 52.138.0.244: ESP(spi=0xac6e0cbe,seq=0x55)
2021-11-03 19:20:35.114577 port7 out 209.15.95.34 -> 52.138.0.244: ESP(spi=0xac6e0cbe,seq=0x56)
Additionally, checking the NPU will show drops over the IPsec engine.
diagnose npu np6 dce 0
IHP0_PKTCHK :0000000000007388 [5a] IHP1_PKTCHK :0000000000301745 [5b]
XHP0_PKTCHK :0000000000003229 [5e] XHP1_PKTCHK :0000000000003405 [5f]
IPSEC0_ENGINB0 :0000000000547093 [80] IPSEC0_ENGINB1 :0000000000031693 [81]
IPSEC0_ENGINB2 :0000000000000655 [82] IPSEC0_ENGINB3 :0000000000000174 [83]
IPSEC0_ENGINB4 :0000000000000074 [84] IPSEC0_ENGINB5 :0000000000000057 [85]
IPSEC0_ENGINB6 :0000000000000022 [86] IPSEC0_ENGINB7 :0000000000000004 [87]
IPSEC1_IQUEUE :0000000000000001 [88] IPSEC1_ENGINB0 :0000000000418401 [89]
IPSEC1_ENGINB1 :0000000000017379 [8a] IPSEC1_ENGINB2 :0000000000000746 [8b]
IPSEC1_ENGINB3 :0000000000000429 [8c] IPSEC1_ENGINB4 :0000000000000117 [8d]
IPSEC1_ENGINB5 :0000000000000086 [8e] IPSEC1_ENGINB6 :0000000000000051 [8f]
diag npu np6 dce 1
IHP0_PKTCHK :0000000000020296 [5a] XHP0_PKTCHK :0000000000000005 [5e]
IPSEC0_ENGINB6 :0000000003712039 [86] IPSEC0_ENGINB7 :0000000006713629 [87]
PDQ_OSW_IPSEC1I :0000000014154116 [a7]
Test by disabling NPU offloading under IPsec phase1 tunnel and check if the inbound traffic will pass as expected.
config vpn ipsec phase1-interfac
edit <phase1-name>
set npu-offload disable
end
If the traffic passes as expected, enable strip padding under system NPU, reboot the FortiGate. Note that if there is an HA cluster, it will be necessary to reboot all of the units in the HA clusters. After, enable npu-offload again.
config system npu
set strip-esp-padding enable
set strip-clear-text-padding enable
end
After, offloading will be enabled and the ESP traffic will pass successfully.
Related article:
Stripping clear text padding an IPsec session esp padding - FortiGate documentation.
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