Technical Tip: IKEv2 negotiation messages and their meaning CREATE_CHILD_SA and INFORMATIONAL

Nivedha · ‎10-03-2024

Description

This article discusses the IKEv2 messages and their meaning.

Scope

FortiGate.

Solution

Below is the overview of IKEv2 messages and their meaning and the IKE debugs seen on two FortiGates:

Topology:

20.0.0.2 is the initiator and 20.0.0.1 is the responder. SHA256- AES256 and DH group 14 are used for both Pahse1 and phase2 negotiations

IKE_SA_INIT: This message exchange begins the process of establishing a secure connection. It involves two messages:
IKE_AUTH: This is the second phase of IKEv2 negotiation, where authentication happens. It also establishes the IPsec SAs for the actual data encryption. The exchange includes two messages. These two messages are mentioned in Understanding the ikev2 debugs SA_INIT and IKE_AUTH article
CREATE_CHILD_SA: This message exchange is used to create or rekey additional Child SAs (additional tunnels) after the initial IKE_AUTH exchange. It is also used for rekeying the IKE SA itself.

Message 5 (Initiator → Responder):
The initiator requests to create a new child SA or rekey an existing SA. This message proposes the new security parameters (encryption and integrity algorithms).

Initiator:

ike V=root:0:hub1-Pri:hub1-Pri: IPsec SA connect 4 20.0.0.2->20.0.0.1:500 negotiating

ike V=root:0:hub1-Pri:145:376 initiating CREATE_CHILD exchange

ike V=root:0:hub1-Pri:145:hub1-Pri:376: PFS enabled

ike V=root:0:hub1-Pri:145:hub1-Pri:376: generate DH public value request queued

ike V=root:0:hub1-Pri:145:376 initiating CREATE_CHILD exchange

ike V=root:0:hub1-Pri:145:hub1-Pri:376: PFS enabled

ike 0:hub1-Pri:145: enc xxxx

ike 0:hub1-Pri:145: out xxxx

ike V=root:0:hub1-Pri:145: sent IKE msg (CREATE_CHILD): 20.0.0.2:500->20.0.0.1:500, len=496, vrf=0, id=2ecbf90464c2e03a/c81df8c224f77489:00000003, oif=4

Responder:

ike V=root:0: comes 20.0.0.2:500->20.0.0.1:500,ifindex=4,vrf=0,len=496.

ike V=root:0: IKEv2 exchange=CREATE_CHILD id=2ecbf90464c2e03a/c81df8c224f77489:00000003 len=496

ike 0: in xxx

ike 0:advpn_1:116: dec xxxx

ike V=root:0:advpn_1:116: received create-child request

ike V=root:0:advpn_1:116: responder received CREATE_CHILD exchange

ike V=root:0:advpn_1:116: responder creating new child

ike V=root:0:advpn_1:116:165: peer proposal:

ike V=root:0:advpn_1:116:165: TSi_0 0:10.255.255.2-10.255.255.2:0

ike V=root:0:advpn_1:116:165: TSi_1 0:0.0.0.0-255.255.255.255:0

ike V=root:0:advpn_1:116:165: TSr_0 0:198.18.1.1-198.18.1.1:0

ike V=root:0:advpn_1:116:165: TSr_1 0:0.0.0.0-255.255.255.255:0

ike V=root:0:advpn_1:116:advpn:165: comparing selectors

ike V=root:0:advpn_1:116:advpn:165: matched by rfc-rule-2

ike V=root:0:advpn_1:116:advpn:165: phase2 matched by subset

ike V=root:0:advpn_1:116:165: local narrowing exactly matches configured selector

ike V=root:0:advpn_1:116:advpn:165: accepted proposal:

ike V=root:0:advpn_1:116:advpn:165: TSi_0 0:0.0.0.0-255.255.255.255:0

ike V=root:0:advpn_1:116:advpn:165: TSr_0 0:0.0.0.0-255.255.255.255:0

ike V=root:0:advpn_1:116:advpn:165: dialup

ike V=root:0:advpn_1:116:advpn:165: incoming child SA proposal:

ike V=root:0:advpn_1:116:advpn:165: proposal id = 1:

ike V=root:0:advpn_1:116:advpn:165: protocol = ESP:

ike V=root:0:advpn_1:116:advpn:165: encapsulation = TUNNEL

ike V=root:0:advpn_1:116:advpn:165: type=ENCR, val=AES_CBC (key_len = 256)

ike V=root:0:advpn_1:116:advpn:165: type=INTEGR, val=SHA256

ike V=root:0:advpn_1:116:advpn:165: type=DH_GROUP, val=MODP2048

ike V=root:0:advpn_1:116:advpn:165: type=ESN, val=NO

ike V=root:0:advpn_1:116:advpn:165: matched proposal id 1

ike V=root:0:advpn_1:116:advpn:165: proposal id = 1:

ike V=root:0:advpn_1:116:advpn:165: protocol = ESP:

ike V=root:0:advpn_1:116:advpn:165: encapsulation = TUNNEL

ike V=root:0:advpn_1:116:advpn:165: type=ENCR, val=AES_CBC (key_len = 256)

ike V=root:0:advpn_1:116:advpn:165: type=INTEGR, val=SHA256

ike V=root:0:advpn_1:116:advpn:165: type=DH_GROUP, val=MODP2048

ike V=root:0:advpn_1:116:advpn:165: type=ESN, val=NO

ike V=root:0:advpn_1:116:advpn:165: lifetime=43200

ike V=root:0:advpn_1:116:advpn:165: PFS enabled, group=14

ike V=root:0:advpn_1:116:advpn:165: generate DH public value request queued

ike V=root:0:advpn_1:116:advpn:165: compute DH shared secret request queued

ike V=root:0:advpn_1:116:advpn:165: replay protection enabled

ike V=root:0:advpn_1:116:advpn:165: set sa life soft seconds=43187.

ike V=root:0:advpn_1:116:advpn:165: set sa life hard seconds=43200.

ike V=root:0:advpn_1:116:advpn:165: IPsec SA selectors #src=1 #dst=1

ike V=root:0:advpn_1:116:advpn:165: src 0 7 0:0.0.0.0-255.255.255.255:0

ike V=root:0:advpn_1:116:advpn:165: dst 0 7 0:0.0.0.0-255.255.255.255:0

ike V=root:0:advpn_1:116:advpn:165: add dynamic IPsec SA selectors a02

ike V=root:0:advpn_1:116:advpn:165: added dynamic IPsec SA proxyids new 2 a02

ike V=root:0:advpn_1:116:advpn:165: add IPsec SA: SPIs=c5c0b59d/f82f8273

ike 0:advpn_1:116:advpn:165: IPsec SA dec spi c5c0b59d key 32:4D5D10F52ECE0B294C1061AA884B4739B57915FD5B06A05B36CCE212D0D3D119 auth 32:53804CE0F186E

B6B2289F52385A8FEE7024E1CAE18123F3555008E741F20B4D6

ike 0:advpn_1:116:advpn:165: IPsec SA enc spi f82f8273 key 32:3E4416D5A2A20EE65539B6906F5D0CE4FD4D9A5CB79B79912AAF79F15B279E09 auth 32:C29471985C023

B28A1D5D6371EF793C8BAF159E8CA2A2E03B6A3323CEAE4832D

ike V=root:0:advpn_1:116:advpn:165: added IPsec SA: SPIs=c5c0b59d/f82f8273

ike V=root:0:advpn_1:116:advpn:165: sending SNMP tunnel UP trap

ike V=root:0:advpn_1: tunnel up event

Message 6 (Responder → Initiator):

The responder replies with its selection of the security parameters for the new child SA or acknowledges the rekeying of the existing SA.

Responder:

ike V=root:0:advpn_1:116:advpn:165: responder preparing CREATE_CHILD message

ike 0:advpn_1:116: enc xxx

ike 0:advpn_1:116: out xxxx

ike V=root:0:advpn_1:116: sent IKE msg (CREATE_CHILD_RESPONSE): 20.0.0.1:500->20.0.0.2:500, len=464, vrf=0, id=2ecbf90464c2e03a/c81df8c224f77489:00000003, oif=4

Initiator logs:

ike V=root:0: comes 20.0.0.1:500->20.0.0.2:500,ifindex=4,vrf=0,len=464.

ike V=root:0: IKEv2 exchange=CREATE_CHILD_RESPONSE id=2ecbf90464c2e03a/c81df8c224f77489:00000003 len=464

ike 0: in xxxx

ike 0:hub1-Pri:145: dec xxxx

ike V=root:0:hub1-Pri:145: received create-child response

ike V=root:0:hub1-Pri:145: initiator received CREATE_CHILD msg

ike V=root:0:hub1-Pri:145:hub1-Pri:376: found child SA SPI f82f8273 state=3

ike V=root:0:hub1-Pri:145:hub1-Pri:376: PFS enabled, group=14

ike V=root:0:hub1-Pri:145:hub1-Pri:376: compute DH shared secret request queued

ike V=root:0:hub1-Pri:145:376: peer proposal:

ike V=root:0:hub1-Pri:145:376: TSr_0 0:0.0.0.0-255.255.255.255:0

ike V=root:0:hub1-Pri:145:376: TSi_0 0:0.0.0.0-255.255.255.255:0

ike V=root:0:hub1-Pri:145:hub1-Pri:376: comparing selectors

ike V=root:0:hub1-Pri:145:hub1-Pri:376: matched by rfc-rule-2

ike V=root:0:hub1-Pri:145:hub1-Pri:376: phase2 matched by subset

ike V=root:0:hub1-Pri:145:hub1-Pri:376: accepted proposal:

ike V=root:0:hub1-Pri:145:hub1-Pri:376: TSr_0 0:0.0.0.0-255.255.255.255:0

ike V=root:0:hub1-Pri:145:hub1-Pri:376: TSi_0 0:0.0.0.0-255.255.255.255:0

ike V=root:0:hub1-Pri:145:hub1-Pri:376: autokey

ike V=root:0:hub1-Pri:145:hub1-Pri:376: incoming child SA proposal:

ike V=root:0:hub1-Pri:145:hub1-Pri:376: proposal id = 1:

ike V=root:0:hub1-Pri:145:hub1-Pri:376: protocol = ESP:

ike V=root:0:hub1-Pri:145:hub1-Pri:376: encapsulation = TUNNEL

ike V=root:0:hub1-Pri:145:hub1-Pri:376: type=ENCR, val=AES_CBC (key_len = 256)

ike V=root:0:hub1-Pri:145:hub1-Pri:376: type=INTEGR, val=SHA256

ike V=root:0:hub1-Pri:145:hub1-Pri:376: type=DH_GROUP, val=MODP2048

ike V=root:0:hub1-Pri:145:hub1-Pri:376: type=ESN, val=NO

ike V=root:0:hub1-Pri:145:hub1-Pri:376: matched proposal id 1

ike V=root:0:hub1-Pri:145:hub1-Pri:376: proposal id = 1:

ike V=root:0:hub1-Pri:145:hub1-Pri:376: protocol = ESP:

ike V=root:0:hub1-Pri:145:hub1-Pri:376: encapsulation = TUNNEL

ike V=root:0:hub1-Pri:145:hub1-Pri:376: type=ENCR, val=AES_CBC (key_len = 256)

ike V=root:0:hub1-Pri:145:hub1-Pri:376: type=INTEGR, val=SHA256

ike V=root:0:hub1-Pri:145:hub1-Pri:376: type=DH_GROUP, val=MODP2048

ike V=root:0:hub1-Pri:145:hub1-Pri:376: type=ESN, val=NO

ike V=root:0:hub1-Pri:145:hub1-Pri:376: lifetime=43200

ike V=root:0:hub1-Pri:145:hub1-Pri:376: replay protection enabled

ike V=root:0:hub1-Pri:145:hub1-Pri:376: set sa life soft seconds=42903.

ike V=root:0:hub1-Pri:145:hub1-Pri:376: set sa life hard seconds=43200.

ike V=root:0:hub1-Pri:145:hub1-Pri:376: IPsec SA selectors #src=1 #dst=1

ike V=root:0:hub1-Pri:145:hub1-Pri:376: src 0 7 0:0.0.0.0-255.255.255.255:0

ike V=root:0:hub1-Pri:145:hub1-Pri:376: dst 0 7 0:0.0.0.0-255.255.255.255:0

ike V=root:0:hub1-Pri:145:hub1-Pri:376: add IPsec SA: SPIs=f82f8273/c5c0b59d

ike 0:hub1-Pri:145:hub1-Pri:376: IPsec SA dec spi f82f8273 key 32:3E4416D5A2A20EE65539B6906F5D0CE4FD4D9A5CB79B79912AAF79F15B279E09 auth 32:C29471985

C023B28A1D5D6371EF793C8BAF159E8CA2A2E03B6A3323CEAE4832D

ike 0:hub1-Pri:145:hub1-Pri:376: IPsec SA enc spi c5c0b59d key 32:4D5D10F52ECE0B294C1061AA884B4739B57915FD5B06A05B36CCE212D0D3D119 auth 32:53804CE0F

186EB6B2289F52385A8FEE7024E1CAE18123F3555008E741F20B4D6

ike V=root:0:hub1-Pri:145:hub1-Pri:376: added IPsec SA: SPIs=f82f8273/c5c0b59d

ike V=root:0:hub1-Pri:145:hub1-Pri:376: sending SNMP tunnel UP trap

ike V=root:0:hub1-Pri: static tunnel up event 10.255.255.1 (dev=14)

ike V=root:0:hub1-Pri: static tunnel up event :: (dev=14)

Meaning:

This exchange is used for rekeying existing SAs (both IKE and Child SAs) or establishing additional IPsec tunnels between the peers.
This exchange is used for exchanging control information, error messages, and notifications.
Message 7 (Initiator → Responder or Responder → Initiator):
One party sends a request with additional information (e.g., error notifications, requests to delete SAs, or NAT detection packets).
Message 8 (Responder → Initiator or Initiator → Responder):
The other party responds to the information message, acknowledging receipt or processing of the information.

Initiator:

ike V=root:0:hub1-Pri:148:383: send informational

ike 0:hub1-Pri:148: enc xxxx

ike 0:hub1-Pri:148: out xxxx

ike V=root:0:hub1-Pri:148: sent IKE msg (INFORMATIONAL): 20.0.0.2:500->20.0.0.1:500, len=80, vrf=0, id=55a418752ab5df75/20c2112f2d151c8b:00000002, oif=4

ike V=root:0: comes 20.0.0.1:500->20.0.0.2:500,ifindex=4,vrf=0,len=80.

ike V=root:0: IKEv2 exchange=INFORMATIONAL_RESPONSE id=55a418752ab5df75/20c2112f2d151c8b:00000002 len=80

ike 0: in xxx

Responder:

ike V=root:0: IKEv2 exchange=INFORMATIONAL id=55a418752ab5df75/20c2112f2d151c8b:00000002 len=80

ike 0: in xxxx

ike 0:advpn_1:119: dec xxxx

ike V=root:0:advpn_1:119: received informational request

ike 0:advpn_1:119: enc xxxx

ike 0:advpn_1:119: out xxxx

ike V=root:0:advpn_1:119: sent IKE msg (INFORMATIONAL_RESPONSE): 20.0.0.1:500->20.0.0.2:500, len=80, vrf=0, id=55a418752ab5df75/20c2112f2d151c8b:00000002, oif=4

Meaning:

INFORMATIONAL messages are typically used to manage the IKE session by sending notifications, deleting SAs, or handling errors like authentication failures.

Summary of IKEv2 Message Types and Meanings:

IKE_SA_INIT: Negotiates cryptographic algorithms and establishes a Diffie-Hellman shared secret.
IKE_AUTH: Authenticates both parties and creates the first IPsec SA (for protecting the actual data).
CREATE_CHILD_SA: Creates or rekeys additional IPsec SAs or rekeys the IKE SA.
INFORMATIONAL: Used for exchanging control information, error messages, or deleting SAs.

In each message exchange, both peers use encryption and authentication to ensure that the messages cannot be tampered with or intercepted by malicious actors.