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Tunnels
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• PRF: A pseudo-random function used to generate further keying info from the Diffie-Hellman key (control
channel only)
• ESN: A flag indicating whether extended sequence numbers are supported for the data channel
Manually-keyed connections do not have a control channel, and use only integrity and encryption algorithms.
Both integrity checking and encryption allow a choice of algorithms. When using IKE the default algorithm
proposals are in most cases a good choice as they allow negotiation with the peer to choose the best mutually
supported algorithms. The supported algorithms are as follows:
Table 11.2. IKE / IPsec algorithm proposals
Name
Type
Channels
Preferred
null
Integrity
Control, Data
Not in default proposal
HMAC-MD5
Integrity
Control, Data
HMAC-SHA1
Integrity
Control, Data
AES-XCBC
Integrity
Control, Data
HMAC-SHA256
Integrity
Control, Data
Yes
null
Encryption
Control, Data
Not in default proposal
3DES-CBC
Encryption
Control, Data
blowfish
Encryption
Control, Data
Yes
blowfish-192
Encryption
Control, Data
blowfish-256
Encryption
Control, Data
AES-CBC
Encryption
Control, Data
Yes
AES-192-CBC
Encryption
Control, Data
AES-256-CBC
Encryption
Control, Data
none
DHGroup
Data
Yes
MODP-1024
DHGroup
Control, Data
MODP-2048
DHGroup
Control, Data
Yes
HMAC-MD5
PRF
Control
HMAC-SHA1
PRF
Control
Yes
AES-XCBC-128
PRF
Control
Yes
HMAC-SHA256
PRF
Control
Yes
ALLOW-ESN
ESN
Data
Yes
ALLOW-SHORT-SN
ESN
Data
Yes
Control items can be specified in IKE-Proposal lists, and Data items can be specified in IPsec-Proposal lists.
If an IKE connection does not have an explicit ike-proposals entry, two default proposals are offered to the
peer. The first includes all the Control entries in the above table marked as preferred, and the second includes
all the implemented entries apart from null. Similarly if no explicit ipsec-proposals entry is given, Data entries
marked preferred are included in the first proposal, and all except null in the second. The IKE negotiation
always picks the first acceptable proposal, so the default proposals will have the effect of selecting from the
preferred algorithms if the peer supports them, and otherwise from all available algorithms. Note that the null
algorithms are never chosen by default; they provide no security and should only be used for testing.
11.1.6. NAT Traversal
Devices performing NAT (Network Address Translation) on the path between the connection peers can cause
difficulties with IPsec operation. Since NAT changes source IP addresses, and these are checked if a type AH
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