Manual:IP/IPsec

Applies to RouterOS: v4.5

RouterOS support IPsec server and client. This article describes it's basic abilities and functionality.

Specifications

• Packages required: security
• License required: Level1
• Submenu level: /ip ipsec
• Standards and Technologies: IPsec
• Hardware usage: high CPU usage
• Encryption algorythms: des; 3des; aes-128; aes-192; aes-256; blowfish; camilla; default: 3des

Description

IPsec (IP Security) supports secure (encrypted, digitally signed) communications over IP networks.

Encryption

After packet is src-natted (if needed), but before putting it into interface queue, IPsec policy database is consulted to find out if packet should be encrypted. Security Policy Database (SPD) is a list of rules that have two parts:

• Packet matching - packet source/destination, protocol and ports (for TCP and UDP) are compared to values in policy rules, one after another
• Action - if rule matches action specified in rule is performed:
  • none - continue with the packet as if there was no IPsec
  • discard - drop the packet
  • encrypt - apply IPsec transformations to the packet

Each SPD rule can be associated with several Security Associations (SA) that determine packet encryption parameters (key, algorithm, SPI).

Note that packet can only be encrypted if there is a usable SA for policy rule. Same SA may be used for different policies, unless especially prohibited by a policy. By setting SPD rule security "level" user can control what happens when there is no valid SA for policy rule:

• use - if there is no valid SA, send packet unencrypted (like accept rule)
• require - drop packet, and ask IKE daemon to establish a new SA.
• unique - same as require, but establish a unique SA for this policy (i.e., this SA may not be shared with other policy)

When encrypted packet is received for local host (after dst-nat and input filter), the appropriate SA is looked up to decrypt it (using packet source, destination, security protocol and SPI value). If no SA is found, the packet is dropped. If SA is found, packet is decrypted. Then decrypted packet's fields are compared to the policy rule that SA is linked to. If the packet does not match the policy rule, it is dropped. If the packet is decrypted fine (or authenticated fine) it is "received once more" - it goes through dst-nat and routing (which finds out what to do - either forward or deliver locally) again.

Note that before forward and input firewall chains, a packet that was not decrypted on local host is compared with SPD reversing its matching rules. If SPD requires encryption (there is valid SA associated with matching SPD rule), the packet is dropped. This is called incoming policy check.

Internet Key Exchange

The Internet Key Exchange (IKE) is a protocol that provides authenticated keying material for Internet Security Association and Key Management Protocol (ISAKMP) framework. There are other key exchange schemes that work with ISAKMP, but IKE is the most widely used one. Together they provide means for authentication of hosts and automatic management of security associations (SA).

Most of the time IKE daemon is doing nothing. There are two possible situations when it is activated:

There is some traffic caught by a policy rule which needs to become encrypted or authenticated, but the policy doesn't have any SAs. The policy notifies IKE daemon about that, and IKE daemon initiates connection to remote host. IKE daemon responds to remote connection. In both cases, peers establish connection and execute 2 phases:

• Phase 1 - The peers agree upon algorithms they will use in the following IKE messages and authenticate. The keying material used to derive keys for all SAs and to protect following ISAKMP exchanges between hosts is generated also.
• Phase 2 - The peers establish one or more SAs that will be used by IPsec to encrypt data. All SAs established by IKE daemon will have lifetime values (either limiting time, after which SA will become invalid, or amount of data that can be encrypted by this SA, or both).

There are two lifetime values - soft and hard. When SA reaches it's soft lifetime treshold, the IKE daemon receives a notice and starts another phase 2 exchange to replace this SA with fresh one. If SA reaches hard lifetime, it is discarded.

IKE can optionally provide a Perfect Forward Secrecy (PFS), which is a property of key exchanges, that, in turn, means for IKE that compromising the long term phase 1 key will not allow to easily gain access to all IPsec data that is protected by SAs established through this phase 1. It means an additional keying material is generated for each phase 2.

Generation of keying material is computationally very expensive. Exempli gratia, the use of modp8192 group can take several seconds even on very fast computer. It usually takes place once per phase 1 exchange, which happens only once between any host pair and then is kept for long time. PFS adds this expensive operation also to each phase 2 exchange.

Diffie-Hellman Groups

Diffie-Hellman (DH) key exchange protocol allows two parties without any initial shared secret to create one securely. The following Modular Exponential (MODP) and Elliptic Curve (EC2N) Diffie-Hellman (also known as "Oakley") Groups are supported:

Diffie-Hellman Group	Name	Reference
Group 1	768 bit MODP group	RFC2409
Group 2	1024 bits MODP group	RFC2409
Group 3	EC2N group on GP(2^155)	RFC2409
Group 4	EC2N group on GP(2^185)	RFC2409
Group 5	1536 bits MODP group	RFC3526

IKE Traffic

To avoid problems with IKE packets hit some SPD rule and require to encrypt it with not yet established SA (that this packet perhaps is trying to establish), locally originated packets with UDP source port 500 are not processed with SPD. The same way packets with UDP destination port 500 that are to be delivered locally are not processed in incoming policy check.

Setup Procedure

To get IPsec to work with automatic keying using IKE-ISAKMP you will have to configure policy, peer and proposal (optional) entries.

For manual keying you will have to configure policy and manual-sa entries.