Manual:IP/IPsec

Applies to RouterOS: v5.0 +

Summary

Sub-menu: /ip ipsec
Package required: security
Standards: RFC 4301

Internet Protocol Security (IPsec) is a set of protocols defined by the Internet Engineering Task Force (IETF) to secure packet exchange over unprotected IP/IPv6 networks such as Internet.

IpSec protocol suite can be divided in following groups:

• Authentication Header (AH) RFC 4302
• Encapsulating Security Payload (ESP) RFC 4303
• Internet Key Exchange (IKE) protocols. Dynamically generates and distributes cryptographic keys for AH and ESP.

Authentication Header (AH)

AH is a protocol that provides authentication of either all or part of the contents of a datagram through the addition of a header that is calculated based on the values in the datagram. What parts of the datagram are used for the calculation, and the placement of the header, depends whether tunnel or transport mode is used.

The presence of the AH header allows to verify the integrity of the message, but doesn't encrypt it. Thus, AH provides authentication but not privacy (Another protocol ESP is used to provide encryption).

RouterOS supports the following authentication algorithms for AH:

• SHA1
• MD5

Transport mode

In transport mode AH header is inserted after IP header. IP data and header is used to calculate authentication value. IP fields that might change during transit, like TTL and hop count, are set to zero values before authentication.

Tunnel mode

In tunnel mode original IP packet is encapsulated within a new IP packet. All of the original IP packet is authenticated.

Encapsulating Security Payload

Encapsulating Security Payload (ESP) uses shared key encryption to provide data privacy. ESP also supports its own authentication scheme like that used in AH, or can be used in conjunction with AH.

ESP packages its fields in a very different way than AH. Instead of having just a header, it divides its fields into three components:

• ESP Header - Comes before the encrypted data and its placement depends on whether ESP is used in transport mode or tunnel mode.
• ESP Trailer - This section is placed after the encrypted data. It contains padding that is used to align the encrypted data.
• ESP Authentication Data - This field contains an Integrity Check Value (ICV), computed in a manner similar to how the AH protocol works, for when ESP's optional authentication feature is used.

Transport mode

In transport mode ESP header is inserted after original IP header. ESP trailer and authentication value is added to the end of the packet. In this mode only IP payload is encrypted and authenticated, IP header is not secured.

Tunnel mode

In tunnel mode original IP packet is encapsulated within a new IP packet thus securing IP payload and IP header.

Encryption algorithms

RouterOS ESP supports various encryption and authentication algorithms.

Authentication:

• SHA1
• MD5

Encryption:

• DES - 56-bit DES-CBC encryption algorithm;
• 3DES - 168-bit DES encryption algorithm;
• AES - 128, 192 and 256-bit key AES-CBC encryption algorithm;
• Blowfish - added since v4.5
• Twofish - added since v4.5
• Camellia - 128, 192 and 256-bit key Camellia encryption algorithm added since v4.5

Hardware encryption

Hardware encryption allows to do faster encryption process by using built-in encryption engine inside CPU. AES is the only algorithm that will be accelerated in hardware.

List of RouterBoards with enabled hardware support:

• RB1000
• RB1100AHx2

For comparison RB1000 with enabled HW support can forward up to 550Mbps encrypted traffic. When HW support is disabled it can forward only 150Mbps encrypted traffic in AES-128 mode.

Some configuration advices on how to get maximum ipsec throughput on multicore RB1100AHx2:

• Avoid using ether12 and ethet13. Since these prots are pci-x they will be slowest ones.
• Fastest forwarding is from switch chip ports (ether1-ether10) to ether11 (directly connected to CPU) and vice versa.
• Set hardware queue on all interfaces

/queue interface set [find] queue=only-hardware-queue

• Disable RPS:

/system resource irq rps disable [find]

• Assign one CPU core to ether11 and other CPU core to everything else. Forwarding over ether11 requires more CPU that is why we are giving one core only for that interface (in IRQ setting ether11 is listed as ether12 tx,rx and error).

/system resource irq
set [find] cpu=1
set [find users="eth12 tx"] cpu=0
set [find users="eth12 rx"] cpu=0
set [find users="eth12 error"] cpu=0

• disable connection tracking

With all above recommendations it is possible to forward 820Mbps (1470byte packets two streams).

With enabled connection tracking 700Mbps (1470 byte packets two streams).

Internet Key Exchange Protocol

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. This phase should match following settings:
  • authentication method
  • DH group
  • encryption algorithm
  • exchange mode
  • hash alorithm
  • NAT-T
  • DPD and lifetime (optional)

• 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). This phase should match following settings:
  • Ipsec protocol
  • mode (tunnel or transport)
  • authentication method
  • PFS (DH) group
  • lifetime

Note: 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	RFC 2409
Group 2	1024 bits MODP group	RFC 2409
Group 3	EC2N group on GP(2^155)	RFC 2409
Group 4	EC2N group on GP(2^185)	RFC 2409
Group 5	1536 bits MODP group	RFC 3526

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.

Warning: Ipsec is very sensitive to time changes. If both ends of the IpSec tunnel are not synchronizing time equally(for example, different NTP servers not updating time with the same timestamp), tunnels will break and will have to be established again.

Peer configuration

Sub-menu: /ip ipsec peer

Peer configuration settings are used to establish connections between IKE daemons ( phase 1 configuration). This connection then will be used to negotiate keys and algorithms for SAs.

Property	Description
address (IP/IPv6 Prefix; Default: 0.0.0.0/0)	If remote peer's address matches this prefix, then the peer configuration is used in authentication and establishment of Phase 1. If several peer's addresses match several configuration entries, the most specific one (i.e. the one with largest netmask) will be used.
auth-method (pre-shared-key | rsa-signature; Default: pre-shared-key)	Authentication method: pre-shared-key - authenticate by a password (secret) string shared between the peers rsa-signature - authenticate using a pair of RSA certificates rsa-key - authenticate using a RSA key imported in Ipsec key menu.
certificate (string; Default: )	Name of a certificate listed in certificate table (signing packets; the certificate must have private key). Applicable if RSA signature authentication method (auth-method=rsa-signature) is used.
comment (string; Default: )	Short description of the peer.
dh-group (ec2n155 | ec2n185 | modp1024 | modp1536 | modp2048 | modp3072 | modp4096 | modp6144 | modp768; Default: modp1024)	Diffie-Hellman group (cipher strength)
disabled (yes | no; Default: no)	Whether peer is used to match remote peer's prefix.
dpd-interval (time | disable-dpd; Default: 2m)	Dead peer detection interval. If set to disable-dpd, dead peer detection will not be used.
dpd-maximum-failures (integer: 1..100; Default: 5)	Maximum count of failures until peer is considered to be dead. Applicable if DPD is enabled.
enc-algorithm (3des | aes-128 | aes-192 | aes-256 | blowfish | camellia-128 | camellia-192 | camellia-256 | des; Default: 3des)	Encryption algorithm.
exchange-mode (aggressive | base | main | main-l2tp; Default: main)	Different ISAKMP phase 1 exchange modes according to RFC 2408. Do not use other modes then main unless you know what you are doing. main-l2tp mode relaxes rfc2409 section 5.4, to allow pre-shared-key authentication in main mode.
generate-policy (yes | no; Default: no)	Allow this peer to establish SA for non-existing policies. Such policies are created dynamically for the lifetime of SA. Automatic policies allows, for example, to create IPsec secured L2TP tunnels, or any other setup where remote peer's IP address is not known at the configuration time.
hash-algorithm (md5 | sha1; Default: md5)	Hashing algorithm. SHA (Secure Hash Algorithm) is stronger, but slower.
key (string; Default: )	Name of the key from key menu. Applicable if auth-method=rsa-key.
lifebytes (Integer: 0..4294967295; Default: 0)	Phase 1 lifetime: specifies how much bytes can be transferred before SA is discarded. If set to 0, SA will not be discarded due to byte count excess.
lifetime (time; Default: 1d)	Phase 1 lifetime: specifies how long the SA will be valid.
my-id-user-fqdn (string; Default: )	By default IP address is used as ID. This parameter replaces ID with specified value. Can be used, for example, in cases if DNS name as ID is required.
nat-traversal (yes | no; Default: no)	Use Linux NAT-T mechanism to solve IPsec incompatibility with NAT routers inbetween IPsec peers. This can only be used with ESP protocol (AH is not supported by design, as it signs the complete packet, including IP header, which is changed by NAT, rendering AH signature invalid). The method encapsulates IPsec ESP traffic into UDP streams in order to overcome some minor issues that made ESP incompatible with NAT.
port (integer:0..65535; Default: 500)	Communication port used for ipsec traffic.
proposal-check (claim | exact | obey | strict; Default: obey)	Phase 2 lifetime check logic: claim - take shortest of proposed and configured lifetimes and notify initiator about it exact - require lifetimes to be the same obey - accept whatever is sent by an initiator strict - if proposed lifetime is longer than the default then reject proposal otherwise accept proposed lifetime
remote-certificate (string; Default: )	Name of a certificate (listed in certificate table) for authenticating the remote side (validating packets; no private key required). Applicable if RSA signature authentication method is used
secret (string; Default: )	Secret string (in case pre-shared key authentication is used). If it starts with '0x', it is parsed as a hexadecimal value
send-initial-contact (yes | no; Default: yes)	Specifies whether to send initial IKE information or wait for remote side.

Note: IPSec phases information is erased, when /ip ipsec peer configuration is modified on the fly, however packets are being encrypted/decrypted because of installed-sa (for example remote-peers information is erased, when peer configuration is modified.

Keys

Sub-menu: /ip ipsec key

This submenu list all imported public/private keys, that can be used for peer authentication. Submenu also has several commands to work with keys.

For example print below shows two imported 1024-bit keys, one public and one private.

[admin@PoETik] /ip ipsec key> print 
Flags: P - private-key, R - rsa 
 #    NAME                                                               KEY-SIZE
 0 PR priv                                                               1024-bit
 1  R pub                                                                1024-bit

Commands

Property	Description
export-pub-key (file-name; key)	Export public key to file from one of existing private keys.
generate-key (key-size; name)	Generate private key. Takes two parameters, name of newly generated key and key size 1024,2048 and 4096.
import (file-name; name)	Import key from file.

Policy

Sub-menu: /ip ipsec policy

Policy table is used to determine whether security settings should be applied to a packet.

Property	Description
action (discard | encrypt | none; Default: encrypt)	Specifies what to do with packet matched by the policy. none - pass the packet unchanged discard - drop the packet encrypt - apply transformations specified in this policy and it's SA
comment (string; Default: )	Short description of the policy
disabled (yes | no; Default: no)	Whether policy is used to match packets.
dst-address (IP/IPv6 prefix; Default: 0.0.0.0/32)	Destination address to be matched in packets.
dst-port (integer:0..65535 | any; Default: any)	Destination port to be matched in packets. If set to any all ports will be matched
ipsec-protocols (ah | esp; Default: esp)	Specifies what combination of Authentication Header and Encapsulating Security Payload protocols you want to apply to matched traffic
level (require | unique | use; Default: require)	Specifies what to do if some of the SAs for this policy cannot be found: use - skip this transform, do not drop packet and do not acquire SA from IKE daemon require - drop packet and acquire SA unique - drop packet and acquire a unique SA that is only used with this particular policy
manual-sa (string | none; Default: none)	Name of the manual SA template
priority (integer:-2147483646..2147483647; Default: 0)	Policy ordering classificator (signed integer). Larger number means higher priority.
proposal (string; Default: default)	Name of the proposal template that will be sent by IKE daemon to establish SAs for this policy.
protocol (all | egp | ggp| icmp | igmp | ...; Default: all)	IP packet protocol to match.
sa-dst-address (ip/ipv6 address; Default: ::)	SA destination IP/IPv6 address (remote peer).
sa-src-address (ip/ipv6 address; Default: ::)	SA source IP/IPv6 address (local peer).
src-address (ip/ipv6 prefix; Default: 0.0.0.0/32)	Source IP prefix
src-port (any | integer:0..65535; Default: any)	Source Port of the packet
tunnel (yes | no; Default: no)	Specifies whether to use tunnel mode

Note: All packets are IPIP encapsulated in tunnel mode, and their new IP header's src-address and dst-address are set to sa-src-address and sa-dst-address values of this policy. If you do not use tunnel mode (id est you use transport mode), then only packets whose source and destination addresses are the same as sa-src-address and sa-dst-address can be processed by this policy. Transport mode can only work with packets that originate at and are destined for IPsec peers (hosts that established security associations). To encrypt traffic between networks (or a network and a host) you have to use tunnel mode.

Policy Stats

Command /ip ipsec policy print stats will show current status of the policy. Additional read-only parameters will be printed.

Property	Description
in-accepted (integer)	How many incoming packets were passed by the policy without an attempt to decrypt.
in-dropped (integer)	How many incoming packets were dropped by the policy without an attempt to decrypt
in-transformed (integer)	How many incoming packets were decrypted (ESP) and/or verified (AH) by the policy
out-accepted (integer)	How many outgoing packets were passed by the policy without an attempt to encrypt.
out-dropped (integer)	How many outgoing packets were dropped by the policy without an attempt to encrypt.
out-transformed (integer)	How many outgoing packets were encrypted (ESP) and/or verified (AH) by the policy.
ph2-state (expired | no-phase2 | established)	Indication of the progress of key establishing.

Dumping Policies

It is possible to dump policies installed into the kernel for debugging purposes with command:

/ip ipsec policy  dump-kernel-policies

After executing this command check the logs to see the result, there should be three policies in the kernel: forward, in and out.

[admin@test-host] >/log print
07:28:34 ipsec,debug,packet policy ipsec fwd: 10.5.101.9[0] - 10.5.101.13[0] 
07:28:34 ipsec,debug,packet policy ipsec in: 10.5.101.9[0] - 10.5.101.13[0] 
07:28:34 ipsec,debug,packet policy ipsec out: 10.5.101.13[0] - 10.5.101.9[0]

Proposal settings

Sub-menu: /ip ipsec proposal

Proposal information that will be sent by IKE daemon to establish SAs for this policy ( Phase 2). Configured proposals are set in policy configuration.

Property	Description
auth-algorithms (md5|sha1|null; Default: sha1)	Allowed algorithms for authorization. sha1 is stronger, but slower algorithm.
comment (string; Default: )	Short description of an item.
disabled (yes | no; Default: no)	Whether item is disabled.
enc-algorithms (null|des|3des|aes-128|aes-192|aes-256|blowfish|camellia-128|camellia-192|camellia-256|twofish; Default: 3des)	Allowed algorithms and key lengths to use for SAs.
lifetime (time; Default: 30m)	How long to use SA before throwing it out.
name (string; Default: )	Name of the proposal template, that will be identified in other parts of ipsec configuration.
pfs-group (ec2n155 | ec2n185 | modp1024 | modp1536 | modp2048 | modp3072 | modp4096 | modp6144 | modp768 | none; Default: modp1024)	Diffie-Helman group used for Perfect Forward Secrecy.

Manual SA

Sub-menu: /ip ipsec manual-sa

Menu is used to configure SAs manually. Created SA template then can be used in policy configuration.

Property	Description
ah-algorithm (in/out in,out = md5|null|sha1; Default: null)	Authentication Header encryption algorithm.
ah-key (string/string; Default: )	Incoming-authentication-key/outgoing-authentication-key
ah-spi (0x100..FFFFFFFF/0x100..FFFFFFFF; Default: 0x100)	Incoming-SA-SPI/outgoing-SA-SPI
disabled (yes | no; Default: no)	Defines whether item is ignored or used
esp-auth-algorithm (in/out in,out = md5|null|sha1; Default: null)	Encapsulating Security Payload authentication encryption algorithm
esp-auth-key (string/string; Default: )	Incoming-authentication-key/outgoing -authentication-key
esp-enc-algorithm (in/out in,out = 3des | aes-128 | aes-192 | aes-256 | des | ...; Default: null)	Incoming-encryption-algorithm
esp-enc-key (string/string; Default: )	Incoming-encryption-key/outgoing-encryption-key
esp-spi (0x100..FFFFFFFF/0x100..FFFFFFFF; Default: 0x100)	Incoming-SA-SPI/outgoing-SA-SPI
lifetime (time; Default: 0s)	Lifetime of this SA
name (string; Default: )	Name of the item for reference from policies

Installed SA

Sub-menu: /ip ipsec installed-sa

This facility provides information about installed security associations including the keys.

Property	Description
AH (yes | no)
ESP (yes | no)
add-lifetime (time/time)	Added lifetime for the SA in format soft/hard soft - time period after which ike will try to establish new SA hard - time period after which SA is deleted
addtime (time)	Date and time when this SA was added.
auth-algorithm (sha1 | md5)	Shows currently used authentication algorithm
auth-key (string)	Shows used authentication key
current-bytes (integer)	Shows number of bytes seen by this SA.
dst-address (IP)
enc-algorithm (des | 3des | aes ...)	Shows currently used encryption algorithm
pfs (yes | no)
replay (integer)
spi (string)
src-address (IP)
state (string)	Shows the current state of the SA ("mature", "dying" etc)

Flushing SAs

Sometimes after incorrect/incomplete negotiations took place, it is required to flush manually the installed SA table so that SA could be renegotiated. This option is provided by the /ip ipsec installed-sa flush command.

This command accepts only one property:

Property	Description
sa-type (ah | all | esp; Default: all)	Specifies SA types to flush: ah - delete AH protocol SAs only esp - delete ESP protocol SAs only all - delete both ESP and AH protocols SAs

Remote Peers

Sub-menu: /ip ipsec remote-peers

This submenu provides you with various statistics about remote peers that currently have established phase 1 connections with this router. Note that if peer doesn't show up here, it doesn't mean that no IPsec traffic is being exchanged with it.

Read only properties:

Property	Description
local-address (ip/ipv6 address)	Local ISAKMP SA address on the router used by the peer
remote-address (ip/ipv6 address)	Remote peer's ip/ipv6 address
side (initiator | responder)	Shows which side initiated the Phase1 negotiation.
state (string)	State of phase 1 negotiation with the peer. For example when phase1 and phase 2 are negotiated it will show state "established".
established (time)	How long peers are in established state.

Closing all IPsec connections

Menu has a command to quickly close all established ipsec connections. This command will clear all installed SAs (Phase2) and remove all entries from remote-peers menu (Phase1).

Usage:

/ip ipsec remote-peers kill-connections 

Statistics

Sub-menu: /ip ipsec statistics

This menu shows various ipsec statistics

Property	Description
in-errors (integer)	All inbound errors that are not matched by other counters.
in-buffer-errors (integer)	No free buffer.
in-header-errors (integer)	Header error
in-no-states (integer)	No state is found i.e. Either inbound SPI, address, or IPsec protocol at SA is wrong
in-state-protocol-errors (integer)	Transformation protocol specific error, for example SA key is wrong or hardware accelerator is unable to handle amount of packets.
in-state-mode-errors (integer)	Transformation mode specific error
in-state-sequence-errors (integer)	Sequence number is out of window
in-state-expired (integer)	State is expired
in-state-mismatches (integer)	State has mismatched option, for example UDP encapsulation type is mismatched.
in-state-invalid (integer)	State is invalid
in-template-mismatches (integer)	No matching template for states, e.g. Inbound SAs are correct but SP rule is wrong
in-no-policies (integer)	No policy is found for states, e.g. Inbound SAs are correct but no SP is found
in-policy-blocked (integer)	Policy discards
in-policy-errors (integer)	Policy errors
out-errors (integer)	All outbound errors that are not matched by other counters
out-bundle-errors (integer)	Bundle generation error
out-bundle-check-errors (integer)	Bundle check error
out-no-states (integer)	No state is found
out-state-protocol-errors (integer)	Transformation protocol specific error
out-state-mode-errors (integer)	Transformation mode specific error
out-state-sequence-errors (integer)	Sequence errors, for example Sequence number overflow
out-state-expired (integer)	State is expired
out-policy-blocked (integer)	Policy discards
out-policy-dead (integer)	Policy is dead
out-policy-errors (integer)	Policy error

Application Examples

Site to Site IpSec Tunnel

Consider setup as illustrated below

Two remote office routers are connected to internet and office workstations behind routers are NATed. Each office has its own local subnet, 10.1.202.0/24 for Office1 and 10.1.101.0/24 for Office2. Both remote offices needs secure tunnel to local networks behind routers.

IP Connectivity

On both routers ether1 is used as wan port and ether2 is used to connect workstations. Also NAT rules are set tu masquerade local networks.

Office1 router:

/ip address
add address=192.168.90.1/24 interface=ether1
add address=10.1.202.1/24 interface=ether2

/ip route 
add gateway=192.168.90.254

/ip firewall nat
add chain=srcnat out-interface=ether1 action=masquerade

Office2 router:

/ip address
add address=192.168.80.1/24 interface=ether1
add address=10.1.101.1/24 interface=ether2

/ip route 
add gateway=192.168.80.254

/ip firewall nat
add chain=srcnat out-interface=ether1 action=masquerade

IpSec Peer's config

Next step is to add peer's configuration. We need to specify peers address and port and pre-shared-key. Other parameters are left to default values.

Office1 router:

/ip ipsec peer
add address=192.168.80.1/32 port=500 auth-method=pre-shared-key secret="test"

Office2 router:

/ip ipsec peer
add address=192.168.90.1/32 port=500 auth-method=pre-shared-key secret="test"

Policy and proposal

It is important that proposed authentication and encryption algorithms match on both routers. In this example we can use predefined "default" proposal

[admin@MikroTik] /ip ipsec proposal> print 
Flags: X - disabled 
 0   name="default" auth-algorithms=sha1 enc-algorithms=3des lifetime=30m 
     pfs-group=modp1024 

As we already have proposal as a next step we need correct IpSec policy. We want to encrypt traffic coming form 10.1.202.0/24 to 10.1.101.0/24 and vice versa.

Office1 router:

/ip ipsec policy
add src-address=10.1.202.0/24 src-port=any dst-address=10.1.101.0/24 dst-port=any \
sa-src-address=192.168.90.1 sa-dst-address=192.168.80.1 \
tunnel=yes action=encrypt proposal=default

Office2 router:

/ip ipsec policy
add src-address=10.1.101.0/24 src-port=any dst-address=10.1.202.0/24 dst-port=any \
sa-src-address=192.168.80.1 sa-dst-address=192.168.90.1 \
tunnel=yes action=encrypt proposal=default

Note that we configured tunnel mode instead of transport, as this is site to site encryption.

NAT Bypass

At this point if you will try to establish IpSec tunnel it will not work, packets will be rejected. This is because both routers have NAT rules that is changing source address after packet is encrypted. Remote router reiceves encrypted packet but is unable to decrypt it because source address do not match address specified in policy configuration. For more information see packet flow ipsec example.

To fix this we need to set up NAT bypass rule.

Office1 router:

/ip firewall nat
add chain=srcnat action=accept  place-before=0 \
 src-address=10.1.202.0/24 dst-address=10.1.101.0/24

Office2 router:

/ip firewall nat
add chain=srcnat action=accept  place-before=0 \
 src-address=10.1.101.0/24 dst-address=10.1.202.0/24

It is very important that bypass rule is placed at the top of all other NAT rules.

Note: If you previously tried to establish tunnel before NAT bypass rule was added, you have to clear connection table from existing connection or restart the routers

Ipsec/L2TP behind NAT

Consider setup as illustrated below

Client needs secure connection to the office with public address 1.1.1.1, but server does not know what will be the source address from which client connects. It is so called road-warrior setup. Our client will also be located behind the router with enabled NAT.

For the setup RouterOS router will be used as the client device behind NAT (it can be any device: Windows PC, Smartphone, Linux PC, etc.)

IP Connectivity

On the server:

/ip address 
add address=1.1.1.1/24 interface=ether1

/ip route
add gateway=1.1.1.2

On the clients router:

/ip address 
add address=2.2.2.2/24 interface=ether1
add address=10.5.8.0/24 interface=ether2

/ip route
add gateway=2.2.2.1

/ip firewall net
add chain=srcnat action=masquerade out-interface=ether1

On the client:

/ip address
add address=10.5.8.120/24 interface=ether1

L2TP Config

On the server:

/interface l2tp-server 
set enabled=yes profil=default

/ip pool 
add name=l2tp-pool ranges=192.168.1.2-192.168.1.20

/ppp profile 
set default local-address=192.168.1.1 remote-address=l2tp-pool

/ppp secret
add name=l2tp-test password=test123456

On the client:

/interface l2tp-client
add connect-to=1.1.1.1 disabled=no name=l2tp-out1 password=password user=l2tp-test

IpSec Config

On server side:

/ip ipsec proposal
set [ find default=yes ] enc-algorithms=3des,aes-128,aes-192,aes-256
/ip ipsec peer
add generate-policy=yes hash-algorithm=sha1 nat-traversal=yes secret=test123456 \
      send-initial-contact=no

RouterOS as client:

/ip ipsec proposal
set [ find default=yes ] enc-algorithms=aes-128
/ip ipsec peer
add address=1.1.1.1/32 hash-algorithm=sha1 nat-traversal=yes secret=test123456

/ip ipsec policy
add dst-address=1.1.1.1/32 protocol=udp sa-dst-address=1.1.1.1 \
      sa-src-address=10.5.8.120 src-address=10.5.8.120/32

Notice that nat-traversal is enabled. This option is required because Ipsec connection will be established through the NAT router otherwise Ipsec will not be able to establish phase2.

Note: Only one L2TP/ipsec connection can be established through the NAT. Which means that only one client can connect to the sever located behind the same router.

Connecting with Shrew Client and allowing only Encrypted traffic

• See example here

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