Interoperability

Although IKE and IPsec are IETF standards, there are often still interoperability issues between different vendors. Below we list known issues with certain vendors, as well as known networking issues of services and cloud providers.

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Amazon EC2

The Amazon cloud has two interoperability concerns. The first issue is that your instance has a temporary RFC1918 IP address. The Amazon cloud NATs this to your permanent public IP address, called "elastic IP". This means that technically speaking, the IPsec server "behind NAT". Some Microsoft Windows operating systems need to set the AssumeUDPEncapsulationContextOnSendRul registry value. When using IPsec, In some configurations, Linux needs to use the elastic IP as source address for packets to be encrypted, but it can only do this properly if the IP is actually configured on the host. In such cases, configure the elastic IP on the loopback device:

ip addr add 184.1.2.3/32 dev lo

The second issue at Amazon is that their internal cloud network does not route IPsec ESP or AH packets. These packets need to be encapsulated in UDP. While normally the NAT detection takes care of this ESPinUDP encapsulation, if NAT is not detected (for instance because this is an IPsec connection between two instances in the Amazon cloud), you can force encapsulation by setting forceencaps=yes.

Below is an example ipsec.conf file for use in the Amazon EC2 cloud

# /etc/ipsec.conf on Amazon EC2 instance
version 2.0 

config setup
     nat_traversal=yes
     # we should exclude ourselves, but that's dynamic.
     # The other end should not be behind NAT anyway. If it is via port forward, avoid 10/8 that Amazon uses
     virtual_private=%v4:10.0.0.0/8,%v4:192.168.0.0/16,%v4:172.16.0.0/12,%v4:25.0.0.0/8,%v4:100.64.0.0/10,%v6:fd00:
:/8,%v6:fe80::/10
     protostack=netkey

conn amazonec2
     # preshared key
     authby=secret
     # load connection and initiate it on startup
     auto=start
     # Amazon does not route ESP/AH packets, so these must be encapsulated in UDP
     forceencaps=yes
     # use %defaultroute to find our local IP, since it is dynamic
     left=%defaultroute
     # set our ID to your (static) elastic IP
     leftid=184.1.2.3
     # set the desired source IP to the Elastic IP. Libreswan will create interface address and route.
     # Configure the elastic IP on loopback, eg: ip addr add 184.1.2.3/32 dev lo
     leftsourceip=184.1.2.3
     # remote endpoint IP
     right=1.2.3.4

Juniper

Although technically not an interop problem, Ryan Waldron <ryanw@phxx.com> contributed a working Juniper configuration that is compatible with libreswan

Juniper endpoint:

set ike gateway "GW-01" address <Your SM IP Here> Main outgoing-zone "V1-Untrust" preshare "Your PSK Here" proposal "pre-g2-3des-md5" 
set ike respond-bad-spi 1
set ike ikev2 ike-sa-soft-lifetime 60
unset ike ikeid-enumeration
unset ike dos-protection
unset ipsec access-session enable
set ipsec access-session maximum 5000
set ipsec access-session upper-threshold 0
set ipsec access-session lower-threshold 0
set ipsec access-session dead-p2-sa-timeout 0
unset ipsec access-session log-error
unset ipsec access-session info-exch-connected
unset ipsec access-session use-error-log
set vpn "VPN-01" gateway "GW-01" no-replay tunnel idletime 0 proposal "g2-esp-3des-md5" 
set vrouter "untrust-vr" 
exit
set vrouter "trust-vr" 
exit
set url protocol websense
exit
set policy id 58 from "V1-Trust" to "V1-Untrust" "10.10.0.0/24" "172.16.0.0/16-VPN-01" "ANY" tunnel vpn "VPN-01" id 0x23 pair-policy 57 log
set policy id 58
set log session-init
exit
set policy id 57 from "V1-Untrust" to "V1-Trust" "172.16.0.0/16-VPN-01" "10.10.0.0/24" "ANY" tunnel vpn "VPN-01" id 0x23 pair-policy 58 log
set policy id 57
set log session-init
exit

And the corresponding libreswan endpoint:

conn NetScreen
        ike=3des-md5
        esp=3des-md5
        authby=secret
        keyingtries=0
        left=<Juniper IP Here>
        leftsubnet=<Remote Subnet Here>
        leftnexthop=%defaultroute
        right=<SW IP Here>
        rightsubnet=<Local Subnet Here>
        rightnexthop=%defaultroute
        compress=no
        auto=start

There is also another example of configuring Juniper with libreswan by Pedro Kiefer

Juniper shows Bad SPI messages in the Event Log

When libreswan and juniper rekey around the same time, the Juniper can get confused. A workaround for this is to increase the ike soft-lifetime-buffer on the Juniper from the default 10 to 40. See also this Juniper Knowledge Base Article