[strongSwan] Encrypting connection between two public IPv6 prefixes

[Marek Szuba]

Hello,

I have been trying to establish an IPSec-encrypted connection between
two IPv6-enabled sites configured as follows:
 - ordinary machines on each site receive either static or
DHCPv6-assigned addresses from their respective /56 prefixes. These
prefixes are globally routeable and, just to make it clear, separate for
each site;
 - site gateways have got two interfaces, with the internal ones
assigned addresses from respective site prefixes and the external ones
sitting in two, again separate, /127 networks assigned to us by the ISP.

The gateways of both sites are Linux servers, running exactly the same
OS and by extension using the same version of StrongSWAN. Needless to
say, the two sites communicate perfectly well over IPv6 when no
encryption is involved.

Unfortunately the tunnel as configured right now does not quite work
right. When running ping6 on one site and tcpdump on the other:
 - incoming ICMPv6 echo requests are visible twice, once as ESP and once
unencrypted; Replies come out fine, i.e. only once as ESP;
 - site-to-site connections do not seem to work, unencrypted ICMPv6 is
visible for both echo-request and echo-reply packets.

Out of curiosity I tried switching the end-to-end connection to
transport mode, in which case I no longer see duplicates - only ESP
packets are seen instead of both incoming echo-request and outgoing
echo-reply. I couldn't get site-to-site connectivity working in this
mode, though.

Note: I should probably mention at this point that while I have used
IPSec on local networks before, this is the first time I have ever tried
using it connections between networks - so chances are I've made some
rather silly mistake. I have also never tried it with IPv6 before either
but at a glance nothing in the configs seems to be IPv6-specific so
perhaps it doesn't matter in this case.

Anyway, ipsec.conf on the two gateway machines looks as follows (with
redactions having been made using exactly the same regexes so am
confident they are consistent):

*** ipsec.conf-sun ***

config setup

ca myca
        cacert=/etc/ssl/certs/ca.pem
        auto=start

conn %default
        ikelifetime=60m
        keylife=20m
        rekeymargin=3m
        keyingtries=1
        keyexchange=ikev2
        fragmentation=yes
        mobike=no

conn tunv6-sites
        also=tunv6-ends
        leftsubnet=2001:1234:5678:100::/56
        rightsubnet=2001:1234:5678:200::/56
        auto=start

conn tunv6-ends
        left=2001:1234:5678::2
        leftcert=/etc/ssl/certs/sun.pem
        leftid=@sun.sunsite.example.com
        leftfirewall=yes
        right=2001:1234:5678::4
        rightcert=/etc/ssl/certs/moon.pem
        rightid=@moon.moonsite.example.com
        auto=start

*** ipsec.conf-moon ***

config setup

ca myca
        cacert=/etc/ssl/certs/ca.pem
        auto=start

conn %default
        ikelifetime=60m
        keylife=20m
        rekeymargin=3m
        keyingtries=1
        keyexchange=ikev2
        fragmentation=yes
        mobike=no

conn tunv6-sites
        also=tunv6-ends
        leftsubnet=2001:1234:5678:200::/56
        rightsubnet=2001:1234:5678:100::/56
        auto=start

conn tunv6-ends
        left=2001:1234:5678::f
        leftcert=/etc/ssl/certs/moon.pem
        leftid=@moon.moonsite.example.com
        leftfirewall=yes
        right=2001:1234:5678::d
        rightcert=/etc/ssl/certs/sun.pem
        rightid=@sun.sunsite.example.com
        auto=start

***

I will greatly appreciate any suggestions which will nudge me towards
getting this to work! If you need any more information, just let me know.

All the best,
-- 
MS