[strongSwan] strongswan/L2TP and NAT-T transport with both NATed

[Ulysse 31]

Hi all,

I have a strongswan with L2TP working with XP roadwarrior clients/ osx
clients and iphone on one gateway with a public IP.Had to enable
--enable-nat-transport but it works great.
Now I have a second configuration, which is like :

    <client>
         |
<NAT Gateway>
         |
  <Internet >
         |
<NAT Gateway (cisco ASA 5505)>
         |
<Strongswan Server>
         |
   <LAN>

It is the almost the same configuration, the main big difference comes
from the strongswan server that is NATed. The cisco ASA as no VPN
feature enable, it is used like a simple NAT gateway, redirecting one
public IP to the internal IP using a static NAT. all IP (TCP/UDP), esp
and AH protocol is allowed. here is the first example of configuration
used :

config setup
      plutodebug="control"
      strictcrlpolicy=no
      overridemtu=1410
      nat_traversal=yes
      charonstart=no
      plutostart=yes

conn L2TP
   authby=psk
   pfs=no
   auto=add
   rekey=no
   type=tunnel
   left=yy.yy.yy.yy                      # Internal private IP
   leftnexthop=XX.XX.XX.XX      # External IP address
   leftprotoport=17/1701
   leftfirewall=yes
   right=%any
   rightprotoport=17/%any
   rightsubnetwithin=0.0.0.0/0
   rightfirewall=yes
   esp=aes128-sha1
   ike=aes128-sha-modp1024
   forceencaps=yes

Here is what I got in the logs (the aa.aa.aa.aa is the IP public
address of the client, and bb.bb.bb.bb is its private address) :


---
Nov 20 11:51:35 src@<stronswanserv> pluto[26142]: "L2TP"[2]
aa.aa.aa.aa:4500 #4: cannot respond to IPsec SA request because no
connection is known for
XX.XX.XX.XX/32===yy.yy.yy.yy:4500:17/1701…aa.aa.aa.aa:4500[bb.bb.bb.bb]:17/%any==={bb.bb.bb.bb/32}
Nov 20 11:51:38 src@<stronswanserv> pluto[26142]: "L2TP"[2]
aa.aa.aa.aa:4500 #4: Quick Mode I1 message is unacceptable because it
uses a previously used Message ID 0xfb7bef8d (perhaps this is a
duplicated packet)
Nov 20 11:51:38 src@<stronswanserv> pluto[26142]: "L2TP"[2]
aa.aa.aa.aa:4500 #4: sending encrypted notification INVALID_MESSAGE_ID
to aa.aa.aa.aa:4500
Nov 20 11:51:38 src@<stronswanserv> pluto[26142]: | next event
EVENT_NAT_T_KEEPALIVE in 13 seconds
The "Quick Mode I1 message is unacceptable" is repeated several times
( retries from client ) and the ISAKMP is never established, then it
times out.
---


So next I tried with adding "leftsubnet=XX.XX.XX.XX/32" on the conn
L2TP config. which allowed me to establish ISAKMP, but then I have on
the logs :


---
Nov 20 11:52:58 src@<stronswanserv> pluto[26339]: |
preparse_isakmp_policy: peer requests PSK authentication
...
Nov 20 11:52:58 src@<stronswanserv> pluto[26339]: "L2TP"[1]
aa.aa.aa.aa #4: responding to Main Mode from unknown peer aa.aa.aa.aa
Nov 20 11:52:59 src@<stronswanserv> pluto[26339]: "L2TP"[1]
aa.aa.aa.aa #4: NAT-Traversal: Result using RFC 3947: both are NATed
Nov 20 11:52:59 src@<stronswanserv> pluto[26339]: "L2TP"[1]
aa.aa.aa.aa #4: Peer ID is ID_IPV4_ADDR: 'bb.bb.bb.bb'
Nov 20 11:52:59 src@<stronswanserv> pluto[26339]: "L2TP"[2]
aa.aa.aa.aa #4: deleting connection "L2TP" instance with peer
aa.aa.aa.aa {isakmp=#0/ipsec=#0}
Nov 20 11:52:59 src@<stronswanserv> pluto[26339]: "L2TP"[2]
aa.aa.aa.aa:4500 #4: sent MR3, ISAKMP SA established
Nov 20 11:52:59 src@<stronswanserv> pluto[26339]: "L2TP"[2]
aa.aa.aa.aa:4500 #4: ignoring informational payload, type
IPSEC_INITIAL_CONTACT
Nov 20 11:53:00 src@<stronswanserv> pluto[26339]: "L2TP"[2]
aa.aa.aa.aa:4500 #5: NAT-Traversal: received 2 NAT-OA. using first,
ignoring others
Nov 20 11:53:00 src@<stronswanserv> pluto[26339]: "L2TP"[2]
aa.aa.aa.aa:4500 #5: responding to Quick Mode
Nov 20 11:53:00 src@<stronswanserv> pluto[26339]: |
kernel_alg_esp_auth_keylen(auth=2, sadb_aalg=3): a_keylen=20
Nov 20 11:53:00 src@<stronswanserv> pluto[26339]: |
install_inbound_ipsec_sa() checking if we can route
Nov 20 11:53:00 src@<stronswanserv> pluto[26339]: | route owner of
"L2TP"[2] aa.aa.aa.aa:4500 unrouted: NULL; eroute owner: NULL
Nov 20 11:53:00 src@<stronswanserv> pluto[26339]: | add inbound eroute
aa.aa.aa.aa/32:57947 -> XX.XX.XX.XX/32:1701 =>
tun.10000 at yy.yy.yy.yy:17
Nov 20 11:53:00 src@<stronswanserv> pluto[26339]: | inserting event
EVENT_RETRANSMIT, timeout in 10 seconds for #5
Nov 20 11:53:00 src@<stronswanserv> pluto[26339]: | next event
EVENT_NAT_T_KEEPALIVE in 6 seconds
...
Nov 20 11:53:00 src@<stronswanserv> pluto[26339]: "L2TP"[2]
aa.aa.aa.aa:4500 #4: ignoring informational payload, type
INVALID_HASH_INFORMATION
Nov 20 11:53:00 src@<stronswanserv> pluto[26339]: "L2TP"[2]
aa.aa.aa.aa:4500 #4: received Delete SA payload: deleting ISAKMP State
#4
---


I have tried different flags on the conn settings that may helped me
(forceencaps=yes, type=transport instead of type=tunnel ). I really
don't see why does it act like this on this machine. this machine (the
strongswan server) is quite particular, since it runs a devil-linux
distribution (livecd), that i had to rebuild in order to add the
--enable-nat-transport on the strongswan script build, it uses
strongswan 4.2.16, is there a way to check if the binary is really
with the --enable-nat-transport ?
Does someone have an hint on what I can do in order to make it work ?
please help !

--
Ulysse31