[strongSwan] IPTables NAT Query

Tormod Macleod TMacleod at paywizard.com
Mon Apr 6 16:36:09 CEST 2015

I'm currently testing a site to site VPN. I need to change both the source and destination address on the left device before forwarding the packets over the VPN to the right device. I believe it all happens in the order below but I may be wrong.
1 IPTables Prerouting
2 Route selected
3 A determination is made on whether the packets should be encapsulated with IPSec
4 IPTables Postrouting
5 Packets are encapsultaed with IPSec where applicable
6 Packets are forwarded
I believe I have a solution but I'm not sure whether it's the best and I'd welcome some ideas...
In order to have the traffic encapsulated I had to create two child SAs on the left side. The first has the original source address and the translated destination address. This is only used in step 3. In step 4 the destination address is translated and by the time it gets to step 5 the traffic source and destination addresses match that of the second child SA which also matches the single child SA on the right side.
I'm concerned that I might run into some problems with this approach that I have not yet foreseen.
Here's my config...
config setup
	    # strictcrlpolicy=yes
	    # uniqueids=no
conn %default
conn SecurityAssociation-1
conn SecurityAssociation-2
Here's the statusall...
[root at localhost ~]# /opt/strongswan522/sbin/ipsec statusall
Status of IKE charon daemon (strongSwan 5.2.2, Linux 2.6.32-504.12.2.el6.x86_64, x86_64):
  uptime: 3 days, since Apr 03 14:47:09 2015
  malloc: sbrk 270336, mmap 0, used 210768, free 59568
  worker threads: 11 of 16 idle, 5/0/0/0 working, job queue: 0/0/0/0, scheduled: 4
  loaded plugins: charon aes des rc2 sha1 sha2 md5 random nonce x509 revocation constraints pubkey pkcs1 pkcs7 pkcs8 pkcs12 pgp dnskey sshkey pem fips-prf gmp xcbc cmac hmac attr kernel-netlink resolve socket-default stroke updown xauth-generic unity
Listening IP addresses:
SecurityAssociation-1:  IKEv2, dpddelay=40s
SecurityAssociation-1:   local:  [] uses pre-shared key authentication
SecurityAssociation-1:   remote: [] uses pre-shared key authentication
SecurityAssociation-1:   child: === TUNNEL, dpdaction=hold
SecurityAssociation-2:   child: === TUNNEL, dpdaction=hold
Security Associations (1 up, 0 connecting):
SecurityAssociation-1[4]: ESTABLISHED 41 minutes ago,[]...[]
SecurityAssociation-1[4]: IKEv2 SPIs: 75498cd903d39dfa_i* 9dca56ab7071039a_r, rekeying in 23 hours
SecurityAssociation-1[4]: IKE proposal: AES_CBC_128/HMAC_MD5_96/PRF_HMAC_SHA1/MODP_1536
SecurityAssociation-2{2}:  INSTALLED, TUNNEL, ESP in UDP SPIs: cb5e661f_i 9add0a95_o
SecurityAssociation-2{2}:  AES_CBC_128/HMAC_SHA1_96, 0 bytes_i, 0 bytes_o, rekeying in 48 minutes
SecurityAssociation-2{2}: === 
And here's the IPTABLES commands I used to send traffic both ways...
  iptables -t nat -A PREROUTING -p tcp -s -d --dport 61001 -j DNAT --to-destination
  iptables -t nat -A POSTROUTING -s -d -j SNAT --to-source
  iptables -t nat -A PREROUTING -p tcp -s -d --dport 61002 -j DNAT --to-destination
  iptables -t nat -A POSTROUTING -s -d -j SNAT --to-source

It's a bit convoluted but it works. I'd love to know if someone has a better idea.

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