[strongSwan] IPTables NAT Query

Noel Kuntze noel at familie-kuntze.de
Mon Apr 6 19:01:42 CEST 2015

Hash: SHA256

Hello Tormod,

There is a graph[1] that describes the path of the traffic in the kernel.

Why do you believe, that you have to apply SNAT/MASQUERADE?
By the way, your tunnel setup is wrong.
You define two IPsec tunnels, but there is only one being used:

>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}: ===

You should look for the reason that causes CHILD_SA of "SecurityAssociation-1" to not come up.
Also, with IKEv2, you can combine the subnets of all SAs into one.

So you can build something like this:


Mit freundlichen Grüßen/Kind Regards,
Noel Kuntze

GPG Key ID: 0x63EC6658
Fingerprint: 23CA BB60 2146 05E7 7278 6592 3839 298F 63EC 6658

Am 06.04.2015 um 16:36 schrieb Tormod Macleod:
> Hello,
> 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
>         ikelifetime=1440m
>         keylife=60m
>         margintime=3m
>         keyingtries=5
>         keyexchange=ikev2
>         authby=secret
>         left=
>         leftid=
>         auto=start
>         ike=aes128-md5-modp1536
>         esp=aes128-sha1
>         reauth=no
>         dpdaction=hold
>         dpddelay=40
> conn SecurityAssociation-1
>         leftsubnet=
>         right=
>         rightsubnet=
>         rightid=
> conn SecurityAssociation-2
>         leftsubnet=
>         right=
>         rightsubnet=
>         rightid=
> 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:
> Connections:
> 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.
> Cheers,
> Tormod
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