[strongSwan] {Disarmed} Re: IPTables NAT Query

Tue Apr 7 13:25:49 CEST 2015

Hi Bryan,

I tried it the way you said and it worked exactly as you said. Heaven knows why it didn't work previously, I must have done something stupid. Thanks for your help.

Incidentally, the two SNAT and DNAT statements are for testing that traffic can be originated from either side of the VPN.

Cheers,

Tormod

>>> Bryan Duff <duff0097 at gmail.com> 06/04/2015 15:47 >>>
If I recall correctly your step 5 is where things matter - make sure at that point (basically after nat POSTROUTING) that the traffic source/dest matches your left/rightsubnets. Don't worry about the routing decision (your step 3). You should only need one DNAT and one SNAT for your traffic.

-Bryan

On Mon, Apr 6, 2015 at 9:36 AM, Tormod Macleod <TMacleod at paywizard.com> wrote:

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=10.180.0.12
leftid=2.2.2.2
auto=start
ike=aes128-md5-modp1536
esp=aes128-sha1
reauth=no
dpdaction=hold
dpddelay=40

conn SecurityAssociation-1
leftsubnet=MailScanner warning: numerical links are often malicious: 10.176.0.0/13
( http://10.176.0.0/13) 
right=3.3.3.3
rightsubnet=MailScanner warning: numerical links are often malicious: 192.168.0.0/16
( http://192.168.0.0/16) 
rightid=3.3.3.3

conn SecurityAssociation-2
leftsubnet=MailScanner warning: numerical links are often malicious: 1.1.1.0/24
( http://1.1.1.0/24) 
right=3.3.3.3
rightsubnet=MailScanner warning: numerical links are often malicious: 192.168.0.0/16
( http://192.168.0.0/16) 
rightid=3.3.3.3
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:
10.180.0.12
Connections:
SecurityAssociation-1: 10.180.0.12...3.3.3.3 IKEv2, dpddelay=40s
SecurityAssociation-1: local: [2.2.2.2] uses pre-shared key authentication
SecurityAssociation-1: remote: [3.3.3.3] uses pre-shared key authentication
SecurityAssociation-1: child: MailScanner warning: numerical links are often malicious: 10.176.0.0/13
( http://10.176.0.0/13)  === MailScanner warning: numerical links are often malicious: 192.168.0.0/16
( http://192.168.0.0/16)  TUNNEL, dpdaction=hold
SecurityAssociation-2: child: MailScanner warning: numerical links are often malicious: 1.1.1.0/24
( http://1.1.1.0/24)  === MailScanner warning: numerical links are often malicious: 192.168.0.0/16
( http://192.168.0.0/16)  TUNNEL, dpdaction=hold
Security Associations (1 up, 0 connecting):
SecurityAssociation-1[4]: ESTABLISHED 41 minutes ago, 10.180.0.12[2.2.2.2]...3.3.3.3[3.3.3.3]
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}: MailScanner warning: numerical links are often malicious: 1.1.1.0/24
( http://1.1.1.0/24)  === MailScanner warning: numerical links are often malicious: 192.168.0.0/16
( http://192.168.0.0/16)  
And here's the IPTABLES commands I used to send traffic both ways...
iptables -t nat -A PREROUTING -p tcp -s MailScanner warning: numerical links are often malicious: 10.176.0.10/32
( http://10.176.0.10/32)  -d MailScanner warning: numerical links are often malicious: 10.180.0.12/32
( http://10.180.0.12/32)  --dport 61001 -j DNAT --to-destination MailScanner warning: numerical links are often malicious: 192.168.1.1:23
( http://192.168.1.1:23) 
iptables -t nat -A POSTROUTING -s MailScanner warning: numerical links are often malicious: 10.176.0.10/32
( http://10.176.0.10/32)  -d MailScanner warning: numerical links are often malicious: 192.168.1.1/32
( http://192.168.1.1/32)  -j SNAT --to-source 1.1.1.2
iptables -t nat -A PREROUTING -p tcp -s MailScanner warning: numerical links are often malicious: 192.168.1.1/32
( http://192.168.1.1/32)  -d MailScanner warning: numerical links are often malicious: 1.1.1.3/32
( http://1.1.1.3/32)  --dport 61002 -j DNAT --to-destination MailScanner warning: numerical links are often malicious: 10.176.0.10:23
( http://10.176.0.10:23) 
iptables -t nat -A POSTROUTING -s MailScanner warning: numerical links are often malicious: 192.168.1.1/32
( http://192.168.1.1/32)  -d MailScanner warning: numerical links are often malicious: 10.176.0.10/32
( http://10.176.0.10/32)  -j SNAT --to-source 10.180.0.12

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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