Saturday, August 10, 2013

Getting Libreswan to connect to Cisco ASA 5500

Here are some notes about problems I had while trying to make Libreswan connect to Cisco ASA. Note that I'm using certificate based authentication in a roadwarrior configuration. The setup is done on Fedora 19.

The procedure, at least in theory, is quite simple. Edit, /etc/ipsec.conf file, modify /etc/ipsec.d/ipsec.secrets file, import certificates into NSS database and then start libreswan daemon. Finally, activate the connection. Well, it turned out that the theory is very far from the practice. So, here we go.

Preparation steps

First, I used the following ipsec.conf file when I started to test connection to the ASA:
version 2.0     # conforms to second version of ipsec.conf specification
# basic configuration
config setup

conn VPN
    # Left side is RoadWarrior
    # Right side is Cisco
    right= # IP address of Cisco VPN
    # config
Note few things about this configuration. First, my client machine (roadwarrior) is a left node, while Cisco is the right one. Next, I didn't arrive to this configuration immediately, I had to experiment with the value of interfaces and left statements. The reason was that I'm assigned dynamic NATed address. So, those settings will cause openswan to automatically select appropriate values (interface and IP address) at the time I'm connecting to VPN. Also, certificate related stuff also took me some time to figure it out.

Into ipsec.secrets file I added the following line:
this will cause openswan to use RSA key for authentication. Finally, I had to import certificates and keys into the NSS database. Note that NSS database is already precreated in /etc/ipsec.d directory. More specifically, the database consists of files cert8.db, key3.db and secmod.db. To see imported certificates (if there are any), use the following command:
# certutil -L -d /etc/ipsec.d/
Certificate Nickname               Trust Attributes
ROOTCA                             CT,C,C
SGROS                              u,u,u
CISCO                              P,P,P
In my case there are three certificates in the database. Mine (SGROS), VPN's (CISCO) and CA that signed them (ROOTCA). Note that I'm referencing those certificates in ipsec.conf file. If you are configuring this database for the first time, it will be empty and you'll have to import all the certificates.

To import CA into your database, use the following command:
certutil -A -i rootca.pem -n ROOTCA -t "TC,TC,TC" -d /etc/ipsec.d/
Note that I'm assuming you have PEM version of certificate stored in the current directory (argument to the -i option). For the rest options, and their meaning, please consult man page.

To import your certificate, with a private key, use the following command:
certutil -A -i certkey.pfx -n SGROS -t "u,u,u" -d /etc/ipsec.d/
Note that this time certificate and private key are stored in PKCS#12 file (named certkey.pfx).

Finally, to import certificate of Cisco ASA, use the following command:
certutil -A -i rootca.pem -n ROOTCA -t "P,P,P" -d /etc/ipsec.d/
Note that the command is very similar to the one used to import ROOTCA, but the trust attributes (option -t) are different. Namely, you don't want ASA's certificate to be CA, i.e. to be able to issue new certificates.

Starting and debugging

To start Libreswan daemon, I used the following command:
ipsec pluto --stderrlog --config /etc/ipsec.conf --nofork
That way I was forcing it not to go to the background (--nofork) and to log to stderr (--stderrlog). Then in another terminal I would trigger VPN establishment using the following command:
ipsec auto --up VPN
The first problem was that Libreswan said it can not determine which end of the connection it is, i.e. I was receiving the following error message.
022 "VPN": We cannot identify ourselves with either end of this connection.
That error message took me some time to resolve. I tried everything possible to let Libreswan knows if it is left or right part in the configuration, which included changing roles several times, changing different parameters and other stuff. In the end, it turned out that that has nothing to do with the configuration file, namely the problem actually was missing kernel module!? NETKEY wasn't loaded, and libreswan couldn't access IPsec stack within the kernel. To be honest, it could be inferred from the log by nothing the following lines:
No Kernel XFRM/NETKEY interface detected
No Kernel KLIPS interface detected
No Kernel MASTKLIPS interface detected
Using 'no_kernel' interface code on 3.10.4-300.fc19.x86_64
But then again, some more informative error message would actually help a lot! In the end, the following command solved that problem:
modprobe af_key
and then, in the logs I saw the following line:
Using Linux XFRM/NETKEY IPsec interface code on 3.10.4-300.fc19.x86_64
that confirmed NETKEY was now accessible, and the error message about not knowing which end of the connection it is, disappeared.

Next, I had problem with peer's public key. The error message I received was:
003 "VPN" #1: no RSA public key known for 'C=HR, L=Zagreb, O=Some ORG, CN=Cisco ASA'
Again, I lost a lot of time trying to figure out why it can not access public key even though it is in the certificate. I also tried to extract public key and write it directly into configuration file. Nothing helped. Then, when I turned on debugging of x509 in ipsec.conf, I found some suspicious messages, like the following one:
added connection description "VPN"
| processing connection VPN
|   trusted_ca called with a=CN=ROOTCA b=
|   trusted_ca returning with failed
|   trusted_ca called with a=CN=ROOTCA b=\001\200\255\373
|   trusted_ca returning with failed
Note the garbage as the second argument of the function trusted_ca!? Googling around for something about this didn't reveal anything useful. But then, out of desperation I tried removing leftca and rightca parameters from ipsec.conf, and guess what! Everything started to work. Checking again at the logging output I saw that now b parameter has the same value as a.

Yet, it still didn't work and after some tinkering I suspected that on the Cisco side XAuth is enabled and required. This I concluded based on the log output where Libreswan says what it received from Cisco:
"VPN" #1: received Vendor ID payload [Cisco-Unity]
"VPN" #1: received Vendor ID payload [XAUTH]
"VPN" #1: ignoring unknown Vendor ID payload [...]
"VPN" #1: ignoring Vendor ID payload [Cisco VPN 3000 Series]
At first, I thought that Libreswan will support XAuth, but obviously, if it is not configured Libreswan can not use it. Also, looking into manual page it says there that Xauth is disabled by default. So, after adding the following statements into ipsec.conf file:
and adding appropriate line into ipsec.secrets file:
@sgros : XAUTH "mypassword"
I managed to get further. Yet, it still didn't work. Looking again in the log output I realised that something was wrong with client configuration. Also, I got segfaults there that I didn't report to upstream for a simple fear that I might send some secret information. But, after adding the following statements into ipsec.conf segmentation fault was solved:
In the logging output of Libreswan I saw that configuration parameters were properly obtained, i.e.:
"VPN" #1: modecfg: Sending IP request (MODECFG_I1)
"VPN" #1: received mode cfg reply
"VPN" #1: setting client address to
"VPN" #1: setting ip source address to
"VPN" #1: Received subnet, maskbits 16
"VPN" #1: transition from state STATE_MODE_CFG_I1 to state STATE_MAIN_I4
Now, it seemed like everything was connected, but ICMP probes were not going through. Using setkey command I checked that policies and associations are correctly installed into the kernel, which they were. I quickly realised that the problem was that Libreswan didn't assign IP address to my local interface, nor did it assign routes. That was easy to check by just listing interface's IP addresses. To see if this is really problem, I manually assigned address and route:
# ip addr add dev wlp3s0
# ip ro add src via
and after that I was able to reach addresses within a destination network. Note that IP address given as argument to via keyword (in my case isn't important since XFRM will change it anyway. So, the problem was why this address isn't added in the first place.

After some poking around I found that the script /usr/libexec/ipsec/_updown.netkey is called to setup all the parameters, and also, looking into that script, I found that it didn't do anything when pluto calls it using up-client parameter! So, no wonder nothing happened. I also found on the Internet post about that problem. The fix is simple, as shown in the post I linked, but it messes something with routes. After some further investigation I discovered that when adding locally assigned IP address, the script messes up netmask. To cut the story, I changed the following line:
-it="ip addr add ${PLUTO_MY_SOURCEIP%/*}/${PLUTO_PEER_CLIENT##*/} dev ${PLUTO_INTERFACE%:*}"
+it="ip addr add ${PLUTO_MY_CLIENT} dev ${PLUTO_INTERFACE%:*}"
and also, I changed the following lines for IP address removal:
-it="ip addr del ${PLUTO_MY_SOURCEIP%/*}/${PLUTO_PEER_CLIENT##*/} dev ${PLUTO_INTERFACE%:*}"
+it="ip addr del ${PLUTO_MY_CLIENT} dev ${PLUTO_INTERFACE%:*}"
You can get the complete patch here.

Some random notes

It might happen that Libreswan suddenly stops working and that you can not access network, i.e. you can only ping you local address, but not local router. In that case try to clear XFRM policy using setkey command:
setkey -FP
you can also check if there is anything with:
setkey -DP

1 comment:

knox2 said...

Would you know the process of connecting via a preshared key

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scientist, consultant, security specialist, networking guy, system administrator, philosopher ;)

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