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Copyright IBM Corporation 2009 Trademarks

Configuring Tomcat and Wireshark to capture and

decode SSL communications

Page 1 of 16

Configuring Tomcat and Wireshark to capture

and decode SSL communications

Debugging secure communications

Dominic Tulley ([email protected])

Leading Architect, DOORS Development Lab

IBM

Skill Level: Introductory

Date: 03 Nov 2009

Investigating client-server communication issues is troublesome at the best of

times, and when the communication is secured with SSL, it becomes much more

difficult. This tutorial takes you through the steps involved in configuring Tomcat

and Wireshark so that the SSL Dissector in Wireshark can decrypt the captured

communication. This allows your investigation to proceed as if SSL was not

involved.

Section 1. Before you start

Developing a system that involves network communication is extremely common in

todays distributed world. During the development of such a system, a communication

problem may arise that is difficult to investigate by instrumenting or debugging your

software. At times like this, you can turn to a tool like Wireshark, which captures

all the network traffic in your system and lets you read it. This often provides

the insight required to resolve your problem. If your system uses SSL to secure

communications, then you can capture the network traffic, but it is all encrypted,

preventing you, the developer, from gaining any useful information. Fortunately,

Wireshark comes with an SSL Dissector that, given the right set of conditions, can

decrypt this captured traffic. This tutorial provides a step-by-step guide to setting up

those conditions using Tomcat as the server and a Web browser as the client.

Objectives

By the end of this tutorial you will have an understanding of the various conditions

that must be met before the Wireshark SSL Dissector can be used. Furthermore, you

will be able to arrange for those conditions to be met in a Tomcat installation.
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Configuring Tomcat and Wireshark to capture anddecode SSL communications Page 2 of 16

Prerequisites

It is assumed that you have a basic understanding of network communication

mechanisms. You dont need to have experience with Tomcat or Wireshark, although

both would be advantageous. In particular, this tutorial does not go into any depth

about how to use Wireshark once it has been configured correctly.

Please note: This tutorial is not intended to provide any significant insight into why

this set of commands works, nor to give any consideration to the many possible

variations on these commands. It is intended to enable you to record and decode

network traffic in a development environment. It is important to note that the

procedures followed here and some of the cryptographic artifacts generated here are

very insecure and should never be applied to a production environment.

System requirements

This tutorial assumes you are using Windows. To follow this tutorial you will need toinstall the following (see Resources for download sites):

Tomcat 5.5

A suitable JVM for Tomcat

Wireshark Version 1.2.2 was used in the development of this tutorial. The

Wireshark download mustbe compiled with GnuTLS and Gcrypt. This can be

checked in the "About" window of Wireshark.

Cygwin including openssl Version 0.9.8i of openssl was used in the

development of this tutorial.

Jetty Version 6.1.21 was used in the development of this tutorial. Otherversions (newer and older) may work, but the location of the required class file

within it may have changed.

In addition, you will need a second machine on which you will use a Web browser to

access your Tomcat server.

Overview

The following key conditions must be met to allow Wireshark to decrypt a secure

connection:

Tomcat must be configured with a server certificate.

The specific cryptographic cipher used between client and server (browser and

tomcat) must be amenable to Wiresharks method of decrypting.

Wireshark must have access to the private key associated with the server

certificate.

This tutorial will work through these steps in this order and then, using a browser

connecting to the Tomcat server, we will capture a request and response and

examine it in Wireshark.


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A note on passwords

Through this tutorial you will be asked to define three separate passwords. Each

password is carefully named and referred to in the tutorial, but to make life easier,

you might like to consider using the same password in all three places.

Section 2. Create a private key and certificate

First I will show you how to create a new private key using openssl. Start Cygwin and

execute the following command:

openssl genrsa -des3 -out ./serverkey.pem 2048

You will be asked to provide a pass phrase for the generated pem file. I will refer to

this pass phrase as later in this tutorial. After providing

and verifying this pass phrase, the command will complete and a new private key will

have been generated and stored in the file serverkey.pem.

Now execute the following command (note the command should all be on one line;

it's not on one line here due to tutorial template restrictions):

openssl req -new -x509 -key ./serverkey.pem -out ./servercert.pem -subj

'/C=GB/ST=XX/O="My Org"/CN="Wireshark dissectable Cert"'

You will be prompted to supply a pass phrase at this point, and you should enter the you selected earlier. This results in a server certificate

stored in servercert.pem which corresponds to the private key you previously

generated.

Convert certificate to JKS keystore

The server certificate that you generated in the previous step is unfortunately not in

the format that Tomcat requires, so you must convert it to be in a JKS keystore. The

first step is to change it to the PKCS12 format with the following command (to be run

in Cygwin):

openssl pkcs12 -export -in ./servercert.pem -inkey ./serverkey.pem

-certfile ./servercert.pem -out ./servercert.p12 -name "cert-name"

This will again prompt for a pass phrase, and you should provide the . It will then prompt for a newpass phrase which I will refer to as the

later in this tutorial. After entering and verifying this new

pass phrase, the command should complete and a new file, servercert.p12, will

have been created.


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Next, we make use of a utility included in the Jetty distribution to convert this

certificate into a JKS keystore. Different versions of Jetty may have this class in

different jar files, or even under different packages in the distribution. In the Jetty

6.1.21 distribution the required class is present in the main jetty-6.1.21 jar in the

org.mortbay.jetty.security package. The command to execute (assuming this jar

file is in the current directory) is:

java -classpath ./jetty-6.1.21.jar org.mortbay.jetty.security.PKCS12Import

./server.p12 ./dissectable.keystore

This will first prompt for the input keystore pass phrase. This is the you selected earlier. You will then be asked to select a new pass phrase

for the new keystore. I will refer to this later as the . Note

that you are not asked to confirm this pass phrase so you must be careful to type

correctly.

Note: If you are not using precisely the same version of Jetty, you may find that thePKCS12Import class is in a different jar file or even under a different package.

Unprotecting the private key

The final piece of work with openssl is to remove the pass phrase being used to

protect your private key. This is necessary so that Wireshark can read the key file

correctly. In cygwin, the following command will remove the pass phrase protection

from the key file:

openssl rsa in ./serverkey.pem out ./open-private-key.pem

This command will prompt you for a pass phrase. You should enter the you selected earlier. This command will result in a new file, open-

private-key.pem, which contains an unprotected copy of your private key.

Section 3. Configure Tomcat

Now that you have the cryptographic artifacts that you need, you can configureTomcat to use them. There are three configuration steps you must complete:

Copy the dissectable.keystore into the Tomcat root directory.

Tell Tomcat to use the server certificate you have constructed.

Restrict the set of ciphers to those that Wireshark can cope with.

The last two actions are achieved by editing the same file. Within the Tomcat

installation, navigate to the conf directory and edit the server.xml file. Locate the

section of the file that defines the SSL connector, as shown in Listing 1:


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Listing 1. Default SSL connector definition

Remove the second line and the last line so that the xml looks like the code in Listing

2.

Listing 2. Uncommented SSL connector definition

Then modify the connector element to include the text in bold below. This assumes

that you copy your dissectable.keystore file to the Tomcat installation root directory

(the same place as the LICENSE and NOTICE files). Where it says "password

here" in the example below, you should enter the you

selected earlier.

Listing 3. Modified SSL connector definition


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keystoreFile="dissectable.keystore"

keystorePass="password here"

ciphers="SSL_RSA_WITH_RC4_128_SHA,

TLS_RSA_WITH_AES_128_CBC_SHA,

SSL_RSA_WITH_3DES_EDE_CBC_SHA"

/>

Verify the Tomcat configuration

At this point it is worth verifying that Tomcat has been configured correctly. To do this,

simply run the startup.bat script in the bin directory. If you have configured everything

correctly and copied the dissectable.keystore file to the correct location, then Tomcat

will start up with no errors.

If Tomcat does not start correctly, either with errors on the screen or with it simply

failing, then you should double check that the xml edits you have made do not have

any typos and that the keystore file is in the correct location.

Once Tomcat has started you can verify that the SSL connection is functioning by

starting a Web browser and navigating to https://localhost:8443/. This will initially

complain about the server certificate and ask you if you wish to trust the server. (The

precise terminology varies depending on which browser you use.) This is caused by

the self-signed certificate you created earlier, and you should respond that you do

trust the server. You should then see the Tomcat welcome page, which means that

you have correctly configured Tomcat to support secure connections.

Section 4. Configure Wireshark

Now that you have Tomcat configured, you need to configure Wireshark. To do this,

start up Wireshark on the machine that you run Tomcat on and in the menus, select

Edit --> Preferences (Ctrl-Shift P).


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Figure 1. Wireshark preferences screen

Expand the Protocols entry on the left side and select SSL from the list that

appears.

Figure 2. Wireshark SSL configuration dialog

Options boxes

You should check both of the options boxes.

RSA keys list

The RSA key list field tells Wireshark which private key to use to decode

conversations depending on which IP address and port the conversation is with. The

format is:


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IPAddress,Port,Protocol,PrivateKey

Multiple entries can be provided in a semicolon-separated list as shown here, but for

this tutorial we only require one entry.

IPAddress1,Port1,Protocol1,PrivateKey1; IPAddress2,Port2,Protocol2,PrivateKey2

For the purposes of this tutorial I assume the IP address I am running Tomcat on is

192.168.45.22, and it is listening on port 8443 for SSL traffic. The traffic will be HTTP

traffic, and the open-private-key.pem file that we generated earlier has been copied

to the root of the C drive. For this, the string to enter in the RSA keys list field is:

192.168.45.22,8443,http,c:\open-private-key.pem

You will need to identify the IP address of your own machine. In Cygwin you can do

this by running:

ipconfig /all

Note: If your machine has multiple network adapters, you may have more than one

IP address for the machine. You must choose the address with which your second

machine can connect to this machines Tomcat server.

SSL debug file

You should provide a filename where Wireshark can produce debug output. This will

be very useful in troubleshooting your Wireshark configuration.

Complete the configuration

Press OK to complete the configuration.

Verify the configuration

You should now open the SSL debug log file that you selected earlier. If your

configuration was successful you will see some text indicating that Wireshark

successfully loaded the private key. See an example below in Listing 4.

Listing 4. SSL debug log contents

ssl_association_remove removing TCP 9443 - http handle 040820A0ssl_init keys string:

9.175.228.55,8443,http,C:\temp\open-private-key.pem

ssl_init found host entry 9.175.228.55,8443,http,C:\temp\open-private-key.pem

ssl_init addr '9.175.228.55' port '8443' filename 'C:\temp\open-private-key.pem'

password(only for p12 file) '(null)'

Private key imported: KeyID 0E:BA:EA:08:5D:FA:FB:85:59:4A:7B:A9:B2:56:C3:16:...

ssl_init private key file C:\temp\open-private-key.pem successfully loaded

association_add TCP port 8443 protocol http handle 040820A0

This confirms that you have carried out all the cryptographic manipulation steps and

Wireshark configuration steps correctly. If you dont see this message you need to


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revisit the steps involved. The mostly likely problems have to do with the path to the

key file in the RSA keys list or the actual generation of the private key file.

Capture a conversation

Once you have Wireshark configured correctly, you are ready to record aconversation. In Wireshark, select Capture --> Interfaces (Ctrl I -- upper case i) from

the menu. This should present a dialog similar to the following:

Figure 3. Wireshark interface listing dialog

You need to decide which of the listed network interfaces is the one that your second

machine will connect to Tomcat through. This should already be clear since you

needed to know the right IP address for the Wireshark configuration step. If you are

unsure, you can use the Web browser on the second machine to access the Tomcat

server and see which interface shows an increased packet count.

Once you have selected the interface you want to listen on, click options and you

should be presented with another dialog:


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Figure 4. Wireshark capture options dialog

You should enter the following in the Capture Filter to reduce the amount of noise in

the captured conversation:

tcp port 8443

Now click Start to begin capturing traffic. At this point you should not see any packets

being captured. You should go to your second machine and access Tomcat with

it using a url such as https://192.168.45.22:8443/, but putting in the IP address or

hostname of the machine that has Tomcat on it. When you do this, you will be told

that there is a certificate problem and asked if you trust the server. As before, youshould confirm that you do trust the server. After this, the Tomcat welcome page

should appear in the browser and lots of rows of data should appear in Wireshark.

You can now stop the capture by selecting Capture --> Stop from the menu.

If you have done this correctly, you will see a screen with some green rows, showing

that they have been decrypted, like this:


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Figure 5. Successful capture output

You are then free to use Wireshark as you normally would to analyse the HTTP

conversation and proceed with your problem investigation.

One final thing to observe here is that if you would normally use follow tcp stream tosee an HTTP conversation, you should now use follow ssl stream instead. This will

show your HTTP conversation in plain text:


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Figure 6. Following an SSL stream

Out of order packets

It is possible that you can encounter Out of Order packets as part of your capture, as

shown here:


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Figure 7. Out of order packets in Wireshark

These can cause the SSL Dissector to partially fail. Observe that there is a green

row near the bottom but the response to that GET has not been decrypted. To fix this

problem you need to save this capture using File --> Save As. Then, in Cygwin you

need to run editcap d capture.pcap fixed.pcap , where editcap is included inthe Wireshark installation, capture.pcap is the saved capture file, and fixed.pcap will

contain the fixed capture after the command has run. After running this command you

can load the fixed packet capture file back in to Wireshark and then you should see

the decrypted packets as expected.

No green rows at all

Wireshark relies on capturing the entire SSL conversation. If you make multiple

requests from your browser over the course of this tutorial it can be that some parts

of the negotiated secure connection are cached and then the captured conversation

does not contain the full negotiation. This will prevent the SSL Dissector from doingits job. Look at the start of the capture for an entry with "Server Hello, Certificate,

Server Hello Done" in the info line. If you cannot find this, then your browser is

reusing part of a previous negotiation. Restarting the browser and trying again should

fix this.

Section 5. Conclusion


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In this tutorial you have learned how to capture and read a secure HTTP

communication between two systems. You have seen how to generate the required

cryptographic artifacts and then how to configure Tomcat and Wireshark to use them

correctly. The steps involved are not complex, and, with some familiarity, can be

performed very easily. This is a very powerful tool in todays world of secured and

distributed systems, although hopefully one that is not required very often.

This mechanism has been demonstrated in the context of a Web browser connecting

to a Tomcat server, but it can easily be applied to any system communicating with

Tomcat over SSL. With some further investigation, the procedure here should be

applicable to other servers with relative ease.


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Resources

Download Tomcat 5.5 from the project Web site.

Download a suitable JVM for Tomcat.

Download Wireshark from the project Web site.

Download Cygwin from the project Web site.

Download Jetty from the project Web site.

The Wireshark Web site has online documentation and a very helpful set of

mailing lists that can quickly help you improve your ability to analyze packet

captures.

This documentation includes more details on configuring the SSL Dissector

within Wireshark.

Check out My developerWorks: Find or create groups, blogs, and activities

about Web development or anything else that interests you.
https://www.ibm.com/developerworks/mydeveloperworks/http://wiki.wireshark.org/SSLhttp://wiki.wireshark.org/SSLhttp://www.wireshark.org/lists/http://www.wireshark.org/http://docs.codehaus.org/display/JETTY/Downloading+Jettyhttp://www.cygwin.com/http://www.wireshark.org/download.htmlhttp://www.ibm.com/developerworks/java/jdk/http://tomcat.apache.org/download-55.cgi


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About the author

Dominic Tulley

Dominic Tulley has more than ten years experience working in server-side Java development, including the intricacies of client-server

communication. He is a leading software architect within the IBM

DOORS Development lab.

Copyright IBM Corporation 2009

(www.ibm.com/legal/copytrade.shtml)

Trademarks

(www.ibm.com/developerworks/ibm/trademarks/)
http://www.ibm.com/developerworks/ibm/trademarks/http://www.ibm.com/legal/copytrade.shtml

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