Configuring Tomcat and Wireshark to capture and decode SSL communications

Debugging secure communications

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.

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Dominic Tulley (dominic.tulley@uk.ibm.com), Leading Architect, DOORS Development Lab, IBM  

Photo of Dominic TulleyDominic 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.



03 November 2009

Also available in Chinese

Before you start

Developing a system that involves network communication is extremely common in today’s 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.

Prerequisites

It is assumed that you have a basic understanding of network communication mechanisms. You don’t 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 to install 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 must be 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. Other versions (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 Wireshark’s 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.

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.


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 <server key pass phrase> 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 <server key pass phrase> 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 <server key pass phrase>. It will then prompt for a new pass phrase which I will refer to as the <server p12 pass phrase> 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.

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 <server p12 pass phrase> 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 <server jks pass phrase>. 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 the PKCS12Import 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 <server key pass phrase> you selected earlier. This command will result in a new file, open-private-key.pem, which contains an unprotected copy of your private key.


Configure Tomcat

Now that you have the cryptographic artifacts that you need, you can configure Tomcat 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:

Listing 1. Default SSL connector definition
    <!-- Define a SSL HTTP/1.1 Connector on port 8443 -->
    <!--
    <Connector port="8443" 
        maxHttpHeaderSize="8192"
        maxThreads="150"
        minSpareThreads="25" 
        maxSpareThreads="75"       
        enableLookups="false" 
        disableUploadTimeout="true"          
        acceptCount="100" 
        scheme="https" secure="true"           
        clientAuth="false" 
        sslProtocol="TLS" />
    -->

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
    <!-- Define a SSL HTTP/1.1 Connector on port 8443 -->
    <Connector 
        port="8443" 
        maxHttpHeaderSize="8192"      
        maxThreads="150" 
        minSpareThreads="25" 
        maxSpareThreads="75"     
        enableLookups="false" 
        disableUploadTimeout="true"        
        acceptCount="100" 
        scheme="https" 
        secure="true"       
        clientAuth="false" 
        sslProtocol="TLS" 
    />

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 <jks server pass phrase> you selected earlier.

Listing 3. Modified SSL connector definition

Click to see code listing

Listing 3. Modified SSL connector definition

    <!-- Define a SSL HTTP/1.1 Connector on port 8443 -->
    <Connector 
        port="8443" 
        maxHttpHeaderSize="8192"      
        maxThreads="150" 
        minSpareThreads="25" 
        maxSpareThreads="75"     
        enableLookups="false" 
        disableUploadTimeout="true"        
        acceptCount="100" 
        scheme="https" 
        secure="true"       
        clientAuth="false" 
        sslProtocol="TLS" 
 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.


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).

Figure 1. Wireshark preferences screen
Screen shot of the Wireshark application preferences screen

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

Figure 2. Wireshark SSL configuration dialog
Screenshot of the 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:

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 machine’s 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 040820A0
ssl_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 don’t see this message you need to 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 a conversation. 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
Screenshot of the Wireshark Capture Interfaces screen showing the different adapters with buttons for each to Start, see Options, or Details.

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:

Figure 4. Wireshark capture options dialog
Screenshot of the 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, you should 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:

Figure 5. Successful capture output
Screenshot of Wireshark packet capture results showing several rows of data highlighted in green to show that they have been correctly decrypted.

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 to see an HTTP conversation, you should now use follow ssl stream instead. This will show your HTTP conversation in plain text:

Figure 6. Following an SSL stream
Screenshot of Wireshark's detail following an SSL stream where the packets are assembled into a coherent whole, showing the header and HTML code. The line beginning 'Host:' and the last five lines of which the first begins 'Cookie:' are blacked out.

Out of order packets

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

Figure 7. Out of order packets in Wireshark
Screenshot of a Wireshark packet capture results screen with some packets highlighted with a black background, indicating that they were received out of order.

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 in the 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 doing its 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.


Conclusion

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 today’s 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.

Resources

Learn

Get products and technologies

  • 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.

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