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.
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.
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.
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.
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
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
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"'
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
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"
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
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
jar in the
org.mortbay.jetty.security package. The
command to execute (assuming this jar file is in the current directory)
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
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.
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
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" />
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
Listing 3. Modified SSL connector definition
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.
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
Expand the Protocols entry on the left side and select SSL from the list that appears.
Figure 2. Wireshark SSL configuration dialog
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:
Multiple entries can be provided in a semicolon-separated list as shown here, but for this tutorial we only require one entry.
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
You will need to identify the IP address of your own machine. In Cygwin you can do this by running:
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: 126.96.36.199,8443,http,C:\temp\open-private-key.pem ssl_init found host entry 188.8.131.52,8443,http,C:\temp\open-private-key.pem ssl_init addr '184.108.40.206' 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
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
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
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
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
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
editcap –d capture.pcap fixed.pcap
editcap is included in the Wireshark
capture.pcap is the saved capture
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.
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.
- 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.
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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