Home > Articles > Security > Network Security

This chapter is from the book

Application Proxy

An application proxy is used to communicate with the Internet on behalf of your host. If you want to visit a Web page, the HTTP proxy (Web proxy) contacts that Web host on your behalf, then sends the information back to your system (Figure 3–10). You need only to communicate with the proxy. In essence, the proxy acts as a middleman between you and the remote host.

Figure 10FIGURE 3–10 Application proxy. The internal client requests a page from http://www.example.com. The client's browser is configured to send all requests to Proxy. Proxy receives request from Client and issues its own request for the page at http://www.example.com. The remote host replies to Proxy. Proxy replies with the data from http://www.example.com to Client.


Proxied communications provide a very powerful layer of protection. The proxy can actually examine the data content of the communication and determine if it is valid for the application being used. For example, some applications like Napster circumvent many firewalls by passing traffic on port 80. Napster, in reality, is providing a file transfer service and should be communicating on some other ports; 7777, 8888, and 8875 are just some of the ports recorded as being used by Napster. If your packet-filtering firewall blocks all normal Napster traffic, but allows HTTP traffic on port 80, you could simply choose to use port 80 for the Napster traffic as well. An application proxy, however, would recognize that the content of the packets traveling through port 80 does not match normal Web traffic and would block the application.

I don't mean to pick on Napster here. Many applications, from streaming audio to email programs, can change the ports used in communications. In fact, anything could really be listening on a given port regardless of the service registered at IANA.

There is a cost associated with using a proxy. The proxy must intercept every packet sent from your host to a remote host, the proxy then generates its own request to the remote host, receives the reply, and generates the reply to be sent to your computer. A benefit of this, however, is that subsequent requests for that same service may be faster as the proxy can cache the information it receives from the remote host. If you request a Web page at http://www.example.com for the first time, the proxy must get the information from the actual remote host. It will then cache that information so that the next time you request the page at http://www.example.com the proxy can simply send you the cached version of the page.

Proxies do not, by definition, provide any packet-filtering services. You do not configure them to allow or disallow certain types of traffic. They only examine the data content to be sure it matches the content expected for a given application.

Application proxies also have the ability to perform a function often called reverse proxying (see Figure 3–11). Let's say you have a Web server you wish to protect; you publish the proxy server's address to the world, and external users connect to the proxy, which then sends a request to the Web server on the external client's behalf. The only host to ever contact your Web server is your proxy server.

Figure 11FIGURE 3–11 Reverse proxy. The remote client requests the page at http://www.example.net. http://www.example.net resolves to 244.1.2.3, the address of the Proxy. Proxy sends a request from itself to the Web server, 172.16.1.3. Web server replies to Proxy. Proxy replies to remote client.


An application proxy is an application, normally run on a separate host. All protected hosts are configured to talk only to the proxy for a given service. It is important to note, though, that you need an application proxy for each application you wish to use. An HTTP/Web proxy is not going to be able to handle SMTP or FTP traffic for you. You will require an SMTP proxy and an FTP proxy if you wish to use these services.

How an Application Proxy Is Implemented in Personal Firewalls

In its most basic form, you run an application proxy for a given service, say HTTP (Web), then you configure the applications—browsers, in this case—on your client computers to use the proxy for all requests. In MS Internet Explorer on a Windows box, you do this by configuring the proxy settings in the Tools, Internet Options dialogue.

In most cases, large companies who want all of their employees to access the Web through their proxy server use this method. Why? Well, for all the same reasons you would consider an application proxy: to hide and protect clients, to monitor and restrict access, and to speed up service for clients (through caching).

The Trusted Information Systems (TIS) Firewall Toolkit (fwtk) was designed to provide the building blocks for a Linux-based firewall solution. You take the components you need and build the system that is right for your environment. It provides proxies for FTP, HTTP, telnet, and others. As the product is no longer officially supported, the TIS team is not building any new proxy applications. However, there is a strong community surrounding the product that has added proxy applications for such things as IRC, UDP traffic, and MS SQL. Source code is also available and users are encouraged to review the code before implementing it.

fwtk uses three files to control access to the new proxy services. These files are /etc/services, which defines the ports that the services listen on; /etc/inetd.conf, which tells the inetd process which program to call for when a service is requested; and /usr/local/etc/netperm-table, which sets out the parameters that fwtk uses to allow or deny access to a service.

The entries in netperm-table for the HTTP proxy may look like the example below. This example is taken directly from the Firewall-HOWTO-10 at linux.org.2

# www and gopher gateway rules:
 http-gw:	userid	root
 http-gw:	directory	/jail
 http-gw:	timeout 90
 http-gw:	default-httpd 	http://www.afs.net
 http-gw:	hosts	192.168.2.* -log { read write ftp }
 http-gw:	deny-hosts   * 

The first two lines simply set out the temporary directory for any files that need to travel through the proxy to the end user. The directory is owned by root and only accessible by root. Next is a short time-out value; since Web connections take little time, this will limit how long a client waits when there is a bad connection. The "default-httpd" line simply sets the default home page for those connecting to the proxy server. The settings for the HTTP proxy allow all hosts on the subnet 192.168.2 to access Web pages and FTP through this proxy, and log the connections. Finally, the last line denies all other hosts access to the proxy service.

In order to get this to work, you must also configure the /etc/services file to associate http-gw with port 80 traffic rather than the standard HTTP service. Finally, /etc/inetd.conf will be set up to call the correct program when traffic is seen on port 80.

Products That Use This Method

When I started to write this book, I figured this section would be very short. I didn't think any personal firewall products made use of proxy technologies. Well, I was wrong. I haven't found any commercial products yet, but, you guessed it, Linux has options. The TIS fwtk seems to be the original, but it is no longer officially supported or developed. There are other products like DeleGate and fw that are actively being developed. Check the Linux.org site for more information.

The nature of application proxy means you end up needing a proxy for every service you wish to run or connect to. So, while you can typically find HTTP and FTP proxies easily it may be more difficult to find an IRC proxy. What does this mean? Well, either you can't do IRC from your host (no proxy, no communication), you can use IRC but it's unprotected, or you install another type of firewall to ensure your host is protected while using IRC.

Advantages

The ability to cache data means that connections appear faster to end users when pages are retrieved from a proxy instead of the Internet. Caching in itself is not a firewall feature, but it is one benefit of using a proxy.

Proponents feel application proxies are the most secure method—the proxy can validate the packet contents against the protocol's expectations, blocking packets that do not comply.

It is possible for an application proxy to be transparent to the user. This means that applications are aware of, and configured for, the proxy. The user does not have to perform any special steps in order to access the services they desire. Unfortunately, transparent proxies do not exist for all applications.

You can perform some level of content or access control. Through the authentication of users and applications, you can restrict access to resources based on a user ID, an IP address, even a MAC address.

Application proxies provide the best logging capabilities. You can see where a host is going and even retrieve the data exchanged when configured appropriately. From a corporate perspective, this could be useful for monitoring employee usage of the Internet.

Disadvantages

The primary issue with application proxies is the need to have one for every application. It can be difficult, if not impossible, to find an application proxy to support every application you wish to use. As a result, you will end up either accessing resources insecurely or using a second product to secure those services.

Application proxies are the slowest of the firewall solutions. They process the packets at the Application layer of the OSI model, which means a lot of resources are used to perform a proxy's tasks. To the user, it appears as if the Internet has slowed.

Not all application proxies are transparent to the user. In some cases, they require software to be installed on client computers, in others users must perform special functions to direct their applications to use the proxy server.

You will likely require a separate computer to host the proxy services. You will need to protect this host, as it is accessible to Internet hosts. The proxy server has protected your internal computers from attack, but that leaves it vulnerable to threats from the Internet.

Active Content

"HTTP is designed_to be readily extensible."3

In English, this means that we can expand the capabilities of Web pages beyond simple static content. Web pages now include programs that run on the client computer. Such features enhance our Web experiences.

Unfortunately, such features can also be used to cause damage to your computer. There are several computer Trojans and viruses that arrive as HTML pages with ActiveX code in them. One such Trojan, Trojan.JS.Offensive, uses ActiveX features to modify your browser's default home page as well as affecting your computer's functionality.

As far as the HTTP protocol is concerned, such applications are valid, the protocol allows for passing executable data. Because the data follow the rules of the protocol, a Web proxy server will forward the data on to the requesting client. When the data includes malicious code, this can be disastrous for the end user. An application proxy can only validate data against the protocol; it cannot determine the intent of the data.

Where an Application Proxy Fits in the Design of a Secure Environment

I guess I have a bias here—for the average home person, this approach is a little over the top. I'd argue there is too much maintenance, setup, overhead, and so on to make it a truly viable personal firewall option. If you have a small office with multiple users, it may make sense to send their communications through a proxy. Then you benefit from the ability to authenticate to the proxy and control who has access to what Internet resources (Figure 3–12).

Figure 12FIGURE 3–12 Network design with application proxy. Server sits between clients and the Internet. All requests flow through proxy server. Must still protect the proxy server from attacks aimed at it, generally exploiting vulnerabilities below the Application layer.

Remember also, that an application proxy does not by definition perform any filtering. When you run an HTTP proxy, for example, your users can access any sites that provide HTTP services. If you want to stop communication with certain hosts, you will require another product to filter them out.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020