TYPES OF PROXY

TYPES OF PROXY

Proxies can be used for several purposes.

The classic use is as a proxy firewall located on the perimeter between

the Internet and your private network. Proxies are not limited to this role

though.

Proxies can be used to accelerate web performance, provide remote

access to internal servers, and provide anonymity for network

conversations. Proxy are of following types:

1. Web Proxies

2. Reverse Proxies

3. Anonymizing Proxies

Web Proxies

Proxies are not just used to implement firewalls. One of their most

popular uses inside a network is increasing web performance.

Web conversations make up a large percentage of the traffic on many

networks, so making the Web more efficient can have a dramatic impact

on network operations.

Proxies can help by monitoring web conversations and eliminating

redundant requests. Web traffic is often characterized by frequent

transmissions of nearly identical information.

Web Proxies

Some studies have shown that as much as half the requests for information

across the Web are duplicates of other recent requests.

Caching frequently requested web pages can dramatically speed up web

browsing.

Proxy servers that provide web caching are often referred to as proxy

caches or web caches.

When a proxy cache is used, browsers are directed to make their HTTP

requests to the proxy cache instead of directly to the destination web server.

Web Proxies

The proxy then has the opportunity to determine whether it already has a copy

of the requested information or if it needs to request a copy from the

destination server.

If the HTTP request is new, the proxy will make a TCP connection and HTTP

request to the destination server, returning the resulting information to the

browser and also storing a copy of the returned result for future use.

Whenever any client of the proxy requests the same information, the proxy can

reply using its local copy, eliminating the need to make a request from the

destination server.

Web Proxies

This reduces network traffic as well as the load on the web server.

However, it can introduce problems.

Web Proxies

Caching works best when the information being retrieved does not

change rapidly.

However, some information is very time sensitive, such as stock quotes.

This can cause problems if the client receives old information from the

cache, when newer, more relevant data is available on the web server.

The term for this is freshness.

A file is "fresh" if the version on the cache is the same as the version on

the web server.

Web Proxies

Web servers can specify when a file should no longer be considered

fresh by placing an "Expires:" header in the returned request.

This tells any caches being used (whether proxy or browser based)

when to discard the file and request a new one.

Many web servers do not provide good expiration guidance though.

Because of this, it is important during the configuration of a proxy cache

to establish good freshness policies.

Web Proxies

Freshness policies are normally developed using several values associated

with the file. The most important, if supplied by the web server, is the

"Expires:" field.

This field is part of the HTTP protocol and, if configured by the web

administrator, is provided in the server's response to a browser request.

It allows the website to provide specific guidance concerning when a file

should be disregarded.

When this information is not available, though, the web proxy server will need

to look at other data to make a freshness decision.

Web Proxies

One simple method would be to set a fixed time to cache all files that

lack "Expires:" headers.

The problem with this approach is that many sites with dynamic content

that do not support "Expires:" will not work correctly when cached.

A better approach is to use the age of the file to determine how long to

cache.

If a file is received that is seconds old, you might not want to cache it

because it is much more likely that it was dynamically generated.

Web Proxies

A file that is weeks old, though, is much less likely to change while its

copy is held in the cache.

Even with files that have not been modified for a long time, it is still a

good idea to periodically refresh the cached files, so most web proxy

servers set a maximum time a file can be considered fresh.

Web Proxies

Another benefit that can be gained through web proxies is control over where

users can browse.

Security and productivity can be increased by limiting access to non-

organization-related web browsing.

It is not uncommon for viruses, worms, and other types of malicious code to

be introduced into a protected network based on files downloaded by users

from inappropriate websites.

By limiting what sites users can reach, you can decrease the chance that this

will happen to your network.

Web Proxies

Placing restrictions on browsing has also been shown to increase

productivity by taking away the temptation to spend excessive time surfing

the Web.

However, not all organizations will want to or be able to place restrictions

on user web behavior. Before considering web filtering, you must examine

your site's policies and procedures regarding user web access.

Often your Human Resources and Legal departments will need to be

involved.

Web Proxies

One last item to discuss with web proxies is the logging they can provide.

Proxy logging can be very useful in detecting malicious activity on your

network. With a web proxy, all the URLs that browsers request can be

used for intrusion analysis.

Looking for requests that do not appear normal can be a powerful method

to discover attacks against your network.

Often your proxy logs will contain the first indications that your network is

under attack.

Web Proxies

Things to look for include excessive requests for files that do not exist on your

web servers (such as those that return 404 errors).

This can indicate that someone is scanning your websites looking for

vulnerable software.

Also looking for excessively long URL requests, or requests that contain special

characters, can indicate that someone is attacking your site.

If you do discover that someone has successfully attacked your site, these logs

can also be invaluable at discovering what weakness led to the compromise,

how extensive the damage is, and (rarely) who is responsible.

Reverse Proxies

Firewalls are frequently thought of as devices that restrict access, not

enable it.

However, proxy techniques can be used for both. If you have a need to

support remote Internet users, reverse proxies can be the answer.

Reverse proxies are used to provide controlled access for external

(normally Internet-based) users to internal servers.

They act as a trusted intermediary that external users must use to gain

access to internal servers that would not normally be Internet accessible.

Reverse Proxies

An external user attempting to gain access to an internal server first

connects and authenticates to the reverse proxy.

Normally this is done over a Secure Sockets Layer (SSL) connection to

provide confidentiality and integrity for the session.

If authentication is successful, the proxy will check its policy to see

whether the user is allowed to access the requested server. If so, it will

begin proxying the connection for the user.

Reverse Proxies

The type of internal servers that can be accessed using a reverse proxy vary

depending on the sophistication of the proxy.

Simple reverse proxies can only support web-based services.

These products are basically normal web proxies that have been enhanced to

support user authentication.

In many cases, they are sufficient because many sites provide a significant

amount of their network content using web systems.

If you are trying to grant access to other applications that do not have a web

interface, you may need to work harder.

Reverse Proxies

One approach is placing a web interface on top of the application you are

trying to proxy.

Once the application is web enabled, normal reverse proxy techniques can

be used to grant remote access.

An example of this is Microsoft's Outlook Web Access (OWA). OWA is part of

Microsoft Exchange and provides a web version of the Outlook mail and

calendaring application.

Any clients who can make a web connection to the OWA application will be

able to use most Outlook functions.

Reverse Proxies

In fact, it can be difficult to recognize that you're accessing Outlook

through a browser because the interface you are interacting with inside

the browser so closely resembles the desktop version of Outlook.

OWA combined with a reverse proxy provides a secure mail and

calendaring solution for remote users.

Reverse Proxies

Alternatively, you can roll the web-enabling technology together with a

reverse proxy.

This is the approach taken by Citrix MetaFrame.

Citrix allows common desktop and server applications to be accessed by web

browsers, including applications such as Microsoft Word and Adobe Acrobat.

In fact, Citrix can proxy an entire user desktop through a browser, giving a

user experience that is highly similar to sitting in front of the actual computer.

Reverse Proxies

Citrix also provides extensive management controls, including role-based

access to internal applications.

Although a capable product, it is not necessarily cheap and simple to

implement.

If you're considering technologies such as Citrix, make sure to include

acquisition and operational costs in your analysis.

In some cases, though, Citrix-like products can actually save you money by

allowing shared access to products too expensive to place on every user's

desktop.

Anonymizing Proxies

Privacy can be an important security service but can be a hard

commodity to come by on the Internet.

Almost all actions taken on a computer leave a digital trail. If you don't

want someone else following that digital trail back to you, an

anonymizing proxy may be the answer.

Anonymizing proxies work exactly like normal proxies, but are used for

the purpose of protecting your identity while you use services across the

Internet.

Anonymizing Proxies

Your requests are forwarded to the anonymizing proxy (usually over an

SSL connection), which hides your identifying details (such as IP

address) by making the request on your behalf.

The destination server you are using only learns about the proxy's

information and does not learn who actually made the request.

This assumes that you do not pass anything identifying in the actual

request.

Anonymizing Proxies

Also assumed is that no one is monitoring the anonymizing proxy.

If they were, they might be able to match incoming requests to outgoing

requests, breaching an important aspect of the connection's privacy.

This is especially easy to do if the proxy is not busy. If yours is the only

IP address connected to the proxy, it's not terribly hard to guess who it

is making requests through the proxy!

Various approaches have been used to solve this problem.

Anonymizing Proxies

One of the most popular is proxy chaining. Tools such as SocksChain (

http://www.ufasoft.com/socks) can be used to build connections through

multiple anonymizing proxies.

An observer at the first proxy in the chain will only see that you are

sending a request to the anonymizer, but will not learn the destination

because the next hop will only be another anonymizer.

In this way, the ultimate destination of your request is hidden from any

outside observers.

Anonymizing Proxies

Another approach along the same lines is Onion routing (

http://www.onion-router.net), which combines proxy chaining with

multiple layers of encryption to ensure that a conversation cannot be

followed through the proxy nodes.

Anonymizing Proxies

If you are in need of an anonymizer service, but do not want to set your

own up, preexisting services are available on the Internet.

Searching on Google for "anonymizers" will return many sites offering

this privacy service.

However, caveat emptor: You should trust that they maintain your

privacy slightly less than you trust them.

Anonymizing Proxies

A perfect case in point is the Java Anonymous Proxy (JAP).

JAP is a anonymizer service run as a joint effort of the Dresden

University of Berlin, the Free University of Berlin, and the Independent

Centre for Privacy Protection, Schleswig-Holstein, Germain.

It is available at http://anon.inf.tu-dresden.de/index_en.html. Back in July

of 2003, it was discovered that they had, as a result of a court order,

added code to JAP that was designed to monitor access to certain IP

addresses.

Anonymizing Proxies

Whenever a user of the service accessed one of these forbidden sites, a

message was generated recording the who, what, and when and sent to

the police.

This hidden behavior was uncovered several days later by an observant

user of the service, but until this discovery was made, users of the JAP

service were getting less privacy than they thought.

For the record, the current version of JAP is supposed to be free of any

tattle-tail code.