Home > Articles > Operating Systems, Server > Linux/UNIX/Open Source

User-Space Redirection—The Illusion of the Facade

The first—and likely the most common—method of redirection is to use a user-space process to listen to networking requests on the desired interface and port, and then to forward these unaltered to the destination system and port. To a large extent, these processes are like proxies. In fact, these processes are often referred to as proxies, although I think redirector is more precise. If all goes well with the redirection, the user can't tell the difference between connecting to the redirecting destination and the true destination. (Note, however, that the destination sees the connection attempt come from the redirecting server.) One positive upshot of all this is that servers with private IP addresses can still be used to provide services to Internet clients. Figure 1 depicts the topology.

Figure 1

User-space redirection of traffic.

Because this type of topology is frequently used (or at least frequently desired), and in many cases admins don't choose to run HTTP daemons on their firewalls or external routers, people have invested some time in developing redirection solutions. Several popular redirection packages are available, some of which are listed in the following paragraphs.

transproxy provides a daemon, tproxy, which is easy to set up and use. It has no configuration file—all options are passed as command-line arguments. It's very simple to configure after you take a look at the man page, so I won't go into detail. But since I've used it a lot, here are a few notes on things that may not be obvious immediately:

  • It can be run either from inetd or as a stand-alone server (the absence of -s port indicates that it should run from inetd).

  • You need to specify -t when you're not proxying HTTP—the default mode is for HTTP translate, which is for use when connecting to HTTP proxy servers Squid or Apache.

  • Use -r username to have the proxy run as a non-root user. This is valid even if the listening port is 1023.

  • You can use -b ipaddress to force the daemon to bind to a particular interface or IP address.

transproxy is available from the upstream source ftp://ftp.nlc.net.au/pub/linux/www/. (There may be a Debian package, too.) Some instructions are floating around out there that incorrectly note that you need to have transparent proxying enabled in the kernel to be able to use the daemon; you can safely ignore this. Note that transproxy is for "simple" TCP connections, so it won't work for every application protocol. You have to use something like the next tool for FTP.

redir, like transproxy, is only for TCP connections. It does everything that transproxy does, and adds some nice capabilities. You can download the sources from http://www.ibiblio.org/linsearch/lsms/redir-2.2.html or use the Debian package redir. The additional functionality above transproxy is listed here by the command-line switches used to invoke it:

  • --ftp handles passive-mode FTP connections.

  • --timeout=n closes the connection after n seconds of inactivity.

  • --syslog enables logging via syslog. It's not in the man page, but it will log to the facility daemon at the level notice.

  • --name=string or --ident=string1 uses string instead of redir in the syslog entries.

  • --debug writes debugging information to stderr, or to syslog if you're using the --syslog switch.

  • --transproxy in conjunction with a kernel running transparent proxying,2 reports the original sender's address and port to the proxied destination.

Here's an example. When machineA connects to the redirector running on routerB, which forwards the traffic to machineC, machineC sees the packet as having originated from machineA. When machineC replies to machineA, the packet is intercepted by routerB, which bound itself to the sender's interface IP and port. This is possible only if redir is running as root and IP: transparent proxy support is compiled into the kernel. Furthermore, works correctly only for connections that are symmetrically routed (in other words, in both directions) by the system running redir. Finally, there's always the chance that the sender's port is already in use on the firewall, and won't be available for use. Nonetheless, it's a neat and useful trick. One benefit is that things like Web server logs will contain the correct source address for generating access statistics. See the transproxy.txt file accompanying the distribution for more information.

rinetd is a multi-headed (and multithreaded) server that acts like redir and transproxy. Like these, it redirects only TCP connections (but not FTP, so keep a copy of redir around!). rinetd sets itself apart with its configuration file. Unlike the previous two, it can listen to an arbitrary number of ports simultaneously, each of which may be bound to a (possibly different) interface address on the router. These port/address tuples are specified in the configuration file with lines in the format of bindaddress bindport connectaddress connectport. The addresses may be hostnames, and the ports may be service names. rinetd's configuration file may also include global and service-specific allow and deny lines, reminiscent of the configuration file used by the TIS FWTK. Any connection from a source that's explicitly denied or isn't found in a global allow will be immediately rejected. The next feature is a choice of logfile formats. You may choose to have no logfile whatsoever, use rinetd's format, or use a common Web server format. Like inetd, rinetd can be made to reload its configuration by sending it a SIGHUP.

rinetd can be found at http://www.boutell.com/rinetd/. There's also a Debian package of the same name. The following is an excerpt from the man page that provides a good synopsis.

rinetd redirects TCP connections from one IP address and port to another. rinetd is a single-process server that handles any number of connections to the address/port pairs specified in the file /etc/rinetd.conf. Since rinetd runs as a single process using nonblocking I/O, it's able to redirect a large number of connections without a severe impact on the machine. This makes it practical to run services on machines inside an IP masquerading firewall.

If none of the packages listed above fits your needs, you can always roll your own. If the nitty-gritty network programming scares you, use one of the general-purpose socket utilities such as socket or netcat. They should be available anywhere, but if you have problems finding them, refer to the socket and nc Debian packages. The next section is about using the kernel to do some of the work of redirection, normally in conjunction with a user-space redirector.

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.


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.


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.


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.


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


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


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.


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.


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