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Linksys Blue Box Router HOWTO

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Linksys Blue Box Router HOWTO

Eric Steven Raymond

[] Thyrsus Enterprises
Revision History                                                             
Revision 2.0           2005-01-18            Revised by: esr                 
Major update to reflect changes in 2.x and 3.x firmware. More firmware       
replacements described. Dropped Hansen Online as it hasn't been updated in a 
Revision 1.6           2004-02-26            Revised by: esr                 
Added Link-n-Log                                                             
Revision 1.5           2003-07-31            Revised by: esr                 
Added the Seattle link.                                         
Revision 1.4           2003-07-03            Revised by: esr                 
Linksys has released source code.                                            
Revision 1.3           2003-06-08            Revised by: esr                 
Added notes about SNMP security problems, casemodding, Linksys tech support. 
The Linksys turns out to have Linux inside.                                  
Revision 1.2           2003-04-29            Revised by: esr                 
Typo corrections.                                                            
Revision 1.1           2003-04-25            Revised by: esr                 
Added link to the linksysmon project. More configuration tips.               
Revision 1.0           2003-04-09            Revised by: esr                 
Initial release, reviewed by LDP.                                            

Linksys makes a line of cheap, ubiquitous router/firewall boxes (models
BEFSR41 and up, including the WRT54G) well-suited for use on a home DSL
connection and popular among Linux hackers. This HOWTO gives hints and tips
for managing Linksys routers from a Linux system, including the firmware
upgrade procedure.

Table of Contents
1. Introduction
    1.1. Why this document?
    1.2. New versions of this document
    1.3. License and Copyright
2. How and where to deploy
3. Lost the manual?
4. Configuration hints
5. Upgrading the firmware
6. Hacking the hardware
7. Hacking the software
8. Utilities
9. Troubleshooting tips
    9.1. Occasional catatonia and epilepsy
    9.2. Mozilla interface quirks under 1.38 and earlier firmware
10. Related Resources

1. Introduction

1.1. Why this document?

Linksys makes a line of cheap, ubiquitous router/firewall boxes well-suited
for use on a home DSL or cable connection and popular among Linux hackers.
This HOWTO gives hints and tips for managing Linksys routers from a Linux

The specific recipes described here are derived from long experience with a
BEFSR41, the 4-port router/firewall box. I have also configured a BEFW11S4v2,
the 4-port router with 80211b wireless, and the WRT54G, which is the same box
with 80211g; I'm currently using a WRT54G. The web interfaces on all these
blue boxes are very similar, and most of the advice should generalize.

In late 2004 the Linksys firmware underwent a major upgrade to 2.x (one easy
way to spot this is the Cisco logo at the lower right). I haven't seen
anything but a WRT54G running the new interface, but I'd be surprised if it
weren't running on the BEFSR41 and kin as well. The changes are largely
cosmetic. Some problematic features in earlier versions have been removed.

This HOWTO describes Linksys firmware version v2.02.7. At time of writing
(January 2005) the current Linksys firmware version is v.3.01.3. I do not
recommend upgrading! I've had a report that enabling WEP on this version
makes the box unable to talk to a Linux machine over a cable.

1.2. New versions of this document

You can also view the latest version of this HOWTO on the World Wide Web via
the URL []  http:

1.3. License and Copyright

Copyright (c) 2003, Eric S. Raymond.

Permission is granted to copy, distribute and/or modify this document under
the terms of the GNU Free Documentation License, Version 1.2 or any later
version published by the Free Software Foundation; with no Invariant
Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the
license is located at []

Feel free to mail any questions or comments about this HOWTO to Eric S.
Raymond, <[email protected]>. But please don't ask me to troubleshoot
your general networking problems; if you do, I'll just ignore you.

2. How and where to deploy

The Linksys BEFSR41, BEFW11, WRT54G and their siblings are designed to be
used as gateway boxes on a home Ethernet. Typically, you'll hook one up to a
DSL or cable modem, which will automatically switch into bridge mode and
simply pass packets between your ISP's router and the Linksys box. Here's a 
recognition chart of these products.

If you want to use a general-purpose PC running Linux as a firewall, have fun
?? but these little boxes are more efficient. The nicest thing about them is
that they run out of firmware and, assuming you take the elementary
precautions we describe, are too stupid to be cracked. Also, they don't
generate fan noise or heat. Finally, they run Linux inside and can be
customized and hacked in useful ways.

Linksys boxes used to have a good reputation for reliability. Something bad
happened to their quality control after Cisco acquired the company in March
2003; I had two go silently dead on me in less than a year, and I heard
grumbling from others about similar problems. Unfortunately when I tried
other low-end brands (Belkin, Buffalo) they proved to have gross design
errors. The Belkin had brain-damage in its firewall rules that interfered
with local SMTP, and the Buffalo intermittently refused connections for no
apparent reason. So I'm back with Linksys. hoping my WRT54G doesn't turn into
a doorstop within a couple of months.

(Building one of these puppies is not rocket science. I can only conjecture
that the competitive pressure is driving the manufacturers to cut costs to
the bone by hiring programmers out of the bottom of the barrel and having the
manufacturing done by some low-end contract house in Indonesia or somewhere.
The results, alas, tend to be unstable crap. Caveat emptor.)

Note another consequence of the Cisco acquisition: Linksys is now what
marketers call a flank guard, a low-end brand designed to protect the margins
and brand image of Cisco's commercial-grade networking products. This means
that Linksys boxes are no longer acquiring new firmware features, and some
old ones like stateful packet inspection almost certainly won't be coming
back. Provided you can live wthin these limits, this is actually good;
simpler firmware is stable firmware.

At minimum, a live Linksys box will do the following things for you:

 1. Act as an Ethernet router. You can plug all your lines and hubs and hosts
    into it to exchange packets even when your outside link is down.
 2. Act as a smart gateway. When you configure the Linksys with a public
    static IP address (or tell it to grab a dynamic IP address from your ISP
    at startup time), it will gateway between hosts on your private network
    and the Internet, performing all the IP masquerading and address
    translation required to route your traffic.
 3. Firewall your connection. You can tell it to block out all but the
    minimum sevice channels you need. You can specify separately, for each
    service, to which of your internal machines the traffic should be routed.

I give my Linksys box the standard private-network gateway address, I then give all my boxes 192.168.1.x addresses and tell them the
Linksys is their gateway. Everything works.

3. Lost the manual?

If you've lost the manual, or acquired a secondhand unit that doesn't have
one with it, never fear. Under the Help tab in older versions there are links
to the PDF and to the Linksys corporate website. Newer versions have
reference documentation built into the firmware, a good thing if your net
connection is down.

Unfortunately, you're in trouble if you have to bring in Linksys tech
support. On the one occasion that I called them, the first tech I raised
couldn't even speak English, and the second was barely competent at it. Both
were complete and utter idiots whose response to any nontrivial question was
to put me on infinite hold while they went off to query someone else ?? and
then garbled the answer. Judging by their accents, my guess is that Linksys
tech support has been outsourced to some particularly benighted corner of the
Third World.

4. Configuration hints

For security, do these things through the Linksys web interface (probably at
[] on your network):

 1. Change your administrative password. On 15 June 2004 it was widely
    reported that turning off the remote admin feature doesn't work ?? you
    can still get at the administration page from the wireless side. This bug
    is still present in the 2.02 firmware, October 2004. It means that if you
    leave your password at default, any script kiddie can break in, steal
    your WEP, and scramble your configuration. The Linksys people get the
    moron medal with oak-leaf cluster for this screwup.
    (I don't know if this bug is still present in the 3.x firmware. It would
    be a good idea to check.)
 2. Make sure the DMZ host feature is disabled, under Applications+Gaming->
    DMZ Host, or in newer versions)Applications & Gaming->DMZ Host. It
    defaults off.
 3. Port-forward specific services instead of setting up a DMZ, and as few of
    those as you can get away with. A good minimum set is 22 (ssh), and 80
    (http). If you want to receive mail add 25 (smtp). If you need to serve
    DNS queries, add 53. To serve identd so remote MTAs can verify your
    identity, enable 113.
 4. Disable Universal Plug and Play. Look under Password. There is a radio
    button for this under the "Password" tab; newer firmware versions put it
    under Administration+Management. UPnP is a notorious security hole in
    Windows, and up to at least firmware version 1.44 there was a lot of Web
    scuttlebutt that the Linksys implementation is flaky. While this won't
    affect operating systems written by competent people, there is no point
    in having traffic from a bunch of script-kiddie probes even reach your

There are two more steps for older firmware versions only. You can ignore
these if you have 2.x firmware.

 1. Disable AOL Parental Controls. Make sure AOL Parental Controls (under 
    Security) is turned off (off is the default); otherwise the Linksys won't
    pass packets for your Unix box at all. Newer versions of the firmware
    don't have this misfeature.
 2. Disable Stateful Packet Inspection. If you want to run a server and are
    running 1.42 or earlier firmware, you also need to make sure stateful
    packet inspection is off ?? this feature restricts incoming packets to
    those associated with an outbound connection and is intended for
    heightened security on client-only systems. On the Filters page, make
    sure SPI is off. If you don't see a radiobutton for SPI, relax ?? the
    feature isn't present in all versions of the firmware, and in fact was
    removed in 1.43 for stability reasons.

5. Upgrading the firmware

Before you upgrade, here is a tip the documentation does not mention:
disconnect all the patch cables except the one from the machine you are using
to upgrade the box. Handling a lot of other network traffic while the
firmware load is going on can corrupt the firmware.

There are three ways you can upgrade your Linksys firmware.

One is to click the "Upgrade firmware" link on the admin page. Download the
firmware image to the machine your browser runs on, fill in the field that
says "Please select a file to upgrade:", click the Upgrade button, and have
the right thing happen. This is the least error-prone procedure and is

Another way is to use one of Linkys's firmware-upgrade floppy images from
their website. This requires that you boot Windows or use WINE. Not

The third way is to use tftp. This is how I did it the first time, before
Linksys added the "Upgrade firmware" to the firmware, and I document it here
for completeness even though I now recommend that method. There is a tftp
client included with Red Hat Linux. To upgrade your firmware this way, do the
following steps:

 1. Write down your settings. The firmware upgrade may wipe some of them.
    Older versions nuked everything back to factory defaults; newer versions
    preserve your basic settings but clear some advanced ones.
 2. Download a copy of the new firmware. You should find it at Firmware
    Upgrades for your Linksys Products on the Linksys site. Note that what
    you get may well be marked "For Windows Users" and be a zip archive. Open
    it in a scratch directory, because it will rudely create several Windows
    files wherever you unpack it. The file you need will be called CODE.BIN.
 3. Disable the router password. Note that every attempt I made to do this
    with Mozilla failed (both under 1.38 and 1.44). Konqueror worked fine,
    and Firefox works fine with the 2.x firmware. Go to the Password tab,
    backspace over both sets of asterisks until both the Password and Confirm
    fields are blank, and click Apply.
 4. Cross your fingers and load the firmware. The command session you want
    will to see will look something like this, with your router's IP address
    substituted for
    tftp> binary                                                             
    tftp> put code.bin                                                       
    Sent 386048 bytes in 10.3 seconds                                        
    Don't panic if the client hangs for a bit before returning and do not
    abort the transfer. The command is writing to firmware, and the Linksys
    hasn't got much of a brain. Wait for it to finish.
 5. Re-enable your router password and other settings. You'll be able to tell
    the upgrade worked because the firmware version number will have changed.

You're done.

6. Hacking the hardware

There is a [] page that tells you how to
casemod the Linksys WAP11 wireless access point.

Linksys boxes have firmware support for a serial console, The circuit board
has traces for two serial ports, but you have to do some fairly serious
modding to get them working. []
This page will show you how.

7. Hacking the software

Linxsys routers run Linux from firmware. Linksys supplies [http://] source code on its site.

There are several replacements for the WRT54G firmware. All add certain commn
features such as (a) the capability to ssh into the Linux running on the box,
(b) European WiFi channels, and (c) VPN service.

[] Wifi-Box
     Supports SNMP/mrtg. Said to have a good interface, convenient for home
[] SveaSoft
     Intended for Wireless ISPs, lots of stuff for routing and repeater
    operation. Open source, but you can buy support and private-release
[] OpenWRT
     Workbench for people who want to experiment with their own
    customizations. Provides a framework and a set of modular packages
    supporting particular features.
[] HyperWRT
     Starts from the Linksys 3.01.3 firmware and adds a handful of features.
    Might be useful for those comfortable with the Linksys interface.
     Another hacker's workbench, this one runs from RAMdisk so you don't have
    to reflash the box. Thus there's no chance of trashing your router. The
    disadvantage is that it has to be reloaded each time after you

Any of these can be installed using the firmware upgrade procedures.

Firmware for other Linksys hardware (notably the WAP54G) can be found [http:/
/] here and [
modules.php?name=Downloads&d_op=viewdownload&cid=15] here.

For a look at the techniques used to develop these firmware alternatives,
there's an interesting site on hacking the Wrt54g by Seattle

8. Utilities

There is a Unix utility called linksysmon that talks with these boxes via
SNMP. There is a [] Linksysmon project site.

Linksysmon is a tool for monitoring Linksys BEFSR41 and BEFSR11 firewalls
under Linux and other Unix-like operating systems. It accepts log messages
from the Linksys, and logs the messages to /var/log/linksys.log. It handles
the standard activity logs, as well as the "secret" extended logging, and can
handle logs from multiple firewalls. When using extended logging, it can
detect external IP address changes (if you are using either DHCP or PPPOE)
and can call an external program to process the change.

Link-n-Log is a similar tool that includes a GUI and logs to an SQL database.
Details at the Link-n-Log project page.

9. Troubleshooting tips

9.1. Occasional catatonia and epilepsy

Linksys boxes freeze up occasionally (once every few months) and have to be
power-cycled. Suspect this is happening if your outside Web access suddenly
stops working; ping the Linksys box to check.

These catatonic episodes may be related to dirty power; at least, they seems
to happen more frequently in association with electrical storms and
brownouts. If you think this has happened, just pull the power connector out
of the back and plug it back in. The Linksys should reboot itself within 30
seconds or so.

There is a more severe failure mode that I've only seen once; it's more like
an epileptic seizure than catatonia, and involves strange blink patterns on
the Link, Collision, and 100Mbit diagnostic lights (the 100Mbit light should
not normally ever blink).

If this happens, power-cycling the Linksys won't suffice; you'll have to
hard-reset the thing. Some versions (like the BEFSR41) have a reset pin that
you poke with a paperclip end through a small hole in the front panel labeled
Reset. Some versions (like the BEFW11S4 and WRT54G) have a reset button on
the back. You have to hold these down for about thirty seconds to hard-reset
the nonvolatile RAM. This will lose your configuration settings.

9.2. Mozilla interface quirks under 1.38 and earlier firmware

Linksys blue boxes have a webserver embedded in their firmware. The normal
way to administer one is to point a browser at its IP address on your
network. You program the box by filling out HTML forms.

This is a nice bit of design that neatly avoids having OS-specific client
software. But some older versions of the webserver firmware have a quirk that
interacts with a bug in Mozilla (at least at release 1.0.1) to make the
interface almost unusable. Fortunately, the recovery procedure is trivial.
This bug was known to be present as late as 1.40, and also interfered with
Netscape; it is absent in 1.44 and a good reason to upgrade. We have a report
that Mozilla 1.3 fails with 1.43, so whatever change fixed the problem likely
came in with 1.44.

The symptom you're likely to see is a broken-image icon at the upper left
hand corner of each page. The broken image is a series of file-folder tabs
for an image map. That image map is how you get to the other web pages.

You can recover by right-clicking on the broken-image icon. Select "View
Image", then back out. This will build the image map correctly.

You will almost always have to do this on the first page, but it often won't
trigger on later page loads.

Here's what's going on. Mozilla tries to stream multiple concurrent requests
at the webservers it talks to in order to speed up page loading. The
dimwitted little firmware webserver in the Linksys is only single-threaded
and doesn't handle concurrent requests. So there's a race condition. When you
hit the window just right, you get an aborted request and a broken graphic.

Most other browsers are immune to this problem. Konqueror doesn't trigger it.
Neither does Internet Explorer.

10. Related Resources

There's a large user-community website at [] It includes news, support forums, and custom firmware

There is a Linksys tips and tricks []
FAQ; it's mostly Windows stuff, but a few of the war stories may be useful.