Jan Kandziora

| jkaptive - a simple captive portal
| Author: Jan Kandziora jjj@gmx.de
| Version: 1.10


I, Jan Kandziora, the author of jkaptive, grant you the right
to use, copy, distribute and modify this software under the terms of the
GNU General Public License(GPL), Version 2 (see file COPYING).


A captive portal is a software which intercepts the transfer of all HTTP
traffic through a router and presents a login page to the user's web browser
instead. Once the user has supplied a token (a special string), transfers
aren't intercepted anymore until the lease the token offered expires.

There is a variety of captive portal software out there, some of them offer
tight security by using a RADIUS server for authentication, while others don't
bother with hackers at all and only intercept traffic from 'john doe'.

Jkaptive belongs to the latter group. The reason behind this is because a lot
of site administrators don't need tight security - the site is just a café
which offers free internet access on an unsecured WLAN access point connected
to the internet and they need a ticketing system to make it cumbersome for
average people to use this offering without actually buying a single coffee.


Aiming at the simple goal above, jkaptive is dead-simple, too. Jkaptive itself
just presents the login page and checks the token. The blocking of unticketed
traffic is done through Linux' netfilter.

As no proxy server is involved, jkaptive has no performance penalty, nor
does it create problems with non-HTTP traffic. Once the token is accepted,
jkaptive is out of the way of any network packets completely.

For presenting the login page, jkaptive has a built-in webserver, so no
additional webserver application is needed.


With the rules shown in section NETFILTER CONFIGURATION all unticketed HTTP
traffic is redirected to jkaptive's login page, while all other unticketed
traffic is rejected. Jkaptive adds a netfilter rule which bypasses redirection
and rejection for any single remote user host once a valid token has been
given, and it automatically revokes that rule once the token expires.

Instead of rejection, all other kinds of netfilter rules can be applied, e.g.
rate-limiting rules, giving full internet access to anyone, but full-speed
only to paying customers. The example rules feature only rejection, though.

Jkaptive can be configured to work on output traffic, too, so it's an easy way
to add ticketing to a stand-alone internet terminal.

User's view

From user's point of view, when he wants to read his favourite web comic, he
clicks on a bookmark in his browser, then it tries to load


Instead of showing the desired page, the browser presents a login page. As
soon the user has entered the token into the login field and that token is
valid, the browser is redirected to the page of the original request and
showing the comic. In the background, full internet access has been allowed
for the user's computer until the token expires. Then, the login page is given


Valid tokens are created using a simple algorithm by putting their expiry date
together with some salt and checksum, permutate it and make a human-typeable
form of it. No communication is needed between the program creating the
token and jkaptive: all needed data is encoded into the token.

A simple implementation of a token generator is supplied with the
jkaptive package, so it can be called by another application or have the
algorithm copied into. The output is intended to be printed on a restaurant
bill or similar.

The same token can't be used simultaneously by more than one user's computer
as jkaptive keeps track of all currently used tokens and the ip address of the
user's computer they belong to.


Jkaptive tries to be secure in the way not compromising the host it runs on.
To change netfilter rules it has to run as the root user but to avoid
exposing root access to a remote user, the built-in httpd used by jkaptive
for serving the login page is run as a non-priviledged user (e.g. nobody).
Both processes communicate via pipes and do only exchange tokens and status

Starting with jkaptive-1.2, the webserver starts a worker process for each
file served and terminates it automatically after a while, so simple
starvation/DOS attacks on it don't work. To work against bruteforce resource
hoggers this isn't enough, though, so you have to create some site-specific
netfilter rules and ressource limits throwing away such packets at netfilter
level. Your Linux distribution might have documentation and templates for it.

Starting with jkaptive-1.3, the webserver protects the token salt against
bruteforcing - a pause of at least 500ms per request for the login page
means that a brute force attack would take hours to succeed. Each simultaneous
connection to the login page from the same ip gets an extra penalty of 500ms.
Note this applies only to the login HTML page. Any other pages or images,
styles and scripts loaded from that or any other page are unaffected. The
whole thing is somewhat against the original idea of jkaptive to let hackers
pass if they really try, but it's only a few lines of code, so why not?


Jkaptive is not a secure captive portal. It can easily be circumvented by
advanced users (aka "hackers") by sniffing for a ticketed IP address and then
mimicing this address on their own computer. If you need a secure captive
portal, look for one featuring RADIUS authentication; don't bother with
others which claim to be "secure" without using RADIUS.

Second, the token algorithm is not cryptographically secure. It doesn't have
to be because no hacker would attack the token algorithm if he can just sniff
the network for some packets in the air.

To say once again, JKAPTIVE IS NOT A SECURE CAPTIVE PORTAL! But if you refuse
to install steel doors into a traditional japanese house, jkaptive is a
perfect fit.

Another limitation is the storage of accepted tokens within the jkaptive
process only. So if that process is terminated (E.g. by power-cycling the
server), the list of accepted tokens is empty on next start: all users have to
authenticate again. They may use their previous token once again, though,
as long as it hasn't expired.


jkaptive is written in the Tcl scripting language so it obviously needs that
interpreter to run. In addition it needs tclx and some sub-packages from the
tcllib. For jkaptive-1.9, the packages are:

* tcl     >= 8.5
* tclx    (8.4 is known to work)
* tcllib  (1.11.1 is known to work)

Starting with jkaptive-1.9, ipsets may be used instead of a netfilter chain.
If your site has a great number of ticketed users at any time, the linear
parsing of netfilter rules inside the jkaptive chain for ticketed ip addresses
may take some more time that you want. With ipsets, the kernel uses a hash
instead of a linear list, which should give you more performance. To use it,
you need

* ipset binary
* ipset-aware kernel (CONFIG_IP_SET=y or m and CONFIG_IP_SET_HASH_IP=y or m).

This is completely optional, though. See CONFIGURATION below.


Very little configuration has to be supplied, as useful defaults apply. One
single configuration item - a site specific salt - has to be supplied
however. Jkaptive will refuse to start if it is not configured. The
configuration file has ini style, one item per line. Please ensure the items
are in the correct section. Lines starting with ; are comments.




Loglevel is 0=no log, 1=critical only, 2=critical and error, etc.
Loglevel 6 and above means all log messages are printed. The default is 4
(down to "notice") which won't clutter your syslog with unneeded messages.

The salt has to be supplied! Please don't use "mysalt" but a site-specific
string. It doesn't need to have more than four characters as it gets shrunk
to a 16-Bit value anyway. The salt has to be the same you use for the token

user and group of the httpd have to be names of user and group of a
least-priviledged account on your machine.

port is the port number the built-in webserver of jkaptive should run. It has
to be an unused unpriviledged port.

timeout is a length in seconds which a httpd worker process may be present
before it gets terminated by the main httpd process. This is a measure against
starvation/DOS attacks to the webserver.

webroot has to point to a directory containing the login and error pages the
build-in webserver of jkaptive should deliver to users.
loginpage, tokenfailpage, tokenoccupiedpage, tokenoccupiedpage,
tokenexpiredpage, tokenexpiredpage, error404page and error500page are the
names of the special HTML files within webroot.

backend is the backend to use for adding ip adresses of ticketed clients to
the netfilter. It's either "iptables" or "ipset".

iptables has to point to the iptables binary jkaptive should use to place its
private rules into the netfilter. If the backend is set to "ipset", this
isn't used by jkaptive.

ipset has to point to the ipset binary jkaptive should use to place its
private rules into the netfilter. If the backend is set to "iptables", this
isn't used by jkaptive.

chain is the private chain/set inside netfilter jkaptive should place its

mark is a packet tracking mark like explained in iptables documentation. In
general, this is a 32-bit integer (written in hex for easier understanding)
with one single bit set. It is not important which bit is set, only that no
other part of the netfilter uses the same bit; if it does, that will result
to a big mess.


In addition to editing the configuration file /etc/jkaptive.conf, which is
explained above, you have to supply some site-specific netfilter rules to make
your captive portal actually work. There is a generic setup which works if no
other firewall is interfering and a special one for use with SuSEfirewall2.
For other distributions you can use the script for SuSEfirewall2 at
/usr/share/jkaptive/SuSEfirewall2 as a template for your own scripts.

Starting with jkaptive-1.9, you can use the "ipset" netfilter framework in
addition to "iptables". The use of ipset is straightforward. Instead of a
private chain "jkaptive" (or whatever you named it), jkaptive will use a
private set "jkaptive" to store the ip addresses of ticketed users. The
netfilter rules are a little different, please see below. The SuSEfirewall2
custom script can use ipset, too.

Generic setup

Without ipset, create an additional chain named "jkaptive".

iptables -t mangle -N jkaptive

With ipset, create a hash:ip set named "jkaptive" instead.

ipset create jkaptive hash:ip

Activate jkaptive connection tracking:
Without ipset, use

iptables -t mangle -A PREROUTING -j jkaptive

With ipset, use

iptables -t mangle -A PREROUTING -m set --match-set jkaptive src -j MARK --set-mark 0x00200000

instead. Redirect all unticketed HTTP traffic from inside (e.g. eth0) to
outside to jkaptive server.

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -m mark ! --mark 0x00200000/0x00200000 -j REDIRECT --to-ports 8088

The "jkaptive" chain name, mark bit and port number have to match the ones
configured in /etc/jkaptive.conf.

It's recommended to rate-limit connections to the jkaptive server. Browsers
try to access a lot of websites on startup simultaneously, and this will
cause hundreds of simultaneous connections to the jkaptive server, making
it fork and eat up hundreds of file descriptors in a short time. Ten
connections per second should be enough to let at least one subwindow display
the login page.

iptables -t filter -A INPUT -p tcp --dport 8088 -m state --state NEW -m recent --set
iptables -t filter -A INPUT -p tcp --dport 8088 -m state --state NEW -m recent --update --seconds 5 --hitcount 50 -j DROP

Only if you have an upstream gateway e.g at which should provide
DHCP/DNS (instead of the host jkaptive is running on), you need additional
rules to let that traffic pass.

iptables -t filter -A FORWARD -d -j ACCEPT
iptables -t filter -A FORWARD -s -j ACCEPT

If your upstream gateway is delivering DHCP, but DNS is directly given by an
internet server, specify

iptables -t filter -A FORWARD -p udp --dport 53 -j ACCEPT
iptables -t filter -A FORWARD -p udp --sport 53 -j ACCEPT

Now you can reject all other unticketed traffic from inside (e.g. eth0) to

iptables -t filter -A FORWARD -i eth0 -m mark ! --mark 0x00200000/0x00200000 --j REJECT --reject-with=icmp-admin-prohibited

The mark has to be the same as above, of course.

If you want to filter traffic originating on the host running jkaptive, e.g.
for an internet terminal, you need additional rules for the output chains:

without ipset: iptables -t mangle -A OUTPUT -j jkaptive
with ipset: iptables -t mangle -A OUTPUT -m set --match-set jkaptive src -j MARK --set-mark 0x00200000
iptables -t nat -A OUTPUT -p tcp --dport 80 -m mark ! --mark 0x00200000/0x00200000 -j REDIRECT --to-ports 8088
iptables -t filter -A OUTPUT -d -j ACCEPT
iptables -t filter -A OUTPUT -m mark ! --mark 0x00200000/0x00200000 --j REJECT --reject-with=icmp-admin-prohibited

Chain, mark and port have to match again, of course.
Now the host itself is filtered. This creates the problem it can't issue DNS
requests to the upstream gateway. If you have such an upstream gateway e.g. at, add

iptables -t filter -A OUTPUT -d -j ACCEPT

before the REJECT line and it is fixed. If your upstream gateway is
delivering DHCP, but DNS is directly given by an internet server, add

iptables -t filter -A OUTPUT -p udp --dport 53 -j ACCEPT

before the REJECT line and it is fixed. If your host is directly connected
to a modem (using e.g. pppoe), both problems don't apply.

jkaptive will place its individual rules like

iptables -t mangle -A jkaptive -s $address -j MARK --set-mark 0x00200000

into the jkaptive chain when a token is accepted and automatically delete
them once the token is expired.

Please note the kernel has to support the icmp-admin-prohibited reject method,
otherwise you get a plain DROP instead.

SuSEfirewall2 setup

For SuSEfirewall2, the generic setup above is put in a file
/usr/share/jkaptive/SuSEfirewall2. You have to edit
/etc/sysconfig/SuSEfirewall2 to activate it.



to activate the custom script. Add lines


or "ipset" instead of "iptables" if you want to use that.


or which input interfaces you consider to filter for forwarding.


or whatever backend, chain name, mark bit and port number you have configured
in /etc/jkaptive.conf.

Only if you want to directly use an upstream gateway for user's DHCP/DNS
instead of the host jkaptive runs on, you need additionally:


or whatever your upstream gateway is. You may supply more than one address.
If your upstream gateway is delivering DHCP, but DNS is directly given by an
internet server, specify


This allows forwarded DNS traffic to and from anywhere.

If you want to filter traffic originating on the host running jkaptive, e.g.
for a single host setup, you additionally need to set


and add a line


Now the host itself is filtered. This creates the problem it can't issue DNS
requests to the upstream gateway. If you have such an upstream gateway add a


or whatever your upstream gateway is. You may supply more than one address.
Don't forget to restart SuSEfirewall2 after configuring it.

# rcSuSEfirewall2 restart


Jkaptive is meant to be run as a daemon started by root. You can test it on
root's command line with

# /usr/sbin/jkaptive --loglevel 6

Log messages are always sent to syslog, with the tag "jkaptive". Messages
from the built-in webserver have the additional tag "httpd". If you want to
start jkaptive automatically on system start, this should be done after the
netfilter/firewall setup is done.

For openSUSE 12.1 and other distributions featuring systemd, a systemd script
is provided. Test it with

# systemctl --system daemon-reload
# systemctl start jkaptive.service
# systemctl status jkaptive.service

Installation as a system service can be done through

# systemctl enable jkaptive.service

jkaptive.service is linked to the multiuser.target (similar to runlevel 3).


A simple utility for generating tokens is provided. Call it with something

# /usr/bin/jkaptive-token mysalt "6 hours"

to create a six-hour token valid from now or

# /usr/bin/jkaptive-token mysalt 19:00

to create a token valid until 19:00 today or

# /usr/bin/jkaptive-token mysalt "2012-12-31 20:00"

to create a token valid until your new year's party starts.

First parameter has to be the same salt as set in /etc/jkaptive.conf. The
second parameter has to be either a date/time specification or distance
accepted by Tcl functions [clock format] and [clock add]. See clock(n) manpage
for details.


The web pages provided by jkaptive at /usr/share/jkaptive/webroot can be seen
as templates for your own creations. You can easily modify them or even add a
whole bunch of new pages, images, scripts and styles.

Modifying templates

There are six files which are served directly by jkaptive's webserver:

    This page is delivered when any url is given to jkaptive, e.g. when the
    user actually wanted to access http://sinfest.net/

    This page is delivered if the user supplied an invalid token, e.g. because
    of typing it wrong.

    This page is delivered if the user supplied a valid token which is used by
    another IP address, e.g. because he wanted to re-use a thrown-away token
    with another computer before it expired. Another reason this page shows up
    is because the DHCP lease expired (See CAVEATS) and the user got a new IP
    address and tried to re-register using the old token.

    This page is delivered if the user supplied a token which has already
    expired, e.g. because of using some old token.

    This page is delivered if the user supplies an URL to jkaptive that points
    somewhere it shouldn't. Usually that means some link on the other pages is
    wrong. The user should never be able to provoke this behaviour.

    This page is delivered if something really bad happened with jkaptive. If
    it ever appears, prepare to file a bug report.

Please note all pages but the loginpage are only delivered after the user
filled out the form on the loginpage - they never appear out of sudden when a
token expires or any other error occurs.

All but the error pages should feature a HTML form which lets the user enter
the token. This form should look like

<form action="%ROOT/login" method="get">
    <input type="text" name="token" size="12" maxlength="12" />
    <input type="submit" value="OK" />
    <input type="hidden" name="ourl" value="%OURL" />

%ROOT and %OURL are special strings which are replaced by a modified request
root and the original URL the user provided before his request was redirected
to jkaptive.

If you don't want to replace the templates, put your files into a directory
and point the "webroot" configuration variable in /etc/jkaptive.conf to that
directory. jkaptive's webroot directory must be readable by the user provided in
its config file. Usually this means it has to be world-read/browseable. Same
applies for all the files which are about to be served.

Adding files

If you want to have custom images, styles or scripts, or a printable PDF
documentation how to obtain a token (or other fancy things) to be included
into your very own captive portal, you can simply add these files into
the webroot directory and use

<img src="%ROOT/warning.png">

in the HTML file to point the browser to the URL where it can load the image
from. The same applies for all other links.

Jkaptive finds out the MIME type of the file by looking at its filename
extension. The following extensions are known to it:

css, gif, html, jpg, jpeg, js, pdf, png

All files with unknown extension are served as text/plain. Please note it
isn't possible to have a file named "login" in the webroot, as this is a
special string to jkaptive triggering the login mechanism.


DNS/DHCP server

Before a web browser on the user's computer tries to load any HTML page
through HTTP, it issues a DNS request for the host part of the address to
find out which IP address to point the HTTP request to. Jkaptive's default
behaviour is to reject all forwarded traffic, even DNS and DHCP. This isn't a
problem if you run a DNS/DHCP server/proxy somewhere inside the local
network or the same host jkaptive runs on.

But if you have an upstream gateway providing this, e.g. a box given by your
telco, it may also serve as the DNS/DHCP server in your network. In that case,
you have to place additional netfilter rules (see NETFILTER CONFIGURATION) to
allow forwarded traffic to that box.

To make it even more complicated, there are telco boxes out there which work
as a router and DHCP server, but not as a local DNS repeater. In that case,
the box will tell the user's computers a DNS server in the internet via DHCP.
As it may change at the will of the telco, there is no other chance but to
allow DNS traffic to and from all adresses. See NETFILTER CONFIGURATION again.

DHCP lease time

As all the filtering is done with ip addresses, is has to be made sure DHCP
always gives the same IP address to the same computer as long the token has
not expired. If you don't honor this, users may not use their full time of
free internet access.

With ISC dhcpd, this is done through /etc/dhcpd.conf


You have to specify a default lease time higher than the expiry time of the
longest jkaptive token you have. Usually a setting of 86400 (one day) is ok.
Specifing max and min lease times may be needed, too.

With the built-in dhcpd of dnsmasq, this is done on the command line through
the -F/--dhcp-range option. See the manpage of dnsmasq and your distribution's
documentation on how dnsmasq is embedded in it.

Packet marking

Make sure the packet tracking mark used by jkaptive (configured in
/etc/jkaptive.conf) is used by jkaptive exclusively. The netfilter/firewall
code already present on your machine may accidentally use the same bit,
causing a lot of random glitches in conjunction with jkaptive.

So before activating jkaptive, list the mangle rules of your machine for
lines having a "MARK" target

iptables -t mangle -nvL | grep MARK

and make sure that the bit used by jkaptive is not used by any of them.

DHCP client (openSUSE specific)

There is a known glitch when the host jkaptive runs on get its own ip address
via DHCP: as soon the lease expires, the network is reconfigured and
SuSEfirewall2 is restarted, flushing all rules in the jkaptive chain. Users
have to relogin with their old tokens after that.

If you want to avoid this glitch, configure the host jkaptive runs on with a
fixed IP address.


All known ones hunted down so far. Please report any new bugs or difficulties
to the author.

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