Content Filter

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Planning

In order to build a successful content filter, some questions need answering. How many users will be filtered? What will be filtered? How will it be filtered?

The DansGuardian wiki has a good over view of filter architecture.

Single machines and small networks

For single machines and small networks I find that Tinyproxy and Dansguardian work best.

Tinyproxy is a very lightweight and easy to configure proxy server. Dansguardian actually does all the filtering work, but is unable to fetch pages by itself, this is where Tinyproxy comes in.

A user will open their web browser and request a web page, this request typically travels over port 80, we'll redirect port 80 to a port that Dansguardian is listening on, then Dansguardian will forward the request onto Tinyproxy, who will then actually go out and fetch the web page.

Installing and configuring tinyproxy

By default a few USE flags are set for Tinyproxy, most are not needed for this guide but don't hurt either. However be sure that the transparent-proxy USE flag is not set. It is not compatible with this guide.

First, install Tinyproxy:

root #emerge --ask tinyproxy

Next, back up the default configuration in case things go astray:

root #cp /etc/tinyproxy.conf /etc/tinyproxy.conf.vanilla

Finally, open /etc/tinyproxy.conf in your favorite text editor, make sure you have suid root privileged. Make sure it looks something like this:

FILE /etc/tinyproxy.conf
User tinyproxy
Group tinyproxy
Port 3128
Timeout 1200
DefaultErrorFile "/usr/share/tinyproxy/default.html"
LogFile "/var/log/tinyproxy/tinyproxy.log"

# change this to Info when debugging issues for more verbose output
LogLevel Error

# these settings can significantly effect the performance of the proxy
MaxClients 100
MinSpareServers 5
MaxSpareServers 20
StartServers 10
MaxRequestsPerChild 0

# for a single computer, chose this, otherwise remove it
Allow 127.0.0.1

# for a small network, chose the subnet in use, otherwise remove it
Allow 10.0.0.0/24

# you can leave this as is or change it to whatever you like, its a privacy thing
ViaProxyName "tinyproxy"

# the easiest, but least secure option for ConnectPort is to leave it commented
# this way all ports are allowed. each port you wish to allow should be listed on its own line
ConnectPort 445 # SSL
ConnectPort 563 # TLS

Start Tinyproxy by issuing this command:

root # /etc/init.d/tinyproxy start

Finally, add Tinyproxy to the start up list:

root # rc-update add tinyproxy default

Installing and configuring DansGuardian

One thing you need to understand about DansGuardian is that it supports multiple filter groups, if you are interested you can read more about them in the large network section below. For the single machine or small network we'll set up a single filter group that everyone will be a member of.

First lets install DansGuardian:

root #emerge --ask dansguardian

You may want/need to enable the pcre USE flag, this enables Perl Compatible Regular Expressions.

First, lets back up the default configuration, just to be safe:

root #cp /etc/dansguardian/dansguardian.conf /etc/dansguardian/dansguardian.conf.vanilla

Now lets start configuring the global settings, for now just change the parameters here, leave everything else, it's a big file:

FILE /etc/dansguardian/dansguardian.conf
# this only logs denied requests to cut back on a large log file
# though you may want to set this parameter to 3 while troubleshooting, to log everything
loglevel = 1

# unless you want to use syslog you probably are better off using DansGuardian's internal logger
loglocation = '/var/log/dansguardian/access.log'

# DansGuardian listen port
filterport = 8080

# if this is a single machine, and DansGuardian is installed on the same machine as Tinyproxy use this IP
# otherwise place the IP address of the proxy server in place of 127.0.0.1
proxyip = 127.0.0.1

# this is the port that the proxy server is listening on
# earlier in the Tinyproxy setting we chose port 3128
proxyport = 3128

# if you have a single machine or a small network you will almost certainly need this set to off
originalip = off

# Portage should have already created a dansguardian user and group
# if not, you should make one, so that DansGuardian can run with its own set of permissions
# for security reasons mainly
daemonuser = 'dansguardian'
daemongroup = 'dansguardian'

Now that the global configuration has been taken care of, lets configure the per-group settings. If there is a conflict between per-group and global settings, per-group settings will always win; just keep that in mind while configuring and troubleshooting.

Again, lets make a backup of the defaults, just in case:

root #cp /etc/dansguardian/dansguardianf1.conf /etc/dansguardian/dansguardianf1.conf.vanilla

Lets configure the first filter group. Again, this is a big file with lots going on, if a parameter isn't mentioned in this outline, usually its ok to leave it at default settings:

FILE /etc/dansguardian/dansguardianf1.conf
groupmode = 1

# this just helps log files parse easier
groupname = 'group_one'

# I found 50 to block far to much, where as 100 would still block adult sites but
# allow for example, the wiki on breast cancer
# this is something you'll need to tweak based on your own needs, the lower the number the more it blocks
naughtynesslimit = 100

# this defaults to 0 or off, I wanted to bring it up anyways though
# if you think you'll need a temporary bypass for whatever reason
# that feature is available
bypass = 0

Under /etc/dansguardian/lists are a set of files that control the mechanics of filtering. There is a lot going on here, usually the defaults are ok. Although a few files are worth noting individually.

  • bannedsitelist - here, there is a section to explicitly list sites to be blocked.
  • exceptionsitelist - similar to bannedsitelist, except for allowing sites rather then blocking.
  • contentregexlist - this is a rather hairy file, especially if you have never worked with regular expressions before, enabling too many of these options often causes over filter problems and performance problems.

Unfortunately, the only official documentation on how to use these lists are the comments in the list files themselves.

Start DansGuardian by issuing this command:

root # /etc/init.d/dansguardian start

Finally, add DansGuardian to the start up list:

root # rc-update add dansguardian default

Large networks

Larger networks pose a few challenges to administrators; the most prevalent being the sheer size of the network. With average enterprise class networks having thousands of users and thousands of machines with many subgroups requiring different levels of service, and the new wave of Bring Your Own Device or BYOD networks, the big question is; how to handle all this traffic without making compromises in what is filtered?

DansGuardian answers this question with filter groups. With filter groups you'll need some external method of authentication so that you can apply different levels of authorization with filter groups, which is unfortunately outside of the scope of this guide. In this guide we'll look at some of the ideas and concepts behind authentication with DansGuardian but the implementation details will be left up to you or another guide.

As you read this guide pay special attention to the default group information located under the Filter Group section. It is a powerful tool that catches everyone who hasn't authenticated.

Installing and configuring squid

Make sure that the tproxy USE flag is not set.

You may wish to conciser setting these USE flags:

  • logrotate - if you are already using logrotate to clean up old logs, you can include Squid's logs in the process via USE flag
  • kerberos, ldap, nis, radius, samba, and sasl - these flags make integrating Squid with existing authentication schemes a little easier, not all are required

To install Squid, execute this command:

root #emerge --ask squid

The Squid config is just as (if not more so) big and scary as the DansGuardian config. Each network has its own set of needs and generalizing all of those into one perfect configuration is impossible. Instead, this configuration is meant to give you the most basic set up to get you running while explaining some core Squid configuration topics. Fine tuning the Squid configuration to suit a specific requirement is up to the individual administrator. For a much closer look at all the configuration parameters that Squid accepts have a look at the official documentation and for some (probably more helpful then raw configuration parameters) example configs check out the Squid wiki.

As per usual, lets back up the default configuration, lest we find ourselves in need of it.

root # mv /etc/squid/squid.conf /etc/squid/squid.conf.vanilla

Keep in mind, this is a very minimal config to get you started:

FILE /etc/squid/squid.conf
# this is the port and interface Squid listens on
# if you do so you also have to change the dansguardian 'proxyip = 172.16.0.5'
# http_port 172.16.0.5:3128
# if you'd rather Squid listen on all interfaces try this instead
http_port 3128

# this option forces the location of the cache log
# the cache log is a kind of general information and error log
# if Squid has trouble running, this should be one of the first places you look
cache_log /var/log/squid/cache.log

# this option forces the location of the access log
# it keeps track of every client request
# if you'd rather not have this be logged change the path to /dev/null
cache_access_log /var/log/squid/access.log

# Squid keeps a cache of requested objects
# this option forces the location of the store log
# which keeps track of what enters and exits the cache
# if you'd rather not log that make the path none as I've done below
cache_store_log none

# strictly speaking this is typically an optional parameter
# it explicitly states what the hostname of the server is to Squid
# at the very least this will help logs be easier to read
visible_hostname squid.mydomain.net

# some common ports that are used, we'll allow these to talk to our Squid
acl Safe_ports port 80 21 443 563 70 210 280 488 591 777 1024-65535

# SSL ports, its nice to have a seporate ACL for them
acl SSL_ports port 443 563

# we'll also make a separate ACL for all traffic over the local loopback interface
acl localhost  src 127.0.0.0/8

# here we'll group all traffic that is attempting an HTTP CONNECT on our Squid into its own ACL, usually only SSL needs to do this
acl CONNECT method CONNECT

# this is the IP of the server running DansGuardian
acl DansGuardian src 172.16.0.6

# here we catch all other hosts not listed above, yes we want any client IP address to fall into this ACL as well
# we'll create a default block rule that will prevent users from bypassing the filter and directly asking Squid to process their request
# you should enforce this with firewall policy too
acl ALL src all

# block access if the destination port doesn't match our Safe_port ACL
http_access deny !Safe_port

# long story short, protocols like SSL need to issue a HTTP CONNECT to be able to tunnel other protocols
# here we're denying HTTP CONNECT to our Squid proxy except for ports we've identified as OK, namely the common SSL ports
http_access deny CONNECT !SSL_port

# since DansGuardian will need to use Squid, we'll let it
http_access allow DansGuardian

# here we catch everything else and deny them access, remember if users directly connect to Squid they'll bypass the DansGuardian content filter
# you really should have firewall rules also enforcing this, but in the event something slips past your firewall, hopefully this stops the access instead
http_access deny ALL

You can check your configuration file for syntax errors by running:

root #squid -k check

Anything it marks as a warning, should be looked into but is not critical to getting Squid to run. Anything marked as an error will need fixed before Squid will even start.

Add Squid to the start up list:

root # rc-update add squid default

And then start Squid:

root # /etc/init.d/squid start

Tweaking DansGuardian for heavier use

The basic global configuration for DansGuardian is pretty much the same as in the previous small networks example. So make sure you read over that section first and then come back here.


Overview of large network filtering

This is essentially a recap of what was presented in the single machine/small network section, with extra content. The real meat and potatoes to enterprise class, per-user filtering is based on authentication methods and filter groups. Basically you sub-divide your users into groups that need different levels of filtering, users (either in the background or directly) authenticate and are placed into the appropriate filter group. So for example, a very basic and minimal set up for a K-12 school might have a filter group for staff, and another for students, and yet another for technical staff who might need access to technical forums and documentation that regular users shouldn't have. Finally, there is a default group. The default group is a powerful tool, a catch all that handles unauthenticated users.

Authentication and authorization

The DansGuardian wiki makes a distinction between authentication and authorization. The long and the short of it is that; authentication is verifying who someone is, and authorization is checking that a given person is allowed to perform some action.

If you'd rather not read the wiki, there is one important gotcha you need to be aware of, that is that DansGuardian itself only cares about a username. The password could be incorrect and DansGuardian wouldn't care. That means to effectively authenticate users, you need to offload the task to another process like Squid.

There are three general categories of authentication processes supported by both DansGuardian and Squid:

  • BASIC - sends credentials in plain text, very insecure (not recomended)
  • Digest - hashes credentials before sending them over the network
  • NTLM - New Technology Lan Manager; A proprietary protocol that also hashes credentials, really only useful if you have a Microsoft network

To use Squid as your authentication process checkout some of their examples and pick one that matches the architecture of your existing network.

Now that you decided how to authenticate your users, you'll need to uncomment one of these lines in your global config:

FILE /etc/dansguardian/dansguardian.conf
#authplugin = '/etc/dansguardian/authplugins/proxy-basic.conf'
#authplugin = '/etc/dansguardian/authplugins/proxy-digest.conf'
#authplugin = '/etc/dansguardian/authplugins/proxy-ntlm.conf'

Filter groups

Before you can have multiple filter groups, you'll need to let DansGuardian know how many you'll have by changing this parameter to however many you want, in this example there will be 3:

FILE /etc/dansguardian/dansguardian.conf
filtergroups = 3

Each filter group will have its own configuration file named /etc/dansguardian/dansguardianfN.conf

Where N is a number assigned to the group, typically you'll need at least a first group or dansguardianf1.conf and by default this is the default group. You should probably make a copy of it to act as a template for further groups:

root # cp -av /etc/dansguardian/dansguardianf1.conf /etc/dansguardian/dansguardian_filtergroup_vanilla.conf

Lets also copy the lists directory as well:

root # cp -aR /etc/dansguardian/lists /etc/dansguardian/filter_lists_vanilla

Now that you have a filter group template made open the first group with your favorite text editor, but first realize that this touches on some of the basic configuration options and is not an all inclusive list:

FILE /etc/dansguaridanf1.conf
# description in the config file is pretty strait-forwards on what the values should be, you may consider blocking all web traffic for the default group
# here we allow some filtered traffic, possibly due to a BYOD style network
groupmode = 1

# technically this is optional, but giving your groups names will make logs easier to parse
groupname = 'DEFAULT'

# if you decide to allow some access to your default group, you'll probably want to set your naughtynesslimit lower
naughtynesslimit = 50

# you can leave this section alone and be ok; but keep in mind you can maintain multiple lists and use these options to configure which set a given group should use
# naturally the more independent lists you manage the more complex it becomes to add a site wide exception or block.
bannedphraselist = '/etc/dansguardian/lists/bannedphraselist'
weightedphraselist = '/etc/dansguardian/lists/weightedphraselist'
exceptionphraselist = '/etc/dansguardian/lists/exceptionphraselist'
bannedsitelist = '/etc/dansguardian/lists/bannedsitelist'
greysitelist = '/etc/dansguardian/lists/greysitelist'
exceptionsitelist = '/etc/dansguardian/lists/exceptionsitelist'
bannedurllist = '/etc/dansguardian/lists/bannedurllist'
greyurllist = '/etc/dansguardian/lists/greyurllist'
exceptionurllist = '/etc/dansguardian/lists/exceptionurllist'
bannedregexpurllist = '/etc/dansguardian/lists/bannedregexpurllist'
bannedextensionlist = '/etc/dansguardian/lists/bannedextensionlist'
bannedmimetypelist = '/etc/dansguardian/lists/bannedmimetypelist'
picsfile = /etc/dansguardian/lists/pics.disabled
contentregexplist = '/etc/dansguardian/lists/contentregexplist'
anonregexplist = '/etc/dansguardian/lists/anonregexplist'
exceptionregexpurllist = '/etc/dansguardian/lists/exceptionregexpurllist'
greyregexpurllist = '/etc/dansguardian/lists/greyregexpurllist'

Transparent filtering with Iptables

So you've decided you'd like to setup your machine to transparently filter web content. The most obvious solution for single computer is to use IPTables to redirect traffic. If you don't already have experience with IPTables have a look at the Gentoo Wiki on the subject before proceeding.

Let's recap: our proxy is listening on port 3128 and address 127.0.0.1, DansGuardian is listening on port 8080 and address 127.0.0.1. We only need to point our web traffic to DansGaurdian which is configured to route traffic through Squid and back. It helps to know that standard HTTP travels over port 80.

Note
It isn't possible to transparently redirect HTTPS traffic over port 443.

Next, make sure your kernel is configured with netfilter support for clients. Also, select the following options:

KERNEL .config-4.4.6
[*]Networking Support --->
        Networking Options --->
            [*] Network packet filtering framework (Netfilter) --->
                [*] Advanced netfilter configuration
                    Core Netfilter Configuration --->
                        *** Xtables targets ***
                        <M> REDIRECT target support
                        *** Xtables matches ***
                        <M> "owner" match support
KERNEL .config-3.7.3
[*]Networking Support --->
        Networking Options --->
            [*] Network packet filtering framework (Netfilter) --->
                    Core Netfilter Configuration --->
                        *** Xtables targets ***
                        <M> REDIRECT target support
                        *** Xtables matches ***
                        <M> "user" match support
KERNEL .config-3.12.13
[*]Networking Support --->
        Networking Options --->
            [*] Network packet filtering framework (Netfilter) --->
                    Core Netfilter Configuration --->
                        *** Xtables matches ***
                        <M> "owner" match support
                    IP: Netfilter Configuration --->
                        <M> IPv4 NAT
                        <M>    REDIRECT target support

We'll also need to find the UID of whatever user your proxy is running under, which should be its own separate UID from both DansGaurdian and any human users:

root #grep squid /etc/passwd | awk -F: {'print $3'}

Finally append your IPTables rule list with something like this (put the number you got from the previous grep of /etc/passwd in place of $SQUID):

root #iptables -t nat -A OUTPUT -p tcp -m owner ! --uid-owner $SQUID --dport 80 -j REDIRECT --to-port 8080

Once you are sure everything is working as it should, save the iptables state with this command:

root # /etc/init.d/iptables save