HTTPS protocol is used more and more in today’s web. While this may be good for privacy, it leaves modern network administrator without any means to prevent questionable or adult contents from entering his/her network. Previously it was assumed that this problem does not have a decent solution. Our how-to guide will try to prove otherwise.
This guide will tell you how to set up Squid on CentOS / RedHat Linux for transparent filtering of HTTP and HTTPS traffic with help of Diladele Web Safety ICAP server, which is a commercial solution for Linux, BSD and MacOS. The Linux installer of Diladele Web Safety used in this tutorial contains fully featured keys which remain valid for 3 month period, so you can test its full features during this trial period.
Assumptions and Requirements
In this tutorial, I will assume the following. You have a network with IP addresses from 192.168.1.0 subnet, network mask is 255.255.255.0, and all workstations are set to use 192.168.1.1 as default gateway. On this default gateway, you have two NICs - one facing LAN with IP address 192.168.1.1, the other is plugged in into ISP network and gets its public Internet address through DHCP. It is also assumed your gateway has CentOS or RedHat Linux up and running.
Step 1. Update and Upgrade
Before going further, run the following script to upgrade your system to the most recent state.
#!/bin/bash set -e # update should be done as root if [[ $EUID -ne 0 ]]; then echo "This script must be run as root" 1>&2 exit 1 fi # update and upgrade yum update && yum upgrade # disable selinux sed -i s/SELINUX=enforcing/SELINUX=disabled/g /etc/selinux/config # and reboot reboot
Step 2. Install Apache Web Server
Diladele Web Safety has sophisticated a web administrator console to easily manage filtering settings and policies. This Web UI is built using Python Django web framework, and requires Apache web server to function correctly. Run the following script to install them.
#!/bin/bash set -e # all web packages are installed as root if [[ $EUID -ne 0 ]]; then echo "This script must be run as root" 1>&2 exit 1 fi # install python libs yum install python-setuptools python-ldap # install python django for web ui easy_install django==1.5 # install apache web server to run web ui yum install httpd php mod_wsgi # make apache autostart on reboot chkconfig httpd on # this fixes some apache errors when working with python-django wsgi echo "WSGISocketPrefix /var/run/wsgi" >> /etc/httpd/conf.d/wsgi.conf # and restart apache service httpd restart
Step 3. Install Diladele Web Safety
Download and install the latest version of Diladele Web Safety using the following script.
#!/bin/bash # all packages are installed as root if [[ $EUID -ne 0 ]]; then echo "This script must be run as root" 1>&2 exit 1 fi # detect current architecture (default assumes x86_64) ARCH_1=`uname -m` ARCH_2="amd64" if [[ $ARCH_1 == 'i686' ]]; then ARCH_1="i386" ARCH_2="i386" fi # bail out on any error set -e # get latest qlproxy curl http://updates.diladele.com/qlproxy/binaries/188.8.131.52CAF/$ARCH_2/release/centos6/qlproxy-3.2.0-4CAF.$ARCH_1.rpm > qlproxy-3.2.0-4CAF.$ARCH_1.rpm # install it yum -y --nogpgcheck localinstall qlproxy-3.2.0-4CAF.$ARCH_1.rpm # qlproxy installed everything needed for apache, so just restart service httpd restart
Step 4. Install Required Build Tools
To be able to perform HTTP/HTTPS transparent filtering, we need to get the latest version of Squid (the one that comes with CentOS / RedHat by default is too outdated), and rebuild it from source. The following script installs all build tools required.
#!/bin/bash # install all build tools if [[ $EUID -ne 0 ]]; then echo "This script must be run as root" 1>&2 exit 1 fi # install development packages required yum install -y gcc-c++ pam-devel db4-devel expat-devel libxml2-devel libcap-devel libtool redhat-rpm-config rpm-build openldap-devel openssl-devel krb5-devel # squid needs perl and needs additional perl modules not present by default in CentOS 6 curl http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm > epel-release-6-8.noarch.rpm rpm -Uvh epel-release-6*.rpm yum install -y perl-Crypt-OpenSSL-X509
Step 5. Build Squid from Source
Rebuild the Squid RPM by running the following script.
#!/bin/bash # stop on any error set -e # rpm build MUST be run as normal user if [[ $EUID -eq 0 ]]; then echo "This script must NOT be run as root" 1>&2 exit 1 fi # get squid sources pushd rpmbuild/SOURCES curl http://www.squid-cache.org/Versions/v3/3.4/squid-3.4.4.tar.xz > squid-3.4.4.tar.xz curl http://www.squid-cache.org/Versions/v3/3.4/squid-3.4.4.tar.xz.asc > squid-3.4.4.tar.xz.asc popd # build the binaries RPMs out of sources pushd rpmbuild/SPECS rpmbuild -v -bb squid.spec popd
Step 6. Install Squid
After build finishes, install Squid. It is advisable to uncomment the lines which generate your own root certification authority. Default installation of Diladele Web Safety does have its own ca, but trusting it may pose serious security risk if your devices are used by users outside of your network.
#!/bin/bash # stop on every error set -e # install RPMs as root if [[ $EUID -ne 0 ]]; then echo "This script must be run as root" 1>&2 exit 1 fi # detect current architecture (default assumes x86_64) ARCH_1=`uname -m` ARCH_2="amd64" ARCH_3="lib64" if [[ $ARCH_1 == 'i686' ]]; then ARCH_2="i386" ARCH_3="lib" fi pushd rpmbuild/RPMS/$ARCH_1 yum localinstall -y squid-3.4.4-0.el6.$ARCH_1.rpm popd # set up the ssl_crtd daemon if [ -f /bin/ssl_crtd ]; then rm -f /bin/ssl_crtd fi ln -s /usr/$ARCH_3/squid/ssl_crtd /bin/ssl_crtd /bin/ssl_crtd -c -s /var/spool/squid_ssldb chown -R squid:squid /var/spool/squid_ssldb # uncomment to regenerate certificates for SSL bumping if you do not like defaults # openssl req -new -newkey rsa:1024 -days 1365 -nodes -x509 -keyout myca.pem -out myca.pem # openssl x509 -in myca.pem -outform DER -out myca.der # then copy certificates # cp myca.pem /etc/opt/quintolabs/qlproxy/ # cp myca.der /etc/opt/quintolabs/qlproxy/ # make squid autostart after reboot chkconfig squid on
Step 7. Integrate Squid with Diladele Web Safety
Integrate Squid and Diladele Web Safety by running the following script.
#!/bin/bash # stop on any error set -e # integration should be done as root if [[ $EUID -ne 0 ]]; then echo "This script must be run as root" 1>&2 exit 1 fi # allow web ui read-only access to squid configuration file chmod o+r /etc/squid/squid.conf # perform integration by replacing squid.conf file mv /etc/squid/squid.conf /etc/squid/squid.conf.original && mv squid.conf /etc/squid/squid.conf # parse the resulting config just to be sure /usr/sbin/squid -k parse # restart squid to load all config /sbin/service squid restart
Step 8. Transparently Redirect HTTPS Traffic to Squid
Transparent filter for HTTP and HTTPS traffic will be implemented by redirecting traffic to ports 80 and 443 to Squid using iptables. This implies that the box with Squid acts as default gateway for your LAN. Please note this is only one way to implementing transparent filtering. Other possible solutions are described in Squid’s Wiki.
#!/bin/bash # firewall setup should be done as root if [[ $EUID -ne 0 ]]; then echo "This script must be run as root" 1>&2 exit 1 fi # check kernel forwarding is enabled enabled=`cat /proc/sys/net/ipv4/ip_forward` if [[ $enabled -ne 1 ]]; then echo "Kernel forwarding seems to be disabled, enable it in /etc/sysctl.conf, reboot and rerun this script" 1>&2 exit 1 fi # set the default policy to accept first (not to lock ourselves out from remote machine) iptables -P INPUT ACCEPT # flush all current rules from iptables iptables -F # allow pings from eth0 and eth1 for debugging purposes iptables -A INPUT -p icmp -j ACCEPT # allow access for localhost iptables -A INPUT -i lo -j ACCEPT # accept packets belonging to established and related connections iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT # allow ssh connections to tcp port 22 from eth0 and eth1 iptables -A INPUT -p tcp --dport 22 -j ACCEPT # allow connection from LAN to ports 3126, 3127 and 3128 squid is running on iptables -A INPUT -i eth0 -p tcp --dport 3126 -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 3127 -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 3128 -j ACCEPT # redirect all HTTP(tcp:80) traffic coming in through eth0 to 3126 iptables -t nat -A PREROUTING -i eth0 -p tcp -m tcp --dport 80 -j REDIRECT --to-ports 3126 # redirect all HTTPS(tcp:443) traffic coming in through eth0 to 3127 iptables -t nat -A PREROUTING -i eth0 -p tcp -m tcp --dport 443 -j REDIRECT --to-ports 3127 # configure forwarding rules iptables -A FORWARD -i eth0 -o eth1 -p tcp --dport 22 -j ACCEPT iptables -A FORWARD -i eth1 -o eth0 -p tcp --sport 22 -j ACCEPT iptables -A FORWARD -p icmp -j ACCEPT iptables -A FORWARD -i eth0 -o eth1 -p tcp --dport 80 -j ACCEPT iptables -A FORWARD -i eth1 -o eth0 -p tcp --sport 80 -j ACCEPT iptables -A FORWARD -i eth0 -o eth1 -p tcp --dport 53 -j ACCEPT iptables -A FORWARD -i eth0 -o eth1 -p udp --dport 53 -j ACCEPT iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A FORWARD -j REJECT --reject-with icmp-host-prohibited # enable NAT for clients within LAN iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE # set default policies for INPUT, FORWARD (drop) and OUTPUT (accept) chains iptables -P INPUT DROP iptables -P FORWARD DROP iptables -P OUTPUT ACCEPT # list created rules iptables -L -v # save the rules so that after reboot they are automatically restored /sbin/service iptables save # enable the firewall chkconfig iptables on # and reboot machine reboot
Check if HTTPS is Transparently Filtered
Please note, in order for HTTPS filtering to function correctly, we must install the proxy certificate from /etc/opt/quintolabs/qlproxy/myca.der into Trusted Root Certification on all workstations in our network. The following screenshots show that HTTPS requests were decrypted and filtered transparently.
Browsing to Google and searching for an adult term (e.g. NSFW), we get the HTTPS request filtered and blocked transparently.
We now have the default gateway in our network capable of transparently filtering HTTP and HTTPS traffic. All workstations in our network trust the root certificate from proxy, and thus get their HTTPS request decrypted and filtered. Browsing environment in our network became much safer.
- Archive with all scripts mentioned in this HOWTO
- Online documentation of Diladele Web Safety
- Squid proxy wiki
Subscribe to Xmodulo
Do you want to receive Linux FAQs, detailed tutorials and tips published at Xmodulo? Enter your email address below, and we will deliver our Linux posts straight to your email box, for free. Delivery powered by Google Feedburner.