Conclusion

We have successfully set up policy based routing of HTTP and HTTPS traffic from our router to separate proxy box. Both HTTP and HTTPS traffic can now be filtered for adult language and unwanted sites.

References

• Original Ubuntu router article at ArsTechnica.
• Explanation how iptables firewall processes packets.
• SquidWiki policy based routing article

Danger

If you are using Google chrome, you are advised to block QUIC protocol on your router, otherwise Chrome will be able to bypass the transparently redirected proxy when going to QUIC enabled sites, like google.com, youtube.com, etc. Adding REJECT rules for UDP protocol on outgoing port 80 and port 443 should be enough.