Fundamentals of Web Filtering & Logging

02 Jul 2018
article 25 minute read

With ever more intrusive, and ever more expensive, web filtering options available - here is a fundamental review of the technology to help you decide what approach suits you best!


We should begin with what should be obvious, but is often ‘clouded’ by marketing claims and sales tactics:

There is no such thing as a perfect web filter.

Expanding this statement even further, web filters aren’t really that much better than each other (despite what their makers may say!). What they may be, though, is better suited to your particular needs and environment. By understanding the fundamentals of how this technology works, a little history and the balance of trade-offs each type of filter requires, you can make an informed choice.

With yearly filtering costs ranging (for a typically sized organisation, like a secondary school) from about £/$/€ 1,000 to tens of thousands (and growing!), your choice needs to help deliver both a suitable service and value for money.


In UK schools, the statuatory requirement for ‘Online Safety’ is found in Annex C of ‘Keeping children safe in education: for schools and colleges’ and the IT Policies section of ‘The Prevent Duty’. Both sections are very brief, and generalised, and neither stipulates a particular technological approach nor level of appropriate filtering.

Whilst filtering and monitoring are an important part of the online safety picture for schools and colleges to consider, it is only one part. [Department of Education, 2018: emphasis added]

Instead, they both encourage schools to think beyond filtering as a ‘solution’ and into the wider area of online safety. They caution against ‘overblocking’ and highlight the importance of staff training and pupil education, alongside appropriate technological measures.

Whilst considering their responsibility to safeguard and promote the welfare of children, and provide them with a safe environment in which to learn, governing bodies and proprietors should consider the age range of their pupils, the number of pupils, how often they access the IT system and the proportionality of costs vs risks. [Department of Education, 2018: emphasis added]

Communication networks are an abundant resource in the modern world, and a school’s IT system is just one, small part of a myriad of ways that pupils can connect with the broader Internet. It cannot protect everywhere, so education, rather than just prohibition, will have a far greater impact on actually safeguarding children.


There are three main areas which a filtering system will need to balance. These traits tend to be mutually exclusive, so if you want to improve/increase performance in one area, that will cause a corresponding drop in the other areas. As with so much in technology, this is pretty much an inviolable rule - however much we may wish it weren’t.

Inspection and Logging Depth

The complexity of the modern web has increased manifoldly over recent years, as has the number of apps and services which use it for communication. Getting inside that traffic, so it can be logged and checked for inappropriate content, is a challenging process because it flies in the face of our concerted efforts to improve web security. If you can see right into web traffic, then others could too …

We have come to rely on HTTPS1 / TLS2 / SSL3 to secure almost everything we do on the web. Recent browser changes have moved it from the exception, to the norm. No competent application developer would even consider launching a service that didn’t encrypt its communications using this standard protocol.

Modern web-security encrypts communication between the client device (laptop, Chromebook, iPad, mobile phone etc.) and the server. This renders both the content of the communication and the destination address (the URL in the web-browser) unreadable to all the systems that exist between the client and server (e.g. ISP, network providers etc.). This is right, and precisely what users want. We want our banking/shopping to be secure and our messaging and email to be only visible to intended recipients. Filtering is in natural tension and conflict with this drive towards security, for to filter content, we want to inspect it - so it has to be readable to the network provider (e.g. your IT department).

Less intrusive inspection (e.g. filtering by site requested, rather than by the content of that request/response) can be achieved without the need to read (or decrypt) the content of all communications, but this gives less depth of control.


The web has thrived because it has emerged as a highly resilient technology. We expect it to work, and except for some high-profile site failures, it broadly does. This reliability is founded on diversity and duplication. There are often multiple, or ‘cloud-based’, servers at the other end of each request, numerous paths to any destination on the web and local caches which can serve up content faster. This reliability is hard won, and the introduction of a intercepting web filter can create a single point of failure in your network, as all the traffic needs to run through it.

As many organisations transition to entirely cloud provided systems, this sort of reliability concern can seriously impact operations if the filtering device fails or is misconfigured. More complex configuration work may be required (together with potentially expensive load balancing equipment) to provide any fault tolerance in an intercepting or on-site proxy filtering device.

Usability and Ease of Deployment

Ideally, a web filtering solution should be entirely transparent to users. They should merely browse the web, safe in the knowledge that viewing inappropriate content is prevented. Deployment should also, ideally, be similarly simple for technical support teams, with just a little bit of configuration and/or hardware installation.

The more complex the functionality offered, concerning interception and HTTPS decryption, the more complex the user experience is probably going to be. They may need to go through the process of installing a wildcard intercepting certificate, or a proxy configuration file, on their device before they can access the internet. More sophisticated logging or policy requirements may require captive portal authentication/identification for BYOD devices at the start of each day’s browsing session. In some environments (particularly schools), these usability speedbumps may lead a significant proportion of users to use their own mobile data services, rather than rely on the organisation provided WiFi. Perversely, this may mean a lower overall filtering effectiveness by reducing the number of users that connect via the filtered system. There is no point in delivering an incredibly complex and hardened filtering service if no-one uses it.


Now that we know the broad concerns we are seeking to address, and the sort of competing areas we wish to balance, we can look at the specific technologies and solutions available.


To provide any logging or customised filtering (allow sites/ content through for some users, but not others) one needs to know who the user is. In traditional IT systems, with rooms full of identical grey PC hardware, this was quite straightforward. A user would log onto the device, using their Windows / Active Directory username and password. Each computer was configured to send all of its web requests to a simple onsite web proxy. Their username was passed through as part of this request, and dependent on who they were, their request was then logged and filtered appropriately.

Now, most organisations will run on a multiplicity of platforms. There will be PCs, Macs, Chromebooks, iPads, Android tablets and Chromebooks. Some of these will belong to the organisation and be tightly managed, others will be supplied by the organisation but configured wholely by the users, others may be brought in by users themselves (BYOD4).

Gathering of identity information from there devices is challenging, so most systems tend to default to a captive portal style of authentication (think hotel or venue wifi). The user is redirected to a login page when they try to access the Internet for the first time in a session (where a session is typically, but not always, a day). This login page verifies user credentials against a centralised store, such as Microsoft Active Directory or Google G-Suite. The limitations here are that apps that use web connections for operating (such as native Google drive apps on iPads, tablets and phones) will not trigger the redirection to a captive portal, nor will requests to secure websites (e.g. Google drive in a web browser - which, we have noted above, are increasingly common!). These requests will fail, leading to confused and frustrated users, and an increase in associated technical support queries.

Other solutions may integrate with an existing network level authentication system, such as RADIUS to gather the authentication information that a user supplied to access the network. This is done via a procedure called accounting, and needs to be supported by both the filtering solution, and the specific network infrastructure that you have in place. This type of authentication is done by the device when it connects to the network, and therefore requires no user input or intervention beyond the initial set-up (and absolutely no user input if RADIUS is already in place). Wireless networks that are secured with a single password (often referred to as PSK or Pre-Shared Key networks) won’t be able to supply this sort of information, as they are not ‘user aware’.

Finally, agent-style apps, software or extensions may be required on devices to help gather identity data - but this needs considerable work to deploy or install, particularly on BYOD devices or devices that are not under centralised administrative control.

Establishing identity is not essential for filtering, but it is required for differentiated filtering, and for logging and monitoring. It is entirely possible to provide simple and safe filtering at different levels by using a BYOD scheme that puts users into different network segments, without the need to individually identify them. These source segments can then be checked when a request is made to the filter, and the appropriate filtering policy applied depending upon your configuration. Again, one has to think about the best balance in any approach. Is the increased visibility of who is making a request actually required, and if it is, does it justify the extra work of installation and support if your network doesn’t provide that sort of information already? Does the pain of having to authenticate through an imperfect captive portal mean that users will avoid using your filtered connection entirely?


An intercepting web-filter is one where all ‘protected’ web traffic must pass through the web-filter itself. In essence, it is a ‘man-in-the-middle’ device, which sits in the path of the traffic flows from your network to the wider Internet.

HTTPS Decryption

As discussed in an earlier section, an increasing amount (as much as 84% for Chrome OS users at time of writing) of web traffic is now encrypted. This means that a rapidly shrinking proportion (likely less than a quarter) of traffic actually ‘filterable’ without full TLS decryption. More modern combinations of browsers and web filters can do some basic level of HTTPS ‘decoding’, which essentially means using Server Name Indication to help understand which actual domain was requested by the client, but this support isn’t completely universal.

To filter and log the actual contents of the web traffic (e.g. block, log and alert when users search for particular terms on a Search Engine), this traffic has to be decrypted by the web filter. This is achieved by way of a Man-In-The-Middle attack (MITM). The intercepting filter ‘pretends’ to be the website, and the client connects securely to this. Then it sets up another encrypted connection to the final endpoint (e.g. Google). This means that the traffic is decrypted and examined/logged at the filter.

How can it do this? Isn’t HTTPS secure, and this sort of eavesdropping should be impossible? Yes - for decryption to occur, each client devices has to essentially authorise it. This is done by way of a wildcard CA certificate, which has to be trusted and installed on each device. HTTPS security is achieved with public key certificates. When HTTPS is enabled for a domain (e.g. the administrator for that domain applies for a certificate (free or paid) and there is some sort of validation process (such as email, domain name checking, DNS record etc) to prove that person has full control of whatever server/service sits at the domain.

That certificate (the private part of it anyway) is then used to secure communications to anyone who wants to visit the website. The browser checks the certificate against the domain name and provided they match, it says the site is secure and everything works (non-secure sites get a huge warning and a pile of steps to go through to get to them - which is enforced by the browser/client). Decrypting web-filters produce their own certification, which then claims to be valid for every site on the internet. Of course, it isn’t, unless each client device is explicitly told that it is!

This certificate deployment can be done automatically for Active Directory controlled PCs and Google Admin Managed Chromebooks. But not for non-managed, BYOD / personal devices, which need manual intervention to do this. Without the certificate being installed, you effectively can’t really browse the web via those filters - as everything is listed as insecure! As for whether this is a good idea? The jury is very much out, as it fundamentally breaks the trust network upon which the secure web is based. There is an advantage to being able to decrypt web searches in some cases but also huge privacy and security implications regarding this for all your users (colleagues and pupils). With this capability, who really knows whether banking or financial information is verifiably secure? Users may need extra training to understand the risks, and to ensure the systems they use can be properly audited to ensure security. It can also risk introducing unintended security problems onto the network. Using decryption in this way undermines a lot of the work that network and security professionals do to help make the web a secure environment for us all.

Connections & Content

A traditional interception device will receive all web traffic (that is, traffic destined for ports 80 or 443). This can be inefficient as once a filtering decision has been made (e.g. this url, or site, is allowed), is is not often changed during the lifetime of the conversation. This means that all traffic has to go through a (sometimes unnecessary) extra stop on it’s journey to/from its final destination. As the perception of speed is normally down to the latency of a connection (e.g. the round trip time for a request).

The alternative approach is to filter the initial connection, rather than the whole conversation. Typically this is done via a DNS filtering, which uses the Domain Name System to provide filtered responses to requests for a particular website. This has drawbacks, as only the domain (e.g. is passed to the filter, rather than the precise URL. So, filtering can only be done at the domain, rather than the page, level. Yet for most scenarios, this is more than good enough, and the advantages are that filtered DNS lookups are much faster than interception and need only be done once for each user/domain/day (meaning only the initial connection request needs to be filtered, rather than all the traffic).

Typically, the infrastructure needed to deliver DNS filtering is much more simple and reliable too, meaning lower total cost. There is no need to rely on technologies like ‘Server Name Indication’, which can be bypassed, to filter HTTPS connections - so, in many ways, a DNS filtering solution is actually better than an intercepting filter for HTTPS traffic if you have no need or desire to fully decrypt the traffic. Like all web filtering systems, logging and identity can be challenging, and is dependent on where filters are located in your network (or in the cloud).



In the end, the choice of approach is going to come down to four basic questions, which echo our balancing act:

  1. How much logging & filter differentiation is really required?

    If this feature is needed, you are going to need to look at onsite/internal filters, or sophisticated external filters with either a complex technology that you need to manage yourself or a disruptive installation and/or identity requirements.

  2. Does HTTPS need to be decrypted?

    If so, an intercepting, rather than DNS-based filtering solution is the only choice, and there needs to be robust and available technical support to help users with problems.

  3. How transparent should the experience for users to be?

    DNS filters or inline transparent web filters (without SSL decryption and captive portal authentication) are the only really transparent options.

  4. How important is reliability & robustness of connectivity?

    Single points of failure need to be avoided here, so cloud-based solutions will be best - or those deployed in a hybrid solution - with the ideal being a logging DNS filter.


Web filtering is typically implemented inside (e.g. onsite) the firewall, or outside (hosted, or in the cloud). Both of these deployment locations have their own set of benefits and drawbacks. Some filtering systems (such as DNS-based approaches) offer a hybrid model, with onsite data collection and cloud filtering.


The good

… and the bad


The up side …

… and the down


Whilst the legal ‘right’ to enforce Internet filtering for students in schools is clear, it is not so for employees or staff. Nor is it clear whether organisations have the right to retain web-traffic logs and if they can, for how long? Opinion 2/2017, authored and published by the European Commission Article 29 Data Protection Working Party is entitled ‘On data processing at work’. This working party, the predeccesor to the European Data Protection Board (EDPB) and was set up to ‘promote consistent application of the Data Protection Directive’ and to provide ‘expert advice’.

As such, organisations (such as schools) would likely claim that the processing and storage of personal data (such as web access logs and/or content) was legally justified under article 7b (the performance of a contract) or article 7f (legitimate interest) for employees, and article 7c (legal obligations) for students. The working party document details an example (page 13 onwards) where HTTPS decryption is proposed at an organisation, with specific recommendations being to respect the fundamental rights of employee privacy over the desire to log and monitor encrypted communications:

Irrespective of the technology concerned or the capabilities it possesses, the legal basis of Article 7(f) is only available if the processing meets certain conditions. Firstly, employers utilising these products and applications must consider the proportionality of the measures they are implementing. [EC Article 29 Data Protection Working Party, 2017]

The appliance should be configured in a way to prevent permanent logging of employee activity. [EC Article 29 Data Protection Working Party, 2017]

If some general logging would nonetheless be deemed strictly necessary, the appliance may also be configured not to store log data unless the appliance signals the occurrence of an incident, with a minimization of the information collected. [EC Article 29 Data Protection Working Party, 2017]

Employers should consider certain types of traffic whose interception endangers the proper balance between their legitimate interests and employee’s privacy—such as the use of private webmail, visits to online banking and health websites—with the aim to appropriately configure the appliance so as not to proceed with interception of communications in circumstances that are not compliant with proportionality. Information on the type of communications that the appliance is monitoring should be specified to the employees. [EC Article 29 Data Protection Working Party, 2017]

Why Monitor?

Monitoring, and storage of web logs, or even decrypted data from secure HTTPS connections are all personal data, and much of it (financial information, health, beliefs, private communication) is highly sensitive. Some monitoring may be required to ensure (and demonstrate) that filtering is at least partly effective. However, any storage and monitoring must be undertaken in a way that is communicated to users and for strictly defined purposes.

The GDPR is very clear that data processing by organisations is only legally justified in cases where there is a clearly stated and clearly communicated purpose, and that in the cases of employees (and students too, one would assume) consent to these operations cannot reasonably be freely given. If that purpose is to ensure the proper operation of a system, then logs should only be retained for a very short period of time, and should never be used in other ways (such as to try to evidence under-performance by an employee). To do this would be a clear breach of duties under this legislation and could open up the organisation to a potentially serious legal challenge.

It is absolutely vital that if any web filtering technology is being used which stores this data, users should be properly informed about what data is being held, why, who can access it and for how long.


Technology is rarely the answer to ‘people problems’, and online safety is clearly an example of that. More time and money should go into educating users about safely using the Internet (particularly children) than goes into prohibition. Filtering is simply one tool, and a small one at that, in ensuring online safety. It covers users for only a proportion of their time online, and probably an ever-shrinking proportion at that. There will always be restrictions to its effectiveness and there will always be ways to circumvent even the most tightly controlled systems. Spending vast sums of money and associated time to chase an unobtainable perfection is rarely a wise plan!


An imperfect technological solution that is lightweight, requires little management, just occasional maintainence and does not impede access to the Internet from any device is likely going to be more successful at filtering harmful content than a complex and heavyweight one. Simply because users are more likely to continue to use it. A lightly filtered wireless connection is always better than an unfiltered mobile data or VPN one. If the aim is truly to improve online safety, this much should be very clear. An inexpensive solution means more resources are available for actual online safety education, and that is never a bad thing. So, think carefully about the best solution - because best often means most workable and most able to integrate with the wider goals!

  1. The secure version of the protocol used to serve up web-content and pages to client devices, see this page for more details. 

  2. A cryptographic protocol which enables communication security, see this page for a more indepth look. 

  3. The predecessor to TLS, now infrequently used. See this video for further explanation. 

  4. Bring your own device, the increasing trend to seperate out services provided by an organisation (e.g. email, storage, applications) from the device required to access them, typically owned by the user themselves. See this page for a more comprehensive description. 

Tagged Education, Network

Related Google Calendars on the Web (01 Sep 2018), Protecting Data with Google Drive (14 Aug 2018), Dealing with confidential information in schools (24 Jul 2018)