The system is based on Nominum's  DNS based filtering system. Subscribers that opt-in to the filtering will have all DNS requests to non-BT name servers either blocked or transparently proxied through a server that restricts DNS responses based on the categories chosen.
Issues from blocking third-party DNS resolution
The BT Home Hub implementation of parental controls blocks the use of third party DNS servers (such as Google or OpenDNS) and returns an error page. 'Internet of Things' devices preconfigured to use other DNS services may be unable to communicate their being blocked leaving the user with no explanation.
Mis-categorised sites can be reported to firstname.lastname@example.org  where BT will pass on the details to their unnamed third-party categorisation supplier.
The blocking page itself provides no links to report incorrectly categorised sites.
I addition to the selectable categories, "all customers opted into Parental Controls are prevented access to sites promoting the use of proxies and anonymisers". Similarly the "Obscene and tasteless" category blocks "Sites with information about illegal manipulation of electronic devices, hacking, fraud and illegal distribution of software".
|Pornography||Sites that contain explicit sexual content||X||X||X|
|Obscene and tasteless||Sites where content is offensive or tasteless, such as criminal activity, bathroom humour, or gruesome or frightening content, such as cruel animal treatment||X||X||X|
|Hate and self-harm||Sites that promote self-harm or encourage the oppression of people or groups||X||X||X|
|Drugs||Sites where content refers to information on illegal drugs or misuse of prescription drugs||X||X||X|
|Alcohol and tobacco||Sites promoting or selling alcohol or tobacco-related products||X||X||X|
|Dating||Sites that promote/facilitate interpersonal relationships - matchmaking, online dating, spousal introduction and escort services||X||X||X|
|Nudity||Sites that contain full or partial nudity but are not necessarily overtly sexual in intent||X||X|
|Weapons and violence||Sites where the main purpose is to delict, sell, review, or describe guns and weapons. Sites that advocate physical assault against humans, animals, or institutions and sites of a gruesome nature||X||X|
|Gambling||Online gambling sites including lottery, casinos and betting agencies||X||X|
|Social networking||Sites that facilitate social networking and chat. Includes forums and newsgroups||X||X|
|Fashion and beauty||Sites concerning fashion, jewellery, glamour, beauty, modelling, cosmetics or related products or services||X|
|File sharing||Sites where the main purpose is to share or distribute illegal content or software, or sites that enable direct exchange of files between users||X|
|Games||Sites where the main purpose is relating to computer or other games and game-related publication sites. Includes access to online multiplayer gaming servers||X|
|Media streaming||Sites where the main purpose is to deliver streaming content, such as internet radio, internet TV or MP3 and live or archived media downloads||X|
|Sex education||Sites relating to sex education, including subjects such as respect for partner, abortion, gay and lesbian lifestyle, contraceptives, sexually transmitted diseases and pregnancy||Personalise only|
|Search engines||Sites enabling the searching of the web, newsgroups, images, directories, and other online content. Includes portal and directory sites such as white/yellow pages||Personalise only|
Source: bt.custhelp.com UPDATE UK ISP BT Launch Parental Controls with Network Level Filtering, ISP Review, 2012-12-13
- No anon pr0n for you: BT's network-level 'smut' filters will catch proxy servers too, Register, 2013-12-17
- Report on Internet safety measures, Ofcom, 2014-07-22 - page 2
- Proxy and DNS workarounds to be foxed by BT's Parental Controls porn block, Recombu, 2013-12-03
- Homework time on BT Parental Controls, BT, retrieved 2013-12-20
- Amazon's Echo struggles to connect to BT's Home Hub, Guardian, 2016-09-29]
- I'm a content or website owner and my site is being blocked incorrectly by BT Parental Controls. How can I report this?, BT, retrieved 2013-12-20
- Blocking categories on Parental Controls, BT, retrieved 2013-12-20
- Mail from bt.com to Open Rights Group, 2013-12-20