OpenNet Initiative: Bulletin 009

Collateral Blocking: Filtering by South Korean Government of Pro-North Korean Websites

January 31, 2005
Last Updated: January 31, 2005

- Background
- Methodology & Results
- Observations
- About the OpenNet Initiative


Major internet service providers in South Korea were ordered by the Ministry of Information and Communication (MIC) to block domestic access to 31 websites considered to be carrying propaganda favoring the North Korean regime. (1) The action was taken under the highly controversial Cold War-era National Security Law, passed in 1948 to protect the country against the threat of communist influence and infiltration. The law has previously been used to persecute political dissidents, and the ruling Roh government claims a desire to see it repealed. (2) The MIC ordered the action just three days after the North launched Kim Il Sung Open University, a website that includes lectures praising North Korean leader Kim Jong Il and the North's Juche ideology. Some of the 31 web sites included in the order espouse strong political views favoring the North, but others focus more on information sharing or business information. Blocked sites include the North Korean book site "Korea Book Center," the Korea Stamp Corporation, and email service provider Silibank. (3) Many of these sites are hosted in Japan and the United States.

This is not the first time South Korea has blocked access to controversial web sites. In June of 2004, the MIC ordered the restriction of hundreds of web sites containing video footage showing the beheading of a South Korean hostage in Iraq. The MIC also called for local search engines to block searches for terms like "beheading," and threatened to prosecute users who posted or distributed the video. In 1998, a 19 year old South Korean citizen was prosecuted for her web site titled, "A Meeting Place for North Korea-Loving People," which had received several thousand visitors. At the same time, 50 other pro-North web sites were shut down or blocked. (4) In 1996, Canadian student David Burgess' web archive of pamphlets and tourist information he obtained on a visit to North Korea was blocked by the South Korean government. (5)

North Korea also controls its citizens' access to the Internet. Access points are few in number and extraordinarily expensive; the country's first cybercafe opened in May 2002, but the cost for 30 minutes of Internet time was equal to one month's wages for the average North Korean citizen. (6)

Tests undertaken by the ONI in December 2004 and January 2005 reveal that South Korean blocking extends to far more than the 31 web sites targeted by the orders. An additional 3,167 unrelated domain names hosted on the same servers as the blocked sites are also blocked. These web sites are completely unrelated to North Korea. This collateral blocking remains in place today.

Methodology & Results

In December 2004 and January 2005, the ONI tested a list of 41 possibly blocked web sites (7) from computers on major South Korean internet service providers, KORnet (Korea Telecom) and Hanaro Telecom Inc., and from a control connection in Toronto. Our tests revealed that 23 of the sites accessible from our control connection were inaccessible from at least one South Korean ISP, 17 were blocked by both the ISP's tested.

The blocking behavior, which resulted in receiving a connection error, was indicative of IP blocking at the router level. In this method, websites are restricted by adding specific blocking rules to access control lists contained in the routing devices that connect South Korean internet users to the global internet backbone. These blocking rules are based on the Internet Protocol addresses of the sites in question.

IP-based blocking has the side effect of restricting access to every other website hosted at the same IP address as the blocked site. Due to the shared nature of most web hosting setups, this type of blocking can potentially result in blocking thousands of additional unrelated sites.

The ONI created a list of all domain names being hosted at 18 of the blocked IP addresses and tested them from our control connection and within South Korea. We found an additional 3,167 unrelated web sites were also being blocked by the South Korean ISP KORnet (Korea Telecom) and 1667 by Hanaro Telecom Inc. (8)

Another disadvantage of IP-based blocking is that it can be easily subverted by changing the IP address of a blocked website. News reports indicate that this is precisely what some of the sites blocked by the South Korean government did.

Because South Korea does not have an equivalent to the "Great Firewall of China," it must rely on individual internet service providers to put the blocks in place. In practical terms, this means that blocking behavior may vary by ISP, and companies with direct connections to internet backbones may not be affected by the technical ban. The threat of prosecution under the National Security Law, however, still looms for anyone who attempts to subvert the spirit of the ban.


The blocking of several dozen pro-North Korean websites has taken place at the same time that the South Korean government has been seeking reconciliation and engagement with the North. However, the collateral blocking of an additional 3,167 unrelated web sites raises serious concerns about the wisdom of using Internet filtering for national security purposes. South Korea's blocking clearly demonstrates one of the important, unintended consequences of implementing Internet filtering: thousands of websites that were never intended to be blocked, and that are completely unrelated to North Korea, have been filtered. Thus, South Korea is preventing its citizens from accessing thousands of sites as a byproduct of its efforts to filter out those that support North Korea. This incident has prompted renewed calls in South Korea for the reform or repeal the 1948 National Security Law.

About the OpenNet Initiative

The OpenNet Initiative is a partnership of the Citizen Lab at the Munk Centre for International Studies, University of Toronto, the Berkman Center for Internet & Society at Harvard Law School, and the Advanced Network Research Group at the Cambridge Security Programme, University of Cambridge.

The OpenNet Initiative releases occasional bulletins based on our ongoing research. These bulletins are meant to be limited responses to current events, policy debates, and/or issues raised by our ongoing research that we feel justify immediate wider circulation. Our more detailed analyses can be found in our major reports.


* The OpenNet Initiative would like to thank Rebecca MacKinnon and Danny Silverman for their contributions to this bulletin.


(2) Additional information about the National Security Law: Full text of the law:


During our testing was blocked by both ISP's tested, was inaccessible from South Korea and our control location in Toronto, and was accessible on KORnet (Korea Telecom) but but inaccessible on Hanaro Telecom Inc.



(6) Economist Intelligence Unit, Country Profile: North Korea 2004 Main Report, Jan. 8, 2004.

(7) The list is comprised of the websites listed at with the addition of the Kim Il Sung Open University (

(8) The inconsistency between the two tests is due to blocking differences between the two ISP's as well as the fact that the tests occurred on different days (and several days after the virtual hosts list was generated) and finally because some of the domains have changed IP addresses since the first test was conducted.

For example, resolved to during tests conducted before January 20, 2005 and was blocked by both KORnet (Korea Telecom) and Hanaro Telecom Inc., but now resolves to and was not blocked in tests conducted on and after January 20, 2005. Others domains such as, which resolves to, or, which resolves to, are blocked by KORnet (Korea Telecom) but are accessible through Hanaro Telecom Inc.