DNS hijacking,DNS poisoning, orDNS redirection is the practice of subverting the resolution ofDomain Name System (DNS) queries.[1] This can be achieved by malware that overrides a computer'sTCP/IP configuration to point at a rogue DNS server under the control of an attacker, or through modifying the behaviour of a trusted DNS server so that it does not comply withinternet standards.
These modifications may be made for malicious purposes such asphishing, for self-serving purposes byInternet service providers (ISPs), by theGreat Firewall of China and public/router-based onlineDNS server providers to direct users' web traffic to the ISP's ownweb servers where advertisements can be served, statistics collected, or other purposes of the ISP; and by DNS service providers to block access to selected domains as a form ofcensorship.
One of the functions of a DNS server is to translate adomain name into anIP address thatapplications need to connect to an Internet resource such as awebsite. This functionality is defined in various formalinternet standards that define theprotocol in considerable detail. DNS servers are implicitly trusted by internet-facing computers and users to correctly resolve names to the actual addresses that are registered by the owners of an internet domain.
A rogue DNS server translates domain names of desirable websites (search engines, banks, brokers, etc.) into IP addresses of sites with unintended content, even malicious websites. Most users depend on DNS servers automatically assigned by theirISPs. A router's assigned DNS servers can also be altered through the remote exploitation of a vulnerability within the router's firmware.[2] When a user tries to visit a website, they are instead sent to a fake page that appears to use the same domain. This attack is termedpharming. If the site they are redirected to is a malicious website, masquerading as a legitimate website, in order to fraudulently obtain sensitive information, it is calledphishing.[3]
A number of consumer ISPs such asAT&T,[4]Cablevision'sOptimum Online,[5]CenturyLink,[6]Cox Communications,RCN,[7]Rogers,[8]Charter Communications (Spectrum),Plusnet,[9]Verizon,[10]Sprint,[11]T-Mobile US,[12]Virgin Media,[13][14]Frontier Communications,Bell Internet,[15]Deutsche Telekom AG,[16]Optus,[17]Mediacom,[18]ONO,[19]TalkTalk,[20]Bigpond (Telstra),[21][22][23][24] TTNET, Türksat, and all Indonesian customer ISPs use or used DNS hijacking for their own purposes, such as displaying advertisements[25] or collecting statistics. Dutch ISPs XS4ALL and Ziggo use DNS hijacking by court order: they were ordered to block access toThe Pirate Bay and display a warning page[26] while all legal customer ISPs in Indonesia do DNS hijacking to comply with the National DNS law[27] which requires every customer Indonesian ISP to hijackport 53 and redirect it to their own server to block website that are listed inTrustpositif byKominfo under the Internet Sehat campaign. These practices violate theRFC standard for DNS (NXDOMAIN) responses,[28] and can potentially open users tocross-site scripting attacks.[25]
The concern with DNS hijacking involves this hijacking of the NXDOMAIN response. Internet andintranet applications rely on the NXDOMAIN response to describe the condition where the DNS has no entry for the specified host. If one were to query the invalid domain name (for example www.example.invalid), one should get an NXDOMAIN response – informing the application that the name is invalid and taking the appropriate action (for example, displaying an error or not attempting to connect to the server). However, if the domain name is queried on one of these non-compliant ISPs, one would always receive a fake IP address belonging to the ISP. In aweb browser, this behavior can be annoying as connections to this IP address display theISP redirect page of the provider, sometimes with advertising, instead of a proper error message. However, other applications that rely on the NXDOMAIN error will instead attempt to initiate connections to this spoofed IP address, potentially exposing sensitive information.
Examples of functionality that breaks when an ISP hijacks DNS:
In some, but not most cases, the ISPs provide subscriber-configurable settings to disable hijacking of NXDOMAIN responses. Correctly implemented, such a setting reverts DNS to standard behavior. Other ISPs, however, instead use a web browsercookie to store the preference. In this case, the underlying behavior is not resolved: DNS queries continue to be redirected, while the ISP redirect page is replaced with a counterfeit DNS error page. Applications other than web browsers cannot be opted out of the scheme using cookies as the opt-out targets only theHTTP protocol, when the scheme is actually implemented in the protocol-neutral DNS.
In the UK, the Information Commissioner's Office has acknowledged that the practice of involuntary DNS hijacking contravenesPECR, and EC Directive 95/46 on Data Protection which require explicit consent for processing of communication traffic.[13] In Germany, in 2019 it was revealed that the Deutsche Telekom AG not only manipulated their DNS servers, but also transmitted network traffic (such as non-secure cookies when users did not useHTTPS) to a third-party company because the web portal T-Online, at which users were redirected due to the DNS manipulation, was not (any more) owned by the Deutsche Telekom. After a user filed a criminal complaint, the Deutsche Telekom stopped further DNS manipulations.[32]
ICANN, the international body responsible for administering top-level domain names, has published a memorandum highlighting its concerns, and affirming:[31]
ICANN strongly discourages the use of DNS redirection, wildcards, synthesized responses and any other form of NXDOMAIN substitution in existing gTLDs, ccTLDs and any other level in the DNS tree for registry-class domain names.
End users, dissatisfied with poor "opt-out" options like cookies, have responded to the controversy by finding ways to avoid spoofed NXDOMAIN responses. DNS software such asBIND andDnsmasq offer options to filter results, and can be run from a gateway or router to protect an entire network. Google, among others, run open DNS servers that currently do not return spoofed results. So a user could useGoogle Public DNS instead of their ISP's DNS servers if they are willing to accept that they use the service underGoogle's privacy policy and potentially be exposed to another method by which Google can track the user. One limitation of this approach is that some providers block or rewrite outside DNS requests.OpenDNS, owned by Cisco, is a similar popular service which does not alter NXDOMAIN responses.
Google in April 2016 launched DNS-over-HTTPS service.[33] This scheme can overcome the limitations of the legacy DNS protocol. It performs remote DNSSEC check and transfers the results in a secure HTTPS tunnel.
There are also application-level work-arounds, such as the NoRedirect[34]Firefox extension, that mitigate some of the behavior. An approach like that only fixes one application (in this example, Firefox) and will not address any other issues caused. Website owners may be able to fool some hijackers by using certain DNS settings. For example, setting a TXT record of "unused" on their wildcard address (e.g. *.example.com). Alternatively, they can try setting the CNAME of the wildcard to "example.invalid", making use of the fact that ".invalid" is guaranteed not to exist per the RFC. The limitation of that approach is that it only prevents hijacking on those particular domains, but it may address some VPN security issues caused by DNS hijacking.
I am not convinced that there is any likelihood of detriment or harm to subscribers or users that would justify taking formal action in this case.[permanent dead link]
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