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A linkback is a method for Web authors to obtain notifications when other authors link to one of their documents. This enables authors to keep track of who is linking to, or referring to, their articles. The four methods (Refback, Trackback, Pingback and Webmention) differ in how they accomplish this task.
"LinkBack" is the generalized term used to reference three methods of communication between websites.
Any of the four terms—Linkback, Trackback, Pingback, or (rarely) Refback—might also refer colloquially to items within a section upon the linked page that display the received notifications, usually along with a reciprocal link; Trackback is used most often for this purpose. Also, the word Trackback is often used colloquially to mean any kind of Linkback.
Dan Magarino, Morgan Stanley Equity Research, is credited with popularizing the term "linkback" and also contributed to its widespread adoption by RiXML.org.
|Trigger mechanism||Visitor to linking site clicks on the link, and his browser takes him to the linked site||Code on linking server examines added or updated documents, extracts links, and sends notification to linked server for each link found||Code on linking server examines added or updated documents, extracts links, and sends notification to linked server for each link found||Code on linking server examines added or updated documents, extracts links, and sends notification to linked server for each link found|
|Notification medium||HTTP referrer value||HTTP POST||XML-RPC call||HTTP POST with source and target parameters|
|Capture mechanism||Examination of incoming HTTP referrer values||Trackback capture script||XML-RPC function||Webmention capture script|
|Information sent by linking server||None||
|Additional information presented to linked server||HTTP referrer sent by a visitor's browser upon clicking the link||IP address of linking server||IP address of linking server||IP address of linking server|
|Autodiscovery mechanism (how the linking server finds out how and where to send the notification)||None||LINK tag in the header of the linked page or Trackback RDF Documents||Special HTTP header or LINK tag on the linked page||HTTP Link header or link element on the linked page|
|Action required when notification is received||
||Verifying that linking page does indeed link to linked page is recommended, not explicitly required|
|Advantages||Requires no special code on linking server (the link itself becomes the notification when someone clicks on it)||All the information desired by the linked server (Linking site name, post title, excerpt) is present in the notification itself||
Despite the potential reduction in value with the IP logging, attackers are still using this technique. Likely because website owners rarely check the user agent logs to derive the real IP address of visitors. [...] Although it is great that WordPress is logging the attacker IP address on newer releases, we still recommend that you disable pingbacks on your site. It won’t protect you from being attacked, but will stop your site from attacking others.
Unless the attacker is very, very naive however, this IP will simply trace back to another infected machine or site. Generally these requesting systems are part of a botnet to mask and distribute the requests. [...] The pingback tool within WordPress still remains an exploitable system for any WordPress site which hasn’t explicitly stopped it. From a web host’s perspective, this is quite frustrating.
This issue arises from the fact that it is possible for an attacker A to impersonate T's blog by connecting to R's blog and sending a link notification that specifies T's blog as the origination of the notification. At that point, K will automatically attempt to connect to T to download the blog post. This is called reflection. If the attacker were careful to select a URL that has a lot of information in it, this would cause amplification. In other words, for a relatively small request from the attacker (A) to the reflector, the reflector (R) will connect to the target (T) and cause a large amount of traffic. [...] On the reflector side for the 200-byte request, the response can easily be thousands of bytes – resulting in a multiplication that starts in the 10x, 20x and more. [...] To avoid overloading the reflector, multiple reflectors can be employed to scale up. Thus, the target will have their outgoing bandwidth, and possibly compute resources, exhausted. [...] Another point to consider is the compute resources tied to the target side. If considering a page that is computationally expensive to produce, it may be more efficient for the attacker to overload the CPU of a system versus the bandwidth of the connection. [...] This is not the first time a CMS, and in particular WordPress, has been used for DDoS or other malicious activity. To a very large extent, this is because WordPress appeals to users that do not have the resources to manage their websites and they often use WordPress to make their job easier. As a result, many users do not have an adequate patch management program or proper monitoring to observe irregularities in their traffic.
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