The underscore problem was a security issue to LJ. To prevent cross-site scripting that could steal cookies and, therefore, passwords, LJ moved to a system where each journal was accessed via a synthetic subdomain name of the form username.livejournal.com. Since the domain name technical standards do not allow underscores, they would be translated to hyphens for this purpose. However, the domain name technical standards allow hyphens only when they are neither the first nor last character of a domain name component. A "component" is one of the things separated by the dots, in this case username.
In other words, user-name.livejournal.com would be a valid domain name, but neither -username.livejournal.com nor username-.livejournal.com would be syntactically valid.
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In other words, user-name.livejournal.com would be a valid domain name, but neither -username.livejournal.com nor username-.livejournal.com would be syntactically valid.
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