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Diffstat (limited to 'lisp/url/url-util.el')
| -rw-r--r-- | lisp/url/url-util.el | 29 |
1 files changed, 29 insertions, 0 deletions
diff --git a/lisp/url/url-util.el b/lisp/url/url-util.el index 85bfb65cb68..77e015068a3 100644 --- a/lisp/url/url-util.el +++ b/lisp/url/url-util.el @@ -627,6 +627,35 @@ Creates FILE and its parent directories if they do not exist." (error "Danger: `%s' is a symbolic link" file)) (set-file-modes file #o0600)))) +(autoload 'dns-query "dns") + +(defvar url--domain-cache (make-hash-table :test 'equal :size 17) + "Cache to minimize dns lookups.") + +;;;###autoload +(defun url-domain (url) + "Return the domain of the host of the url, or nil if url does +not contain a registered name." + ;; Determining the domain of a name can not be done with simple + ;; textual manipulations. a.b.c is either host a in domain b.c + ;; (www.google.com), or domain a.b.c with no separate host + ;; (bbc.co.uk). Instead of guessing based on tld (which in any case + ;; may be inaccurate in the face of subdelegations), we look for + ;; domain delegations in DNS. + ;; + ;; Domain delegations change rarely enough that we won't bother with + ;; cache invalidation, I think. + (let* ((host-parts (split-string (url-host url) "\\.")) + (result (gethash host-parts url--domain-cache 'not-found))) + (when (eq result 'not-found) + (setq result + (cl-loop for parts on host-parts + for dom = (mapconcat #'identity parts ".") + when (dns-query dom 'SOA) + return dom)) + (puthash host-parts result url--domain-cache)) + result)) + (provide 'url-util) ;;; url-util.el ends here |