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1;;; srv.el --- perform SRV DNS requests
2
3;; Copyright (C) 2005, 2007, 2018 Magnus Henoch
4
5;; Author: Magnus Henoch <magnus.henoch@gmail.com>
6;; Keywords: comm
7;; Version: 0.2
8;; Package-Requires: ((emacs "24.3"))
9;; URL: https://github.com/legoscia/srv.el
10
11;; This file is free software; you can redistribute it and/or modify
12;; it under the terms of the GNU General Public License as published by
13;; the Free Software Foundation; either version 2, or (at your option)
14;; any later version.
15
16;; This file is distributed in the hope that it will be useful,
17;; but WITHOUT ANY WARRANTY; without even the implied warranty of
18;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
19;; GNU General Public License for more details.
20
21;; You should have received a copy of the GNU General Public License
22;; along with GNU Emacs; see the file COPYING. If not, write to
23;; the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
24;; Boston, MA 02111-1307, USA.
25
26;;; Commentary:
27
28;; This code implements RFC 2782 (SRV records). It was originally
29;; written for jabber.el <http://emacs-jabber.sf.net>, but is now a
30;; separate package.
31;;
32;; It is used to look up hostname and port for a service at a specific
33;; domain. There might be multiple results, and the caller is supposed
34;; to attempt to connect to each hostname+port in turn. For example,
35;; to find the XMPP client service for the domain gmail.com:
36;;
37;; (srv-lookup "_xmpp-client._tcp.gmail.com")
38;; -> (("xmpp.l.google.com" . 5222)
39;; ("alt3.xmpp.l.google.com" . 5222)
40;; ("alt4.xmpp.l.google.com" . 5222)
41;; ("alt1.xmpp.l.google.com" . 5222)
42;; ("alt2.xmpp.l.google.com" . 5222))
43
44
45;;; Code:
46
47(require 'dns)
48
49;;;###autoload
50(defun srv-lookup (target)
51 "Perform SRV lookup of TARGET and return list of connection candidiates.
52TARGET is a string of the form \"_Service._Proto.Name\".
53
54Returns a list with elements of the form (HOST . PORT), where HOST is
55a hostname and PORT is a numeric port. The caller is supposed to
56make connection attempts in the order given, starting from the beginning
57of the list. The list is empty if no SRV records were found."
58 (let* ((result (srv--dns-query target))
59 (answers (mapcar #'(lambda (a)
60 (cadr (assq 'data a)))
61 (cadr (assq 'answers result))))
62 answers-by-priority weighted-result)
63 (if (or (null answers)
64 ;; Special case for "service decidedly not available"
65 (and (eq (length answers) 1)
66 (string= (cadr (assq 'target (car answers))) ".")))
67 nil
68 ;; Sort answers into groups of same priority.
69 (dolist (a answers)
70 (let* ((priority (cadr (assq 'priority a)))
71 (entry (assq priority answers-by-priority)))
72 (if entry
73 (push a (cdr entry))
74 (push (cons priority (list a)) answers-by-priority))))
75 ;; Sort by priority.
76 (setq answers-by-priority
77 (sort answers-by-priority
78 #'(lambda (a b) (< (car a) (car b)))))
79 ;; Randomize by weight within priority groups. See
80 ;; algorithm in RFC 2782.
81 (dolist (p answers-by-priority)
82 (let ((weight-acc 0)
83 weight-order)
84 ;; Assign running sum of weight to each entry.
85 (dolist (a (cdr p))
86 (cl-incf weight-acc (cadr (assq 'weight a)))
87 (push (cons weight-acc a) weight-order))
88 (setq weight-order (nreverse weight-order))
89
90 ;; While elements remain, pick a random number between 0 and
91 ;; weight-acc inclusive, and select the first entry whose
92 ;; running sum is greater than or equal to this number.
93 (while weight-order
94 (let* ((r (random (1+ weight-acc)))
95 (next-entry (cl-dolist (a weight-order)
96 (if (>= (car a) r)
97 (cl-return a)))))
98 (push (cdr next-entry) weighted-result)
99 (setq weight-order
100 (delq next-entry weight-order))))))
101 ;; Extract hostnames and ports
102 (mapcar #'(lambda (a) (cons (cadr (assq 'target a))
103 (cadr (assq 'port a))))
104 (nreverse weighted-result)))))
105
106(defun srv--dns-query (target)
107 "Perform DNS query for TARGET.
108On Windows, call `srv--nslookup'; on all other systems, call `dns-query'."
109 ;; dns-query uses UDP, but that is not supported on Windows...
110 (if (featurep 'make-network-process '(:type datagram))
111 (dns-query target 'SRV t)
112 ;; ...so let's call nslookup instead.
113 (srv--nslookup target)))
114
115(defun srv--nslookup (target)
116 "Call the `nslookup' program to make an SRV query for TARGET."
117 (with-temp-buffer
118 (call-process "nslookup" nil t nil "-type=srv" target)
119 (goto-char (point-min))
120 (let (results)
121 ;; This matches what nslookup prints on Windows. It's unlikely
122 ;; to work for other systems, but on those systems we use DNS
123 ;; directly.
124 (while (search-forward-regexp
125 (concat "[\s\t]*priority += \\(.*\\)\r?\n"
126 "[\s\t]*weight += \\(.*\\)\r?\n"
127 "[\s\t]*port += \\(.*\\)\r?\n"
128 "[\s\t]*svr hostname += \\(.*\\)\r?\n")
129 nil t)
130 (push
131 (list
132 (list 'data
133 (list
134 (list 'priority (string-to-number (match-string 1)))
135 (list 'weight (string-to-number (match-string 2)))
136 (list 'port (string-to-number (match-string 3)))
137 (list 'target (match-string 4)))))
138 results))
139 (list (list 'answers results)))))
140
141(provide 'srv)
142;;; srv.el ends here