dns.el: forget DNS servers in new network

[Magnus Henoch <magnus.henoch@gmail.com>]

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Hi all,

I'm using Gnus' dns.el to look up SRV records for Jabber servers in
jabber.el.  It works fine except when I move my laptop from one network
to another, because dns.el caches the list of DNS servers (obtained from
/etc/resolv.conf) in the variable dns-servers, but sometimes those DNS
servers aren't appropriate/reachable for the new network.

Thus the patch below.  It checks whether the names and IP addresses of
all network interfaces have changed, and if so, rereads /etc/resolv.conf
instead of using the value of dns-servers.  What do you think about it?

(I'm calling fboundp for network-interface-list as I'm not sure if the
relevant XEmacs versions support it.)

Regards,
Magnus


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diff --git a/lisp/dns.el b/lisp/dns.el
index 1c456eb..ccf0617 100644
--- a/lisp/dns.el
+++ b/lisp/dns.el
@@ -31,6 +31,12 @@
   "List of DNS servers to query.
 If nil, /etc/resolv.conf and nslookup will be consulted.")
 
+(defvar dns-servers-valid-for-interfaces nil
+  "The return value of `network-interface-list' when `dns-servers' was set.
+If the set of network interfaces and/or their IP addresses
+change, then presumably the list of DNS servers needs to be
+updated.  Set this variable to t to disable the check.")
+
 ;;; Internal code:
 
 (defvar dns-query-types
@@ -297,6 +303,15 @@ If TCP-P, the first two bytes of the package with be the length field."
            (t string)))
       (goto-char point))))
 
+(defun dns-servers-up-to-date-p ()
+  "Return false if we need to recheck the list of DNS servers."
+  (and dns-servers
+       (or (eq dns-servers-valid-for-interfaces t)
+	   ;; `network-interface-list' was introduced in Emacs 22.1.
+	   (not (fboundp 'network-interface-list))
+	   (equal dns-servers-valid-for-interfaces
+		  (network-interface-list)))))
+
 (defun dns-set-servers ()
   "Set `dns-servers' to a list of DNS servers or nil if none are found.
 Parses \"/etc/resolv.conf\" or calls \"nslookup\"."
@@ -314,7 +329,9 @@ Parses \"/etc/resolv.conf\" or calls \"nslookup\"."
 	  (goto-char (point-min))
 	  (re-search-forward
 	   "^Address:[ \t]*\\([0-9]+\\.[0-9]+\\.[0-9]+\\.[0-9]+\\)" nil t)
-	  (setq dns-servers (list (match-string 1)))))))
+	  (setq dns-servers (list (match-string 1))))))
+  (when (fboundp 'network-interface-list)
+    (setq dns-servers-valid-for-interfaces (network-interface-list))))
 
 (defun dns-read-txt (string)
   (if (> (length string) 1)
@@ -378,7 +395,7 @@ Parses \"/etc/resolv.conf\" or calls \"nslookup\"."
 If FULLP, return the entire record returned.
 If REVERSEP, look up an IP address."
   (setq type (or type 'A))
-  (unless dns-servers
+  (unless (dns-servers-up-to-date-p)
     (dns-set-servers))
 
   (when reversep