William Ahern:

Make UDP sockets non-blocking. I've confirmed that at least on Linux 2.4 a
  read event can come back from poll() on a valid SOCK_DGRAM socket but
  recv(2) will still block. This patch doesn't ignore EAGAIN in
  read_udp_packets(), though maybe it should. (This patch was edited by Daniel
  Stenberg and a new configure test was added (imported from curl's configure)
  to properly detect what non-blocking socket approach to use.)

ares/acinclude.m4

@@ -1,3 +1,126 @@
+dnl Check for how to set a socket to non-blocking state. There seems to exist
+dnl four known different ways, with the one used almost everywhere being POSIX
+dnl and XPG3, while the other different ways for different systems (old BSD,
+dnl Windows and Amiga).
+dnl
+dnl There are two known platforms (AIX 3.x and SunOS 4.1.x) where the
+dnl O_NONBLOCK define is found but does not work. This condition is attempted
+dnl to get caught in this script by using an excessive number of #ifdefs...
+dnl
+AC_DEFUN([CURL_CHECK_NONBLOCKING_SOCKET],
+[
+  AC_MSG_CHECKING([non-blocking sockets style])
+
+  AC_TRY_COMPILE([
+/* headers for O_NONBLOCK test */
+#include <sys/types.h>
+#include <unistd.h>
+#include <fcntl.h>
+],[
+/* try to compile O_NONBLOCK */
+
+#if defined(sun) || defined(__sun__) || defined(__SUNPRO_C) || defined(__SUNPRO_CC)
+# if defined(__SVR4) || defined(__srv4__)
+#  define PLATFORM_SOLARIS
+# else
+#  define PLATFORM_SUNOS4
+# endif
+#endif
+#if (defined(_AIX) || defined(__xlC__)) && !defined(_AIX4)
+# define PLATFORM_AIX_V3
+#endif
+
+#if defined(PLATFORM_SUNOS4) || defined(PLATFORM_AIX_V3) || defined(__BEOS__)
+#error "O_NONBLOCK does not work on this platform"
+#endif
+  int socket;
+  int flags = fcntl(socket, F_SETFL, flags | O_NONBLOCK);
+],[
+dnl the O_NONBLOCK test was fine
+nonblock="O_NONBLOCK"
+AC_DEFINE(HAVE_O_NONBLOCK, 1, [use O_NONBLOCK for non-blocking sockets])
+],[
+dnl the code was bad, try a different program now, test 2
+
+  AC_TRY_COMPILE([
+/* headers for FIONBIO test */
+#include <unistd.h>
+#include <stropts.h>
+],[
+/* FIONBIO source test (old-style unix) */
+ int socket;
+ int flags = ioctl(socket, FIONBIO, &flags);
+],[
+dnl FIONBIO test was good
+nonblock="FIONBIO"
+AC_DEFINE(HAVE_FIONBIO, 1, [use FIONBIO for non-blocking sockets])
+],[
+dnl FIONBIO test was also bad
+dnl the code was bad, try a different program now, test 3
+
+  AC_TRY_COMPILE([
+/* headers for ioctlsocket test (cygwin?) */
+#include <windows.h>
+],[
+/* ioctlsocket source code */
+ int socket;
+ unsigned long flags = ioctlsocket(socket, FIONBIO, &flags);
+],[
+dnl ioctlsocket test was good
+nonblock="ioctlsocket"
+AC_DEFINE(HAVE_IOCTLSOCKET, 1, [use ioctlsocket() for non-blocking sockets])
+],[
+dnl ioctlsocket didnt compile!, go to test 4
+
+  AC_TRY_LINK([
+/* headers for IoctlSocket test (Amiga?) */
+#include <sys/ioctl.h>
+],[
+/* IoctlSocket source code */
+ int socket;
+ int flags = IoctlSocket(socket, FIONBIO, (long)1);
+],[
+dnl ioctlsocket test was good
+nonblock="IoctlSocket"
+AC_DEFINE(HAVE_IOCTLSOCKET_CASE, 1, [use Ioctlsocket() for non-blocking sockets])
+],[
+dnl Ioctlsocket didnt compile, do test 5!
+  AC_TRY_COMPILE([
+/* headers for SO_NONBLOCK test (BeOS) */
+#include <socket.h>
+],[
+/* SO_NONBLOCK source code */
+ long b = 1;
+ int socket;
+ int flags = setsockopt(socket, SOL_SOCKET, SO_NONBLOCK, &b, sizeof(b));
+],[
+dnl the SO_NONBLOCK test was good
+nonblock="SO_NONBLOCK"
+AC_DEFINE(HAVE_SO_NONBLOCK, 1, [use SO_NONBLOCK for non-blocking sockets])
+],[
+dnl test 5 didnt compile!
+nonblock="nada"
+AC_DEFINE(HAVE_DISABLED_NONBLOCKING, 1, [disabled non-blocking sockets])
+])
+dnl end of fifth test
+
+])
+dnl end of forth test
+
+])
+dnl end of third test
+
+])
+dnl end of second test
+
+])
+dnl end of non-blocking try-compile test
+  AC_MSG_RESULT($nonblock)
+
+  if test "$nonblock" = "nada"; then
+    AC_MSG_WARN([non-block sockets disabled])
+  fi
+])
 
 dnl We create a function for detecting which compiler we use and then set as
 dnl pendantic compiler options as possible for that particular compiler. The