http://blog.powerdns.com/2014/02/06/related-to-recent-dos-attacks-recursor-configuration-file-guidance/
https://github.com/Habbie/pdns/commit/e24b124a4c7b49f38ff8bcf6926cd69077d16ad8

diff -Naurp pdns-3.3.1/pdns/misc.cc pdns-3.3.1.oden/pdns/misc.cc
--- pdns-3.3.1/pdns/misc.cc	2013-12-17 14:47:33.000000000 +0100
+++ pdns-3.3.1.oden/pdns/misc.cc	2014-06-12 13:02:29.847155292 +0200
@@ -1,6 +1,6 @@
 /*
     PowerDNS Versatile Database Driven Nameserver
-    Copyright (C) 2002 - 2010  PowerDNS.COM BV
+    Copyright (C) 2002 - 2014  PowerDNS.COM BV
 
     This program is free software; you can redistribute it and/or modify
     it under the terms of the GNU General Public License version 2
@@ -22,6 +22,7 @@
 #include <netdb.h>
 #include <sys/time.h>
 #include <time.h>
+#include <sys/resource.h>
 #include <netinet/in.h>
 #include <unistd.h>
 #endif // WIN32
@@ -784,3 +785,22 @@ Regex::Regex(const string &expr)
   if(regcomp(&d_preg, expr.c_str(), REG_ICASE|REG_NOSUB|REG_EXTENDED))
     throw AhuException("Regular expression did not compile");
 }
+
+unsigned int getFilenumLimit(bool hardOrSoft)
+{
+  struct rlimit rlim;
+  if(getrlimit(RLIMIT_NOFILE, &rlim) < 0)
+    unixDie("Requesting number of available file descriptors");
+  return hardOrSoft ? rlim.rlim_max : rlim.rlim_cur;
+}
+
+void setFilenumLimit(unsigned int lim)
+{
+  struct rlimit rlim;
+
+  if(getrlimit(RLIMIT_NOFILE, &rlim) < 0)
+    unixDie("Requesting number of available file descriptors");
+  rlim.rlim_cur=lim;
+  if(setrlimit(RLIMIT_NOFILE, &rlim) < 0)
+    unixDie("Setting number of available file descriptors");
+}
diff -Naurp pdns-3.3.1/pdns/misc.hh pdns-3.3.1.oden/pdns/misc.hh
--- pdns-3.3.1/pdns/misc.hh	2013-12-17 14:47:33.000000000 +0100
+++ pdns-3.3.1.oden/pdns/misc.hh	2014-06-12 13:02:33.184155478 +0200
@@ -507,4 +507,6 @@ private:
   regex_t d_preg;
 };
 
+unsigned int getFilenumLimit(bool hardOrSoft=0);
+void setFilenumLimit(unsigned int lim);
 #endif
diff -Naurp pdns-3.3.1/pdns/pdns_recursor.cc pdns-3.3.1.oden/pdns/pdns_recursor.cc
--- pdns-3.3.1/pdns/pdns_recursor.cc	2013-12-17 14:47:33.000000000 +0100
+++ pdns-3.3.1.oden/pdns/pdns_recursor.cc	2014-06-12 13:02:39.467155829 +0200
@@ -1816,7 +1816,22 @@ int serviceMain(int argc, char*argv[])
   
   g_tcpTimeout=::arg().asNum("client-tcp-timeout");
   g_maxTCPPerClient=::arg().asNum("max-tcp-per-client");
-  g_maxMThreads=::arg().asNum("max-mthreads");
+  g_maxMThreads=::arg().asNum("max-mthreads");	
+  unsigned int availFDs=getFilenumLimit();
+  if(g_maxMThreads * g_numThreads > availFDs) {
+    if(getFilenumLimit(true) >= g_maxMThreads * g_numThreads) {
+      setFilenumLimit(g_maxMThreads * g_numThreads);
+      L<<Logger::Warning<<"Raised soft limit on number of filedescriptors to "<<g_maxMThreads * g_numThreads<<" to match max-mthreads and threads settings"<<endl;
+    }
+    else {
+      int newval = getFilenumLimit(true) / g_numThreads;
+      L<<Logger::Warning<<"Insufficient number of filedescriptors available for max-mthreads*threads setting! ("<<availFDs<<" < "<<g_maxMThreads*g_numThreads<<"), reducing max-mthreads to "<<newval<<endl;
+      g_maxMThreads = newval;
+      setFilenumLimit(g_maxMThreads * g_numThreads);
+    }
+
+    
+  }
 
   if(g_numThreads == 1) {
     L<<Logger::Warning<<"Operating unthreaded"<<endl;