diff --git a/acinclude.m4 b/acinclude.m4
index 0e5e611c0..6eb0e6de6 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -87,6 +87,27 @@ int main() {
   [AC_MSG_RESULT(cross-compiling -- assuming yes); $3])
 ])
 
+dnl Checks if PCAP_NETMASK_UNKNOWN is defined (has been since libpcap 1.1.1)
+dnl Sets it to 0 (no checking) if it's not defined.
+AC_DEFUN([PCAP_DEFINE_NETMASK_UNKNOWN],
+[
+  AC_MSG_CHECKING(if PCAP_NETMASK_UNKNOWN is defined/handled by libpcap)
+  AC_CACHE_VAL(ac_cv_have_pcap_netmask_unknown,
+    AC_TRY_COMPILE(
+      [
+      #include <pcap.h>
+      ],
+      [
+      int i = PCAP_NETMASK_UNKNOWN;
+      ],
+      ac_cv_have_pcap_netmask_unknown=yes,
+      ac_cv_have_pcap_netmask_unknown=no))
+  if test $ac_cv_have_pcap_netmask_unknown = no; then
+    AC_DEFINE(PCAP_NETMASK_UNKNOWN, 0, [Possibly using libpcap prior to 1.1.0.])
+  fi
+  AC_MSG_RESULT($ac_cv_have_pcap_netmask_unknown)
+])
+
 dnl Checks if IPPROTO_RAW induces IP_HDRINCL-like behavior in AF_INET6 sockets.
 dnl Defines HAVE_IPV6_IPPROTO_RAW if so. So far I only know this happens on
 dnl Linux.
diff --git a/configure b/configure
index c1318a2df..b0d7b5c6b 100755
--- a/configure
+++ b/configure
@@ -6526,6 +6526,44 @@ fi
 
 
 
+
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking if PCAP_NETMASK_UNKNOWN is defined/handled by libpcap" >&5
+$as_echo_n "checking if PCAP_NETMASK_UNKNOWN is defined/handled by libpcap... " >&6; }
+  if ${ac_cv_have_pcap_netmask_unknown+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+      #include <pcap.h>
+
+int
+main ()
+{
+
+      int i = PCAP_NETMASK_UNKNOWN;
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  ac_cv_have_pcap_netmask_unknown=yes
+else
+  ac_cv_have_pcap_netmask_unknown=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+
+  if test $ac_cv_have_pcap_netmask_unknown = no; then
+
+$as_echo "#define PCAP_NETMASK_UNKNOWN 0" >>confdefs.h
+
+  fi
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_have_pcap_netmask_unknown" >&5
+$as_echo "$ac_cv_have_pcap_netmask_unknown" >&6; }
+
+
 have_pcre=no
 requested_included_pcre=no
 LIBPCREDIR=libpcre
diff --git a/configure.ac b/configure.ac
index cc4183008..721a88f49 100644
--- a/configure.ac
+++ b/configure.ac
@@ -458,6 +458,8 @@ AC_SUBST(PCAP_CLEAN)
 AC_SUBST(PCAP_DIST_CLEAN)
 AC_SUBST(LIBPCAP_LIBS)
 
+PCAP_DEFINE_NETMASK_UNKNOWN
+
 have_pcre=no
 requested_included_pcre=no
 LIBPCREDIR=libpcre
diff --git a/nsock/src/acinclude.m4 b/nsock/src/acinclude.m4
index 9a972cc97..696cd3414 100644
--- a/nsock/src/acinclude.m4
+++ b/nsock/src/acinclude.m4
@@ -118,3 +118,24 @@ $1],[AC_MSG_RESULT([no])
 $2])
 ])dnl
 
+dnl Checks if PCAP_NETMASK_UNKNOWN is defined (has been since libpcap 1.1.1)
+dnl Sets it to 0 (no checking) if it's not defined.
+AC_DEFUN([PCAP_DEFINE_NETMASK_UNKNOWN],
+[
+  AC_MSG_CHECKING(if PCAP_NETMASK_UNKNOWN is defined/handled by libpcap)
+  AC_CACHE_VAL(ac_cv_have_pcap_netmask_unknown,
+    AC_TRY_COMPILE(
+      [
+      #include <pcap.h>
+      ],
+      [
+      int i = PCAP_NETMASK_UNKNOWN;
+      ],
+      ac_cv_have_pcap_netmask_unknown=yes,
+      ac_cv_have_pcap_netmask_unknown=no))
+  if test $ac_cv_have_pcap_netmask_unknown = no; then
+    AC_DEFINE(PCAP_NETMASK_UNKNOWN, 0, [Possibly using libpcap prior to 1.1.0.])
+  fi
+  AC_MSG_RESULT($ac_cv_have_pcap_netmask_unknown)
+])
+
diff --git a/nsock/src/configure b/nsock/src/configure
index f0d3cd22c..8e811ded8 100755
--- a/nsock/src/configure
+++ b/nsock/src/configure
@@ -3625,6 +3625,44 @@ if test "$have_libpcap" != "no"; then
 fi
 
 
+
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking if PCAP_NETMASK_UNKNOWN is defined/handled by libpcap" >&5
+$as_echo_n "checking if PCAP_NETMASK_UNKNOWN is defined/handled by libpcap... " >&6; }
+  if ${ac_cv_have_pcap_netmask_unknown+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+      #include <pcap.h>
+
+int
+main ()
+{
+
+      int i = PCAP_NETMASK_UNKNOWN;
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  ac_cv_have_pcap_netmask_unknown=yes
+else
+  ac_cv_have_pcap_netmask_unknown=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+
+  if test $ac_cv_have_pcap_netmask_unknown = no; then
+
+$as_echo "#define PCAP_NETMASK_UNKNOWN 0" >>confdefs.h
+
+  fi
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_have_pcap_netmask_unknown" >&5
+$as_echo "$ac_cv_have_pcap_netmask_unknown" >&6; }
+
+
   ax_have_epoll_cppflags="${CPPFLAGS}"
   ac_fn_c_check_header_mongrel "$LINENO" "linux/version.h" "ac_cv_header_linux_version_h" "$ac_includes_default"
 if test "x$ac_cv_header_linux_version_h" = xyes; then :
diff --git a/nsock/src/configure.ac b/nsock/src/configure.ac
index 1772e9da8..2a0ba2034 100644
--- a/nsock/src/configure.ac
+++ b/nsock/src/configure.ac
@@ -168,6 +168,8 @@ if test "$have_libpcap" != "no"; then
 fi
 AC_SUBST(LIBPCAP_LIBS)
 
+PCAP_DEFINE_NETMASK_UNKNOWN
+
 AX_HAVE_EPOLL([AC_DEFINE(HAVE_EPOLL)], )
 AX_HAVE_POLL([AC_DEFINE(HAVE_POLL)], )
 AC_CHECK_FUNCS(kqueue kevent, [AC_DEFINE(HAVE_KQUEUE)], )
diff --git a/nsock/src/nsock_pcap.c b/nsock/src/nsock_pcap.c
index aa977a5f9..04980cb18 100644
--- a/nsock/src/nsock_pcap.c
+++ b/nsock/src/nsock_pcap.c
@@ -107,7 +107,7 @@ static int nsock_pcap_set_filter(struct npool *nsp, pcap_t *pt, const char *devi
   struct bpf_program fcode;
   int rc;
 
-  rc = pcap_compile(pt, &fcode, (char *)bpf, 1, 0);
+  rc = pcap_compile(pt, &fcode, (char *)bpf, 1, PCAP_NETMASK_UNKNOWN);
   if (rc) {
     nsock_log_error(nsp, "Error compiling pcap filter: %s", pcap_geterr(pt));
     return rc;