diff --git a/dll/windivert_helper.c b/dll/windivert_helper.c
index 833f79a..84f7d59 100644
--- a/dll/windivert_helper.c
+++ b/dll/windivert_helper.c
@@ -423,8 +423,14 @@ extern UINT WinDivertHelperCalcChecksums(PVOID pPacket, UINT packetLen,
     PWINDIVERT_UDPHDR udp_header;
     UINT payload_len, checksum_len;
     UINT count = 0;
+    UINT64 flags_all =
+        (WINDIVERT_HELPER_NO_IP_CHECKSUM |
+         WINDIVERT_HELPER_NO_ICMP_CHECKSUM |
+         WINDIVERT_HELPER_NO_ICMPV6_CHECKSUM |
+         WINDIVERT_HELPER_NO_TCP_CHECKSUM |
+         WINDIVERT_HELPER_NO_UDP_CHECKSUM);
 
-    if ((flags & 0x1F) == 0x1F)
+    if ((flags & flags_all) == flags_all)
     {
         return 0;
     }
@@ -433,7 +439,8 @@ extern UINT WinDivertHelperCalcChecksums(PVOID pPacket, UINT packetLen,
         &icmp_header, &icmpv6_header, &tcp_header, &udp_header, NULL,
         &payload_len);
 
-    if (ip_header != NULL && !(flags & WINDIVERT_HELPER_NO_IP_CHECKSUM))
+    if (ip_header != NULL && !(flags & WINDIVERT_HELPER_NO_IP_CHECKSUM) &&
+        (!(flags & WINDIVERT_HELPER_NO_REPLACE) || ip_header->Checksum == 0))
     {
         ip_header->Checksum = 0;
         ip_header->Checksum = WinDivertHelperCalcChecksum(NULL, 0,
@@ -443,7 +450,9 @@ extern UINT WinDivertHelperCalcChecksums(PVOID pPacket, UINT packetLen,
 
     if (icmp_header != NULL)
     {
-        if (flags & WINDIVERT_HELPER_NO_ICMP_CHECKSUM)
+        if ((flags & WINDIVERT_HELPER_NO_ICMP_CHECKSUM) ||
+            ((flags & WINDIVERT_HELPER_NO_REPLACE) &&
+                icmp_header->Checksum != 0))
         {
             return count;
         }
@@ -456,7 +465,9 @@ extern UINT WinDivertHelperCalcChecksums(PVOID pPacket, UINT packetLen,
 
     if (icmpv6_header != NULL)
     {
-        if (flags & WINDIVERT_HELPER_NO_ICMPV6_CHECKSUM)
+        if ((flags & WINDIVERT_HELPER_NO_ICMPV6_CHECKSUM) ||
+            ((flags & WINDIVERT_HELPER_NO_REPLACE) &&
+                icmpv6_header->Checksum != 0))
         {
             return count;
         }
@@ -472,7 +483,9 @@ extern UINT WinDivertHelperCalcChecksums(PVOID pPacket, UINT packetLen,
 
     if (tcp_header != NULL)
     {
-        if (flags & WINDIVERT_HELPER_NO_TCP_CHECKSUM)
+        if ((flags & WINDIVERT_HELPER_NO_TCP_CHECKSUM) ||
+            ((flags & WINDIVERT_HELPER_NO_REPLACE) &&
+                tcp_header->Checksum != 0))
         {
             return count;
         }
@@ -499,7 +512,9 @@ extern UINT WinDivertHelperCalcChecksums(PVOID pPacket, UINT packetLen,
 
     if (udp_header != NULL)
     {
-        if (flags & WINDIVERT_HELPER_NO_UDP_CHECKSUM)
+        if ((flags & WINDIVERT_HELPER_NO_UDP_CHECKSUM) ||
+            ((flags & WINDIVERT_HELPER_NO_REPLACE) &&
+                udp_header->Checksum != 0))
         {
             return count;
         }
diff --git a/doc/windivert.html b/doc/windivert.html
index 640bef6..d7ff45a 100644
--- a/doc/windivert.html
+++ b/doc/windivert.html
@@ -614,7 +614,7 @@ WinDivert sometimes captures outbound or loopback packets before the
 IP/TCP/UDP checksum fields have been calculated.
 For outbound packets, this occurs when <i>checksum offloading</i> is enabled,
 which defers checksum calculation to a compatible NIC card.
-If the checksum is absent the corresponding 
+If the checksum is absent then the corresponding 
 checksum field will be set to zero.
 Correct checksums can be reconstructed using the
 <a href="#divert_helper_calc_checksums"><tt>WinDivertHelperCalcChecksums()</tt></a>
@@ -730,6 +730,15 @@ fields are currently ignored and may be arbitrary values.
 Injecting an inbound packet on the outbound path <i>may</i> work (for some
 types of packets), however this should be considered "undocumented" behavior,
 and may be changed in the future.
+</p><p>
+Injected packets should have correct checksums.
+Correct checksums can be calculated using the
+<a
+href="#divert_helper_calc_checksums"><tt>WinDivertHelperCalcChecksums()</tt></a>
+function.
+Note that packets returned by
+<a href="#divert_recv"><tt>WinDivertRecv()</tt></a> are not
+guaranteed to have correct checksums.
 </p>
 </dd></dl>
 
@@ -1246,6 +1255,8 @@ UINT <b>WinDivertHelperCalcChecksums</b>(
     checksum.</li>
 <li> <tt>WINDIVERT_HELPER_NO_UDP_CHECKSUM</tt>: Do not calculate the UDP
     checksum.</li>
+<li> <tt>WINDIVERT_HELPER_NO_REPLACE</tt>: Do not replace non-zero
+    checksums.</li>
 </ul></li>
 </ul>
 </p><p>
@@ -1259,12 +1270,19 @@ Individual checksum calculations may be disabled via the appropriate flag.
 Typically this function should be invoked on a modified packet before it is
 injected with <a href="#divert_send"><tt>WinDivertSend()</tt></a>.
 </p><p>
-This function will calculate each checksum from scratch, even if the existing
-checksum is correct.
+By default this function will calculate each checksum from scratch, even if
+the existing checksum is correct.
 This may be inefficient for some applications.
 For better performance, incremental checksum calculations should be used
 instead (not provided by this API).
-<p>
+</p><p>
+If the <tt>WINDIVERT_HELPER_NO_REPLACE</tt> flag is used, this function will
+assume that all non-zero checksum fields are already valid and will not
+replace them.
+This is useful for reconstructing checksums for packets returned by
+<a href="#divert_recv"><tt>WinDivertRecv()</tt></a>, where
+non-zero checksum fields do not need to be recalculated.
+</p>
 </dd></dl>
 
 <a name="divert_helper_check_filter"><h3>6.11 WinDivertHelperCheckFilter</h3></a>
diff --git a/examples/netdump/netdump.c b/examples/netdump/netdump.c
index 39ac27d..93f5902 100644
--- a/examples/netdump/netdump.c
+++ b/examples/netdump/netdump.c
@@ -118,11 +118,9 @@ int __cdecl main(int argc, char **argv)
             continue;
         }
 
-        // Calculate checksums for outbound packets.
-        if (addr.Direction == WINDIVERT_DIRECTION_OUTBOUND)
-        {
-            WinDivertHelperCalcChecksums(packet, packet_len, 0);
-        }
+        // Calculate checksums.
+        WinDivertHelperCalcChecksums(packet, packet_len,
+            WINDIVERT_HELPER_NO_REPLACE);
        
         // Print info about the matching packet.
         WinDivertHelperParsePacket(packet, packet_len, &ip_header,
diff --git a/examples/passthru/passthru.c b/examples/passthru/passthru.c
index fe52562..7f862d5 100644
--- a/examples/passthru/passthru.c
+++ b/examples/passthru/passthru.c
@@ -107,14 +107,11 @@ static DWORD passthru(LPVOID arg)
             continue;
         }
 
-        // NOTE: Since WinDivert1.2 outbound packets are not guaranteed
-        //       to have correct checksums.
-        if (addr.Direction == WINDIVERT_DIRECTION_OUTBOUND)
-        {
-            WinDivertHelperCalcChecksums(packet, packet_len, 0);
-        }
-
         // Re-inject the matching packet.
+        // NOTE: Only use the WINDIVERT_HELPER_NO_REPLACE flag if the packet
+        //       was not modified.
+        WinDivertHelperCalcChecksums(packet, packet_len,
+            WINDIVERT_HELPER_NO_REPLACE);
         if (!WinDivertSend(handle, packet, packet_len, &addr, NULL))
         {
             fprintf(stderr, "warning: failed to reinject packet (%d)\n",
diff --git a/examples/webfilter/webfilter.c b/examples/webfilter/webfilter.c
index d162ef1..2895cb4 100644
--- a/examples/webfilter/webfilter.c
+++ b/examples/webfilter/webfilter.c
@@ -188,7 +188,8 @@ int __cdecl main(int argc, char **argv)
             !BlackListPayloadMatch(blacklist, payload, (UINT16)payload_len))
         {
             // Packet does not match the blacklist; simply reinject it.
-            WinDivertHelperCalcChecksums(packet, packet_len, 0);
+            WinDivertHelperCalcChecksums(packet, packet_len, 
+                WINDIVERT_HELPER_NO_REPLACE);
             if (!WinDivertSend(handle, packet, packet_len, &addr, NULL))
             {
                 fprintf(stderr, "warning: failed to reinject packet (%d)\n",
diff --git a/include/windivert.h b/include/windivert.h
index 23d327d..0655c82 100644
--- a/include/windivert.h
+++ b/include/windivert.h
@@ -313,6 +313,7 @@ typedef struct
 #define WINDIVERT_HELPER_NO_ICMPV6_CHECKSUM                 4
 #define WINDIVERT_HELPER_NO_TCP_CHECKSUM                    8
 #define WINDIVERT_HELPER_NO_UDP_CHECKSUM                    16
+#define WINDIVERT_HELPER_NO_REPLACE                         2048
 
 /*
  * Parse IPv4/IPv6/ICMP/ICMPv6/TCP/UDP headers from a raw packet.