Add documentation for GH-1829

This adds documentation that clarifies that the `ignore_checksums`
option now also allows IPv4 packets with a length of 0.

NEWS

@@ -53,6 +53,13 @@ Changed Functionality
 - The ``SYN_packet`` record now records TCP timestamps (TSval/TSecr)
   when available.
 
+- The ``ignore_checksums`` options and the ``-C`` command-line option now additionally cause
+  Zeek to accept IPv4 packets that provide a length of zero in the total-length IPv4 header
+  field. When the length is set to zero, the capture length of the packet is used instead.
+  This can be used to replay traces, or analyze traffic when TCP sequence offloading is enabled
+  on the local NIC - which typically causes the total-length of affected packets to be set to
+	zero.
+
 Removed Functionality
 ---------------------
 

man/zeek.8

@@ -66,7 +66,7 @@ print version and exit
 print contents of state file
 .TP
 \fB\-C\fR,\ \-\-no\-checksums
-ignore checksums
+When this option is set, Zeek ignores invalid packet checksums and does process the packets. Furthermore, if this option is set Zeek also processes IP packets with a zero total length field, which is typically caused by TCP (TCP Segment Offloading) on the NIC.
 .TP
 \fB\-F\fR,\ \-\-force\-dns
 force DNS

scripts/base/init-bare.zeek

@@ -1016,9 +1016,16 @@ const TCP_RESET = 6;	##< Endpoint has sent RST.
 const UDP_INACTIVE = 0;	##< Endpoint is still inactive.
 const UDP_ACTIVE = 1;	##< Endpoint has sent something.
 
-## If true, don't verify checksums.  Useful for running on altered trace
+## If true, don't verify checksums, and accept packets that give a length of
-## files, and for saving a few cycles, but at the risk of analyzing invalid
+## zero in the IPv4 header. This is useful when running against traces of local
-## data. Note that the ``-C`` command-line option overrides the setting of this
+## traffic and the NIC checksum offloading feature is enabled. It can also
+## be useful for running on altered trace files, and for saving a few cycles
+## at the risk of analyzing invalid data.
+## With this option, packets that have a value of zero in the total-length field
+## of the IPv4 header are also accepted, and the capture-length is used instead.
+## The total-length field is commonly set to zero when the NIC sequence offloading
+## feature is enabled.
+## Note that the ``-C`` command-line option overrides the setting of this
 ## variable.
 const ignore_checksums = F &redef;