[Bug] tcprewrite provides incorrect checksum for certain ipv4 packets

[ChuckCottrill]

The tcprewrite program changes packet length to an undesirable (incorrect) value.

Describe the bug
TCP rewrite appears to change packet length incorrectly, due to incorrect handling of minimum length packets;
this could cause incorrect checksum (which could be interpreted as a spoofing attempt and discard packet).

Expected behavior:
TCP rewrite should correctly change packet length (more to follow).

To Reproduce

Steps to reproduce the behavior:

1. uncompress packet captures:

mkdir -p pcaps
unzip tcprewrite-pcaps.zip
cp tcprewrite-pcaps/pcap-original-packet-3.pcap pcaps/.

2. Run tcprewrite version 4.4.0 and observe the output, as follows

# version 4.4.0
VERSION="4.4.0"
# prepare
tcpreplay-4.4.0/src/tcpprep \
    --cidr=0.0.0.0/0 \
    --pcap=pcaps/pcap-original-packet-3.pcap \
    --cachefile=pcaps/pcap.cache
# use tcprewrite to rewrite packet addresses
tcpreplay-4.4.0/src/tcprewrite \
    --cachefile=pcaps/pcap.cache \
    --infile=pcaps/pcap-original-packet-3.pcap \
    --outfile=pcaps/cap-4.4.0-packet-out.pcap \
    --endpoints=10.200.1.1:10.200.1.2

3. Run tcprewrite version 4.4.1 and observe the output, as follows

# version 4.4.1
# prepare
tcpreplay-4.4.1/src/tcpprep \
    --cidr=0.0.0.0/0 \
    --pcap=pcaps/pcap-original-packet-3.pcap \
    --cachefile=pcaps/pcap.cache
# use tcprewrite to rewrite packet addresses
tcpreplay-4.4.1/src/tcprewrite \
    --cachefile=pcaps/pcap.cache \
    --infile=pcaps/pcap-original-packet-3.pcap \
    --outfile=pcaps/cap-4.4.1-packet-out.pcap \
    --endpoints=10.200.1.1:10.200.1.2

4. compare files, should be identical

bdiff pcaps/cap-4.4.0-packet-out.pcap pcaps/cap-4.4.1-packet-out.pcap

Packet Captures

Packet Captures to Reproduce:

• pcap-original-packet-3.pcap
• pcap-4.4.0-packet-3.pcap
• pcap-4.4.1-packet-3.pcap

Examine packets

Use Wireshark to examine and compare both packets.

• Note that the ver 4.4.1 reports incorrect checksum.
• Note also that packet length was changed, which is different behavior from desired.
• Perhaps a flag to specify whether length change is needed or desired?

Screenshots
N/A - use Wireshark to view packets

System (please complete the following information):

• OS: Linux
• OS version
  • Linux hostname 5.15.0-71-generic #78-Ubuntu SMP datetime x86_64 x86_64 x86_64 GNU/Linux
• Tcpreplay Version [4.4.1] versus [4.4.0]

Additional context
The changed length results in an erroneous checksum which results in dropped packet(s).