+# Modify TCP payload without corrupting the TCP packet
+#
+# This rule inverts a 8-bit word at byte offset 148 for the 2nd TCP ACK packets
+# carrying enough data.
+# Once it is done, the TCP Checksum field is updated so the packet is still
+# considered as valid at the TCP level.
+# Because the MPTCP checksum, covering the TCP options and data, has not been
+# updated, the modification will be detected and an MP_FAIL will be emitted:
+# what we want to validate here without corrupting "random" MPTCP options.
+#
+# To avoid having tc producing this pr_info() message for each TCP ACK packets
+# not carrying enough data:
+#
+# tc action pedit offset 162 out of bounds
+#
+# Netfilter is used to mark packets with enough data.
+reset_with_fail()
+{
+ reset "${1}" || return 1
+
+ ip netns exec $ns1 sysctl -q net.mptcp.checksum_enabled=1
+ ip netns exec $ns2 sysctl -q net.mptcp.checksum_enabled=1
+
+ check_invert=1
+ validate_checksum=1
+ local i="$2"
+ local ip="${3:-4}"
+ local tables
+
+ tables="iptables"
+ if [ $ip -eq 6 ]; then
+ tables="ip6tables"
+ fi
+
+ ip netns exec $ns2 $tables \
+ -t mangle \
+ -A OUTPUT \
+ -o ns2eth$i \
+ -p tcp \
+ -m length --length 150:9999 \
+ -m statistic --mode nth --packet 1 --every 99999 \
+ -j MARK --set-mark 42 || exit 1
+
+ tc -n $ns2 qdisc add dev ns2eth$i clsact || exit 1
+ tc -n $ns2 filter add dev ns2eth$i egress \
+ protocol ip prio 1000 \
+ handle 42 fw \
+ action pedit munge offset 148 u8 invert \
+ pipe csum tcp \
+ index 100 || exit 1
+}
+