diff --git a/tools/testing/selftests/net/forwarding/lib.sh b/tools/testing/selftests/net/forwarding/lib.sh
index 9fe3004af6dc6..733bee34631ad 100755
--- a/tools/testing/selftests/net/forwarding/lib.sh
+++ b/tools/testing/selftests/net/forwarding/lib.sh
@@ -1709,6 +1709,62 @@ ipv6_to_bytes()
 	expand_ipv6 "$IP" :
 }
 
+u16_to_bytes()
+{
+	local u16=$1; shift
+
+	printf "%04x" $u16 | sed 's/^/000/;s/^.*\(..\)\(..\)$/\1:\2/'
+}
+
+# Given a mausezahn-formatted payload (colon-separated bytes given as %02x),
+# possibly with a keyword CHECKSUM stashed where a 16-bit checksum should be,
+# calculate checksum as per RFC 1071, assuming the CHECKSUM field (if any)
+# stands for 00:00.
+payload_template_calc_checksum()
+{
+	local payload=$1; shift
+
+	(
+	    # Set input radix.
+	    echo "16i"
+	    # Push zero for the initial checksum.
+	    echo 0
+
+	    # Pad the payload with a terminating 00: in case we get an odd
+	    # number of bytes.
+	    echo "${payload%:}:00:" |
+		sed 's/CHECKSUM/00:00/g' |
+		tr '[:lower:]' '[:upper:]' |
+		# Add the word to the checksum.
+		sed 's/\(..\):\(..\):/\1\2+\n/g' |
+		# Strip the extra odd byte we pushed if left unconverted.
+		sed 's/\(..\):$//'
+
+	    echo "10000 ~ +"	# Calculate and add carry.
+	    echo "FFFF r - p"	# Bit-flip and print.
+	) |
+	    dc |
+	    tr '[:upper:]' '[:lower:]'
+}
+
+payload_template_expand_checksum()
+{
+	local payload=$1; shift
+	local checksum=$1; shift
+
+	local ckbytes=$(u16_to_bytes $checksum)
+
+	echo "$payload" | sed "s/CHECKSUM/$ckbytes/g"
+}
+
+payload_template_nbytes()
+{
+	local payload=$1; shift
+
+	payload_template_expand_checksum "${payload%:}" 0 |
+		sed 's/:/\n/g' | wc -l
+}
+
 igmpv3_is_in_get()
 {
 	local igmpv3