Added note about system TCP interference

Includes a script to filter interfering TCP segments using nftables.

tcpreq-nft.conf

@@ -0,0 +1,12 @@
+# Delete afterwards with: nft delete table inet tcpreq
+# The kernel-out chain filters all TCP segments from the ports
+# used by tcpreq unless the socket was created by the tcpreq user
+# Whitelisting is necessary because the kernel skips meta skuid checks
+table inet tcpreq {
+    chain kernel-out {
+        type filter hook output priority 10; policy drop;
+        meta skuid tcpreq accept
+        meta l4proto != tcp accept
+        tcp sport < 49152 accept
+    }
+}

tcpreq/__main__.py

@@ -59,6 +59,8 @@ def _process_results(test: Type[BaseTest], targets: Sequence[Tuple[AnyIPAddress,
             print(out)
 
 
+# Make sure to prevent the kernel TCP stack from interfering
+# See e.g. tcpreq-nft.conf for an nfttables script
 def main() -> None:
     args = parser.parse_args()
     loop = asyncio.SelectorEventLoop()