Fix default reject rule to correctly handle packets that should be reassembled

When using an L7 NetworkPolicy that allows egress HTTP requests, the corresponding
Suricata rules may look like the following example:

```
reject ip any any -> any any (msg: "Reject by AntreaNetworkPolicy:default/egress-allow-http"; flow: to_server, established; sid: 1;)
pass http any any -> any any (msg: "Allow http by AntreaNetworkPolicy:default/egress-allow-http"; http.method; content:"GET"; sid: 2;)`
```

If an HTTP request exceeds the MTU, it will be split into multiple packets. The packets
should be reassembled and allowed by the corresponding Suricata rule for the L7
NetworkPolicy.

However, there is a default reject rule which is to reject packets which are not matched
by the `pass` rule, which will take effect before packets are reassembled and matched by
the `pass` rule, causing the connection to fail.

To address the issue, the keyword `only_stream` is added to the default reject rule. This
ensures that only reassembled packets are matched, preventing premature rejection of
packets that should be allowed after reassembly.

Signed-off-by: Hongliang Liu <hongliang.liu@broadcom.com>

Author: hongliangl

pkg/agent/controller/networkpolicy/l7engine/reconciler.go

@@ -173,8 +173,10 @@ func generateTenantRulesData(policyName string, protoKeywords map[string]sets.Se
 	rulesData := bytes.NewBuffer(nil)
 	sid := 1
 
-	// Generate default reject rule.
-	allKeywords := fmt.Sprintf(`msg: "Reject by %s"; flow: to_server, established; sid: %d;`, policyName, sid)
+	// Generate default reject rule. The keyword `only_stream` is used to match on packets that have been reassembled by
+	// the Suricata stream engine. Without this keyword, reassembled packets, such as those from HTTP requests split
+	// across multiple packets, would be rejected by the default rule, causing an otherwise allowed connection to fail.
+	allKeywords := fmt.Sprintf(`msg: "Reject by %s"; flow: to_server, established, only_stream; sid: %d;`, policyName, sid)
 	rule := fmt.Sprintf("reject ip any any -> any any (%s)\n", allKeywords)
 	rulesData.WriteString(rule)
 	sid++