Nftables Connection Tracking Issue: Counter Triggering Incorrectly

Oct 22, 2025 by Dimemap Team 67 views

Hey guys, having some weirdness with my nftables setup and hoping someone can shed some light on it! I've set up a counter that's supposed to trigger when the connection count goes over 2, but it seems to be firing even when I only have one connection active. I've been using conntrack -L to peek at the connections and their states, and things just aren't adding up. My rules should be working, but they aren't behaving as they should. Let's dive deeper into the specifics and see if we can figure out what's going on.

Understanding Nftables and Connection Tracking

Before we get into the nitty-gritty of my specific issue, let's quickly recap what Nftables and connection tracking are all about. Think of Nftables as the modern evolution of the classic iptables firewall. It's a powerful framework within the Linux kernel for filtering and manipulating network packets. The key here is its flexibility and efficiency. Nftables allows you to define rulesets using a more streamlined syntax and data structures, which can lead to better performance, especially in complex network setups.

Connection tracking, often abbreviated as ct, is a crucial component of stateful firewalls like Nftables. It's the mechanism that allows the firewall to remember details about network connections, such as their source and destination addresses and ports, the protocol being used (TCP, UDP, etc.), and the state of the connection (new, established, related, etc.). This memory enables the firewall to make intelligent decisions about packets based on the context of the connection they belong to. For example, instead of just blindly allowing or blocking packets based on IP addresses and ports, a stateful firewall can allow packets that are part of an already established connection, even if they wouldn't normally be allowed based on the firewall's basic rules. This capability is essential for many common network services, like FTP or SIP, which use multiple connections or dynamically negotiated ports.

The connection tracking system maintains a table – the connection tracking table – where it stores information about each active connection. When a new packet arrives, the firewall consults this table to see if the packet belongs to an existing connection. If it does, the firewall can apply the appropriate rules based on the connection's state. If it's a new connection, the firewall creates a new entry in the table and applies the rules for new connections.

So, in my case, I'm using Nftables' connection tracking capabilities to count the number of connections and trigger a counter when it exceeds a certain threshold. The idea is to limit the number of concurrent connections to protect my server from potential abuse. But, as I mentioned earlier, the counter seems to be triggering prematurely, even when I'm sure the actual connection count is lower than the limit. This is where things get interesting, and where I need your help to debug!

Diving into the Problem: Counter Triggering Issues

Okay, so let's break down the specific issue I'm facing. I've set up an Nftables rule that utilizes the connection tracking (ct) functionality to monitor the number of active connections. The goal is to trigger a counter when the number of connections exceeds 2. The problem is, the counter seems to be triggering even when there's only 1 connection established, which is definitely not the intended behavior.

I've been using the conntrack -L command to inspect the active connections and their states. This command provides a detailed view of the connection tracking table, showing information like the source and destination IP addresses and ports, the protocol being used, and the connection state. When I run this command, I can clearly see that there's only one connection active, yet the counter in Nftables is still being incremented beyond my defined limit. This discrepancy between what conntrack -L shows and what the Nftables counter is reporting is what's causing the confusion.

Here's a potential scenario to illustrate the issue. Let's say I open a single SSH connection to my server. I would expect conntrack -L to show one established connection, and the Nftables counter should reflect this. However, what I'm observing is that the counter quickly jumps to 3 or higher, even though only one SSH connection is active. This suggests that something is either misconfigured in my Nftables rules or there might be an underlying issue with how connection tracking is behaving.

This issue is particularly concerning because it could lead to false positives. My intention is to use this counter to implement rate limiting or other security measures. If the counter is triggering incorrectly, it could block legitimate connections, which would obviously be a major problem. So, it's crucial to get to the bottom of this and understand why the counter is behaving this way.

To further investigate this, I've started by carefully examining my Nftables rules to see if there are any logical errors or misconfigurations. I've also been researching potential issues with connection tracking itself, such as table size limits or timeouts. I'll share my Nftables rules in the next section so you guys can take a look and offer your insights.

Examining the Nftables Rules

Alright, let's get down to the code! I'm going to share the relevant parts of my Nftables configuration so you can get a better understanding of how I've set things up. This will hopefully make it easier to spot any potential issues or misconfigurations that might be causing the counter to trigger incorrectly. Remember, the goal is to have the counter accurately reflect the number of active connections and only trigger when that number exceeds 2.

# This is a simplified example, your actual rules might be more complex
table inet filter {
 chain input {
  ct state new, untracked counter name input_new_connections
  ct count over 2 counter name input_limit_exceeded
  # ... other rules ...
 }
}

This snippet shows a simplified version of my Nftables ruleset. The key parts to focus on are the ct state and ct count expressions.

ct state new, untracked counter name input_new_connections: This rule is intended to count new and untracked connections. It increments the input_new_connections counter for each new connection attempt.
ct count over 2 counter name input_limit_exceeded: This is the rule that's causing the problem. It's supposed to trigger the input_limit_exceeded counter when the connection count, as tracked by the connection tracking system, goes over 2. However, as we've discussed, this counter seems to be triggering even when there's only one active connection.

Now, you might be thinking, "Hey, maybe the problem is in the order of the rules!" And that's a valid point. Rule order in Nftables (and iptables before it) is crucial. Rules are processed sequentially, and the first rule that matches a packet determines the action. However, in this simplified example, the order shouldn't be the primary cause of the issue, but we should definitely consider the implications of rule order in more complex scenarios.

Another thing to consider is the scope of the ct count expression. It's important to understand what exactly Nftables is counting when it evaluates this expression. Is it counting all connections regardless of their state? Is it considering connections that are in a transient state, like those in the process of being established or closed? These are the kinds of questions we need to answer to fully understand what's going on.

I'm also wondering if there might be some interaction between this rule and other rules in my ruleset that I haven't included in this simplified example. It's possible that some other rule is inadvertently affecting the connection count or the state of connections, leading to the incorrect counter triggering. This highlights the importance of carefully considering the interactions between different rules when designing your Nftables configuration.

So, what do you guys think? Do you see any obvious issues with these rules? Are there any other aspects of my configuration that I should be sharing to help diagnose this problem? Let's brainstorm and see if we can figure this out!

Potential Causes and Troubleshooting Steps

Let's put our detective hats on and explore some potential causes for this counter misbehavior. We need to systematically investigate the various factors that could be contributing to the problem. This involves not only scrutinizing the Nftables rules themselves but also digging into the underlying connection tracking mechanisms and the overall network environment.

One of the first things that comes to mind is the scope of the ct count expression. As I mentioned earlier, it's crucial to understand exactly what Nftables is counting when it evaluates this expression. Is it counting all connections regardless of their state? Is it including connections that are in a transient state, such as those that are in the process of being established or torn down? If the ct count expression is too broad, it might be including connections that we wouldn't normally consider as "active," leading to the counter triggering prematurely.

To narrow down the scope, we could try adding more specific criteria to the ct count expression. For example, we could try filtering by connection state, such as only counting connections that are in the ESTABLISHED state. This would exclude connections that are in the process of being established or closed, which might be contributing to the inflated count. We can achieve this by adding a ct state established condition before the ct count condition in the rule.

Another potential cause could be related to connection tracking timeouts. The connection tracking system has timeouts for different connection states. If a connection remains idle for a certain period, it's removed from the connection tracking table. However, if these timeouts are not configured correctly, it's possible that connections are being prematurely removed from the table or, conversely, lingering for too long. This could lead to inconsistencies between the actual number of active connections and the count maintained by the connection tracking system.

To investigate this, we can examine the connection tracking timeout settings using the sysctl command. The relevant settings are typically located under the net.netfilter namespace. We can also use the conntrack -L command to observe the state and timeouts of individual connections. This might give us clues as to whether timeouts are playing a role in the issue.

Furthermore, we need to consider the possibility of network address translation (NAT) interfering with connection tracking. If NAT is being used, it can complicate the connection tracking process, as multiple internal connections might appear to originate from the same external IP address and port. This could potentially lead to the ct count expression miscounting connections. We need to carefully examine our NAT configuration and ensure that it's not conflicting with our Nftables rules.

Finally, let's not forget the basics of troubleshooting: logging and debugging. We can add logging rules to our Nftables configuration to get more insights into which packets are triggering the ct count expression. This can help us pinpoint the specific connections that are causing the issue. We can also use tools like tcpdump or wireshark to capture network traffic and analyze the packets in detail. This can be invaluable for understanding the flow of traffic and identifying any anomalies.

Next Steps and Seeking Your Expertise

So, where do we go from here? I've outlined a few potential causes and troubleshooting steps, but I'm still feeling a bit stuck. I'm hoping that by sharing this with you guys, we can collectively brainstorm and come up with a solution. Your experience and insights are incredibly valuable, and I'm eager to hear your thoughts.

Here are some specific questions I have for you:

Have you encountered similar issues with Nftables connection tracking counters before? If so, what were the causes and how did you resolve them?
Do you see any potential problems with my Nftables rules that I might have overlooked?
Are there any specific troubleshooting steps or tools that you would recommend?
What are your thoughts on the potential role of connection tracking timeouts or NAT in this issue?

I'm committed to getting to the bottom of this, and I believe that by working together, we can crack this case. I'll be actively monitoring this discussion and responding to your suggestions. Please don't hesitate to share your ideas, even if they seem far-fetched. Sometimes, the most unconventional ideas lead to the biggest breakthroughs.

Let's get this counter counting correctly!