TensorRT inference API server unreachable in staging after firewall update

Problem

After a scheduled firewall rule update, the TensorRT inference API server in the staging environment stopped accepting TCP connections on port 8000. All client services that invoke /v1/predict receive a connection timed out error, while the same request works fine in the production environment.

Typical client error:

requests.exceptions.ConnectTimeout: HTTPSConnectionPool(host='staging-infer.example.com', port=8000): Max retries exceeded with url: /v1/predict (Caused by ConnectTimeoutError(, 'Connection to staging-infer.example.com timed out. (connect timeout=5)'))

Root Cause

The firewall update introduced a new rule that inadvertently blocked inbound traffic to the 10.42.3.0/24 subnet on TCP port 8000. The rule was intended to restrict access to the 8080 management port, but the CIDR mask was mis‑typed as 10.42.3.0/28, which overlaps the inference server’s IP range and drops packets before they reach the TensorRT process.

Key observations:

• Network trace on the staging host shows SYN packets being sent but no SYN‑ACK received.
• Firewall logs (/var/log/ufw.log) contain entries like:

Oct 10 14:23:11 firewall kernel: [UFW BLOCK] IN=eth0 OUT= MAC=aa:bb:cc:dd:ee:ff SRC=203.0.113.45 DST=10.42.3.12 PROTO=TCP SPT=54321 DPT=8000

Because the rule was applied at the DROP chain before the allow rule for the inference service, the packets never reach the server process.

Debug

1. Verify connectivity from a client pod

# From a staging client container
nc -zv staging-infer.example.com 8000
Connection timed out

2. Check server listening socket

# On the inference host
ss -ltnp | grep 8000
LISTEN 0      128          0.0.0.0:8000          0.0.0.0:*    users:(("trt_server",pid=3124,fd=5))

The server is listening on all interfaces, confirming the issue is not local binding.

3. Inspect firewall rules

# On the firewall VM (using iptables)
iptables -S INPUT | grep 8000
-A INPUT -p tcp -s 10.42.3.0/28 --dport 8000 -j DROP
-A INPUT -p tcp -s 10.42.3.0/24 --dport 8000 -j ACCEPT

The -A INPUT -s 10.42.3.0/28 --dport 8000 -j DROP rule precedes the allow rule, causing the block.

4. Capture packets on the server

# tcpdump on the inference host
tcpdump -i eth0 tcp port 8000 -nn -vv
14:23:45.123456 IP 203.0.113.45.54321 > 10.42.3.12.8000: Flags [S], seq 123456789, win 29200, length 0

No SYN‑ACK is observed, confirming the packet is dropped before reaching the kernel TCP stack.

Solution

Correct the firewall rule order and CIDR mask so that the allow rule for the inference subnet is evaluated before any generic drop rule.

Before

# iptables-save (excerpt)
*filter
:INPUT ACCEPT [0:0]
-A INPUT -p tcp -s 10.42.3.0/28 --dport 8000 -j DROP
-A INPUT -p tcp -s 10.42.3.0/24 --dport 8000 -j ACCEPT
COMMIT

After

# iptables-save (excerpt) – corrected
*filter
:INPUT ACCEPT [0:0]
-A INPUT -p tcp -s 10.42.3.0/24 --dport 8000 -j ACCEPT
-A INPUT -p tcp -s 10.42.3.0/28 --dport 8000 -j DROP   # optional, kept for other sub‑ranges
COMMIT

Steps performed:

1. Delete the offending rule:

iptables -D INPUT -p tcp -s 10.42.3.0/28 --dport 8000 -j DROP

2. Insert the correct allow rule at the top of the INPUT chain:

iptables -I INPUT 1 -p tcp -s 10.42.3.0/24 --dport 8000 -j ACCEPT

3. Persist the updated rules (example for iptables-persistent on Ubuntu):

netfilter-persistent save

After the change, the firewall now permits traffic from the entire staging subnet to the inference port.

Verify

1. Re‑run client connectivity test

# From a staging client container
nc -zv staging-infer.example.com 8000
Connection to staging-infer.example.com 8000 port [tcp/*] succeeded!

2. Confirm server logs show successful connections

# On the inference host
journalctl -u trt_server -f | grep "Incoming request"
2026-10-10 14:25:01,123 INFO  [trt_server] Incoming request from 203.0.113.45:54321

3. Health‑check endpoint

curl -s http://staging-infer.example.com:8000/healthz
{"status":"ok","uptime_seconds":342}

Prevent

• Rule Auditing: Enforce a CI/CD gate that runs iptables -S diff against a baseline and fails if a DROP rule precedes an intended ACCEPT for the same port/subnet.
• Change Management: Tag firewall changes with the affected services and require a post‑deployment connectivity test script that pings all known API endpoints.
• Monitoring: Add a Prometheus blackbox exporter probe for tcp://staging-infer.example.com:8000 with alerts on failure > 30 seconds.
• Documentation: Keep a version‑controlled firewall rule matrix that maps service ports to allowed CIDR blocks, reducing manual entry errors.

Related Topic Hub: Model Serving Troubleshooting Hub

FAQ

1. Why does the production environment still work?
   Production traffic originates from a different CIDR (10.42.1.0/24) that is not covered by the erroneous /28 drop rule, so its packets bypass the block.
2. Can I use ufw instead of raw iptables?
   Yes. With ufw, the equivalent fix is:

# ufw delete deny from 10.42.3.0/28 to any port 8000
# ufw allow from 10.42.3.0/24 to any port 8000

3. What if the inference server runs behind a load balancer?
   Ensure the load balancer’s health‑check IP range is also included in the allow rule, otherwise the LB will mark the backend as unhealthy.
4. Is there a way to automatically rollback firewall changes?
   Deploy firewall rules via an immutable configuration management tool (e.g., Terraform). If terraform apply fails health checks, the CI pipeline can automatically revert to the previous state.
5. How do I differentiate between a firewall block and an application crash?
   A firewall block shows SYN packets without SYN‑ACK in tcpdump and appears as “DROP” entries in firewall logs. An application crash would result in a RST packet or “connection refused” from the OS.