Wireshark Packet Analysis: Diagnosing Spoofed TCP RST Attack Disruption

“When application logs lie, raw Wireshark PCAP packet captures provide undisputed truth.”

The Setup

In July 2008 during my initial Network Administrator training at HCL Infosystems, we supported enterprise remote access infrastructure protecting sensitive client networks.

Remote employees connected via IPsec and SSL VPN tunnels to access internal file servers and intranet applications.

The Mess

Users reported that active SSH and RDP sessions were abruptly disconnecting every 15 to 20 minutes with generic client errors: Connection reset by peer.

Firewall logs reported normal connection teardowns without security alarms. Server logs indicated clients initiated the teardown, while client logs indicated the server dropped the connection.

Both application teams blamed the network. I connected a mirror port on the core switch and initiated a raw Wireshark PCAP packet capture:

[WIRESHARK PACKET DISSECTION LOG] 2008-07-18 11:05:42.104
No.  Time       Source        Destination   Protocol Length Info
104  12.410200  10.10.4.15    192.168.1.50  TCP      66     443 -> 51200 [ACK] Seq=1420 Ack=501 Win=64240
105  12.410850  192.168.1.50  10.10.4.15    TCP      54     51200 -> 443 [RST] Seq=501 Win=0
106  12.410880  10.10.4.15    192.168.1.50  TCP      54     443 -> 51200 [RST] Seq=1420 Win=0

Deep packet inspection revealed the anomaly: Packet #105 contained a TCP RST flag with an IP Time-To-Live (TTL) of 255, whereas legitimate client packets from 192.168.1.50 arrived with a TTL of 128 (Windows host).

An intermediate misconfigured stateful WAN optimizer was injecting out-of-sequence spoofed TCP RST packets whenever session idle timers reached 900 seconds.

The Solution

We used the Wireshark packet capture analysis to isolate and reconfigure the WAN optimizer:

1. TTL Anomaly Identification: Proved the TCP RST packets originated from the WAN optimizer inline appliance based on Ethernet MAC address and IP TTL discrepancy.
2. TCP State Bypass Configuration: Configured TCP stateful bypass rules on the WAN optimizer for encrypted SSL/IPsec VPN tunnels.
3. Session Keep-Alive Tuning: Reduced TCP keep-alive timers on enterprise gateways from 15 minutes to 5 minutes.

# Cisco ASA Stateful TCP Normalizer Rules
class-map TCP_BYPASS_CMAP
 match access-list VPN_TRAFFIC_ACL
policy-map GLOBAL_POLICY
 class TCP_BYPASS_CMAP
  set connection random-sequence-number disable
  set connection timeout tcp 0:30:00

Key Takeaway

When troubleshooting mystery connection drops, analyze IP TTL fields and TCP sequence numbers in Wireshark. Discrepancies in TTL prove an intermediate appliance or middlebox is actively injecting RST packets into your TCP stream.

Architecture and decisions: mine. Debugging sessions at odd hours: mine. AI assistance: structure, syntax, first draft. — Sachin