“When BGP sessions tear down without interface errors, don’t guess—capture raw pcap traces. Wireshark reveals TCP sequence numbers, TTL deltas, and forged RST flags instantly.”

While inspecting core ISP backbone links at Spectranet in 2010, an enterprise client reported intermittent 5-minute outages on their dedicated Internet leased line.

Physical interface statistics showed 0 CRC errors, but internal BGP neighbor sessions flapped unpredictably 3 to 4 times a day.

Methodology: Wireshark Wireshark Capture & TTL Analysis

We mirrored the physical transit port and captured raw .pcap packet streams using Wireshark / tcpdump.

Filtering for tcp.flags.reset == 1, we observed incoming TCP RST packets targeting port 179 (BGP). Crucially, the Time-To-Live (TTL) value on valid BGP packets was 254 (direct hop), whereas the unexpected TCP RST packets arrived with a TTL of 52, proving they originated from an external spoofing source outside the local transit segment.

[!IMPORTANT] Enable BGP GTSM (Generalized TTL Security Mechanism - RFC 5082) on eBGP peer sessions to force the router to drop any incoming BGP packet with a TTL value less than 254.

# # Tcpdump Packet Capture Command on Core Router Transit Interface
tcpdump -i eth0 -n -s 0 -w /tmp/bgp_flap_debug.pcap "tcp port 179"

# # Cisco IOS Command to Enable BGP GTSM (TTL Security)
router bgp 65000
 neighbor 192.168.1.1 ttl-security hops 1

The Verdict

Key Takeaway

Enable BGP GTSM TTL Security Mechanism to Protect Border Routers from Forged TCP RST Attacks.

Analyzing Wireshark pcap traces and enforcing GTSM TTL filtering protects core BGP router peering sessions from spoofed packet tearing attacks.