How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP Challenge ACK Drops in Linux Using eBPF

High-throughput Linux production systems handling thousands of concurrent inbound connections can experience mysterious connection delays when SYN-ACK retransmissions coincide with TCP challenge ACK rate-limiting. When an unexpected packet sequence or out-of-window segment arrives, RFC 5961 mandates the transmission of challenge ACKs. If Linux exceeds its global rate limit configured in net.ipv4.tcp_challenge_ack_limit, the kernel drops these packets silently, causing tcpi_retrans_synack counters to escalate. Extended Berkeley Packet Filters (eBPF) provide the precise, non-intrusive observability required to isolate these kernel drops in real time.

Kernel Mechanics of Challenge ACK Throttling

Under default kernel parameters, the Linux network stack enforces a maximum burst of challenge ACKs per second to mitigate off-path blind reset and data injection attacks. When an incoming SYN or connection acknowledgement violates window validation during a connection handshake, the kernel calls tcp_send_challenge_ack(). Inside this function, a token bucket algorithm checks current challenge ACK generation against the global ceiling.

When the rate limit is exceeded, challenge ACKs are discarded. Consequently, the remote endpoint fails to receive handshakes or resets promptly, forcing the local socket into exponential SYN-ACK backoff visible in the struct tcp_info field tcpi_retrans_synack. Traditional tools like netstat or ss -tie show cumulative socket counts but cannot correlate drops with specific processes or network paths.

Authoring an eBPF Probe with bpftrace

To capture challenge ACK generation and throttling events directly inside the kernel, construct a targeted bpftrace script attached to the kernel function tcp_send_challenge_ack and the TCP drop tracepoint:

#!/usr/bin/env bpftrace
/* trace_challenge_ack_drops.bt - Trace Linux TCP challenge ACK drops */

#include <net/sock.h>
#include <linux/tcp.h>

kprobe:tcp_send_challenge_ack
{
    $sk = (struct sock *)arg0;
    $inet = (struct inet_sock *)$sk;
    $saddr = ntop($sk->__sk_common.skc_rcv_saddr);
    $daddr = ntop($sk->__sk_common.skc_daddr);
    $dport = $sk->__sk_common.skc_dport;
    $dport = ($dport >> 8) | (($dport & 0xff) << 8);

    @challenge_requests[$saddr, $daddr, $dport] = count();
}

kprobe:tcp_v4_send_reset,
kprobe:tcp_v6_send_reset
{
    @reset_triggers[probe] = count();
}

tracepoint:tcp:tcp_retransmit_synack
{
    $sk = (struct sock *)args->skaddr;
    $saddr = ntop(args->saddr);
    $daddr = ntop(args->daddr);

    printf("SYN-ACK Retransmit: %s -> %s (Total retrans counter incremented)
",
           $saddr, $daddr);
    @synack_retransmits[$saddr, $daddr] = count();
}

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP Challenge ACK & SYN-ACK Activity (5s Window) ---
");
    print(@challenge_requests);
    print(@synack_retransmits);
    clear(@challenge_requests);
    clear(@synack_retransmits);
}

Deploying and Executing the Kernel Trace

Execute the script on your target Linux system with root privileges. Ensure that kernel headers and the bpftrace package are installed:

# Verify BPF capabilities and install bpftrace on Debian/Ubuntu
sudo apt-get update && sudo apt-get install -y bpftrace linux-headers-$(uname -r)

# Run the eBPF trace script
sudo bpftrace trace_challenge_ack_drops.bt

While the trace is running, simulate or monitor production inbound connection attempts. If challenge ACK bursts exceed the kernel limit, you will observe rising @challenge_requests without matching outgoing segment activity in your packet captures, accompanied by SYN-ACK Retransmit tracepoint logs.

Analysing Global Drop Metrics and Kernel Tuning

To verify if the kernel dropped challenge ACKs at the system level during your observation window, cross-examine Linux SNMP network statistics:

# Inspect kernel TCP challenge ACK counters
nstat -az TcpExtTCPChallengeACK
nstat -az TcpExtTCPSYNChallenge

# Check current kernel challenge ACK rate limit
sysctl net.ipv4.tcp_challenge_ack_limit

If TcpExtTCPChallengeACK increments rapidly while tcpi_retrans_synack spikes on critical services, adjust the limit dynamically to accommodate legitimate high-density handshake validation:

# Temporarily increase challenge ACK limit to 5000 per second
sudo sysctl -w net.ipv4.tcp_challenge_ack_limit=5000

# Persist configuration in sysctl.d
echo "net.ipv4.tcp_challenge_ack_limit = 5000" | sudo tee /etc/sysctl.d/99-tcp-challenge-ack.conf
sudo sysctl --system

By pairing eBPF kernel instrumentation with calibrated sysctl adjustments, network engineers can eliminate elusive SYN-ACK stalls while preserving essential protection against off-path spoofing attacks.