NAME

tcpdrop.bt - Trace kernel-based TCP packet drops with details. Uses Linux bpftrace/eBPF

SYNOPSIS

tcpdrop.bt

DESCRIPTION

This tool traces TCP packets or segments that were dropped by the kernel, and shows details from the IP and TCP headers, the socket state, and the kernel stack trace. This is useful for debugging cases of high kernel drops, which can cause timer-based retransmits and performance issues.

This tool works using dynamic tracing of the tcp_drop() kernel function, which requires a recent kernel version.

This tool is limited to ipv4, and cannot parse tcpflags as bpftrace currently cannot parse socket buffers in the way that bcc can.

Since this uses BPF, only the root user can use this tool.

REQUIREMENTS

CONFIG_BPF and bpftrace.

EXAMPLES

Trace all tcp drops:: # tcpdrop.bt

FIELDS

TIME: Time of the call, in HH:MM:SS format.
PID: Process ID that was on-CPU during the drop. This may be unrelated, as drops can occur on the receive interrupt and be unrelated to the PID that was interrupted.
COMM: Process name
SADDR: Source IP address.
SPORT: Source TCP port.
DADDR: Destination IP address.
DPORT: Destionation TCP port.
STATE: TCP session state ("ESTABLISHED", etc).

OVERHEAD

This traces the kernel tcp_drop() function, which should be low frequency, and therefore the overhead of this tool should be negligible.

As always, test and understand this tools overhead for your types of workloads before production use.

SOURCE

This is from bpftrace

: https://github.com/iovisor/bpftrace

Also look in the bpftrace distribution for a companion _examples.txt file containing example usage, output, and commentary for this tool.

This is a bpftrace version of the bcc tool of the same name. The bcc tool may provide more options and customizations.

: https://github.com/iovisor/bcc

OS

Linux

STABILITY

Unstable - in development.

AUTHOR

Brendan Gregg, adapted for bpftrace by Dale Hamel