dropwatch - Man Page

kernel dropped packet monitoring utility


dropwatch [-l method | list]


dropwatch is an interactive utility for monitoring and recording packets that are dropped by the kernel.


-l method | list, --lmethod method | list

Select the translation method to use when a drop alert arrives.  By default the raw instruction pointer of a drop location is output, but by the use of the -l option, we can assign a translation method so that the instruction pointer can be translated into function names.  Currently supported lookup methods are:


Show all supported methods.


Use /proc/kallsyms to lookup instruction pointers to function mappings.

Interactive Commands


Tells the kernel to start reporting dropped packets.


Tells the kernel to discontinue reporting dropped packets.


Exits the dropmonitor program.


Displays summary of all commands.

set alertlimit value

Sets a triggerpoint to stop monitoring for dropped packets after value alerts have been received.

set alertmode { summary | packet }

summary - Default mode. A summary of recent packet drops is sent to user space. The alert includes drop locations and number of drops in each location.

packet - Each dropped packet is sent to user space. The alert includes the packet itself and various metadata such as drop location and timestamp.

set trunc len

Sets the truncation length. Reported packets will be truncated to length len. This setting is only applicable when alertmode is set to packet. By default packets are not truncated. A value of 0 disables truncation.

set queue len

Sets the queue length in the kernel. When alertmode is set to packet, the kernel queues dropped packets in a per-CPU drop list before preparing a netlink message for each. This setting controls the queue's length. By default this is limited by the kernel to 1,000 packets. Increasing this value will increase the memory utilization of the system.

set sw { true | false }

Enables or disables the monitoring of software originated drops. By default software originated drops are monitored.

set hw { true | false }

Enables or disables the monitoring of hardware originated drops. By default hardware originated drops are not monitored.


Query existing configuration from the kernel and show it.


Query statistics from the kernel and show it.

Tail dropped - Number of packets that could not be enqueued to the per-CPU drop list(s).

Referenced By


Mar 2009 Neil Horman