Other tools


mcdump is a tool similar to tcpdump, but specialised for high-speed capture of multicast UDP traffic using hardware that supports the Infiniband Verbs API. It has only been tested on Mellanox ConnectX-3 NICs. Like gulp, it uses a separate thread for disk I/O and CPU core affinity to achieve reliable performance. With a sufficiently fast disk subsystem, it is able to capture line rate from a 40Gb/s adapter.

It is not limited to capturing SPEAD data. It is included with spead2 rather than released separately because it reuses a lot of the spead2 code.


The tool is automatically compiled and installed with spead2, provided that libibverbs support is detected at configure time.

It may also be necessary to configure the system to work with ibverbs. See Support for ibverbs for more information.


The simplest incantation is

mcdump -i xx.xx.xx.xx output.pcap yy.yy.yy.yy:zzzz

which will capture on the interface with IP address xx.xx.xx.xx, for the multicast group yy.yy.yy.yy on UDP port zzzz. mcdump will take care of subscribing to the multicast group. Note that only IPv4 is supported. Capture continues until interrupted by Ctrl-C. You can also list more group:port pairs, which will all stored in the same pcap file.

You can also specify - in place of the filename to suppress the write to file. This is useful to simply count the bytes/packets received without being limited by disk throughput.

Unfortunately, unlike tcpdump, it is not possible to directly tell whether packets were dropped. NIC counters (on Linux, accessed with ethtool -S) can give an indication, although sometimes packets are dropped during the shutdown process.

These options are important for performance:

-N <cpu>, -C <cpu>, -D <cpu>

Set CPU core IDs for various threads. The -D option can be repeated multiple times to use multiple threads for disk I/O. By default, the threads are not bound to any particular core. It is recommended that these cores be on the same CPU socket as the NIC.


Use the O_DIRECT flag to open the file. This bypasses the kernel page cache, and can in some cases yield higher performance. However, not all filesystems support it, and it can also reduce performance when capturing a small enough amount of data that it will fit into RAM.

--count <count>

Stop after <count> packets have been received. Without this option, mcdump will run until SIGINT (Ctrl-C) is received.


  • Timestamps are only collected if Mellanox extensions to the verbs API are detected at compile time. Otherwise, all packets have a zero timestamp in the file.
  • Only IPv4 multicast is supported.
  • It is not optimised for small packets (below about 1KB). Packet capture rates top out around 6Mpps for current hardware.