dormando edited this page Feb 27, 2018 · 1 revision

Memcached 1.5.6 Release Notes

Date: 2018-2-27



This is a bugfix release, but it primarily disables the UDP protocol by default.

In the last few days reports of UDP amplification attacks utilizing inesure memcached instances have surfaced. Attackers are able to set large values into memcached, then send requests via spoofed UDP packets. Memcached will then send a very large number of very large UDP packets back in response.

12 years ago, the UDP version of the protocol had more widespread use: TCP overhead could be very high. In the last few years, I've not heard of anyone using UDP anymore. Proxies and special clients allow connection reuse, which lowers the overhead. Also, RAM values are so large that TCP buffers just don't add up as much as they used to.

That said, I don't have any way of knowing how many UDP installations there are. Everyone who uses UDP and upgrades past this version, will find the UDP protocol disabled unless they explicitly enable it via -U 11211. Hopefully this one-time pain is acceptable.

Thanks for everyone who reached out in the last couple days to help understand the problem and coordinate patches sent to linux and BSD distro's.


  • disable UDP port by default
  • systemd instancing support & rpm build improvements
  • fix gcc warnings in beta GCC
  • fix build with clang
  • fix for dtrace compilation on freebsd

New Features

The RPM specfiles have been greatly improved for systemd installations, see: for full detail.


The following people contributed to this release since 1.5.5.

Note that this is based on who contributed changes, not how they were done. In many cases, a code snippet on the mailing list or a bug report ended up as a commit with your name on it.

Note that this is just a summary of how many changes each person made which doesn't necessarily reflect how significant each change was. For details on what led up into a branch, either grab the git repo and look at the output of git log 1.5.5..1.5.6 or use a web view.

     4	dormando
     1	David Carlier
     1	J. Grizzard
     1	Manish Katiyar
     1	Miroslav Lichvar
     1	Qian Li

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