středa, dubna 01, 2015

Limits of ssh-agent on Mac OS

When executing task using our custom SSH-based tool from some kind of (linux) jump box, I got "lost ssh-agent" error from about 1/3 hosts and the tool could not connect to those hosts.  Found out that this happens only when using ssh-agent from Mac OS machine; it works fine on Ubuntu and Fedora machines. Logs and even running ssh-agent in debug mode don't show anything suspicious. Use of powerful weapons sudo dtruss -p `pgrep ssh-agent` was much more helpful: "accept from AUTH_SOCKET: Too many open files". So let's have look at Mac OS limits... Short version, there are three levels of limits:  ulimit, launchd and sysctl.

$ ulimit -n
  2560
$ launchctl limit maxfiles
  maxfiles    256            unlimited
$ sysctl -a | grep files
  kern.maxfiles: 12288
  kern.maxfilesperproc: 10240
  kern.num_files: 4681

All values looks fine, except launchctl's maxfiles. After increasing to reasonable value using sudo launchctl limit maxfiles 1024 unlimited (and restarting ssh-agent) it finally works! To persists this settings, use for example this:

cat >> EOF | sudo tee /Library/LaunchDaemons/limit.maxfiles.plist
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
  <plist version="1.0">
  <dict>
  <key>Label</key>
  <string>limit.maxfiles</string>
  <key>ProgramArguments</key>
  <array>
    <string>/bin/launchctl</string>
    <string>limit</string>
    <string>maxfiles</string>
    <string>2048</string>
    <string>unlimited</string>
  </array>
  <key>RunAtLoad</key>
  <true/>
  <key>ServiceIPC</key>
  <false/>
</dict>
</plist>
EOF

(use of /etc/launchd.conf doesn't work in 10.10.2 anymore)

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