Adding section on OpenBSD to doc/TUNING

  o Documentation:

    - Adding section on OpenBSD to our TUNING document. Thanks to

      mmcc for writing the OpenBSD-specific tips. Resolves ticket

      13702.

kern.ipc.maxsockets is determined dynamically by the system and thus is 

read-only on OS X.

OpenBSD

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For recent versions of OpenBSD (5.5 and 5.6, and probably older releases

as well), the maximum number of file descriptors that can be opened is

7030:

http://unix.stackexchange.com/questions/104929/does-openbsd-have-a-limit-to-the-number-of-file-descriptors/104948#104948

The maximum number of file descriptors that an OpenBSD machine can have

open is stored in the sysctl variable kern.maxfiles. This value defaults

to 7030 - to verify this, run sysctl kern.maxfiles.

To immediately change a running system's file descriptor limit to, for

example, 20,000 files, run sudo sysctl kern.maxfiles=20000. All sysctl

variables are reset upon reboot using defaults and /etc/sysctl.conf, so

to make your change permanent you must add the line kern.maxfiles=20000

to /etc/sysctl.conf.

One can also change a maximum number of allowed file descriptors for Tor

daemon alone by editing /etc/rc.d/tor and adding the following lines:

tor:\

        :openfiles-max=8192:\

        :tc=daemon:

However, there are stricter limits set on users. This is a security

feature intended to prevent one user from choking out others by opening

all possible file descriptors.

The stricter limits are set in /etc/login.conf. This config file sets

resource access rules for user classes. You should be running

Tor as a non-privileged daemon user '_tor', which belongs to the 'daemon'

class. It will therefore be subject to the 'default' and 'daemon' rules.

There are two relevant rules: openfiles-cur and openfiles-max. The prior

is the initial limit upon login - the soft limit. The latter is the maximum

limit that can be set using 'ulimit -n' or setrlimit() without editing

/etc/login.conf and rebooting. This is known as the hard limit.

Without editing /etc/login.conf, daemon-owned processes have

soft limit of 512 open files and a hard limit of 1024 open files.

Tor can increase the soft limit as needed, so you will therefore

eventually get warnings about running out of available file descriptors

once Tor reaches ~1024 open files.

To increase the hard limit, add the following line to the daemon class

rules in /etc/login.conf:

tor:\

        :openfiles-max=8192:\

        :tc=daemon:

Upon restarting the machine, Tor will be able to open up to 6500 file

descriptors.

Be aware that, by doing this, you are bypassing a security and stability

feature of the OS. If you are running your relay on a weak or old system,

watch your system load to ensure that it can handle this many open files.

Also, Tor may interfere with any other programs that open many files.

Disclaimer

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