No file descriptors available

Why do I get a bunch of No file descriptors available (os error 24) errors?

There are a few potential causes of this error. The simplest is that your operating system sets an open file limit that is aggressively low. Through personal testing, I’ve found that 4096 is a reasonable open file limit (this will vary based on your exact setup).

There are quite a few options to solve this particular problem, of which a handful are shown below.

Increase the Number of Open Files

We’ll start by increasing the number of open files the OS allows. On my Kali install, the default was 1024, and I know some MacOS installs use 256 😕.

Edit `/etc/security/limits.conf`

One option to up the limit is to edit /etc/security/limits.conf so that it includes the two lines below.

* represents all users
hard and soft indicate the hard and soft limits for the OS
nofile is the number of open files option.

/etc/security/limits.conf
-------------------------
...
*        soft nofile 4096
*        hard nofile 8192
...

Use `ulimit` directly

A faster option, that is not persistent, is to simply use the ulimit command to change the setting.

ulimit -n 4096

Additional Tweaks (may not be needed)

If you still find yourself hitting the file limit with the above changes, there are a few additional tweaks that may help.

This section was shamelessly stolen from this stackoverflow answer. More information is included in that post and is recommended reading if you end up needing to use this section.

✨ Special thanks to HTB user @sparkla for their help with identifying these additional tweaks ✨

Increase the ephemeral port range, and decrease the tcp_fin_timeout.

The ephermal port range defines the maximum number of outbound sockets a host can create from a particular I.P. address. The fin_timeout defines the minimum time these sockets will stay in TIME_WAIT state (unusable after being used once). Usual system defaults are

net.ipv4.ip_local_port_range = 32768 61000
net.ipv4.tcp_fin_timeout = 60

This basically means your system cannot consistently guarantee more than (61000 - 32768) / 60 = 470 sockets per second.

sudo sysctl net.ipv4.ip_local_port_range="15000 61000"
sudo sysctl net.ipv4.tcp_fin_timeout=30

Allow socket reuse while in a `TIME_WAIT` status

This allows fast cycling of sockets in time_wait state and re-using them. Make sure to read post Coping with the TCP TIME-WAIT from Vincent Bernat to understand the implications.

sudo sysctl net.ipv4.tcp_tw_reuse=1

No file descriptors available