When running Asterisk PBX on Ubuntu, you might encounter situations where the default file descriptor limit (1024) becomes restrictive. This is particularly common in high-call-volume environments. The challenge arises because Asterisk typically runs as a non-login system user, making traditional ulimit commands ineffective.

Ubuntu has two levels of file descriptor limits:

1. System-wide limit (check with: cat /proc/sys/fs/file-max)
2. Per-process limit (check with: cat /proc/$(pidof asterisk)/limits | grep "Max open files")

Here's the proper sequence to implement permanent changes:

# 1. Edit systemd service file (for Ubuntu 16.04+)
sudo systemctl edit --full asterisk

# Add these lines under [Service]:
LimitNOFILE=2048
LimitNPROC=4096

# 2. Update pam limits (works for both systemd and sysvinit)
sudo nano /etc/security/limits.conf

Add these entries:

asterisk soft nofile 2048
asterisk hard nofile 2048
@asterisk soft nofile 2048
@asterisk hard nofile 2048

For older systems using sysvinit or to ensure complete coverage:

# Edit sysctl configuration
sudo nano /etc/sysctl.conf

# Add these lines:
fs.file-max = 65536
fs.nr_open = 65536

# Apply changes immediately
sudo sysctl -p

After making changes, verify with:

# Check current process limits
cat /proc/$(pidof asterisk)/limits | grep "Max open files"

# Check system-wide limit
cat /proc/sys/fs/file-max

# Check user limits (as root)
su - asterisk -s /bin/bash -c "ulimit -n"

If changes don't take effect:

1. Ensure you've restarted Asterisk AND the server (some changes require full reboot)
2. Check for apparmor/selinux restrictions
3. Verify asterisk is running under the correct user
4. Check for conflicting configurations in /etc/security/limits.d/

For high-performance installations, consider these additional tweaks:

# Increase epoll limits
echo 1048576 > /proc/sys/fs/epoll/max_user_watches
echo "fs.epoll.max_user_watches=1048576" >> /etc/sysctl.conf

# Optimize TCP settings
echo "net.ipv4.tcp_max_syn_backlog=4096" >> /etc/sysctl.conf
echo "net.core.somaxconn=4096" >> /etc/sysctl.conf

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Here's a comprehensive guide to properly configure file descriptor limits for the Asterisk PBX daemon on Ubuntu systems. The standard approaches often fail because Asterisk runs as a system service with specific initialization requirements.

The default 1024 file descriptor limit becomes problematic for Asterisk when handling high call volumes or multiple concurrent connections. Even when modifying /etc/security/limits.conf, the changes don't persist because:

• Systemd services ignore traditional limits.conf settings
• Asterisk typically runs as a non-login service account
• Ubuntu's PAM configuration may not apply to daemon processes

For modern Ubuntu systems using systemd, we need a multi-layer approach:

1. Systemd Service Configuration

Create or modify the Asterisk service override file:

sudo mkdir -p /etc/systemd/system/asterisk.service.d
sudo nano /etc/systemd/system/asterisk.service.d/limits.conf

Add these contents:

[Service]
LimitNOFILE=2048
LimitNPROC=4096

2. Asterisk Built-in Limits

Configure Asterisk's internal file handling in /etc/asterisk/asterisk.conf:

[options]
maxfiles = 2048

3. Sysctl Configuration (Optional)

For systems expecting very high loads, adjust kernel-level limits:

sudo nano /etc/sysctl.conf

Add these lines:

fs.file-max = 65536
fs.nr_open = 65536

After applying all changes, execute:

sudo systemctl daemon-reload
sudo systemctl restart asterisk

Then verify with:

pid=$(pgrep asterisk)
cat /proc/$pid/limits | grep "Max open files"

If you still see 1024:

• Check for conflicting settings in /etc/pam.d/common-session
• Verify Asterisk isn't being started via init.d (use systemd instead)
• Ensure no SELinux/AppArmor restrictions exist (uncommon on Ubuntu)

For a high-volume server handling 500 concurrent calls:

# /etc/systemd/system/asterisk.service.d/limits.conf
[Service]
LimitNOFILE=8192
LimitNPROC=16384

# /etc/asterisk/asterisk.conf
[options]
maxfiles = 8192
maxload = 1.4

# /etc/sysctl.conf
fs.file-max = 131072