How to Reach One Million TCP Connections on a Single Linux Server – Full Guide

1. Server preparation

Increase system-wide file descriptor limits and per-process limits to support >1 000 000 open sockets.

# vi /etc/sysctl.conf
fs.file-max=1100000
fs.nr_open=1100000

Apply with sysctl -p and verify with sysctl -a. Then raise user limits in /etc/security/limits.conf for all users:

# vi /etc/security/limits.conf
* soft nofile 1010000
* hard nofile 1010000

Hard nofile must be smaller than fs.nr_open , otherwise login may fail.

Confirm new limits with ulimit -n (should show 1010000).

Start the PHP server listening on all interfaces:

php server.php 0.0.0.0 8090

Verify with netstat -nlt | grep 8090.

2. Client preparation

Adjust the kernel's local port range because the default (~30 000 ports) is insufficient for 50 000 concurrent connections per client process.

# vi /etc/sysctl.conf
net.ipv4.ip_local_port_range = 5000 65000

Apply with sysctl -p. Increase file‑descriptor limits: system-wide to 1 100 000, per‑process to 60 000.

# vi /etc/sysctl.conf
fs.file-max=1100000
fs.nr_open=60000

Set per‑user limits to 55 000 (soft and hard) in /etc/security/limits.conf:

# vi /etc/security/limits.conf
* soft nofile 55000
* hard nofile 55000

Confirm with ulimit -n (should show 55000).

Configure multiple IP aliases on the same NIC (e.g., eth0:0 … eth0:19) so that up to 20 source IPs can be used simultaneously:

ifconfig eth0:1 CIP1 netmask 255.255.248.0 up

Update the $ips array in clientd.php with the chosen IPs and subnet mask. Verify that none of the IPs conflict with other machines using the provided ping helper:

php clientd.php ping

When all pings return false, bring the interfaces up:

php clientd.php ifup

Check the configuration with ifconfig (should list eth0, eth0:0 … eth0:19).

3. Running the connection test

Edit clientd.php to point to the server’s IP and port, then start the clients:

php clientd.php start

In another terminal, monitor the number of ESTABLISHED sockets in real time:

watch "ss -ant | grep ESTABLISHED"

When the count exceeds one million, the experiment succeeded. Stop the clients with:

php clientd.php stop

Terminate the server with Ctrl+C. If the port remains busy, wait briefly for the OS to release it before restarting.

Inspect memory usage, especially the slab allocator, to understand kernel overhead:

cat /proc/meminfo
# Example excerpt
MemTotal:        3922956 kB
MemFree:           96652 kB
Slab:           3241244 kB

Use slabtop to see which kernel objects consume the most memory.

4. Cleanup

Terminate the server ( Ctrl+C) and stop the client processes:

php clientd.php stop

Remove the temporary IP aliases:

php clientd.php ifdown

Source code for the experiment is available at:

https://github.com/yanfeizhang/coder-kung-fu/tree/main/tests/network/test01