Setting Up a Million‑Connection TCP Test on Linux (Server and Client Configuration)

After reading previous articles on achieving a million TCP connections with a single server and client, the author provides the complete source code and step‑by‑step instructions for reproducing the experiment.

The first method described runs a single server process while using many client IPs; the second method (multiple server processes) will be covered later.

1. Server Preparation

1.1 Increase maximum file handles

Edit /etc/sysctl.conf to set system‑wide limits:

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

Apply with sysctl -p and verify using sysctl -a.

Then raise user‑level limits in /etc/security/limits.conf (applied to all users):

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

Note: hard nofile must be smaller than fs.nr_open to avoid login failures.

Open a new terminal and confirm with ulimit -n (should show 1010000).

1.2 Start the server php server.php 0.0.0.0 8090 Verify listening with netstat -nlt | grep 8090.

2. Client Preparation

2.1 Adjust usable port range

Linux default ports are ~30,000; increase to cover 50,000‑65,000 connections:

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

Apply with sysctl -p.

2.2 Increase file limits for the client

Set system‑wide limits to 1,100,000 and per‑process fs.nr_open to 60,000:

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

Apply and verify as before.

Configure user limits in /etc/security/limits.conf for each client process (5 × 10⁴ files per process, set to 55 000):

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

Confirm with ulimit -n (should show 55000).

2.3 Choose new IP addresses for the client

Because a single client IP can open at most ~64 k connections, configure multiple IP aliases on one machine using ifconfig (e.g., eth0:1, eth0:2, … up to eth0:20). ifconfig eth0:1 CIP1 netmask 255.255.248.0 up Ensure the chosen IPs do not conflict with other devices on the LAN.

2.4 Apply the new IPs in the test code

Edit clientd.php to fill the $ips array with the selected addresses. Verify they are unreachable in the LAN with the provided ping tool: php clientd.php ping When all pings return false, bring the interfaces up: php clientd.php ifup Check with ifconfig that the aliases appear.

3. Run the Connection Test

Update clientd.php with the server IP and port, then start the connections: php clientd.php start In another terminal, monitor the number of ESTABLISHED sockets: watch "ss -ant | grep ESTABLISH" If the count exceeds 1,000,000, the experiment succeeded.

During the run, you may need to stop and restart the client ( php clientd.php stop) or the server (Ctrl‑C) and wait for the port to be released.

After success, examine memory usage:

$ cat /proc/meminfo
... (focus on Slab memory)

Use slabtop to see kernel object allocations.

4. Clean Up

Terminate the server with Ctrl‑C and stop the client: php clientd.php stop Remove the temporary IP aliases: php clientd.php ifdown The source code for the whole experiment is available at:

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

Additional related articles are linked at the end of the original post.