Chapter 8. Tuning applications with a large number of incoming requests

Procedure

1. To verify whether tuning is required, display the statistics for the affected port:

   # ss -ntl '( sport = :443 )'
   State    Recv-Q   Send-Q   Local Address:Port   Peer Address:Port  Process
   LISTEN   650      500      192.0.2.1:443        0.0.0.0:*

   If the current number of connections in the backlog (Recv-Q) is larger than the socket backlog (Send-Q), the listen backlog is still not large enough and tuning is required.

2. Optional: Display the current TCP listen backlog limit:

   # sysctl net.core.somaxconn
   net.core.somaxconn = 4096

3. Create the /etc/sysctl.d/10-socket-backlog-limit.conf file, and set a larger listen backlog limit:

   net.core.somaxconn = 8192

   Note that applications can request a larger listen backlog than specified in the net.core.somaxconn kernel parameter but the kernel limits the application to the number you set in this parameter.

4. Load the setting from the /etc/sysctl.d/10-socket-backlog-limit.conf file:

   # sysctl -p /etc/sysctl.d/10-socket-backlog-limit.conf

5. Reconfigure the application to use the new listen backlog limit:

   • If the application provides a config option for the limit, update it. For example, the Apache HTTP Server provides the ListenBacklog configuration option to set the listen backlog limit for this service.
   • If you cannot configure the limit, recompile the application.
   Important

   You must always update both the net.core.somaxconn kernel setting and the application’s settings.

6. Restart the application.

Verification

1. Monitor the Systemd journal for further occurrences of possible SYN flooding on port <port_number> error messages.

2. Monitor the current number of connections in the backlog and compare it with the socket backlog:

   # ss -ntl '( sport = :443 )'
   State    Recv-Q   Send-Q   Local Address:Port   Peer Address:Port  Process
   LISTEN   0        500      192.0.2.1:443        0.0.0.0:*

   If the current number of connections in the backlog (Recv-Q) is larger than the socket backlog (Send-Q), the listen backlog is not large enough and further tuning is required.