somaxconn

somaxconn is a kernel parameter that controls the maximum length of the listen backlog for TCP sockets on many Unix-like systems, notably Linux. The backlog represents the queue of pending connections that have completed the TCP three-way handshake (or are in progress) and are waiting to be accepted by the application.

When a program calls listen(fd, backlog), the kernel enforces a cap: the effective backlog is the minimum of the requested backlog and somaxconn. If the queue becomes full, new connection attempts can be refused or dropped by the kernel depending on the state of the handshake and the kernel version in use.

In Linux, somaxconn is accessible at /proc/sys/net/core/somaxconn and can be changed at runtime using sysctl net.core.somaxconn or by writing to the proc file. The value is system-wide rather than per-socket. The C header defines a constant named SOMAXCONN, commonly set to 128, which serves as a conventional upper bound that applications may use as a guideline for backlog sizes.

There is also a separate parameter that affects the queue of incomplete connections in the TCP stack: net.ipv4.tcp_max_syn_backlog (tcp_max_syn_backlog). The overall behavior is shaped by the combination of somaxconn and the SYN backlog limit, so increasing one without the other may have limited effect in practice.

Impact and considerations: Raising somaxconn can help servers tolerate bursts of connection attempts by increasing the number of connections that can be queued, but it increases kernel memory usage and may conceal bottlenecks in the application’s accept loop. Adjustments are typically guided by observed connection rates, backlog events, and accept latency.

See also: listen backlog, net.core.somaxconn, tcp_max_syn_backlog, SOMAXCONN.