Using TCP keepalive under Linux

Linux has built-in support for keepalive. You need to enable TCP/IP networking in order to use it. You also need procfs support and sysctl support to be able to configure the kernel parameters at runtime.

The procedures involving keepalive use three user-driven variables:


tcp_keepalive_time
the interval between the last data packet sent (simple ACKs are not considered data) and the first keepalive probe; after the connection is marked to need keepalive, this counter is not used any further

tcp_keepalive_intvl
the interval between subsequential keepalive probes, regardless of what the connection has exchanged in the meantime

tcp_keepalive_probes
the number of unacknowledged probes to send before considering the connection dead and notifying the application layer


Remember that keepalive support, even if configured in the kernel, is not the default behavior in Linux. Programs must request keepalive control for their sockets using the setsockopt interface. There are relatively few programs implementing keepalive, but you can easily add keepalive support for most of them following the instructions explained later in this document.

3.1. Configuring the kernel
There are two ways to configure keepalive parameters inside the kernel via userspace commands:


procfs interface

sysctl interface


We mainly discuss how this is accomplished on the procfs interface because it's the most used, recommended and the easiest to understand. The sysctl interface, particularly regarding the sysctl(2) syscall and not the sysctl(8) tool, is only here for the purpose of background knowledge.

3.1.1. The procfs interface
This interface requires both sysctl and procfs to be built into the kernel, and procfs mounted somewhere in the filesystem (usually on /proc, as in the examples below). You can read the values for the actual parameters by "catting" files in /proc/sys/net/ipv4/ directory: