Dynamically configuring the network stack

To change the default network stack parameters, use the sysctl() function or sysctlbyname() function that are declared in the header file sysroot-*-kos/include/sys/sysctl.h from the KasperskyOS SDK. The parameters that can be changed are presented in the table below.

Configurable network stack parameters

Parameter name	Parameter description
`net.inet.ip.ttl`	Maximum time to live (TTL) of sent IP packets. It does not affect the ICMP protocol.
`net.inet.ip.mtudisc`	If its value is set to `1`, "Path MTU Discovery" (RFC 1191) mode is enabled. This mode affects the maximum size of a TCP segment (Maximum Segment Size, or MSS). In this mode, the MSS value is determined by the limitations of network nodes. If "Path MTU Discovery" mode is not enabled, the MSS value does not exceed the value defined by the `net.inet.tcp.mssdflt` parameter.
`net.inet.tcp.mssdflt`	MSS value (in bytes) that is applied if only the communicating side failed to provide this value when opening the TCP connection, or if "Path MTU Discovery" mode (RFC 1191) is not enabled. This MSS value is also forwarded to the communicating side when opening a TCP connection.
`net.inet.tcp.minmss`	Minimum MSS value, in bytes.
`net.inet.tcp.mss_ifmtu`	If its value is set to `1`, the MSS value is calculated for an opened TCP connection based on the maximum size of a transmitted data block (Maximum Transmission Unit, or MTU) of the employed network interface. If its value is set to `0`, the MSS value for an opened TCP connection is calculated based on the MTU of the network interface that has the highest value for this parameter among all available network interfaces (except the loopback interface).
`net.inet.tcp.keepcnt`	Number of times to send test messages (Keep-Alive Probes, or KA) without receiving a response before the TCP connection will be considered closed. If its value is set to `0`, the number of sent keep-alive probes is unlimited.
`net.inet.tcp.keepidle`	TCP connection idle period, after which keep-alive probes begin. This is defined in conditional units, which can be converted into seconds via division by the `net.inet.tcp.slowhz` parameter value.
`net.inet.tcp.keepintvl`	Time interval between recurring keep-alive probes when no response is received. This is defined in conditional units, which can be converted into seconds via division by the `net.inet.tcp.slowhz` parameter value.
`net.inet.tcp.recvspace`	Size of the buffer (in bytes) for data received over the TCP protocol.
`net.inet.tcp.sendspace`	Size of the buffer (in bytes) for data sent over the TCP protocol.
`net.inet.udp.recvspace`	Size of the buffer (in bytes) for data received over the UDP protocol.
`net.inet.udp.sendspace`	Size of the buffer (in bytes) for data sent over the UDP protocol.

MSS configuration example:

static const int mss_max = 1460;

static const int mss_min = 100;

static const char* mss_max_opt_name = "net.inet.tcp.mssdflt";

static const char* mss_min_opt_name = "net.inet.tcp.minmss";

int main(void)

{

...

if ((sysctlbyname(mss_max_opt_name, NULL, NULL, &mss_max, sizeof(mss_max)) != 0) ||

(sysctlbyname(mss_min_opt_name, NULL, NULL, &mss_min, sizeof(mss_min)) != 0))

{

ERROR(START, "Can't set tcp default maximum/minimum MSS value.");

return EXIT_FAILURE;

}

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