How does I increase backlog length tcp_max_syn_backlog value


I tried that, but it didnt increase from 512.
is its max value 512.I make 1024 this variable


To increase it instantly (will be lost after reboot) use this:

echo 1024 > /proc/sys/net/ipv4/tcp_max_syn_backlog

To increase it permanently, use this:

echo "net.ipv4.tcp_max_syn_backlog=1024" >> /etc/sysctl.conf


actually it dont affect Linux half open connection number.its default value 256 in centos.
ı would like increase half open connection number.


Code Location:Linux/net/core/request_sock.c
23 /*
24 * Maximum number of SYN_RECV sockets in queue per LISTEN socket.
25 * One SYN_RECV socket costs about 80bytes on a 32bit machine.
26 * It would be better to replace it with a global counter for all sockets
27 * but then some measure against one socket starving all other sockets
28 * would be needed.
29 *
30 * The minimum value of it is 128. Experiments with real servers show that
31 * it is absolutely not enough even at 100conn/sec. 256 cures most
32 * of problems.
33 * This value is adjusted to 128 for low memory machines,
34 * and it will increase in proportion to the memory of machine.
35 * Note : Dont forget somaxconn that may limit backlog too.
36 */
37 int sysctl_max_syn_backlog = 256;
38 EXPORT_SYMBOL(sysctl_max_syn_backlog);

beside suggestion !


Is there no one to help ?
I cannot make it


A tcp_max_syn_backlog variable sets how many half-open connections to backlog queue. For example, by default 256 half-open connections can be handled in memory. So, to increase this variable you can use standard sysctl to change the default size of the backlog queue as shown:

sysctl -w net.ipv4.tcp_max_syn_backlog=“2048”


this command dont affect, max backlog list always is 512, how a solution ???
Note:Centos 6.7


Try to reload all defined kernel variables in sysctl.conf file using following sysctl command as shown without reboot.

/sbin/sysctl -p


and result

netstat -ant|grep SYN_RECV|wc -l





Yes sir, I tried again again reboot and /sbin/sysctl -p


While running /sbin/sysctl -p command, do you see any errors?


I don’t see any error, interesting case, what can I do


What’s the output of this command:

cat /proc/sys/net/ipv4/tcp_max_syn_backlog


this value is 2048
I wold like to increase half open connection list length, its name is backlog or syn queue



the limit of 512 depends on the 511 limit in the apache2.conf.

Read the article below to get full understanding of how the value of half open connetions is generated.