Kernel params used for synfloods

echo 1 > /proc/sys/net/ipv4/tcp_syncookies
echo 45 > /proc/sys/net/netfilter/nf_conntrack_tcp_timeout_syn_recv
echo 3 > /proc/sys/net/ipv4/tcp_synack_retries
echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter
echo 1 > /proc/sys/net/ipv4/tcp_tw_recycle

I tried to use these common values to help deter a syn flood from occurring. Syn-cookies challenges the other opponent. While you change the default timing of syn_recv from 60 seconds to 45 seconds which would also be the equivalent of 3 tcP_synack_retries.

Last but not least, reuse that old ttimewait port. 🙂

Getting tired of a Synflood?

This dirty last ditch modification to the kernel will force close connections after so many seconds. The default is 60.

echo 30 > /proc/sys/net/ipv4/tcp_fin_timeout

This will tell everyone that you have 30 seconds to finish or else I will drop your connection. This is good in a pinch, but do not recommend running this at all times.

Instant kernel panic

After researching what could cause an instant kernel panic with no documentation or messages in kdump, I found that if manually execute the following code (warning: will cause system disruption, I am not held responsible for your copy and pasting):

echo c > /proc/sysrq-trigger

More information regarding this can be found on the Old Internet Archives at

while loop (true)

Need to execute a set of commands in a loop, non-stop?

while true; do killall -9 program; sleep 3; done

This will tell your Linux O/S do killall -9, sleep for 3 seconds, and start it again. This is great if you don’t have access to a crontab or just need it for a quick fix.


Do you need to check the zone file syntax via command line after making a change. You can run the following:

named-checkzone /var/named/

Results should be similar to

zone loaded serial 20190xxxxx