The TCP Syn is DoS (Denial of Service) attack. It consumes resources on your Linux server. The attacker begin with the TCP connection handshake sending the SYN packet, and
| Tutorial details | |
|---|---|
| Difficulty level | Intermediate |
| Root privileges | Yes |
| Requirements | /etc/sysctl.conf |
| Est. reading time | N/A |
See the current settings
Use sysctl command to configure or see kernel parameters at runtime. To see the current settings for net.ipv4.tcp_syncookies kernel parameter, enter:
# sysctl -n net.ipv4.tcp_syncookies
OR
# cat /proc/sys/net/ipv4/tcp_syncookies
Sample outputs:
Fig.01: View current TCP SYN cookie protection
Enable TCP SYN cookie protection
Edit the file /etc/sysctl.conf, run:
# vi /etc/sysctl.conf
Append the following entry:
net.ipv4.tcp_syncookies = 1
Save and close the file. To reload the change, type:
# sysctl -p
Recommended readings
- How to set advanced security options of the TCP/IP stack and virtual memory to improve security and performance of your Linux based system.
- Twenty Linux server hardening security tips
- Linux firewall tutorial and shorewall firewall tutorial for more information.
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🐧 5 comments so far... add one ↓
🐧 5 comments so far... add one ↓
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What’s the positive and negative aspects of this command? I’m interested what exactly the setting does do :)
this will create a timeout to drop this open connections. preventing a syn flood attack.
regards
When i run command
netstat -an | grep :80 | grep -i syn | wc -l
I see more than 250 connections, that cause Apache timeout error, is this a fix for this?
I did like shown in this post, but i can not check will it work because attack passed….
HELLO,
What this person described above is NOT CORRECT!
This person showed both of these commands;
sysctl -n net.ipv4.tcp_syncookies
cat /proc/sys/net/ipv4/tcp_syncookies
If you NOTICED they have 1 showing, that means it’s ENABLED, so you don’t need to do anything, which they did not explain!
ONLY do; net.ipv4.tcp_syncookies = 1 if you don’t get a 1 in the cmds!
I’m not too sure why you’re shouting (see RFC1855 about upper case), there are lots of reasons why sysctl -n (…) may report the services as enabled at runtime, it doesn’t mean it’s enabled or disabled in any file (yet). Adding the line in /etc/sysctl.conf won’t have any impact until you reload with sysctl –system or until you reboot, which doesn’t guarantee /proc/sys/net/ipv4/tcp_syncookies will still be enabled.
From what I can see in the output, `sysctl –system` only sets the value if it is different from the current value.
Now, if you really wanna be a purist, you’ll check net.ipv4.tcp_syncookies is not already present in /etc/sysctl.conf (it’s commented out in Ubuntu 16) and uncomment the line instead of appending it.