1. Find.. to which IP address in the server is targeted by the ddos attack
netstat -plan | grep :80 | awk ‘{print $4}’ | cut -d: -f1 |sort |uniq -c
2. Find… from which IPs, the attack is coming
netstat -plan | grep :80 | awk ‘{print $5}’ | cut -d: -f1 |sort |uniq -c
In csf:
vi /etc/csf/csf.conf
SYNFLOOD
SYNFLOOD is disabled by default. If you are not receiving any sort of attack, there is no need to enable it. If you are expecting an attack, enable it and set the rules a bit strict, like
SYNFLOOD = “1”
SYNFLOOD_RATE = “30/s”
SYNFLOOD_BURST = “10”
i.e. if 30 connections are received from an IP/sec for 10 times, block it. Make sure don’t keep it too strict if you are not receiving an attack else it will generate false positives and will block legit connections.
PORTFLOOD
PORTFLOOD = 80;tcp;100;5,22;tcp;5;300
ie, If an IP makes 100 connections in 5 sec to port 80 (tcp), then it will be blocked from the server and if 5 connections in 300 sec to 22 port.
In /etc/sysctl.conf
Paste the following into the file, you can overwrite the current information.
#Kernel sysctl configuration file for Red Hat Linux
#
# For binary values, 0 is disabled, 1 is enabled. See sysctl(8) and
# sysctl.conf(5) for more details.
# Disables packet forwarding
net.ipv4.ip_forward=0
# Disables IP source routing
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.lo.accept_source_route = 0
net.ipv4.conf.eth0.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
# Enable IP spoofing protection, turn on source route verification
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.lo.rp_filter = 1
net.ipv4.conf.eth0.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1
# Disable ICMP Redirect Acceptance
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.lo.accept_redirects = 0
net.ipv4.conf.eth0.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
# Enable Log Spoofed Packets, Source Routed Packets, Redirect Packets
net.ipv4.conf.all.log_martians = 0
net.ipv4.conf.lo.log_martians = 0
net.ipv4.conf.eth0.log_martians = 0
# Disables IP source routing
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.lo.accept_source_route = 0
net.ipv4.conf.eth0.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
# Enable IP spoofing protection, turn on source route verification
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.lo.rp_filter = 1
net.ipv4.conf.eth0.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1
# Disable ICMP Redirect Acceptance
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.lo.accept_redirects = 0
net.ipv4.conf.eth0.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
# Disables the magic-sysrq key
kernel.sysrq = 0
# Decrease the time default value for tcp_fin_timeout connection
net.ipv4.tcp_fin_timeout = 15
# Decrease the time default value for tcp_keepalive_time connection
net.ipv4.tcp_keepalive_time = 1800
# Turn off the tcp_window_scaling
net.ipv4.tcp_window_scaling = 0
# Turn off the tcp_sack
net.ipv4.tcp_sack = 0
# Turn off the tcp_timestamps
net.ipv4.tcp_timestamps = 0
# Enable TCP SYN Cookie Protection
net.ipv4.tcp_syncookies = 1
# Enable ignoring broadcasts request
net.ipv4.icmp_echo_ignore_broadcasts = 1
# Enable bad error message Protection
net.ipv4.icmp_ignore_bogus_error_responses = 1
# Log Spoofed Packets, Source Routed Packets, Redirect Packets
net.ipv4.conf.all.log_martians = 1
# Increases the size of the socket queue (effectively, q0).
net.ipv4.tcp_max_syn_backlog = 1024
# Increase the tcp-time-wait buckets pool size
net.ipv4.tcp_max_tw_buckets = 1440000
# Allowed local port range
net.ipv4.ip_local_port_range = 16384 65536
Run /sbin/sysctl -p and sysctl -w net.ipv4.route.flush=1 to enable the changes without a reboot.
TCP Syncookies
echo 1 > /proc/sys/net/ipv4/tcp_syncookies
Some IPTABLES Rules:
iptables -A INPUT -p tcp –syn -m limit –limit 1/s –limit-burst 3 -j RETURN
ServerAdminz Linux Blog 
How would those limit rules help???, If i get a flood of fake connections and start limiting after the burst has reached, then legitimate connects will be filtered too, causing users to not browse my web site or sending to my mail server (what ever the service is)
Yes the server will not be down, but the service is still down for users trying to access it as I am blocking them from my firewall.
Please explain more if possible as I am confused here, Thanks
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Hi there, i read your blog from time to time and i own a similar one and i was just wondering if
you get a lot of spam remarks? If so how do you protect against it, any plugin or anything you can advise?
I get so much lately it’s driving me mad so any support is very much appreciated.
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Hello
Any idea to block tcp withou flag?
–tcp-option 64
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Reblogged this on thebashline and commented:
Good info on tuning sysctl conf for syn
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thank you for this information it is a great nesesity these days to be able to protect ourselves from these large scale attacks.
for this iam greatful.
@other person who wanted to block spam from blogs..
I would look to stopforumspam.com for a plugin/mod which can check IP/email addresses against a DB of known Spammers and Auto-Block these posts immediately redirecting the scammers to another site ie; scambusters.whatever 😉
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Hello! That works great! Does it work with MBN too?
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