Posts Tagged ‘SYN’

Check and Restart Apache if it is malfunctioning (not returning HTML content) shell script

Monday, March 19th, 2012

Check and Restart Apache Webserver on Malfunction, Apache feather logo

One of the company Debian Lenny 5.0 Webservers, where I'm working as sys admin sometimes stops to properly server HTTP requests.
Whenever this oddity happens, the Apache server seems to be running okay but it is not failing to return requested content

I can see the webserver listens on port 80 and establishing connections to remote hosts – the apache processes show normally as I can see in netstat …:

apache:~# netstat -enp 80
Active Internet connections (w/o servers)
Proto Recv-Q Send-Q Local Address Foreign Address State User Inode PID/Program name
tcp 0 0 xxx.xxx.xxx.xx:80 46.253.9.36:5665 SYN_RECV 0 0 -
tcp 0 0 xxx.xxx.xxx.xx:80 78.157.26.24:5933 SYN_RECV 0 0 -
...

Also the apache forked child processes show normally in process list:

apache:~# ps axuwwf|grep -i apache
root 46748 0.0 0.0 112300 872 pts/1 S+ 18:07 0:00 \_ grep -i apache
root 42530 0.0 0.1 217392 6636 ? Ss Mar14 0:39 /usr/sbin/apache2 -k start
www-data 42535 0.0 0.0 147876 1488 ? S Mar14 0:01 \_ /usr/sbin/apache2 -k start
root 28747 0.0 0.1 218180 4792 ? Sl Mar14 0:00 \_ /usr/sbin/apache2 -k start
www-data 31787 0.0 0.1 219156 5832 ? S Mar14 0:00 | \_ /usr/sbin/apache2 -k start

In spite of that, in any client browser to any of the Apache (Virtual hosts) websites, there is no HTML content returned…
This weird problem continues until the Apache webserver is retarted.
Once webserver is restarted everything is back to normal.
I use Apache Check Apache shell script set on few remote hosts to regularly check with nmap if port 80 (www) of my server is open and responding, anyways this script just checks if the open and reachable and thus using it was unable to detect Apache wasn't able to return back HTML content.
To work around the malfunctions I wrote tiny script – retart_apache_if_empty_content_is_returned.sh

The scripts idea is very simple;
A request is made a remote defined host with lynx text browser, then the output of lines is counted, if the output returned by lynx -dump http://someurl.com is less than the number returned whether normally invoked, then the script triggers an apache init script restart.

I've set the script to periodically run in a cron job, every 5 minutes each hour.
# check if apache returns empty content with lynx and if yes restart and log it
*/5 * * * * /usr/sbin/restart_apache_if_empty_content.sh >/dev/null 2>&1

This is not perfect as sometimes still, there will be few minutes downtime, but at least the downside will not be few hours until I am informed ssh to the server and restart Apache manually

A quick way to download and set from cron execution my script every 5 minutes use:

apache:~# cd /usr/sbin
apache:/usr/sbin# wget -q http://www.pc-freak.net/bscscr/restart_apache_if_empty_content.sh
apache:/usr/sbin# chmod +x restart_apache_if_empty_content.sh
apache:/usr/sbin# crontab -l > /tmp/file; echo '*/5 * * * *' /usr/sbin/restart_apache_if_empty_content.sh 2>&1 >/dev/null

 

How to prevent SSH and FTP bruteforce attacks with iptables on Linux

Friday, December 30th, 2011

Earlier I've blogged about how to prevent brute force attacks with fail2ban, denohosts and blockhosts , however there is easier way to secure against basic brute force attacks by not installing or configuring any external programs.
The way I'm talking about uses simple iptables rules to filter out brute force attacks.

Here is a small script to stop ssh and FTP invaders which try to initiate more than 3 consequential connections in 5 minutes time to port 22 or port 23:

SERVER_MAIN_IP='AAA.BBB.CCC.DDD';/sbin/iptables -N SSH_WHITELIST
/sbin/iptables -A INPUT -p tcp --dport 22 --syn -m recent --name sshbr --set
/sbin/iptables -A INPUT -p tcp --dport 22 --syn -j SSH_WHITELIST
/sbin/iptables -A INPUT -p tcp --dport 22 --syn -m recent --name sshbr \
--update --rttl --hitcount 3 --seconds 300 -j REJECT --reject-with tcp-reset
/sbin/iptables -A SSH_WHITELIST -s $SERVER_MAIN_IP -p tcp --dport 22 --syn -m recent --rttl --remove

The only thinIf the rules are matched iptables filter rules will be added to the iptables CHAIN SSH_WHITELIST
In case if you want to add some more truested IPs add some more iptables rules, like:

ALLOW_IP='BBB.CCC.DDD.EEE';
/sbin/iptables -A SSH_WHITELIST -s $ALLOW_IP -p tcp --dport 22 --syn -m recent --rttl --remove

Each filtered IP that matches the rules will be filtered for 5 minutes, if 5 minutes is enough, the 300 value has to be increased.
 

How to check if your Linux WebServer is under a DoS attack

Friday, July 22nd, 2011

There are few commands I usually use to track if my server is possibly under a Denial of Service attack or under Distributed Denial of Service

Sys Admins who still have not experienced the terrible times of being under a DoS attack are happy people for sure …

1. How to Detect a TCP/IP Denial of Service Attack This are the commands I use to find out if a loaded Linux server is under a heavy DoS attack, one of the most essential one is of course netstat.
To check if a server is under a DoS attack with netstat, it’s common to use:

linux:~# netstat -ntu | awk '{print $5}' | cut -d: -f1 | sort | uniq -c | sort -n|wc -l

If the output of below command returns a result like 2000 or 3000 connections!, then obviously it’s very likely the server is under a DoS attack.

To check all the IPS currently connected to the Apache Webserver and get a very brief statistics on the number of times each of the IPs connected to my server, I use the cmd:

linux:~# netstat -ntu | awk '{print $5}' | cut -d: -f1 | sort | uniq -c | sort -n
221 80.143.207.107 233 145.53.103.70 540 82.176.164.36

As you could see from the above command output the IP 80.143.207.107 is either connected 221 times to the server or is in state of connecting or disconnecting to the node.

Another possible way to check, if a Linux or BSD server is under a Distributed DoS is with the list open files command lsof
Here is how lsof can be used to list the approximate number of ESTABLISHED connections to port 80.

linux:~# lsof -i TCP:80
litespeed 241931 nobody 17u IPv4 18372655 TCP server.pc-freak.net:http (LISTEN)
litespeed 241931 nobody 25u IPv4 18372659 TCP 85.17.159.89:http (LISTEN)
litespeed 241931 nobody 30u IPv4 29149647 TCP server.pc-freak.net:http->83.101.6.41:54565 (ESTABLISHED)
litespeed 241931 nobody 33u IPv4 18372647 TCP 85.17.159.93:http (LISTEN)
litespeed 241931 nobody 34u IPv4 29137514 TCP server.pc-freak.net:http->83.101.6.41:50885 (ESTABLISHED)
litespeed 241931 nobody 35u IPv4 29137831 TCP server.pc-freak.net:http->83.101.6.41:52312 (ESTABLISHED)
litespeed 241931 nobody 37w IPv4 29132085 TCP server.pc-freak.net:http->83.101.6.41:50000 (ESTABLISHED)

Another way to get an approximate number of established connections to let’s say Apache or LiteSpeed webserver with lsof can be achieved like so:

linux:~# lsof -i TCP:80 |wc -l
2100

I find it handy to keep track of above lsof command output every few secs with gnu watch , like so:

linux:~# watch "lsof -i TCP:80"

2. How to Detect if a Linux server is under an ICMP SMURF attack

ICMP attack is still heavily used, even though it’s already old fashioned and there are plenty of other Denial of Service attack types, one of the quickest way to find out if a server is under an ICMP attack is through the command:

server:~# while :; do netstat -s| grep -i icmp | egrep 'received|sent' ; sleep 1; done
120026 ICMP messages received
1769507 ICMP messages sent
120026 ICMP messages received
1769507 ICMP messages sent

As you can see the above one liner in a loop would check for sent and recieved ICMP packets every few seconds, if there are big difference between in the output returned every few secs by above command, then obviously the server is under an ICMP attack and needs to hardened.

3. How to detect a SYN flood with netstat

linux:~# netstat -nap | grep SYN | wc -l
1032

1032 SYNs per second is quite a high number and except if the server is not serving let’s say 5000 user requests per second, therefore as the above output reveals it’s very likely the server is under attack, if however I get results like 100/200 SYNs, then obviously there is no SYN flood targetting the machine 😉

Another two netstat command application, which helps determining if a server is under a Denial of Service attacks are:

server:~# netstat -tuna |wc -l
10012

and

server:~# netstat -tun |wc -l
9606

Of course there also some other ways to check the count the IPs who sent SYN to the webserver, for example:

server:~# netstat -n | grep :80 | grep SYN |wc -l

In many cases of course the top or htop can be useful to find, if many processes of a certain type are hanging around.

4. Checking if UDP Denial of Service is targetting the server

server:~# netstat -nap | grep 'udp' | awk '{print $5}' | cut -d: -f1 | sort |uniq -c |sort -n

The above command will list information concerning possible UDP DoS.

The command can easily be accustomed also to check for both possible TCP and UDP denial of service, like so:

server:~# netstat -nap | grep 'tcp|udp' | awk '{print $5}' | cut -d: -f1 | sort |uniq -c |sort -n
104 109.161.198.86
115 112.197.147.216
129 212.10.160.148
227 201.13.27.137
3148 91.121.85.220

If after getting an IP that has too many connections to the server and is almost certainly a DoS host you would like to filter this IP.

You can use the /sbin/route command to filter it out, using route will probably be a better choice instead of iptables, as iptables would load up the CPU more than simply cutting the route to the server.

Here is how I remove hosts to not be able to route packets to my server:

route add 110.92.0.55 reject

The above command would null route the access of IP 110.92.0.55 to my server.

Later on to look up for a null routed IP to my host, I use:

route -n |grep -i 110.92.0.55

Well hopefully this should be enough to give a brief overview on how, one can dig in his server and find if he is under a Distributed Denial of Service, hope it’s helpful to somebody out there.
Cheers 😉

How to harden Linux Security and imprpove network efficiency on Kernel sysctl Level to Stop SYN flood

Friday, July 8th, 2011

Power up Linux and protect against DDoS with sysctl var optimization

Some long time ago I’ve written an article Optimizing Linux tcp/ip networking

In the article I’ve examined a number of Linux kernel sysctl variables, which significantly improve the way TCP/IP networking is handled by a non router Linux based servers.

As the time progresses I’ve been continuing to read materials on blogs and internet sites on various tips and anti Denial of Service rules which one could apply on newly installed hosting (Apache/MySql/Qmail/Proxy) server to improve webserver responce times and tighten the overall security level.

In my quest for sysctl 😉 I found a few more handy sysctl variables apart from the old ones I incorporate on every Linux server I adminstrate.
The sysctl variables improves the overall network handling efficiency and protects about common SYN/ACK Denial of service attacks.

Here are the extra sysctl variables I started incorporating just recently:

############ IPv4 Sysctl Settings ################
#Enable ExecShield protection (randomize virtual assigned space to protect against many exploits)
kernel.randomize_va_space = 1
#Increase the number of PIDs processes could assign this is very needed especially on more powerful servers
kernel.pid_max = 65536
# Prevent against the common 'syn flood attack'
net.ipv4.tcp_syncookies = 1
# Controls the use of TCP syncookies two is generally a better idea, though you might experiment
#net.ipv4.tcp_syncookies = 1
net.ipv4.tcp_synack_retries = 2
##################################################
#
############## IPv6 Sysctl Settings ################
# Number of Router Solicitations to send until assuming no routers are present.
net.ipv6.conf.default.router_solicitations = 0
# Accept Router Preference in RA? Again not necessery if the server is not a router
net.ipv6.conf.default.accept_ra_rtr_pref = 0
# Learn Prefix Information in Router Advertisement (Unnecessery) for non-routers
net.ipv6.conf.default.accept_ra_pinfo = 0
# disable accept of hop limit settings from other routers (could be used for DoS)
net.ipv6.conf.default.accept_ra_defrtr = 0
# disable ipv6 global unicasts server assignments
net.ipv6.conf.default.autoconf = 0
# neighbor solicitations to send out per address (better if disabled)
net.ipv6.conf.default.dad_transmits = 0
# disable assigning more than 1 address per network interface
net.ipv6.conf.default.max_addresses = 1
#####################################################

 

To use this settings paste the above sysctl variables in /etc/sysctl.conf and ask sysctl command to read and apply the newly added conf settings:

server:~# sysctl -p
...

Hopefully you should not get errors while applying the sysctl settings, if you get some errors, it’s possible some of the variable is differently named (depending on the Linux kernel version) or the Linux distribution on which sysctl’s are implemented.

For some convenience I’ve created unified sysctl variables /etc/sysct.conf containing the newly variables I started implementing to servers with the ones I already exlpained in my previous post Optimizing Linux TCP/IP Networking

Here is the optimized / hardened sysctl.conf file for download

I use this exact sysctl.conf these days on both Linux hosting / VPS / Mail servers etc. as well as on my personal notebook 😉

Here is also the the complete content of above’s sysctl.conf file, just in case if somebody wants to directly copy/paste it in his /etc/sysctl.conf

# Sysctl kernel variables to improve network performance and protect against common Denial of Service attacks
# It's possible that not all of the variables are working on all Linux distributions, test to make sure
# Some of the variables might need a slight modification to match server hardware, however in most cases it should be fine
# variables list compiled by hip0
### http://www.pc-freak.net
#### date 08.07.2011
############ IPv4 Sysctl Kernel Settings ################
net.ipv4.ip_forward = 0
# ( Turn off IP Forwarding )
net.ipv4.conf.default.rp_filter = 1
# ( Control Source route verification )
net.ipv4.conf.default.accept_redirects = 0
# ( Disable ICMP redirects )
net.ipv4.conf.all.accept_redirects = 0
# ( same as above )
net.ipv4.conf.default.accept_source_route = 0
# ( Disable IP source routing )
net.ipv4.conf.all.accept_source_route = 0
# ( - || - )net.ipv4.tcp_fin_timeout = 40
# ( Decrease FIN timeout ) - Useful on busy/high load server
net.ipv4.tcp_keepalive_time = 4000
# ( keepalive tcp timeout )
net.core.rmem_default = 786426
# Receive memory stack size ( a good idea to increase it if your server receives big files )
##net.ipv4.tcp_rmem = "4096 87380 4194304"
net.core.wmem_default = 8388608
#( Reserved Memory per connection )
net.core.wmem_max = 8388608
net.core.optmem_max = 40960
# ( maximum amount of option memory buffers )
# tcp reordering, increase max buckets, increase the amount of backlost
net.ipv4.tcp_max_tw_buckets = 360000
net.ipv4.tcp_reordering = 5
##net.core.hot_list_length = 256
net.core.netdev_max_backlog = 1024
#Enable ExecShield protection (randomize virtual assigned space to protect against many exploits)
kernel.randomize_va_space = 1
#Increase the number of PIDs processes could assign this is very needed especially on more powerful servers
kernel.pid_max = 65536
# Prevent against the common 'syn flood attack'net.ipv4.tcp_syncookies = 1
# Controls the use of TCP syncookies two is generally a better idea, though you might experiment
#net.ipv4.tcp_syncookies = 1
net.ipv4.tcp_synack_retries = 2
###################################################
############## IPv6 Sysctl Settings ################
# Number of Router Solicitations to send until assuming no routers are present.
net.ipv6.conf.default.router_solicitations = 0
# Accept Router Preference in RA? Again not necessery if the server is not a router
net.ipv6.conf.default.accept_ra_rtr_pref = 0
# Learn Prefix Information in Router Advertisement (Unnecessery) for non-routersnet.
ipv6.conf.default.accept_ra_pinfo = 0
# disable accept of hop limit settings from other routers (could be used for DoS)
net.ipv6.conf.default.accept_ra_defrtr = 0
# disable ipv6 global unicasts server assignmentsnet.
ipv6.conf.default.autoconf = 0
# neighbor solicitations to send out per address (better if disabled)
net.ipv6.conf.default.dad_transmits = 0
# disable assigning more than 1 address per network interfacenet.
ipv6.conf.default.max_addresses = 1
#####################################################
# Reboot if kernel panic
kernel.panic = 20

These sysctl settings will tweaken the Linux kernel default network settings performance and you will notice the improvements in website responsiveness immediately in some cases implementing this kernel level goodies will make the server perform better and the system load might decrease even 😉

This optimizations on a kernel level are not only handy for servers, their implementation on Linux Desktop should also have a positive influence on the way the network behaves and could improve significantly the responce times of opening pages in Firefox/Opera/Epiphany Torrent downloads etc.

Hope this kernel tweakenings are helpful to someone.
Cheers 😉