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

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 😉

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5 Responses to “How to check if your Linux WebServer is under a DoS attack”

  1. admin says:
    Epiphany 2.30.6 Epiphany 2.30.6 Debian GNU/Linux x64 Debian GNU/Linux x64
    Mozilla/5.0 (X11; U; Linux x86_64; en-us) AppleWebKit/531.2+ (KHTML, like Gecko) Version/5.0 Safari/531.2+ Debian/squeeze (2.30.6-1) Epiphany/2.30.6

    Another way to NULL ROUTE an IP address is via the command:
    /sbin/route add xxx.xxx.xxx.xxx gw 127.0.0.1 lo

    Cheers 😉

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  2. admin says:
    Epiphany 2.30.6 Epiphany 2.30.6 Debian GNU/Linux x64 Debian GNU/Linux x64
    Mozilla/5.0 (X11; U; Linux x86_64; en-us) AppleWebKit/531.2+ (KHTML, like Gecko) Version/5.0 Safari/531.2+ Debian/squeeze (2.30.6-1) Epiphany/2.30.6

    Or another way to route an IP address is with:

    route add xxx.xxx.xxx.xxx 0.0.0.0

    I’m not sure which way is the best.

    I’ll be glad if somebody clears it up for me and the readers 😉

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  3. Josh says:
    Google Chrome 12.0.742.122 Google Chrome 12.0.742.122 Windows 7 x64 Edition Windows 7 x64 Edition
    Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/534.30 (KHTML, like Gecko) Chrome/12.0.742.122 Safari/534.30

    Great article!
    We use http://www.Jumpple.com to notify us of any change to our website , or if its down – so this is useful after you get notified .
    Thx a lot!
    Josh.

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  4. Pol says:
    Firefox 30.0 Firefox 30.0 GNU/Linux GNU/Linux
    Mozilla/5.0 (X11; Linux i686; rv:30.0) Gecko/20100101 Firefox/30.0

    Hi boy! Wonderful article! Very thanks. Only a precisation on 4th point: the line should be:

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

    (not udp|udp)

    🙂

    Pol

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  5. Audrie Masek says:
    Google Chrome 62.0.3202.94 Google Chrome 62.0.3202.94 Windows 10 x64 Edition Windows 10 x64 Edition
    Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.94 Safari/537.36

    Another obvious way to gauge the health of a site is to see which pages on the site get zero traffic from Google over a certain period of time. I do this by merging analytics data with crawl data – as analytics doesn’t give you data on pages it sends no traffic to.

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