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Joyce Babu
Asked: 2013-07-04 08:53:33 +0800 CST

Is this DoS attack [duplicate]

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I am seeing a huge number of connections from a single IP.

# netstat -alpn | grep :80 | grep 92.98.64.103

tcp        0      0 my.ip.address.x:80            92.98.64.103:45629          TIME_WAIT   -                   
tcp        0      0 my.ip.address.x:80            92.98.64.103:44288          TIME_WAIT   -                   
tcp        0      0 my.ip.address.x:80            92.98.64.103:48783          TIME_WAIT   -                   
tcp        0      0 my.ip.address.x:80            92.98.64.103:40531          TIME_WAIT   -                   
tcp        0      0 my.ip.address.x:80            92.98.64.103:54094          TIME_WAIT   -                   
tcp        0      0 my.ip.address.x:80            92.98.64.103:47394          TIME_WAIT   -                   
tcp        0      0 my.ip.address.x:80            92.98.64.103:43495          TIME_WAIT   -                   
tcp        0      0 my.ip.address.x:80            92.98.64.103:55429          TIME_WAIT   -                   
tcp        0      0 my.ip.address.x:80            92.98.64.103:42993          TIME_WAIT   -                   
tcp        0      0 my.ip.address.x:80            92.98.64.103:49542          TIME_WAIT   -                   
tcp        0      0 my.ip.address.x:80            92.98.64.103:54812          TIME_WAIT   -

There are 419 such lines. But I see only 1 request from 92.98.64.103 in my access log. Is this DoS attack?

UPDATE - 419 Connections are from a single IP. There are several such IPs with over 100 connections.

The normal RAM usage of my server is around 2-3 GB. But at that time it was using 15GB RAM.

root@mars [~]# netstat -alpn | grep ':80 .*TIME_WAIT' | wc -l
6728

UPDATE 2 - TIME_WAIT increased to over 10000 in a few seconds

root@mars [~]# netstat -nat | awk '{print $6}' | sort | uniq -c | sort -n
      1 established)
      1 Foreign
      4 CLOSE_WAIT
      9 FIN_WAIT2
     34 LAST_ACK
     39 SYN_RECV
     44 LISTEN
     45 CLOSING
     68 FIN_WAIT1
    128 ESTABLISHED
  10261 TIME_WAIT

But the web server is loading fine without any problem.

ddos

1 Answers

  1. 419 measly connections is not a DoS.

    On one particular medium-usage webserver at the moment I have:

    $ netstat -alpn | grep ':80 .*TIME_WAIT' | wc -l
384

    384 connections in this state.

    How many per IP?

    $ netstat -alpn | awk 'BEGIN {FS="[ :]+";} /:80 .*TIME_WAIT/ {print $6}' | sort | uniq -c | sort -g | tail -n5
      6 xx.xxx.xx.xx
      6 xx.xxx.xx.xx
      9 xxx.xxx.xx.xx
     13 xxx.xx.xx.xx
     48 xxx.xxx.xxx.xx

    It's unusual to have so many from one IP - my guess is a PMTU problem resulting in terminated connections without requests being made.

    If you're concerned, dump the traffic.

    I'd recommend dropping down the number of seconds the socket stays in TIME_WAIT. Add the following to /etc/sysctl.conf:

    net.netfilter.nf_conntrack_tcp_timeout_time_wait=15

    and run:

    sysctl -w net.netfilter.nf_conntrack_tcp_timeout_time_wait=15
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