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I started seeing this odd sort of effect that resembles denial-of-service attack against a Linux server. The effect is that the network becomes at least partially unusable very much the same as what you see with a traditional DOS or DDOS attack.

Here's a trimmed netstat dump from during the "attack" (assuming that's what it is):

Proto Recv-Q Send-Q Local Address  Foreign Address       State       PID
tcp        1      0 1.2.3.1:80     50.128.251.184:1768   CLOSE_WAIT  18482/httpd         
tcp        0      1 1.2.3.4:80     71.75.22.31:52323     LAST_ACK    -                   
tcp        0  18980 1.2.3.4:80     98.180.31.210:60499   ESTABLISHED 18016/nginx: worker 
tcp        0  11709 1.2.3.4:80     98.180.31.210:60498   ESTABLISHED 18016/nginx: worker 
tcp        0  55743 1.2.3.4:80     71.75.22.31:52239     LAST_ACK    -                   
tcp        0      0 1.2.3.5:80     75.190.139.103:58265  ESTABLISHED 16808/httpd         
tcp        0  32814 1.2.3.4:80     71.75.22.31:52279     LAST_ACK    -                   
tcp        0  48029 1.2.3.4:80     71.75.22.31:52284     LAST_ACK    -                   
tcp        1  33581 1.2.3.4:80     71.75.22.31:52285     LAST_ACK    -                   
tcp        0  23582 1.2.3.4:80     71.75.22.31:52283     LAST_ACK    -                   
tcp        0    684 1.2.3.5:80     123.125.71.31:57865   FIN_WAIT1   -                   
tcp        0  37621 1.2.3.4:80     71.75.22.31:52218     LAST_ACK    -                   
tcp        0  18980 1.2.3.4:80     174.106.209.104:39937 ESTABLISHED 18016/nginx: worker 
tcp        0      0 1.2.3.1:80     95.140.125.125:60078  ESTABLISHED 18377/httpd         
tcp        0      0 1.2.3.2:39509  2.2.3.1:3306          ESTABLISHED 18379/httpd         
tcp        0    174 1.2.3.2:33029  2.2.3.1:3306          ESTABLISHED 18482/httpd         
tcp        0  44538 1.2.3.4:80     72.230.205.217:58271  FIN_WAIT1   -                   
tcp        0  64812 1.2.3.2:80     184.35.67.238:49173   ESTABLISHED 1251/httpd          
tcp        1      0 1.2.3.1:80     174.96.155.77:59167   CLOSE_WAIT  18379/httpd         
tcp        0      1 1.2.3.4:80     174.110.137.71:61496  FIN_WAIT1   -                   
tcp        1  31751 1.2.3.4:80     99.25.112.12:55747    CLOSING     -                   
tcp        0  33396 1.2.3.4:80     99.25.112.12:55764    ESTABLISHED 18016/nginx: worker 

Notice primarily the high usage of Send-Q buffer space by connections that are essentially closed, or partially closed. By keeping these connections open, it appears that an attacker can burn though the allowable send queue and bring traffic largely to a halt. This doesn't appear to be a sophisticated attack, but just a few attackers can apparently bring down a server with minimal traffic.

Does anyone recognize this attack pattern and know how to counter it?

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1 Answer

Looks like a kind of resource-exhaustion attack. Would need more data to give a more specific answer but are you able to get isolated packet captures of the attack traffic?

Not sure what you're protecting but do you need to receive many connections from the internet at large? My first attempt at countering would be that you may be able to limit how many connections are being setup - to a reasonable threshold per (host/netrange/etc).

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