Intentando detener un DDoS

v 2.32

Porque un null-route a una ip no es una solución, es una chapuza.

Si unos script-kiddies están DDoSeando tu web.....

1) Se basa en ataques reales.
2) No hay nada de teoría, solo parte práctica.

A) Detectando el ataque

1) Usando el comando netstat

Código:

netstat -an | grep :80 | sort

Código:

netstat -n -p | grep SYN_REC | awk '{print $5}' | awk -F: '{print $1}'

Código:

netstat -n -p|grep SYN_REC | wc -l

Código:

netstat -lpn|grep :80 |awk '{print $5}'|sort

Código:

netstat -an | grep :80 | awk '{ print $5 }' | awk -F: '{ print $1 }' | sort | uniq -c | sort -n

Ejemplo de ataque SYN_RECV o SYN Flooding al Apache (puerto 80).

192.168.0.3 es la ip del servidor apache y 192.168.0.105 es la ip del "atacante".

Código:

tcp 0 0 192.168.0.3:80 192.168.0.5:60808 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60761 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60876 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60946 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60763 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60955 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60765 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60961 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60923 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61336 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61011 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60911 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60758 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60828 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61114 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61074 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60826 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60959 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60900 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60940 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60920 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60825 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60945 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60913 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61009 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60755 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60904 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61583 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60910 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60915 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60827 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61458 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60908 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61007 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60927 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60951 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60942 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61113 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60909 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60822 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60894 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60952 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60928 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60936 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60906 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61466 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60919 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60914 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60926 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60939 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60931 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60831 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60823 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60954 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60916 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60963 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60947 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61006 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60933 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60950 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60895 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60917 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61480 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60935 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60960 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60767 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60918 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60821 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61077 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60905 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61517 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60893 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60953 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60903 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61439 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61337 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61545 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61299 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61010 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60930 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60744 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60929 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60754 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61008 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61116 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60811 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60807 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60938 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60764 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60873 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60817 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61550 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60748 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60956 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60753 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61115 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60741 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61075 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60948 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60829 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60943 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61338 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60762 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60824 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60830 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61535 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60898 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60815 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60962 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60957 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60944 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60921 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60759 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60897 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61518 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60958 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60922 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60937 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60875 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60766 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60751 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60768 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60743 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:61076 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60912 SYN_RECV
tcp 0 0 192.168.0.3:80 192.168.0.5:60816 SYN_RECV

Claro ejemplo de SYN Attack al Apache.

2) Mirando el server-status del Apache

Si miramos el server-status del apache veremos conexiones en estado "Reading" ("R" Reading Request).

El problema es que cuando el número de conexiones "Reading" llena el "MaxClients" del Apache no acepta nuevas peticiones, por lo que los nuevos clientes, aunque sean legítimos, no serán aceptados.

Podemos aumentar el valor del "MaxClients" para que no se llene la cola de peticiones y acepte a todos los clientes, sean atacantes o no.

Otra buena medida es bajar el valor del "Timeout" del Apache para que las peticiones "Reading" sean "matadas" rápidamente, antes que pueda llenarse el MaxClients a su tope.

Para aumentar el MaxClients en el Apache 2 basta con añadir la directiva:

ServerLimit 350 antes de MaxClients y ya dejará.

<IfModule prefork.c>
[..]
ServerLimit 500
MaxClients 450
[..]
</IfModule>

3) Mirando los logs del mod_evasive

Citar

Jun 22 18:24:04 lan mod_evasive[3835]: Blacklisting address 82.228.169.50: possible attack.
Jun 22 18:24:45 lan mod_evasive[3600]: Blacklisting address 81.206.164.163: possible attack.
Jun 22 18:25:46 lan mod_evasive[3589]: Blacklisting address 155.232.250.19: possible attack.
Jun 22 18:27:23 lan mod_evasive[3671]: Blacklisting address 83.227.217.2: possible attack.
Jun 22 18:28:10 lan mod_evasive[3673]: Blacklisting address 68.187.171.89: possible attack.
Jun 22 18:29:57 lan mod_evasive[3605]: Blacklisting address 70.143.2.130: possible attack.
Jun 22 18:30:45 lan mod_evasive[3803]: Blacklisting address 69.157.93.88: possible attack.
Jun 22 18:31:45 lan mod_evasive[10397]: Blacklisting address 146.64.81.22: possible attack.
Jun 22 18:35:01 lan mod_evasive[3794]: Blacklisting address 66.38.192.134: possible attack.
Jun 22 18:35:15 lan mod_evasive[3553]: Blacklisting address 81.190.204.64: possible attack.
Jun 22 18:40:10 lan mod_evasive[16602]: Blacklisting address 64.231.39.129: possible attack.
Jun 22 18:48:04 lan mod_evasive[16479]: Blacklisting address 84.99.195.100: possible attack.
Jun 22 18:48:12 lan mod_evasive[16467]: Blacklisting address 201.0.10.142: possible attack.
Jun 22 18:52:57 lan mod_evasive[16573]: Blacklisting address 219.95.39.242: possible attack.
Jun 22 18:53:07 lan mod_evasive[16534]: Blacklisting address 86.129.3.91: possible attack.
Jun 22 18:53:26 lan mod_evasive[16527]: Blacklisting address 62.254.0.32: possible attack.
Jun 22 18:54:41 lan mod_evasive[30473]: Blacklisting address 24.196.199.191: possible attack.
Jun 22 18:55:17 lan mod_evasive[30520]: Blacklisting address 142.161.157.227: possible attack.
Jun 22 18:55:24 lan mod_evasive[30461]: Blacklisting address 65.92.145.133: possible attack.
Jun 22 18:55:33 lan mod_evasive[30509]: Blacklisting address 88.111.227.200: possible attack.
Jun 22 18:56:13 lan mod_evasive[30473]: Blacklisting address 69.199.94.227: possible attack.
Jun 22 18:57:45 lan mod_evasive[30517]: Blacklisting address 86.125.135.212: possible attack.
Jun 22 18:57:54 lan mod_evasive[30479]: Blacklisting address 84.192.141.65: possible attack.
Jun 22 18:58:46 lan mod_evasive[30527]: Blacklisting address 83.140.97.106: possible attack.
Jun 22 18:59:31 lan mod_evasive[30469]: Blacklisting address 82.173.216.196: possible attack.
Jun 22 19:00:33 lan mod_evasive[30517]: Blacklisting address 80.176.157.245: possible attack.
Jun 22 19:00:38 lan mod_evasive[30470]: Blacklisting address 86.133.102.51: possible attack.
Jun 22 19:01:35 lan mod_evasive[30870]: Blacklisting address 24.42.134.253: possible attack.
Jun 22 19:01:48 lan mod_evasive[30509]: Blacklisting address 62.254.0.34: possible attack.
Jun 22 19:02:57 lan mod_evasive[31009]: Blacklisting address 81.227.219.125: possible attack.
Jun 22 19:03:29 lan mod_evasive[31056]: Blacklisting address 172.209.173.153: possible attack.
Jun 22 19:05:07 lan mod_evasive[31385]: Blacklisting address 84.6.12.110: possible attack.
Jun 22 19:06:52 lan mod_evasive[31008]: Blacklisting address 85.227.144.249: possible attack.
Jun 22 19:06:56 lan mod_evasive[31263]: Blacklisting address 213.222.156.222: possible attack.
Jun 22 19:07:13 lan mod_evasive[31393]: Blacklisting address 62.163.143.166: possible attack.
Jun 22 19:07:37 lan mod_evasive[31021]: Blacklisting address 62.135.101.73: possible attack.
Jun 22 19:08:03 lan mod_evasive[31251]: Blacklisting address 82.201.249.69: possible attack.
Jun 22 19:08:17 lan mod_evasive[31200]: Blacklisting address 81.62.65.53: possible attack.
Jun 22 19:11:04 lan mod_evasive[31263]: Blacklisting address 82.39.148.204: possible attack.
Jun 22 19:12:37 lan mod_evasive[31241]: Blacklisting address 213.222.154.13: possible attack.
Jun 22 19:13:54 lan mod_evasive[31027]: Blacklisting address 81.51.79.4: possible attack.
Jun 22 19:24:04 lan mod_evasive[31041]: Blacklisting address 84.221.118.156: possible attack.
Jun 22 19:48:47 lan mod_evasive[3400]: Blacklisting address 62.135.101.192: possible attack.
Jun 22 19:53:04 lan mod_evasive[31031]: Blacklisting address 62.30.33.13: possible attack.
Jun 22 19:54:32 lan mod_evasive[31016]: Blacklisting address 72.14.194.18: possible attack.
Jun 22 19:56:10 lan mod_evasive[31067]: Blacklisting address 198.96.34.58: possible attack.
Jun 22 20:03:24 lan mod_evasive[5144]: Blacklisting address 172.213.33.242: possible attack.
Jun 22 20:08:31 lan mod_evasive[5137]: Blacklisting address 83.241.11.16: possible attack.
Jun 22 20:21:59 lan mod_evasive[6645]: Blacklisting address 201.23.193.20: possible attack.
Jun 22 20:32:28 lan mod_evasive[7801]: Blacklisting address 212.38.134.172: possible attack.
Jun 22 20:45:46 lan mod_evasive[7836]: Blacklisting address 81.247.11.48: possible attack.
Jun 22 20:48:03 lan mod_evasive[7796]: Blacklisting address 70.245.98.186: possible attack.
Jun 22 20:49:38 lan mod_evasive[7832]: Blacklisting address 61.8.138.203: possible attack.
Jun 22 20:51:21 lan mod_evasive[7801]: Blacklisting address 201.132.197.161: possible attack.
Jun 22 20:57:18 lan mod_evasive[10426]: Blacklisting address 82.201.249.67: possible attack.
Jun 22 20:57:51 lan mod_evasive[7822]: Blacklisting address 81.77.26.162: possible attack.
Jun 22 21:00:25 lan mod_evasive[7817]: Blacklisting address 200.39.202.243: possible attack.
Jun 22 21:12:04 lan mod_evasive[7794]: Blacklisting address 84.27.139.25: possible attack.
Jun 22 21:22:27 lan mod_evasive[7816]: Blacklisting address 217.208.98.254: possible attack.

Si es un DDoS muy distribuido enseguida notaremos que muchas ip's diferente DoSean el Apache.

4) Mirando los logs del syslog (del kernel)

Citar

May 17 13:39:01 lan kernel: possible SYN flooding on port 80. Sending cookies.
May 17 13:39:02 lan kernel: ip_conntrack: table full, dropping packet.
May 17 13:39:35 lan kernel: NET: 4 messages suppressed.
May 17 13:39:35 lan kernel: ip_conntrack: table full, dropping packet.
May 17 13:39:38 lan kernel: NET: 1 messages suppressed.
May 17 13:39:38 lan kernel: ip_conntrack: table full, dropping packet.
May 17 13:39:43 lan kernel: NET: 6 messages suppressed.
May 17 13:39:43 lan kernel: ip_conntrack: table full, dropping packet.
May 17 13:39:48 lan kernel: NET: 4 messages suppressed.
May 17 13:39:48 lan kernel: ip_conntrack: table full, dropping packet.
May 17 13:39:52 lan kernel: NET: 9 messages suppressed.
May 17 13:39:52 lan kernel: ip_conntrack: table full, dropping packet.
May 17 13:39:57 lan kernel: NET: 15 messages suppressed.
May 17 13:39:57 lan kernel: ip_conntrack: table full, dropping packet.
May 17 13:40:01 lan kernel: possible SYN flooding on port 80. Sending cookies.

Líneas a mirar:

Citar

possible SYN flooding on port 80. Sending cookies.

"Sending Cookies" si lo tenemos activado en el /etc/sysctl.conf
# Enable TCP SYN Cookie Protection

Código:

net.ipv4.tcp_syncookies = 1

A veces es mejor deshabilitarlo:

Código:

net.ipv4.tcp_syncookies = 0

De esta manera podemos ver las ip's del ataque:

Citar

Jul 14 12:46:50 lan kernel: TCP: drop open request from 80.171.45.81/63069
Jul 14 12:46:55 lan kernel: NET: 1401 messages suppressed.
Jul 14 12:46:55 lan kernel: TCP: drop open request from 80.103.166.148/4403
Jul 14 12:46:59 lan kernel: NET: 1772 messages suppressed.
Jul 14 12:46:59 lan kernel: TCP: drop open request from 200.127.62.215/4019
Jul 14 12:47:05 lan kernel: NET: 2362 messages suppressed.
Jul 14 12:47:05 lan kernel: TCP: drop open request from 85.57.169.142/19899
Jul 14 12:47:11 lan kernel: NET: 2618 messages suppressed.
Jul 14 12:47:11 lan kernel: TCP: drop open request from 83.19.73.122/2710
Jul 14 12:47:14 lan kernel: NET: 898 messages suppressed.
Jul 14 12:47:14 lan kernel: TCP: drop open request from 80.235.39.64/3554
Jul 14 12:47:19 lan kernel: NET: 1120 messages suppressed.
Jul 14 12:47:19 lan kernel: TCP: drop open request from 80.171.45.81/62095
Jul 14 12:47:24 lan kernel: NET: 1714 messages suppressed.
Jul 14 12:47:24 lan kernel: TCP: drop open request from 84.62.152.44/34014
Jul 14 12:47:29 lan kernel: NET: 2274 messages suppressed.
Jul 14 12:47:29 lan kernel: TCP: drop open request from 200.127.62.215/3207
Jul 14 12:47:34 lan kernel: NET: 1552 messages suppressed.
Jul 14 12:47:34 lan kernel: TCP: drop open request from 80.103.166.148/4797
Jul 14 12:47:39 lan kernel: NET: 4044 messages suppressed.
Jul 14 12:47:39 lan kernel: TCP: drop open request from 80.235.39.64/2678
Jul 14 12:47:44 lan kernel: NET: 4360 messages suppressed.
Jul 14 12:47:44 lan kernel: TCP: drop open request from 80.103.166.148/1312
Jul 14 13:04:15 lan kernel: TCP: drop open request from 200.14.237.83/4787
Jul 14 13:04:22 lan kernel: NET: 147 messages suppressed.
Jul 14 13:04:22 lan kernel: TCP: drop open request from 81.38.172.161/4892
Jul 14 13:04:30 lan kernel: NET: 6 messages suppressed.
Jul 14 13:04:30 lan kernel: TCP: drop open request from 200.14.237.83/4934
Jul 14 13:04:30 lan kernel: TCP: drop open request from 200.14.237.83/4935
Jul 14 13:04:38 lan kernel: NET: 76 messages suppressed.
Jul 14 13:04:38 lan kernel: TCP: drop open request from 81.84.212.34/2861
Jul 14 13:04:40 lan kernel: NET: 269 messages suppressed.
Jul 14 13:04:40 lan kernel: TCP: drop open request from 200.14.237.83/3070
Jul 14 13:04:45 lan kernel: NET: 287 messages suppressed.
Jul 14 13:04:45 lan kernel: TCP: drop open request from 81.203.228.102/4400
Jul 14 13:04:50 lan kernel: NET: 98 messages suppressed.
Jul 14 13:04:50 lan kernel: TCP: drop open request from 81.84.212.34/3961
Jul 14 13:04:54 lan kernel: NET: 245 messages suppressed.
Jul 14 13:04:54 lan kernel: TCP: drop open request from 200.84.169.200/1183
Jul 14 13:05:00 lan kernel: NET: 1787 messages suppressed.
Jul 14 13:05:00 lan kernel: TCP: drop open request from 81.203.228.102/2050
Jul 14 13:05:04 lan kernel: NET: 3208 messages suppressed.
Jul 14 13:05:04 lan kernel: TCP: drop open request from 86.212.167.27/4720
Jul 14 13:05:09 lan kernel: NET: 2031 messages suppressed.
Jul 14 13:05:09 lan kernel: TCP: drop open request from 81.203.228.102/1794
Jul 14 13:05:14 lan kernel: NET: 2221 messages suppressed.
Jul 14 13:05:14 lan kernel: TCP: drop open request from 81.38.172.161/4908
Jul 14 13:05:21 lan kernel: NET: 730 messages suppressed.
Jul 14 13:05:21 lan kernel: TCP: drop open request from 81.203.228.102/1430
Jul 14 13:05:25 lan kernel: NET: 234 messages suppressed.
Jul 14 13:05:25 lan kernel: TCP: drop open request from 81.203.228.102/2939
Jul 14 13:05:30 lan kernel: NET: 1594 messages suppressed.
Jul 14 13:05:30 lan kernel: TCP: drop open request from 200.14.237.83/3876
Jul 14 13:05:36 lan kernel: NET: 633 messages suppressed.
Jul 14 13:05:36 lan kernel: TCP: drop open request from 86.212.167.27/1116
Jul 14 13:05:39 lan kernel: NET: 970 messages suppressed.
Jul 14 13:05:39 lan kernel: TCP: drop open request from 81.38.172.161/3040
Jul 14 13:05:45 lan kernel: NET: 548 messages suppressed.
Jul 14 13:05:45 lan kernel: TCP: drop open request from 81.203.228.102/2119
Jul 14 13:05:50 lan kernel: NET: 421 messages suppressed.
Jul 14 13:05:50 lan kernel: TCP: drop open request from 81.203.228.102/2478
Jul 14 13:05:56 lan kernel: NET: 379 messages suppressed.
Jul 14 13:05:56 lan kernel: TCP: drop open request from 81.203.228.102/4005
Jul 14 13:05:59 lan kernel: NET: 891 messages suppressed.
Jul 14 13:05:59 lan kernel: TCP: drop open request from 81.38.172.161/3568
Jul 14 13:06:04 lan kernel: NET: 2221 messages suppressed.
Jul 14 13:06:04 lan kernel: TCP: drop open request from 81.203.228.102/4532
Jul 14 13:06:09 lan kernel: NET: 243 messages suppressed.
Jul 14 13:06:09 lan kernel: TCP: drop open request from 81.203.228.102/1939
Jul 14 13:06:14 lan kernel: NET: 2166 messages suppressed.
Jul 14 13:06:14 lan kernel: TCP: drop open request from 81.38.172.161/2137
Jul 14 13:06:19 lan kernel: NET: 2071 messages suppressed.
Jul 14 13:06:19 lan kernel: TCP: drop open request from 81.38.172.161/3136
Jul 14 13:06:24 lan kernel: NET: 2069 messages suppressed.
Jul 14 13:06:24 lan kernel: TCP: drop open request from 81.84.212.34/4600
Jul 14 13:06:29 lan kernel: NET: 1797 messages suppressed.
Jul 14 13:06:29 lan kernel: TCP: drop open request from 86.212.167.27/3171
Jul 14 13:06:35 lan kernel: NET: 1292 messages suppressed.
Jul 14 13:06:35 lan kernel: TCP: drop open request from 81.203.228.102/1394
Jul 14 13:06:39 lan kernel: NET: 715 messages suppressed.

Citar

May 17 14:13:24 lan kernel: ip_conntrack: table full, dropping packet.

Tabla llena. Tenemos un problema porque no admitiremos más conexiones aunque sean legítimas.

Podemos aumentar el valor de dicha tabla si nuestra red da para más.

Directamente:

Código:

echo "65535" > /proc/sys/net/ipv4/ip_conntrack_max

Para que el valor quede guardardo y no se pierda al reiniciar, debemos añadirlo en el sysctl.conf

Código:

net.ipv4.ip_conntrack_max = 65535

Recuerda reiniciar la red para aplicar los cambios en el /proc (service network restart).

Paquetes Marcianos:

Citar

Aug 31 12:41:29 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 12:45:07 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 12:52:57 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 12:58:55 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 13:08:12 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 13:12:03 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 13:34:38 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 13:37:38 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 13:52:42 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 13:56:18 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 13:59:54 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 14:13:32 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 14:38:08 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 14:43:42 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 14:50:05 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 14:51:05 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 14:57:58 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 15:05:27 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 15:06:14 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0
Aug 31 15:09:08 lan kernel: martian source 192.168.0.10 from 0.0.0.0, on dev eth0

Son paquetes inesperados que llegan por un camino por el cual no pueden llegar indica algún problema de audacia (cracker).

Usando paquetes como éstos se pueden atacar vulnerabilidades remotas en stacks TCP/IP

5) Mirando las gráficas del MRTG, RRDtool

Si ves que el tráfico inbound sube hasta los 100mbps es que te están doseando hehehe.

B) Intentar parar el ataque

1) - mod_evasive

Web Oficial:
http://www.nuclearelephant.com/projects/mod_evasive/

Consideramos que 50 conexiones por segundo a 2 páginas es suficiente motivo como para bloquear esa ip:

Código:

<IfModule mod_evasive.c>
DOSHashTableSize 3097
DOSPageCount 2
DOSSiteCount 50
DOSPageInterval 1
DOSSiteInterval 1
DOSBlockingPeriod 900
</IfModule>

Igual que el anterior pero con 50 peticiones en un segundo a 1 sola página:

Código:

<IfModule mod_evasive.c>
DOSHashTableSize 3097
DOSPageCount 1
DOSSiteCount 50
DOSPageInterval 1
DOSSiteInterval 1
</IfModule>

Si queremos bloquear las ips que floodean, podemos usar el iptables:

DOSSystemCommand "sudo -u root -c '/sbin/iptables -A INPUT -s %s -j DROP"

Recordar mirar el syslog por si hay posibles falsos positivos (ip's que no hacian flood).

Para evitar falsos positivos:

Código:

<IfModule mod_evasive.c>
# añadir estas líneas que corresponden a rangos de los bots de google
DOSWhitelist 66.249.65.*
DOSWhitelist 66.249.66.*
</IfModule>

Importante:

Para que el mod_evasive funcione correctamente deberás modificar el:

Citar

MaxRequestsPerChild 0

Para poner un valor alto pero nunca ilimitado (0).

Citar

MaxRequestsPerChild 10000

Config ejemplo:
http://www.eth0.us/mod_evasive

2 - mod_security

El único problema del mod_security es que necesitamos al menos un argumento para detectar el ataque.

En el ejemplo usamos en http_referer y el User Agent para detectar el DDoS:

Bloqueando un ataque Iframe
http://foro.elhacker.net/index.php/topic,127481.0.html

3- tcplimit, ipdrop, ipblock

Usando firewalls dinámicos.

4- Optimizando y asegurando la red con el sysctl.conf

cat /proc/sys/net/ipv4/tcp_syncookies

# Enable IP spoofing protection, turn on Source Address Verification

net.ipv4.conf.all.rp_filter = 1

# Enable TCP SYN Cookie Protection

net.ipv4.tcp_syncookies = 1

# Enable ignoring broadcasts request

net.ipv4.icmp_echo_ignore_broadcasts = 1

1). Activate SynCookies protection

It works by sending out 'syncookies' when the
syn backlog queue of a socket overflows.

=> echo 1 >/proc/sys/net/ipv4/tcp_syncookies

or

=> /sbin/sysctl -w net.ipv4.tcp_syncookies=1

2). Disable source routing

=> for f in /proc/sys/net/ipv4/conf/*/accept_source_route; do
echo 0 > $f
done

or

=> /sbin/sysctl -w net.ipv4.conf.all.accept_source_route=0

3). Reverse Path Filtering

Reject incoming packets if their source address doesn't match
the network interface that they're arriving on

=> for f in /proc/sys/net/ipv4/conf/*/rp_filter; do
echo 1 > $f
done

or

=> /sbin/systcl -w net.ipv4.conf.all.rp_filter=1

4). Log RP filter dropped packets (martians)

=> for f in /proc/sys/net/ipv4/conf/*/log_martians; do
echo 1 > $f
done

or

=> /sbin/sysctl -w net.ipv4.conf.all.log_martians=1

5). Maximal number of remembered connection requests

=> /sbin/sysctl -w net.ipv4.tcp_max_syn_backlog=256

6). How may times to retry before killing TCP connection

(default 7 on most systems)

=> /sbin/sysctl -w net.ipv4.tcp_orphan_retries=4

7). Number of SYN packets the kernel will send before giving up

=> /sbin/sysctl -w net.ipv4.tcp_syn_retries=5

8). Disable broadcast icmp reply

=> /sbin/sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=1

9). Ignore Bogus icmp packets

=> /sbin/sysctl -w net.ipv4.icmp_ignore_bogus_error_responses=1

10). Disable ICMP redirect

=> echo 0 >/proc/sys/net/ipv4/conf/all/accept_redirects
=> echo 0 >/proc/sys/net/ipv4/conf/all/send_redirects

or

=> /sbin/sysctl -w net.ipv4.conf.all.accept_redirects=0
=> /sbin/sysctl -w net.ipv4.conf.all.send_redirects=0

11). Disable timestamps

=> echo 0 >/proc/sys/net/ipv4/tcp_timestamps

or

=> /sbin/sysctl -w net.ipv4.tcp_timestamps=0

12). Reduce DOS ability by reducing timeouts

=> echo 30 >/proc/sys/net/ipv4/tcp_fin_timeout
=> echo 1800 >/proc/sys/net/ipv4/tcp_keepalive_time
=> echo 0 >/proc/sys/net/ipv4/tcp_window_scaling
=> echo 0 >/proc/sys/net/ipv4/tcp_sack

or

=> /sbin/sysctl -w net.ipv4.tcp_fin_timeout=30
=> /sbin/sysctl -w net.ipv4.tcp_keepalive_time=1800
=> /sbin/sysctl -w net.ipv4.tcp_window_scaling=0
=> /sbin/sysctl -w net.ipv4.tcp_sack=0

- Lista de todas las variables del TCP:
http://ipsysctl-tutorial.frozentux.net/chunkyhtml/tcpvariables.html

- Lista de Variables del /proc/sys/net/ipv4/* (con varlores por defecto y explicaciones)
http://ipsysctl-tutorial.frozentux.net/other/ip-sysctl.txt

Más ejemplos de configuración completa del sysctl.conf en las referencias del documento.

5- APF Firewall con el módulo anti-ddos

Código:

wget http://www.rfxnetworks.com/downloads/apf-current.tar.gz
tar xvzf apf-current.tar.gz
cd apf-0.9.6-1/
./install.sh

service apf start
/usr/local/sbin/apf -s

Fichero de configuración:

Citar

/etc/apf/conf.apf

Despues de hacer las pruebas dejar:
DEVEL_MODE="0"

Si nos sale un error parecido a este:
apf(9413): unable to load iptables module (ip_tables), aborting.

Cambiamos esto:
SET_MONOKERN="1"

Puertos que queremos abrir (inbound)
IG_TCP_CPORTS="21,22,25,53,80,110"

Si queres bloquear todo el tráfico de salida lo ponemos en 1 (outbound)
EGF="0"

Si queremos usar el módulo antddos poner a 1:
USE_AD="0"

Log:
/var/log/apf_log

Para ver los paquetes que dropeamos:
LOG_DROP="1"

Lo guardará en el syslog, ejemplo:

Proto= Protocolo
SRC= ip origen
SPT= Source Port (puerto d origen)
DST= Destination Port (puerto destino)

Citar

Oct 20 13:59:27 ns2 kernel: ** SANITY ** IN=eth0 OUT= SRC=213.27.201.254 DST=192.168.0.3 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=18779 PROTO=TCP SPT=11629 DPT=80 WINDOW=0 RES=0x00 RST FIN URGP=0
Oct 20 14:00:16 ns2 kernel: ** SANITY ** IN=eth0 OUT= SRC=213.27.201.254 DST=192.168.0.3 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=20376 PROTO=TCP SPT=27734 DPT=80 WINDOW=0 RES=0x00 RST FIN URGP=0
Oct 20 14:00:17 ns2 kernel: ** SANITY ** IN=eth0 OUT= SRC=213.27.201.254 DST=192.168.0.3 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=20382 PROTO=TCP SPT=25943 DPT=80 WINDOW=0 RES=0x00 RST FIN URGP=0
Oct 20 14:00:17 ns2 kernel: ** SANITY ** IN=eth0 OUT= SRC=213.27.201.254 DST=192.168.0.3 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=20387 PROTO=TCP SPT=19026 DPT=80 WINDOW=0 RES=0x00 RST FIN URGP=0
Oct 20 14:00:17 ns2 kernel: ** SANITY ** IN=eth0 OUT= SRC=213.27.201.254 DST=192.168.0.3 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=20397 PROTO=TCP SPT=2155 DPT=80 WINDOW=0 RES=0x00 RST FIN URGP=0
Oct 20 14:00:17 ns2 kernel: ** SANITY ** IN=eth0 OUT= SRC=213.27.201.254 DST=192.168.0.3 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=20407 PROTO=TCP SPT=9294 DPT=80 WINDOW=0 RES=0x00 RST FIN URGP=0
Oct 20 14:00:22 ns2 kernel: ** SANITY ** IN=eth0 OUT= SRC=213.27.201.254 DST=192.168.0.3 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=20687 PROTO=TCP SPT=9269 DPT=80 WINDOW=0 RES=0x00 RST FIN URGP=0
Oct 20 14:00:22 ns2 kernel: ** SANITY ** IN=eth0 OUT= SRC=213.27.201.254 DST=192.168.0.3 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=20694 PROTO=TCP SPT=27223 DPT=80 WINDOW=0 RES=0x00 RST FIN URGP=0
Oct 20 14:00:23 ns2 kernel: ** SANITY ** IN=eth0 OUT= SRC=213.27.201.254 DST=192.168.0.3 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=20830 PROTO=TCP SPT=30938 DPT=80 WINDOW=0 RES=0x00 RST FIN URGP=0
Oct 20 14:00:25 ns2 kernel: ** SANITY ** IN=eth0 OUT= SRC=213.27.201.254 DST=192.168.0.3 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=21038 PROTO=TCP SPT=5377 DPT=80 WINDOW=0 RES=0x00 RST FIN URGP=0
Oct 20 14:00:27 ns2 kernel: ** SANITY ** IN=eth0 OUT= SRC=213.27.201.254 DST=192.168.0.3 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=21219 PROTO=TCP SPT=13341 DPT=80 WINDOW=0 RES=0x00 RST FIN URGP=0
Oct 20 14:00:42 ns2 kernel: ** SANITY ** IN=eth0 OUT= SRC=213.27.201.254 DST=192.168.0.3 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=21990 PROTO=TCP SPT=22960 DPT=80 WINDOW=0 RES=0x00 RST FIN URGP=0
Oct 20 14:02:32 ns2 kernel: ** SANITY ** IN=eth0 OUT= SRC=213.27.201.254 DST=192.168.0.3 LEN=40 TOS=0x00 PREC=0x00 TTL=54 ID=26386 PROTO=TCP SPT=2826 DPT=80 WINDOW=0 RES=0x00 RST FIN URGP=0

Recuerda que para usar el antidos debes añadir el cron job:

*/8 * * * * root /etc/apf/ad/antidos -a >> /dev/null 2>&1

http://www.r-fx.org/apf/README.antidos

KISS My Firewall es una alternativa.

Script PHP
http://www.prism-hosting.com/AntiDoS

6- Parar el botnet

ZmbScap - Zombie Scapper - Stoopt DDoS Programs
http://www.metaeye.org/projects/zmbscap/

Tracking Botnets - Bot-Commands
http://www.honeynet.org/papers/bots/botnet-commands.html

Tracking Botnets
http://www.honeynet.org/papers/bots/

Tracking Botnets - DDoS-attacks
http://www.honeynet.org/papers/bots/botnet-ddos.html

Phatbot Trojan Analysis
http://www.lurhq.com/phatbot.html

F-Bot by f-secure- Elimina el Agobot y todas sus variantes
http://www.f-secure.com/tools/f-bot.zip

Nepenthes - Autoinfecarse sin peligro para analizar
http://nepenthes.mwcollect.org/

honeytrap – trap attacks against tcp services
http://honeytrap.sourceforge.net/

7- Usando reglas del iptables

Código:

# todo el trafico syn
-P INPUT DROP
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p tcp ! --syn -j REJECT --reject-with tcp-reset
-A INPUT -m state --state INVALID -j DROP
-P OUTPUT DROP
-A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A OUTPUT -p tcp ! --syn -j REJECT --reject-with tcp-reset
-A OUTPUT -m state --state INVALID -j DROP
-P FORWARD DROP
-A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
-A FORWARD -p tcp ! --syn -j REJECT --reject-with tcp-reset
-A FORWARD -m state --state INVALID -j DROP
-A INPUT -i lo -j ACCEPT
-A OUTPUT -o lo -j ACCEPT
-A FORWARD -i lo -o lo -j ACCEPT

# sube las cargas pero muchos wwww buena señal
-A INPUT -p tcp --syn -j REJECT --reject-with icmp-port-unreachable

# la que mejor va
-N syn-flood
-A syn-flood -m limit --limit 100/second --limit-burst 150 -j RETURN
-A syn-flood -j LOG --log-prefix "SYN flood: "
-A syn-flood -j DROP

# igual que el de arriba pero muy bestia
-N syn-flood
-A INPUT -i eth0:2 -p tcp --syn -j syn-flood
-A syn-flood -m limit --limit 1/s --limit-burst 4 -j RETURN
-A syn-flood -j DROP

-A INPUT -p tcp -m tcp --tcp-flags SYN,RST,ACK SYN -m limit --limit
1/sec -j ACCEPT
-A INPUT -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK RST -m limit
--limit 1/sec -j ACCEPT

# no es muy efectivo
-A INPUT -s 0/0 -p tcp --syn --source-port 1000:5000
--destination-port 80 -j DROP

# no es muy efectivo
-A INPUT -p tcp -m tcp --dport 80 --sport 1000:5000 --tcp-flags SYN SYN -j DROP

# Descartar paquetes mal formados

-N PKT_FAKE
-A PKT_FAKE -m state --state INVALID -j DROP
-A PKT_FAKE -p tcp --dport 80 --tcp-flags ALL ACK,RST,SYN,FIN -j DROP
-A PKT_FAKE -p tcp --dport 80 --tcp-flags SYN,FIN SYN,FIN -j DROP
-A PKT_FAKE -p tcp --dport 80 --tcp-flags SYN,RST SYN,RST -j DROP
-A PKT_FAKE -p tcp --dport 80 ! --syn -m state --state NEW -j DROP
-A PKT_FAKE -f -j DROP
-A PKT_FAKE -j RETURN

# syn-flood
-N syn-flood
-A INPUT -i eth+ -p tcp --tcp-flags SYN,ACK,FIN,RST RST -j syn-flood
-A FORWARD -i eth+ -p tcp --tcp-flags SYN,ACK,FIN,RST RST -j syn-flood
-A syn-flood -m limit --limit 4/s --limit-burst 16 -j RETURN
-A syn-flood -m limit --limit 75/s --limit-burst 100 -j RETURN
-A syn-flood -j LOG --log-prefix "SYN FLOOD " --log-tcp-sequence --log-tcp-options --log-ip-options -m limit --limit 1/second
-A syn-flood -j DROP

# By pepel. Requiere módulo "recent"
-A INPUT -p tcp --dport 80 -m state --state NEW -m recent --set
-A INPUT -p tcp --dport 80 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROP

#explicación:
Se añade cada ip que se conecte a la tabla de recent
Por por cada ip en la tabla de recent si hace mas de x hits en x segundos, se dropea.

iptables -I INPUT -p tcp –syn -m recent –set
iptables -I INPUT -p tcp –syn -m recent –update –seconds 10 –hitcount 30 -j DROP

Lo que hace es contar el número de paquetes SYN (inicio de conexión TCP) para
cada dirección IP en los últimos 10 segundos. Si llega a 30 descarta ese paquete por
lo que no se establecerá la conexión (el TCP volverá a intentar varias veces,
cuando baje del límite podrá establecerse).

8- Usando el mod_throttle

http://www.snert.com/Software/mod_throttle/

Otros:

Mod_Throttle, mod_bandwidth, mod_iplimit, mod_tsunami, mod_limitipconn.c

Para Apache 2:
mod_cband

Código:

cd /usr/src
wget http://www.snert.com/Software/mod_throttle/mod_throttle312.tgz
tar zxvf mod_throttle312.tgz
cd mod_throttle-3.1.2
pico Makefile
Then edit the line that reads:
APXS=apxs
And change it to read:
APXS=/usr/local/apache/bin/apxs
make
make install
service httpd restart

Citar

<IfModule mod_throttle.c>
ThrottlePolicy Volume 10G 30d
</IfModule>
<Location /throttle-me>
SetHandler throttle-me
</Location>

http://www.webhostgear.com/160.html

C) Referencias

- Opciones de seguridad en Linux a través de /proc (I) y (II)
http://www.elhacker.net/opciones-seguridad-linux-proc.html

- Syctl.conf Hardening
http://www.eth0.us/sysctl

- Ipsysctl tutorial 1.0.4
http://ipsysctl-tutorial.frozentux.net/ipsysctl-tutorial.html

- Hardening the TCP/IP stack to SYN attacks
http://www.securityfocus.com/infocus/1729

- DDOS and SYN_Recv Attacks And some SOlutions
http://www.vbulletin.com/forum/showthread.php?t=126699

- Distributed Reflection Denial of Service
http://www.grc.com/dos/drdos.htm

- Dynamic iptables firewalls
http://www-128.ibm.com/developerworks/library/l-fw/

- Preventing DDoS Attacks
http://www.linuxsecurity.com/content/view/121960/49/

- Distributed Denial of Service (DDoS) Attacks/tools
http://staff.washington.edu/dittrich/misc/ddos/

- Linux firewall rulesets and snippets of rule sets
http://www.gotroot.com/tiki-index.php?page=Linux%20Firewall%20rules