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Sunday, October 24, 2010
Question 148
How would you prevent a router from returning its IP address during a Reconnaissance Attack.
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ip unreachables - allows the generation of ICMP unreachable messages - This is the default behavior.
To prevent your router from generating unreachable messages use "no ip unreachables" interface configuration command.
quick test of this command, and a little insight on how cisco(unix) udp based traceroute works with the icmp unreachables
R1 <-> R2
R1#traceroute 1.1.1.2
Type escape sequence to abort.
Tracing the route to 1.1.1.2
1 1.1.1.2 188 msec 108 msec *
R2(config-if)#do sh ip interface f0/0 | i ICMP
ICMP redirects are always sent
ICMP unreachables are always sent
ICMP mask replies are never sent
R2(config-if)#
*Mar 1 00:07:35.787: ICMP: dst (1.1.1.2) port unreachable sent to 1.1.1.1
*Mar 1 00:07:44.935: ICMP: dst (1.1.1.2) port unreachable sent to 1.1.1.1
R2(config-if)#no ip unreachables
R2(config-if)#do sh ip interface f0/0 | i ICMP
ICMP redirects are always sent
ICMP unreachables are never sent
ICMP mask replies are never sent
R1#traceroute 1.1.1.2
Type escape sequence to abort.
Tracing the route to 1.1.1.2
1 * * *
2 * * *
3 * * *
4 * * *
5 * * *
6 * * *
7 * * *
8 * * *
9 * * *
10 * * *
11 * * *
12 * * *
13 * * *
and on and on...
Good stuff
Examples provide so much more insight.
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