6.5

Table Of Contents
2 Congure the host system to ignore IPv4 ICMP redirect messages.
a Open the /etc/sysctl.conf le.
b If the values are not set to 0, add the following entries to the le or update the existing entries
accordingly. Set the value to 0.
net.ipv4.conf.all.accept_redirects=0
net.ipv4.conf.default.accept_redirects=0
c Save the changes and close the le.
Configure the Host System to Ignore IPv6 ICMP Redirect Messages
As a security best practice, verify that the host system ignores IPv6 Internet Control Message Protocol
(ICMP) redirect messages. A malicious ICMP redirect message might allow a man-in-the-middle aack to
occur. Routers use ICMP redirect messages to tell hosts that a more direct route exists for a destination.
These messages modify the host's route table and are unauthenticated.
Procedure
1 Run the # grep [01] /proc/sys/net/ipv6/conf/*/accept_redirects|egrep "default|all" command
on the host system and check whether it ignores IPv6 redirect messages.
2 Congure the host system to ignore IPv6 ICMP redirect messages.
a Open the /etc/sysctl.conf to congure the host system to ignore the IPv6 redirect messages.
b If the values are not set to 0, add the following entries to the le or update the existing entries
accordingly. Set the value to 0.
net.ipv6.conf.all.accept_redirects=0
net.ipv6.conf.default.accept_redirects=0
c Save the changes and close the le.
Configure the Host System to Deny IPv4 ICMP Redirects
As a security best practice, verify that the host system denies IPv4 Internet Control Message Protocol (ICMP)
redirects. Routers use ICMP redirect messages to inform servers that a direct route exists for a particular
destination. These messages contain information from the system's route table that might reveal portions of
the network topology.
Procedure
1 Run the # grep [01] /proc/sys/net/ipv4/conf/*/send_redirects|egrep "default|all" on the host
system to verify whether it denies IPv4 ICMP redirects.
2 Congure the host system to deny IPv4 ICMP redirects.
a Open the /etc/sysctl.conf le to congure the host system.
b If the values are not set to 0, add the following entries to the le or update the existing entries
accordingly. Set the value to 0.
net.ipv4.conf.all.send_redirects=0
net.ipv4.conf.default.send_redirects=0
c Save the changes and close the le.
Chapter 4 Network Security and Secure Communication
VMware, Inc. 43