6.6

Table Of Contents
Configure the Host System to Use IPv4 TCP Syncookies
As a security best practice, verify that the host system uses IPv4 Transmission Control Protocol (TCP)
Syncookies. A TCP SYN ood aack might cause a denial of service by lling a system's TCP connection
table with connections in the SYN_RCVD state. Syncookies are used so as not to track a connection until a
subsequent ACK is received, verifying that the initiator is aempting a valid connection and is not a ood
source.
This technique does not operate in a fully standards-compliant manner, but is only activated when a ood
condition is detected, and allows defence of the system while continuing to service valid requests.
Procedure
1 Run the # cat /proc/sys/net/ipv4/tcp_syncookies command to verify whether the host system uses
IPv4 TCP Syncookies.
2 Congure the host system to use IPv4 TCP syncookies.
a Open the /etc/sysctl.conf to congure the host system.
b If the value is not set to 1, add the following entry to the le or update the existing entry
accordingly. Set the value to 1.
net.ipv4.tcp_syncookies=1
c Save the changes and close the le.
Configure the Host System to Deny IPv6 Router Advertisements
As a security best practice, verify that the host system denies the acceptance of router advertisements and
Internet Control Message Protocol (ICMP) redirects unless necessary. A feature of IPv6 is how systems can
congure their networking devices by automatically using information from the network. From a security
perspective, it is preferable to manually set important conguration information rather than accepting it
from the network in an unauthenticated way.
Procedure
1 Run the # grep [01] /proc/sys/net/ipv6/conf/*/accept_ra|egrep "default|all" command on the
host system to verify whether the system denies the acceptance of router advertisements and ICMP
redirects unless necessary.
2 Congure the host system to deny IPv6 router advertisements.
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.ipv6.conf.all.accept_ra=0
net.ipv6.conf.default.accept_ra=0
c Save the changes and close the le.
Secure Configuration
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