6.5

Table Of Contents
2 Set the queue size for TCP backlog.
a Open the /etc/sysctl.conf le in a text editor.
b Set the default TCP backlog queue size by adding the following entry to the le.
net.ipv4.tcp_max_syn_backlog=1280
c Save your changes and close the le.
Deny ICMPv4 Echoes to Broadcast Address
Responses to broadcast Internet Control Message Protocol (ICMP) echoes provide an aack vector for
amplication aacks and can facilitate network mapping by malicious agents. Conguring your system to
ignore ICMPv4 echoes provides protection against such aacks.
Procedure
1 Run the # cat /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts command to verify that the system is
not sending responses to ICMP broadcast address echo requests.
2 Congure the host system to deny ICMPv4 broadcast address echo requests.
a Open the /etc/sysctl.conf le in a text editor.
b If the value for this entry is not set to 1, add the net.ipv4.icmp_echo_ignore_broadcasts=1 entry.
c Save the changes and close the le.
Configure the Host System to Disable IPv4 Proxy ARP
IPv4 Proxy ARP allows a system to send responses to ARP requests on one interface on behalf of hosts
connected to another interface. You must disable IPv4 Proxy ARP to prevent unauthorized information
sharing. Disable the seing to prevent leakage of addressing information between the aached network
segments.
Procedure
1 Run the # grep [01] /proc/sys/net/ipv4/conf/*/proxy_arp|egrep "default|all" command to verify
whether the Proxy ARP is disabled.
2 Congure the host system to disable IPv4 Proxy ARP.
a Open the /etc/sysctl.conf le in a text editor.
b If the values are not set to 0, add the entries or update the existing entries accordingly. Set the value
to 0.
net.ipv4.conf.all.proxy_arp=0
net.ipv4.conf.default.proxy_arp=0
c Save any changes you made and close the le.
Configure the Host System to Ignore IPv4 ICMP Redirect Messages
As a security best practice, verify that the host system ignores IPv4 Internet Control Message Protocol
(ICMP) redirect messages. A malicious ICMP redirect message can allow a man-in-the-middle aack to
occur. Routers use ICMP redirect messages to notify 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/ipv4/conf/*/accept_redirects|egrep "default|all" command
on the host system to check whether the host system ignores IPv4 redirect messages.
Secure Configuration
42 VMware, Inc.