HP-UX IPSec Version A.03.00 Administrator's Guide

Maximizing Security
A system can have both “public” interface IP addresses and “private” interface IP addresses. A
public interface IP address is an IP address configured on a Network Interface Card (NIC)
connected to a public network. A private interface IP address is an IP address configured on a
NIC connected to a private internal network. If you have a system with both a public interface
IP address and a private interface IP address, do not assume that all packets processed by the
private interface originated from the private network. Do not configure any “open” IPsec policies
that allow most or all packets sent to the private interface IP address to pass in clear text.
If you configure an open IPsec policy for a private interface IP address on a system that also has
public interfaces, intruders may be able to access services or ports bound to the private interface
IP address from other NICs on the system, even if the other interface IP addresses are secured
by IPsec policies. Intruders may access services or ports bound to the private interface IP address,
even if the intruders are not directly connected to the private interface.
Bypass List
Configuring an entry in the bypass list has the same effect as configuring an open IPsec policy,
so the same conditions exist. Intruders may be able to access services or ports bound to the
address in the bypass list from other interfaces on the system, even if the other interfaces are
secured by IPsec policies. Intruders may access services or ports bound to the address in the
bypass list even if the intruders are not directly connected to the interface in the bypass list.
HP recommends that you do not configure open IPsec policies, or entries in the bypass list for
private interfaces on systems that also have public interfaces, or on systems on which you are
using HP-UX IPSec as a filter or firewall to protect your network.
Strong End System Model
To maximize security when using open policies or the bypass list, HP recommends that you
enable the strong end system (ES) model, which is described in RFC 1122. When the strong ES
model is enabled, a system cannot act as an IP router. A system with the strong ES model enabled
silently drops incoming IP packets with destination IP addresses that do not match the interface
address. The source IP address of an outbound packet must match the address of the interface
used to transmit the packet.
To enable the strong ES model, use the ndd utility. For example, the following ndd command
enables the strong ES model until the system is shut down or restarted:
ndd -set /dev/ip ip_strong_es_model 1
You can also enable the strong ES model at system startup time by editing the /etc/
rc.config.d/nddconf file. See ndd(1M) for more information.
66 Configuring HP-UX IPSec