Tcp Keepalive; Firewall Configuration (Ip Filtering); Structure Of The Iptables - Avocent Cyclades ACS Command Reference Manual

30 ACS Advanced Console Server Command Reference Guide

TCP keepalive

The objective of this feature is to allow the ACS console server to recognize when the socket client,
SSH or Telnet goes down without closing the connection properly. The TCP engine of the ACS
console server sends a TCP keepalive message (ACK) to the client. If the maximum retry number is
reached without an answer from the client, the connection is closed.
To configure TCP keepalive:
1. Configure the pool interval in milliseconds.
cli> config physicalports all other tcpkeepalive <number>
2. Activate and save your configuration.

Firewall Configuration (IP Filtering)

IP filtering consist of blocking the passage of IP packets based on rules defined in the
characteristics of the packets, such as the contents of the IP header, the input/output interface or the
protocol. This feature is used mainly in firewall applications, which filter the packets that could
crack the network system or generate unnecessary traffic.
Network Address Translation (NAT) allows the IP packets to be translated from local network to
global network and vice-versa. This feature is particularly useful when there is demand for more IP
addresses in the local network than available as global IP addresses. In the ACS console server, this
feature is used mainly for clustering (one master console server works as the interface between the
global network and the slave console servers).
The ACS console server uses the Linux utility iptables to set up, maintain and inspect both the filter
and the NAT tables of IP packet rules in the Linux kernel. Besides filtering or translating packets,
the iptables utility is able to count the packets which match a rule and to create logs for
specific rules.

Structure of the iptables

The iptables are structured in three levels: table, chain and rule. A table may contain several chains
and each chain may contain several rules.
Table
The table indicates how the iptables works. There are currently three independent tables supported
by the iptables but only two are used.
• filter: This is the default table.
• nat: This table is consulted when a packet that creates a new connection is encountered.