Lantronix SCS Reference Manual page 202

Security
Denies X-Windows traffic, but permits incoming TCP/IP traffic to ports 1023 and higher.
Permits DNS queries to the local Domain Name Server, 192.0.1.101
Permits ICMP (Internet Control Message Protocol) messages
Permits outgoing finger requests
The firewall will be named fw_i. Packets that do not specifically match the filters in fw_i will be denied
passage through the SCS.
Due to the length of the commands in the following examples, the keywords
Note:
Define and Filter are shortened to Def and Filt.
The Set/Define Filter Create command is used to create the firewall.
To prevent IP spoofing, the Define Filter Add Deny IP SRC command is used. This filter will block any
packets from an outside network that claim to have originated from the local network. This filter is placed
at the beginning of the filter list; if it were not, spoofed IP packets could be permitted passage by filters
positioned before this rule.
Local>> DEF FILT fw_i ADD DENY IP SRC 255.255.255.0 192.0.1.0
The CERT advisory on IP spoofing is available from ftp://cert.org/pub/
Note:
cert_advisories/CA-95:01.IP.spoofing.
To permit outgoing Telnet connections initiated from the local network, the following command is used:
Local>> DEF FILT fw_i ADD ALLOW IP TCP SPORT EQ TELNET DPORT GT 1023 ACK
To permit SMTP traffic between the SCS and the local and backup SMTP servers, the following commands
are required:
Local>> DEF FILT fw_i ADD ALLOW IP TCP DPORT EQ SMTP SPORT GT 1023 DST 255.255.255.255 192.0.1.102
Local>> DEF FILT fw_i ADD ALLOW IP TCP SPORT EQ SMTP DPORT GT 1023 ACK DST 255.255.255.255. 192.0.1.102
Local>> DEF FILT fw_i ADD ALLOW IP TCP DPORT EQ SMTP SPORT GT 1023 DST 255.255.255.255 192.0.1.103
Local>> DEF FILT fw_i ADD ALLOW IP TCP SPORT EQ SMTP DPORT GT 1023 ACK DST 255.255.255.255 192.0.1.103
Figure 11-59: Creating the Filter List
Local>> DEF FILT fw_i CREATE
Figure 11-60: Preventing IP Spoofing
Figure 11-61: Permitting Outgoing Telnet Connections
Figure 11-62: Permitting SMTP Traffic to SMTP Servers
11-31
Examples