Configuring and Managing MPE/iX Internet Services (August 2002)

Chapter 10
Sendmail for MPE/iX
Firewall Issues
189
Firewall Issues
The number two cause of Sendmail installation problems is due to a firewall or other network security device
blocking your HP e3000 from being able to send and receive packets on port 53 (DNS) and port 25 (SMTP).
Sendmail uses port 53 (DNS) to resolve hostnames into IP addresses and IP addresses into hostnames.
Sendmail may do multiple DNS resolutions for every e-mail message sent or received, and if a firewall is
blocking these DNS packets, Sendmail may experience long delays and/or generate various error messages
logged to syslog.
Sendmail may need to contact external DNS servers if you are attempting to exchange e-mail with the
Internet. Some intranet environments may require you to reference a “forwarding DNS server” (which can
traverse your border firewall to talk to the Internet) via a nameserver statement in /SYS/NET/RESLVCNF.
Consult your local network administrator for advice on how to choose a proper DNS server.
Port 25 (SMTP) is used to connect to remote mail servers to deliver outgoing e-mail, and is also used on the
HP e3000 to listen for incoming e-mail. If a firewall is blocking outbound port 25 packets, Sendmail may
experience long delays and generate various error messages logged to syslog as well as bounce messages
returned to the e-mail originator. If a firewall is blocking inbound port 25 packets, Sendmail will not be able
to receive any incoming e-mail, and there will be no extra syslog messages.