How can I look up IP addresses under UNIX or Linux operating systems using command line options?
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Q. I’m running Apache 2 server under FreeBSD jail. However, I see lots of warning error messages in /var/log/httpd-error.log as follows:

[Sat Sep 20 20:47:09 2008] [warn] (61)Connection refused: connect to listener on 0.0.0.0:80
[Sat Sep 20 20:47:10 2008] [warn] (61)Connection refused: connect to listener on 0.0.0.0:80
[Sat Sep 20 20:47:11 2008] [warn] (61)Connection refused: connect to listener on 0.0.0.0:80

How do stop all these warning messages flooding my Apache log files?
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Q. Can you explain DNS name resolution concept? Why I need to set DNS resolution for my Windows or Linux server? How do I setup DNS name resolution under Linux / UNIX server system?
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Q. Can one Linux / BSD system respond to multiple IP address on a single Ethernet interface? I need to run multiple web sites and mod_ssl with a single server. How do I configure my box for multiple IP address?
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Q. I’m experiencing decreased network performance due to ARP broadcasts coming from one system. What would cause a lot of ARP broadcasts from one server / computer?
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Q. How do I configure my Postfix mail server to scan incoming mail for spam using DNS Blackhole List such as:
a) The Spamhaus
b) Open Relay Database etc

A. To discard spam or garbage email you can use 3rd party services such as Spamhaus. These are realtime database of IP addresses of verified spam sources and spam operations (including spammers, spam gangs and spam support services), maintained by the Spamhaus or other project team and supplied as a free service to help email administrators better manage incoming email streams.

Postfix MTA DNS Blackhole Lists Configuration

Under Postfix mail server you need to define DNSRBLs in main.cf file using the smtpd_recipient_restrictions configuration directive. Open main.cf file:
# vi /etc/postfix/main.cf
Locate smtpd_recipient_restrictions line and setup reject_rbl_client as follows:

smtpd_recipient_restrictions =
....
.....
   reject_rbl_client list.dsbl.org,
   reject_rbl_client sbl-xbl.spamhaus.org,
   reject_rbl_client cbl.abuseat.org,
   reject_rbl_client dul.dnsbl.sorbs.net,
....
   permit

Here is my complete configuration:

smtpd_recipient_restrictions =
   reject_invalid_hostname,
   reject_non_fqdn_hostname,
   reject_non_fqdn_sender,
   reject_non_fqdn_recipient,
   reject_unknown_sender_domain,
   reject_unknown_recipient_domain,
   reject_unauth_destination,
   permit_mynetworks,
   reject_rbl_client list.dsbl.org,
   reject_rbl_client sbl-xbl.spamhaus.org,
   reject_rbl_client cbl.abuseat.org,
   reject_rbl_client dul.dnsbl.sorbs.net,
   permit

Save and close the file. Restart / reload postfix mail server:
# /etc/init.d/postfix restart