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Configuring Linux Mail Servers

Configuring Linux Mail Servers. Objectives This chapter will show you how to install and use Mailservers Contents An Overview Of How Sendmail Works Sendmail configruation files Relaying Sendmail Masquerading Using Sendmail to Change the Sender's Email Address Practical

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Configuring Linux Mail Servers

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  1. Configuring Linux Mail Servers • Objectives • This chapter will show you how to install and use Mailservers • Contents • An Overview Of How Sendmail Works • Sendmail configruation files • Relaying • Sendmail Masquerading • Using Sendmail to Change the Sender's Email Address • Practical • Setting up mailserver

  2. Getting SENDMAIL • Installing Sendmail • You will need to make sure that the sendmail, sendmail-cf and m4 software RPMs are installed. rpm –ivh sendmail-8.12.8-4.i386.rpm (The client/server) rpm –ivh sendmail-cf-8.12.8-4.i386.rpm (Config files) rpm –ivh sendmail-devel-8.12.8-4.i386.rpm (Optional) rpm –ivh sendmail-doc-8.12.8-4.i386.rpm (Optional) • Starting/Stopping Sendmail • You can use the chkconfig command to get Sendmail configured to start at boot: • To start/stop/restart sendmail after booting • You need to restart sendmail after changing sendmail.cf # chkconfig sendmail on # service sendmail start # service sendmail stop # service sendmail restart

  3. The /etc/mail/sendmail.mc File • How to Put Comments in sendmal.mc • The sendmail.mc file doesn't use the "#" for commenting, but instead uses the string "dnl". • Configuring the DNS for sendmail • Make Your Mail Server The Mail Server For Your Domain in DNS Disabled statements due to "dnl" commenting dnl DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA') dnl # DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA') Incorrectly disabled statement # DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA') Active statement DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA') IN MX 10 mail.my-site.com # host mail.my-site.com mail.my-site.com has address 192.168.0.1

  4. Sendmail & name resolution • All hosts that are not the nameserver should have /etc/resolv.conf file like this: • And incorrectly configured resolv.conf • file can lead to errors like this when running the m4 • The /etc/hosts File must have loopback address domain my-site.com nameserver 192.168.0.1 WARNING: local host name (smallfry) is not qualified; fix $j in config file 127.0.0.1  bigboy.my-site.com  bigboy  localhost.localdomain \ localhost

  5. How To Configure Linux Sendmail • All Linux mail clients in your home or company need to know which server is the mail server. • In /etc/mail/sendmail.mc file: • Converting From a Mail Client to a Mail Server • Determine Which NICs Sendmail Is Running On • We can verify that sendmail is running by first using the pgrep command define(`SMART_HOST',`mail.my-site.com') # pgrep sendmail22131 # netstat -an | grep :25 | grep tcp tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN

  6. Convert the sendmail client to server • Edit sendmail.mc To Make Sendmail Listen On All Interfaces dnl This changes sendmail to only listen on the loopback device 127.0.0.1dnl and not on any other network devices. Comment this out if you wantdnl to accept email over the network. -> dnl DAEMON_OPTIONS(`Port=smtp,Addr=mail.my-site.com, Name=MTA')dnl NOTE: binding both IPv4 and IPv6 daemon to the same port requiresdnl a kernel patchdnl DAEMON_OPTIONS(`port=smtp,Addr=::1, Name=MTA-v6, Family=inet6')dnl We strongly recommend to comment this one out if you want to protectdnl yourself from spam. However, the laptop and users on computers that dodnl not have 24x7 DNS do need this. -> dnl FEATURE(`accept_unresolvable_domains')dnldnl FEATURE(`relay_based_on_MX')dnl

  7. Convert the sendmail client to server • Comment out the "SMART_HOST" Entry In sendmail.mc • Regenerate The sendmail.cf File & Restart sendmail • This step can be accomplished by running the script we created at the beginning of the chapter. • Now Make Sure Sendmail Is Listening On All Interfaces dnl define(`SMART_HOST',`mail.my-site.com') # ./smmake # netstat -an | grep :25 | grep tcp tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN

  8. A General Guide To Using The sendmail.mc File • Primary rule, nice and clean sendmail.mc with comments • Masquerade rewrites all mail from hosts to coming from domain • FEATURE adds functionallity to sendmail dnl ***** Customised section 1 start ***** Dnl Dnl FEATURE(delay_checks)dnl FEATURE(masquerade_envelope)dnl FEATURE(allmasquerade)dnl FEATURE(masquerade_entire_domain)dnl dnl dnl dnl ***** Customised section 1 end *****

  9. Sendmail feature files • The /etc/mail/relay-domains File • The /etc/mail/access File • Keywords include RELAY, REJECT, OK (not ACCEPT) and DISCARD • The /etc/mail/local-host-names File • Also recieve mail from my other site • In DNS we need to enter: my-other-site.com my-site.com localhost.localdomain           RELAYlocalhost                       RELAY127.0.0.1                       RELAY192.168.1.16                    RELAY192.168.1.17                    RELAY 192.168.2                       RELAY my-site.com                     RELAY my-site.com my-other-site.com my-other-site.com. MX 10 mail.my-site.com.

  10. Which User Should Really Receive The Mail? • The /etc/mail/virtusertable file • The /etc/aliases File • Allways run command newaliases after working with aliases webmaster@my-other-site.com   webmasters @my-other-site.com            marc sales@my-site.com             sales@my-other-site.com paul@my-site.com              paul finance@my-site.com           paul @my-site.com                  error:nouser User unknown . . . manager:              rootabuse:                root# trap decode to catch security attacksdecode:               root# Person who should get root's mailroot:                 marc,webmaster@my-site.com # My mailing list file admin-list:     ":include:/home/mailings/admin-list"

  11. Sendmail Masquerading Explained • If you want your mail to appear to come from • user@mysite.com and not user@bigboy.mysite.com You can in that case: • Configure your email client, such as Outlook Express, to set your email address to user@mysite.com • Set up masquerading to modify the domain name of all traffic originating from and passing trough your mail server • Configuring masquerading • This can be solved by editing your sendmail.mc configuration file and adding some masquerading commands: FEATURE(always_add_domain)dnl FEATURE(`masquerade_entire_domain')dnl FEATURE(`masquerade_envelope')dnl FEATURE(`allmasquerade')dnl MASQUERADE_AS(`my-site.com')dnl MASQUERADE_DOMAIN(`my-site.com.')dnl MASQUERADE_DOMAIN(localhost)dnl MASQUERADE_DOMAIN(localhost.localdomain)dnl

  12. Sendmail Masquerading Explained •  Testing Masquerading • You should also tail the /var/log/maillog file to verify that the masquerading is operating • Other Masquerading Notes • By default, user "root" will not be masqueraded. This is achieved with the: • in /etc/mail/sendmail.mc mail -v username EXPOSED_USER(`root')dnl

  13. Using Sendmail to Change the Sender's Email Address • Add these statements to your /etc/mail/sendmail.mc • In order to rewrite emails origin address, you need some features: • In /etc/mail/sendmail.mc • Create a /etc/mail/generics-domains • Create your /etc/mail/genericstable FEATURE(`genericstable',`hash -o /etc/mail/genericstable.db')dnl GENERICS_DOMAIN_FILE(`/etc/mail/generics-domains')dnl my-site.com my-other-site.com bigboy.my-site.com #linux-username       username@new-domain.com alert          security-alert@my-site.com peter          urgent-message@my-site.com

  14. Fighting SPAM, add features • RFC-Ignorant valid IP address checker. • Easynet open proxy list. • The Open Relay Database open mail relay list. • Spamcop spammer blacklist. • Spamhaus spammer blacklist. FEATURE(`dnsbl', `ipwhois.rfc-ignorant.org',`"550 Mail from " $&{client_addr} " refused. Rejected for bad WHOIS info on IP of your SMTP server - see http://www.rfc-ignorant.org/"') FEATURE(`dnsbl', `proxies.blackholes.easynet.nl', `"550 5.7.1 ACCESS DENIED to OPEN PROXY SERVER "$&{client_name}" by easynet.nl DNSBL  (http://proxies.blackholes.easynet.nl/errors.html)"', `')dnl FEATURE(`dnsbl', `relays.ordb.org', `"550 Email rejected due to sending server misconfiguration - see http://www.ordb.org/faq/\#why_rejected"')dnl FEATURE(`dnsbl', `bl.spamcop.net', `"450 Mail from " $`'&{client_addr} " refused - see http://spamcop.net/bl.shtml"') FEATURE(`dnsbl',`sbl.spamhaus.org',`Rejected - see http://spamhaus.org/')dn

  15. Spamassassin • Downloading & Installing Spamassassin • Starting Spamassassin • Configuring Procmail for Spamassassin • Startup Spamassassin • Combine spamassasin with sendmail features # rpm –ivh spamassassin-2.60-2.i386.rpm. # chkconfig --level 35 spamassassin on cp /etc/mail/spamassassin/spamassassin-spamc.rc /etc/procmailrc # /etc/init.d/spamassassin start

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