Lesson 16: Email¶
- Email service
- How it works
- Configuration Postfix
- Planning
Email: System Components¶
- Mail User Agent (MUA)
- Mail Transport Agent (MTA)
- Delivery Agent (MDA)
- Access Agent (MAA)
Note
MUA: |
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MTA: |
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MDA: |
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MAA: | access to mail store (i.e IMAP, POP) |
Email: System Components¶
Note
The most confusing part about email is understanding the routing. Knowing the different components is important to fully grasping it.
Transport Agents¶
Accept mail form user agent
Postfix: | More common, easier to configure & use |
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Sendmail: | Highly configurable, steep learning curve |
Exim: | Similar to Postfix |
Qmail: | Logging is horrid, but some people like it |
Note
Postfix is the easiest to learn and understand, but queue management is a “black box”
Sendmail & qmail is great for high volume sites, but postfix/exim still perform great.
Sendmail has great options for queue management
- Features to look out for:
- SASL (authenticated SMTP)
- Queue Management
Delivery Agents & Message Stores¶
procmail – great filtering
maildrop – newer procmail-like
mail.local
- Message Stores
- mbox – one large file, locking problems
- maildir – one file per message, great for IMAP
Note
Consider scaling issues for the mailstore.
Generally maildir is the best & most compatible option
Anatomy of a Mail Message¶
- Envelope
- Destination email address
- Headers
- Record of variety of important information
- Great for tracking down problems
Body of the message
Note
- Headers:
- Know how to identify and track queue id’s
- Originator starts at the bottom
- Headers can be forged
- X- Headers non-RFC headers
- Message ID is always unique
MTA Log Files¶
- Track emails via queue ID
- Look something like: 03CE18819A
Tracking via message ID
- Informational fields
- to, from, status, relay, etc
Log files differ between each MTA
Note
Being able to read log files is important.
Configuring Postfix¶
- /etc/postfix
- main.cf – main config file
- master.cf – postfix process config file
- /etc/aliases – local email forwarding
- Set to relay email to central MTA
- relayhost = [smtp.osuosl.org]
- myorigin = osuosl.org
- /etc/aliases – root: foo@gmail.com
Note
- relayhost: [smtp.osuosl.org] vs. osuosl.org
- [smtp.osuosl.org] goes directly to smtp.osuosl.org
- ‘osuosl.org’ does DNS lookup and uses MX
Make sure you run “newaliases” after updating /etc/aliases
Reloading postfix is ideal too
- To test email:
- echo “this is a test” | mail root@localhost
Sendmail¶
- Config files created via m4
- Makefile
Always edit the .mc files not the .cf files
Remember to rebuild .cf files with make
Extremely configurable
Note
Config files in /etc/mail usually Primary file to edit should be sendmail.mc
Email: Viruses & Spam¶
- Virus
- Clamav
- Ensure freshclam is running too
- Spam
- Spamassassin
- All-in-one
- Amavis
Check abuse emails
Note
Make sure you have enough CPU & RAM for Spam checking Neglecting abuse emails may get you blacklisted For larger infrastructures, have dedicated machines to process spam Important to keep these updated
Email: Infrastructure Implementation¶
- Small sites
- Can have MTA/MDA/etc all on the same server
- Medium sites
- Separate MTA from MDA
- Large sites
- Split outgoing mail and incoming
Note
Consider resources, redundancy, & scalability. MDA is hardest to scale.
- Look at Cyrus Murder for large scalability
- dovecot is another option
Email: Security¶
- On General servers:
- Only listen on localhost
- Don’t allow other hosts to relay through it
- Relay all outbound mail through central host
- On Email servers:
- Restrict relaying to trusted networks
- Implement antivirus & spam protection
Note
Always test new configurations to ensure spammers can’t relay mail through your server Having dedicate outbound servers will ensure they always catch spam/viruses/etc