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Agenda DNS & mail

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InternetWeek ’98 Tutorial 1998/12/15 Motonori Nakamura [email protected] [email protected]

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Internet mail basics DNS structure and management Mail system design SPAM countermeasures

E- mail System n

MUA (Mail User Agent) MTA (Mail Transfer Agent) n DNS (Domain Name System) n

1. Internet Mail Basics

DNS SMTP MUA

MTA SMTP

MTA

MUA

POP/IMAP/... MB mailbox 4

MUA (Mail User Agent)

MTA (Mail Transfer Agent) n

User application

Mail receipt Determination of delivery point n Delivery of mail

– reading mail – writing mail – saving/searching mail n

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– to remote, to local, to sender (error)

UNIX – ucbmail ucbmail,, RMAIL, mush, MH (mh (mh-- e), mew,....

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Windows

Store and Forward – After receiving, attempt forwarding to next host

– OutLook OutLook,, Netscape Mail, Eudora,.... 5

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Sending and Receiving Mail on the Internet

MTA Programs n n n n n n n n

sendmail http://www.sendmail.org http:// www.sendmail.org// qmail http://www.qmail.org http:// www.qmail.org// SMAIL (GNU) MMDF (Multi (Multi--channel Memo Distribution, CSNET) exim http://www.exim.org http:// www.exim.org// VMail http://wzv.win.tue.nl/vmail http:// wzv.win.tue.nl/vmail// LSMTP http://www.lsoft.com/LSMTP.html http:// www.lsoft.com/LSMTP.html PP (X.400)

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SMTP - Simple Mail Transfer Protocol RFC821(S) n TCP port number 25 n

Most MTAs implement SMTP – has coordination function with DNS

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Method of Determining Mail Destination on the Internet

SMTP Action 220 r.domain SMTP Server ready (Message from server) HELO s.domain (Message to server) 250 r.domain Hello s.domain MAIL FROM:<[email protected]> (sender (sender’’s address) 250 sender ok RCPT TO: (recipient (recipient’’s address) 250 recipient ok DATA 354 Enter mail, end with "." on a line by itself e - mail data comes here . (indicates end of data) 250 Message accepted for delivery QUIT 221 r.domain closing connection

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Extracting host name from destination mail address user@host

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Retrieval of IP address from host name host → 12.34.56.78 – /etc/hosts – NIS (YP) – DNS (Domain Name System)

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DNS (Domain Name System)

Terminology n

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Wide--area distributed directory service Wide

delivery – local delivery → mailbox – remote delivery → pass to another MTA

– Distributed allocation – Decentralized management

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transfer

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acceptance (probably not a universal term)

– remote delivery

Host name → IP address n Mail address → MXHost name → IP address n

– local delivery n

– Sharing same domain space

– delivered from remote 11

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Mail Address

%-Hack

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Used as sender information/receiver information n User @ domain

user % host @ relay

– motonori @ wide.ad.jp n

RFC1123(S)

sender → relay → host çwhen relay is reached, rewritten as user @ host

Other formats – %- Hack – Route Address – UUCP addressing

user % host % relay2 @ relay1 sender → relay1 → relay2 → host 13

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Route Address n

UUCP Addressing n

RFC822(S)

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host ! user relay ! host ! user

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host ! user @ domain interpretation

@relay: user @ host sender → relay → host çwhen relay is reached, rewritten as user @ host

– “ host ! user” user” @ domain

(in terms of Internet)

» sender → domain → host

– host ! “ user @ domain” domain ”

@relay1, @relay2: user @ host

(in terms of UUCP)

» sender → host → domain

sender → relay1 → relay2 → host 15

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Comment Format n

Full Name <user@domain>

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user@domain (Full Name)

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user(User Name)@domain(Company Name) – The ( ) comment may be inserted anywhere

Domain Part n

Fully Qualified Domain Name – A complete host/domain name in the Internet domain format

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Fully Qualified Mail Address – [email protected] – means it ’ s not user@mailhost

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Not Qualified Mail Address

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Generic Address

– user 17

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– [email protected]

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Message Format n

Sender and Recipient

Header and body n

RFC822(S): Standard for the format of arpa internet text messages n

Sender – one person – the senders in the header may be plural

The first blank line is the divider

» representing the senders

From: [email protected] To: [email protected] Subject: InternetWeek ’98

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Recipient – one or more persons

← blank line (no space either)

InternetWeek ’ 98 announcement 19

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Header and Envelope (cont.)

Header and Envelope (cont ’d)

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Looks like an envelope n Envelope

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– Sender/destination – Sender/destination as indicated on front

– person who wrote body/person intended for – sender/recipient of enclosed text – generally cannot be rewritten

» person who actually does procedure

– Rewritten upon delivery n

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RFC821(S): Simple Mail Transfer Protocol

» to individuals

UUCP – Envelope is specified in rmail command line

– may be different » mailing lists etc.

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When the Envelope is Created n

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Extracted from the header

Delivery error notice reply (automatic) – Sender of envelope – “ Errors Errors--To: To:”” header » For systems that don’ don’ t have the envelope concept （do these still exist?） exist?）

The envelope is rewritten during the delivery process

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Response to content (person intervention) – Sender in header

– transfer – mailing list

» From:, ReplyReply-To: » (To:, Cc:) 23

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Address Used for Reply

– The sending MUA does it – The MTA which processes it first does it n

header and envelope sender/recipient – may be the same

– Envelope is specified with command n

header

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From the Mailbox to MUA

The 3 Points of Mail Delivery 1) Receipt (delivery from remote)

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Local mailbox

– Sent from remote mail server

– UNIX etc.

2) Acceptance (delivery to local) 3) Sending/Forwarding (delivery to remote)

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POP n IMAP

– Sent to recipient’ recipient ’ s mail server DNS

DNS MTA

receive

MTA

accept Range of configuration

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Send/transfer

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DNS Records referred to for Mail Delivery

1) Settings for Mail Receipt How to convey destination to sender n Internet

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A (Address) RR (Resource Record)

– Direct delivery by SMTP → Define delivery destination to DNS

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MX (Mail eXchanger eXchanger)) RR

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– IP address extracted from host name – Destination host name extracted from mail address

Bucket relay system – UUCP etc. (From JUNET era) → Configure delivery destination in (all) hosts along path – Usage of mailconf » sendmail.cf creation tool

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CNAME (Canonical NAME) RR – Alternative host name extracted

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Confirm A with nslookup (1)

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Hosts with Multiple IP addresses mail.x.co.jp

IN A 12.34.56.78 IN A 12.34.54.32 n If delivery to the first address doesn’ doesn ’t work, it tries all addresses one by one (implementation dependent) n With the DNS roundround -robin function, the address obtained through search is different each time

% nslookup sh.wide.ad.jp sh.wide.ad.jp.. Server: localhost Address: 127.0.0.1 Name: sh.wide.ad.jp Address: 203.178.137.73

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– Load sharing – Even it only tries first address, it may work eventually after several time trial(?)

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Confirm A with nslookup (2)

Generic Mail Address n

% nslookup jp jp--gate.wide.ad.jp Server: localhost Address: 127.0.0.1

No host name part – Not dependent on host reorganization

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Name: jp jp--gate.wide.ad.jp gate.wide.ad.jp.. Addresses: 203.178.137.17, 203.178.136.81, 203.178.137.75, 203.178.136.89

Uses MX (Mail eXchanger eXchanger)) RR n Mail to [email protected] is sent to specified host – Look up MX, and with obtained host name around the right, look up A and obtain IP address

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Confirm MX with nslookup

Preparing for Failures (for MX)

% nslookup - q= q=mx mx wide.ad.jp wide.ad.jp.. Server: localhost Address: 127.0.0.1 wide.ad.jp preference = 10, mail exchanger = sh.wide.ad.jp : (additional information) sh.wide.ad.jp internet address = 203.178.137.73 n Please note: for destination when MX can’ can’t be found, it follows A, and if both are found, MX has priority. – Therefore it’ it’ s possible for mail to go to another host using MX setting.

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Mail receipt backback-up x.co.jp preference= preference=10 10,, mx mx=mail1.x.co.jp =mail1.x.co.jp preference=50 preference= 50,, mx mx=mail2.x.co.jp =mail2.x.co.jp

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Smaller the number, Higher the priority (cost value) – Until the sender succeeds in sending, higher cost attempts are gradually made

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Mail2 transfers to mail1 upon its recovery – Be aware of mail saving period for mail2 mail2

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sender

Lower MX Conditions

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Load Sharing

(Conditions to avoid mail loop) x.co.jp n

mail1

Awareness of own name on right of MX RR

preference=10, mx mx=mail1.x.co.jp. =mail1.x.co.jp. Preference=10, mx mx=mail2.x.co.jp. =mail2.x.co.jp.

– Prevent connection to oneself » confirm with $=w at sendmail -bt » automatic registration of interface address names » qmail is confirmed by IP address n

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When cost is the same, sender chooses destination randomly n In the end, sent to one mailbox

Connection is not made to IP address of oneself

RR costs higher than the MX RR preference – Prevent pingping- pong between Lower MX

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– recipient needs some settings » static delivery definition, etc.

for oneself are thrown out 35

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Configuration of Accepting Address

2) Acceptance of Mail n

Recognize that received mail is to oneself – local delivery (acceptance) – not “ received = to oneself” oneself”

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Sendmail (CF)

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qmail

– set as ACCEPT_ADDRS – set as /var/qmail/control/locals /var/qmail/control/locals

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If decided that it’ it ’s not to oneself – search transfer destination

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Summary of Receive Mail Settings n

3) Mail Delivery Settings

Convey destination to sender

Variations of delivery methods

– Define MX record n n

Recognize that received mail is to oneself

Delivery by reference to DNS MX RR – Prepare MTA to refer to MX

– Delivery to local (acceptance)

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Delivery based on host name only n Delivery based on set rules

Separate configurations are necessary

– Consider need of referring to DNS 39

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Basic Configuration for Referring to DNS n n

/etc/ etc/resolv.conf resolv.conf n

/etc/ etc/resolv.conf resolv.conf

Designation of name server nameserver 0.0.0.0 (interpreted as localhost - 127.0.0.1) nameserver 12.34.56.78 nameserver 12.34.56.79

service switch file

– up to 3 (MAXNS in resolv.h resolv.h)) » time out is same regardless of how many (75s) domain sub.x.co.jp search sub1.x.co.jp sub2.x.co.jp x.co.jp

– Used for address supplement 41

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Service Switch file n

When referring to DNS MX n

Solaris – /etc/ /etc/nsswitch.conf nsswitch.conf

– sendmail.mx

» hosts: files dns n

» link to libresolv.a

– sendmail.cf for reference to MX

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» MX_SENDMAIL=yes (CF) » (Actually Wildcard MX strategy only)

– /etc/ /etc/svc.conf svc.conf n

MTA referring to MX

Others

→ Address supplement

– ServiceSwitchFile option (sendmail.cf (sendmail.cf)) – Default: /etc/service.switch hosts dns files nis

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Delivery Based on Set Rules n

Confirmation for Delivery

Write set rules in sendmail.cf – mailconf – CF

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Is the address interpreted correctly?

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Is MX able to search normally?

– sendmail - bv or sendmail - bt /parse

» STATIC_ROUTE_FILE

– use sendmail - bt for /mx /mx command n

Is it able to actually send? – sendmail - v

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Summary of Delivery Settings n

Host should be able to refer to DNS – resolv.conf – Service Switch file

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Consider destination according to mail address – Refer to DNS (MX) and deliver as is » which name server should be looked at (more later)

– Destination is statically set

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