10-3

DNS & mail

1997/12/17

Contents

Mail System Design l

Domain master l NULL client l PPP client l Back-up mail server l Firewall and mail server l Virtual host

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Domain Master

Domain Master Configuration ACCEPT_ADDRS='x.co.jp'

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Accepts [email protected] in addition to [email protected] l When sending mail, [email protected] is set as the sender address

– address part which should be accepted (Important!)

FROM_ADDRESS='x.co.jp' – domain part of send default – host name assigned to administrator address

u For mail that should specify computer itself

(i.e. mail from root), [email protected] is preferred

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root, daemon, postmaster, etc.

Accept for multiple domains – ACCEPT_ADDRS='sub1.co.jp sub2.x.co.jp'

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NULL Client

PPP Client

MS l

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No spooling l All mail goes to mail server

NULL

NULL

– only definition of mail server address is needed

CF_TYPE=R8V7 -null SPOOL_HOST=mail.x.co.jp l

Dial-up environment, so not always connected l IP address is assigned upon connection l DNS host name not fixed either l Not good for internal host name to be public – mail reply will not be returned l

List addresses that reach mail server only – enclose with [] (refer to A RR when there ’s lower MX) – list [IP address] 5

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Sending address of provider mail server is used l Mail receipt is POP (i.e. popclient)

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DNS & mail

1997/12/17

PPP Client Configuration

Advanced PPP Client

When provider mail server can be used for sending

DIRECT_DELIVER_DOMAINS=none DEFAULT_RELAY=mail.provider.ne.jp FROM_ADDRESS=po.provider.ne.jp CON_EXP=True tips to not send SMTP_MAILER_FLAG_ADD=e immediately l l l

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Rewriting of sender address – conversion of local and contract user name – use of userdb and usertable

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Curb sending from non-contracted addresses – use of check_compat rule set

Accumulate initially at mqueue When connected send altogether with sendmail-q Daemon sendmail launches only with -bd (when necessary)

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Time out for automatic dial-up O DialDelay=15s

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Back-- up Mail Server (cont.) Back l

Back-- up Mail Server (cont’ Back (cont’ d)

Receives on behalf of 1st-MX during failures

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– operates as 2nd-MX l

– use sharing system such as NIS – also possible to do rdist and newaliases – local aliases and shared aliases separated

If possible, send direct independently of 1st-MX – share alias files with 1st-MX – accept same addresses l

all users

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designated users

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ACCEPT_ADDRS=

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Problem of ML delivery back-up – what to do with sequence number

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Firewall and Name Server (cont.)

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Firewall and Name Server (cont’’ d) (cont l

Name server for outside organization

Name server for inside an organization – Methods to hide outside organization addresses internally

– When defining Wildcard MX $ORIGIN x.co.jp. * IN MX 10 ext-mail.x.co.jp. l doesn’t show existence of internal host

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– Define all existing mail addresses

all services are via proxy root server is provided within organization to avoid DNS timeout

– Methods to get outside organizations addresses with forwarders

method that doesn’t use Wildcard MX l mail that should be an error doesn’t reach gateway l

when direct connection can be established from inside to outside l SOCKS l

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R8 sendmail can handle multiple-file aliases OA/etc/aliases, nis: mail.aliases

– what to do with archive

SECONDARY_*= – those non-designated wait for recovery of 1st-MX

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Share aliases

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2

DNS & mail

1997/12/17

With 1 Mail Server

Firewall and Mail Server (1)

a

(To prevent leaking of internal info to outside)

External NS

Structure of name servers and mail servers Internet l

To cover with 1 mail server

split-brain DNS (has internal-target zone)

internal delivery

external delivery

Mail server

a. Method of referring outside search name server with internal-target zone l

external search

b External NS

split-brain DNS

static delivery rule for internal

Internet

b. Method of delivering internal mails according to set rules

internal delivery

external delivery

Mail server

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With 2 Mail Servers

Firewall and Mail Server (2)

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NS for externalStatic sending rule NS for internal

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With 2 mail servers Internet

– mail server for referring to external DNS – mail server for referring to internal DNS l

Formula a

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Formula b

Mail server for external

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– static route configuration in between

Mail server for internal

NS for external NS for internal

– dedicated receiving mail server – dedicated sending mail server

Send to internal Mail server for receiving

Internet Send to external

Mail server for sending

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Configuration of Mail Server for Internal

Configuration of Mail Server for External

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Static rule for external delivery DIRECT_DELIVER_DOMAINS=x.co.jp DEFAULT_RELAY=external.x.co.jp

Static rule for internal delivery STATIC_ROUTE_FILE=x.static x.static content:

– mail server for external

GW [12.34.56.78] # (internal.x.co.jp) DOM x.co.jp – able to accept mail addressed to mail server 17

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3

DNS & mail

1997/12/17

With 1 Name Server (1) l

With 1 Name Server (2) c. Map internal to separate branch

To make do with 1 NS and MS each...

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a. Place first MX facing internal by setting NS inner-host IN MX 10 inner-host IN MX 20 gw l direct communication is not possible from outside with 1st-MX; time-out occurs

– convert address with sendmail.cf • MAP line in STATIC_ROUTE_FILE (CF)

d. Multiple daemon launched on 1 server l

– stressful for sender, so not recommended

b. At GW, A RR referred to deliver mails to internal inner-host IN A 12.34.56.78 IN MX 10 gw

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configuration of so-called virtual host

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With 1 Name Server (4)

Problems with a, b method

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– although direct communication between internal and external is impossible, – internal host information can be seen from outside l

bind external and internal IP address – External named and internal named • listen-on, query -source, transfer-source (bind8.1.2) – External sendmail and internal sendmail • O DaemonPortOptions=Address=12.34.56.78

With 1 Name Server (3) l

inner.domain.jp → inner.domain.jp.local

Method b specific configuration – sending from gateway to internal l l

static route definition look at A RR – if 1st-MX is itself, adjust behavior TRY_NULL_MX_LIST=True (CF) O TryNullMXList=True (sendmail.cf)

bind8 allow-query alone is insufficient

– if configuration is incorrect local configuration error will be returned 21

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Hosts within Gateway l

Virtual Host (cont.) l

Everything goes to gateway

a) Sharing of user space

DIRECT_DELIVER_DOMAINS=none DEFAULT_RELAY=internal.x.co.jp l

Multiple addresses used on one host USERTABLE_MAPS='domain1=hash:/etc/map1 ¥ domain2=hash:/etc/map2'

Direct delivery internally

b) Separation of user space (1)

DIRECT_DELIVER_DOMAINS=x.co.jp DEFAULT_RELAY=internal.x.co.jp

– Multiple IP addresses for one host – Bind sendmail for each address

u For qmail, defined at control/smtproutes

O DaemonPortOptions=Address=1.2.3.4 23

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– Good to separate environment also with chroot24

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DNS & mail

1997/12/17

Virtual Host (cont’ (cont’ d)

System Design WrapWrap- up

c) Separation of user space (2)

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What addresses would be accepted

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Choosing sending method by address

– Work with sendmail.cf l

Switch local mailer with each address

– Use separate database from /etc/passwd – Dedicated service such as POP l

– Delivery destination statically defined – Refer to name server (MX)

Authenticate user with identifier including domain name

Clarify these conditions 25

Supplement：useful sendmail Supplement： techniques l

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To gather mail waiting to send at one host l

Gather mail waiting to send at one host Reject huge messages l Change process depending on sender address l If failed with SMTP, send with UUCP l Designation of mailer processing sequence l Configure ML on personal level

Use FallBackMX option – when DNS can’t be checked – when mail can’t be sent to all MX, transfer mail to designated host

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Administration etc. of mqueue is easier

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Route trouble is easier to catch

– adjustment of longest storage life

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Change process depending on sender address

Reject Huge Messages l

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When using MaxMessageSize option – reject message if surpassing size upon receipt – when using ESMTP, size is notified at point of MAIL FROM, so reject there

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When designating M= in mailer definition – receive message initially – check size immediately before sending l

due to possibility of different acceptable size range depending on mailer

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Carry out categorization of error mail and spam at sendmail.cf level

CT root news postmaster MAILER-DAEMON uucp cron S0 : Envelope sender R $* $: $1 $| $>3 $&f R motonori $| <@> $: trash when <> R motonori $| $=T<@$*> $: trash review at class T R $* $| $* $: $1 : 30

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DNS & mail

1997/12/17

If SMTP doesn’ doesn’t work, send with UUCP (3.1W patch) l

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Use function to launch multiple mailers sequentially

S0 : R $*<@x.co.jp>$* (space) :

$# smtp $@ x.co.jp $: $1<@x.co.jp>$2 $# uucp $@ uucp-x $: $1<@x.co.jp>$2

Configure ML on personal level

Mlocal …, %=0 Msmtp …, %=10 (will be processed after local mailer)

– Not only user@host, user+opt@host also may be used – [email protected] is also supported – User can use .forward as well as .forward+opt and .forward+default – By configuring .forward+ML, [email protected] can be employed – Reference：Samples/virt-domain+.def

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EightBitMode=pass8

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SendMimeErrors

– 8bit data that is not MIME may pass through – Whether or not to return error notification as according to RFC1894 - DSN (Delivery Status Notification) l

Important sendmail options (2) – Send header only of error mail to postmaster also – to prevent fatal troubles before they happen

MeToo – Whether or not to send back if sender is included in alias deployment

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PrivacyOption

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QueueSortOrder

DoubleBounceAddress

– Forbidding of SMTP EXPN, VRFY, etc.

– Destination address in case error mail can’t be returned to sender – Take note that body of mail will also be sent

– sort sequence of mail accumulated in mqueue – by order of time might be good (order of arrival) 35

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Important sendmail options (3) l

PostMasterCopy

ConnectionCacheSize – Number of SMTP to stay connected – Helpful for run queue

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Important sendmail options (1)

Use CF local deliver function

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With large-scale ML etc., when local delivery processing before SMTP(to oneself, archive)is preferred – Designate priority (cost) in mailer definition with %=

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Designation of Mailer Processing Sequence (3.1W patch)

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DNS & mail

1997/12/17

Important sendmail options (4)

Important sendmail options (5)

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MinQueueAge – Minimum resend interval for mail accumulated in mqueue – To override when resending manually, use -qI

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– From later half of 8.7, for better host internal security, became stricter towards file modes etc. – Designate when you want to ease checks – Details at http://www.jpcert.or.jp/tech/98 -0001/

ConnectionRateThrottle – Number of receiving connections allowed per second – Default is 0 (unlimited) – Might be effective against DoS attacks

DontBlameSendmail

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DontProbeInterface – prohibits reverse look-up of interface address upon launching

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Important sendmail options (6) l

Timeout.queuereturn .* – Maximum time limit for resending mails due to not reaching destination – Once exceeded, error is returned to sender – Consider long holidays when deciding limit

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Timeout.queuewarn .* – Time until notifying that mail is in resend mode – Set at 0 when unneeded 39

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