Mail agen ts. Introduction to Internet Mail Abridged & Up dated by Hervey Allen. Message form at (1) Message form at (2)

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1 Mail agen ts Introduction to Internet Mail Abridged & Up dated by Hervey Allen Noah Sem atim ba Based on Materials by Philip Hazel MUA = Mail User Agent Interacts directly with the en d user Pine, MH, Elm, m utt, m ail, Eu dora, Marcel, Mailstrom, Mulberry, Pegasu s, Sim eon, Netscape, Outlook,... Multip le MUAs on on e system - en d user choice = Mail Tran sfer Agen t Receives an d delivers m essages Sendm ail, Sm ail, PP, MMDF, Charon, Exim, qm ail, Postfix,... One per system - sysadm in ch oice Message form at (1) Cc: Mark Anthony <MarkA@cleo.co.uk> Subject: How Internet mail works Julius, I'm going to be running a course on... Form at was origin ally defined by RFC 822 in 1982 Now sup erseded by RFC 2822 Message con sists of Header lin es A blan k line Body lines Message form at (2) An address consists of a local part an d a dom ain julius@ancient-rome.net A basic m essage body is un stru ctured Other RFCs (MIME, 2045) add addition al h eaders which defin e structure for the body MIME sup ports attach m ents of various kinds an d in variou s en codings Creatin g/ decodin g attach m ents is the MUA's job

2 Authen ticating senders log in MUA MUA POP / IMAP Server Mailbox Mailbox Em bedded MUA uses in ter-process call to send to May use p ip e, file, or in tern al over a p ipe knows th e iden tity of the sen der Norm ally in serts Sen der: h eader if differs from From : Freestanding MUA u ses to sen d m ail cann ot easily distin guish local/ rem ote clien ts No authen tication in basic p rotocol AUTH com m an d in exten ded Use of security addition s (TLS/ SSL) MUA can p oin t at an y wh atsoever Need for relay con trol Host and n etwork blocks A m essage in tran sit (1) Headers added by the MUA before sen din g cc: Mark Anthony <MarkA@cleo.co.uk> Subject: How Internet mail works Date: Fri, 10 May :29: (BST) Message-ID: <Pine.SOL A @taurus.cus.cam.ac.uk> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=us-ascii Julius, I'm going to be running a course on... A m essage in transit (2) Headers added by s Received: from taurus.cus.cam.ac.uk ([ ] ident=exim) by mauve.csi.cam.ac.uk with esmtp (Exim 4.00) id 101qxX-00011X-00; Fri, 10 May :50: Received: from ph10 (helo=localhost) by taurus.cus.cam.ac.uk with local-smtp (Exim 4.10) id 101qin-0005PB-00; Fri, 10 May :50: cc: Mark Anthony <MarkA@cleo.co.uk>...

3 A m essage in tran sit (3) A m essage is tran sm itted with an en velope: MAIL FROM:<ph10@cus.cam.ac.uk> RCPT TO:<julius@ancient-rome.net> The en velope is separate from the RFC 2822 m essage En velope (RFC 2821) fields n eed not be the sam e as the header (RFC 2822) fields s are (m ain ly) con cern ed with en velopes Ju st like the Post Office... Error ( boun ce ) m essages have n ull sen ders MAIL FROM:<> An session (1) telnet relay.ancient-rome.net relay.ancient-rome.net E Exim... EHLO taurus.cus.cam.ac.uk 250-relay.ancient-rome.net SIZE PIPELINING 250 HELP MAIL FROM:<ph10@cus.cam.ac.uk> 250 OK RCPT TO:<julius@ancient-rome.net> 250 Accepted DATA 354 Enter message, ending with. Received: from... (continued on next slide) An session (2) From:... To:... etc OK id=10spdr-00034h-00 quit 221 relay.ancient-rome.net closing conn... return codes 2xx OK 3xx send m ore data 4xx tem porary failure 5xx p erm anent failure Use of the DNS for em ail (1) Two DNS record typ es are used for rou tin g m ail Mail Exch an ge (MX) records m ap m ail dom ains to host n am es, an d p rovide a list of hosts with p referen ces: hermes.cam.ac.uk. MX 5 green.csi.cam.ac.uk. MX 7 ppsw3.csi.cam.ac.uk. MX 7 ppsw4.csi.cam.ac.uk. Address (A) records m ap host n am es to IP addresses: green.csi.cam.ac.uk. A ppsw3.csi.cam.ac.uk. A ppsw4.csi.cam.ac.uk. A

4 Use of the DNS for em ail (2) MX records were added to the DNS after its in itial deploym en t Backwards com patibility rule: If n o MX records found, look for an A record, and if fou nd, treat as an MX with 0 preference MX records were inven ted for gateways to other m ail system s, but are n ow heavily u sed for handling gen eric m ail dom ain s Com m on DNS errors Fin al dots m issin g on RHS host n am es in MX record s MX records point to aliases in stead of can on ical nam es This should work, but is in efficien t an d dep recated MX records point to n on-existen t hosts MX records con tain an IP address in stead of a h ost nam e on the righthand side Un fortu nately som e s accept this MX records do n ot con tain a preferen ce value Som e broken n am e servers give a server error when asked for a n on -existent MX record Process local ad d resses Alias lists Forwarding files Routin g a m essage Recognize special rem ote addresses e.g. local clien t hosts Look up MX records for rem ote addresses If self in list, ign ore all MX records with preferen ces greater than or equ al to own preferen ce For each MX record, get IP address(es) Perform local delivery Deliverin g a m essage For each rem ote delivery Try to con n ect to each rem ote host un til on e succeeds If it accep ts or p erm an ently reject the m essage, th at's it After tem p orary failures, try again at a later tim e Tim e out after deferring too m an y tim es Addresses are often sorted to avoid sending m ultip le copies

5 Checking incom ing sen ders A lot of m essages are sen t with bad envelope sen ders Miscon figured m ail software Un registered dom ains Misconfigured nam e servers Forgers Forgery seem s to be th e largest category n owadays Man y s check the sen der's dom ain It is harder to check the local p art Uses m ore resources, and can be quite slow Boun ce m essages have n o envelope sender Checkin g in com ing recipients Som e s check each local recip ien t du rin g the transaction Errors are h an dled by th e sen din g The receivin g avoids p roblem s with bad sen ders Other s accept m essages without ch ecking, an d look at th e recipien ts later Errors are h an dled by th e receivin g More detailed error m essages can be gen erated The curren t proliferation of forged sen ders h as m ade the first app roach m uch m ore pop ular Relay con trol In com in g: From any host to sp ecified dom ain s e.g. in com in g gateway or backup Outgoing: From sp ecified hosts to an ywhere e.g. ou tgoing gateway on local network From au th enticated hosts to an ywhere e.g. travelling em ployee or ISP custom er connected to rem ote network En cryption can be used for password protection durin g authen tication Authentication can also be don e using certificates Policy con trols on incom ing m ail Block kn own m iscreant hosts an d n etworks Realtim e Blackhole List (RBL), Dialup list (DUL), etc. (n ow a ch arged service) an d others Block kn own m iscreant sen ders Not as effective as it on ce was for SPAM Refuse m alform ed m essages Recognize junk m ail Discard Annotate

Mail agents. Introduction to Internet Mail. Message format (1) Message format (2)

Mail agents. Introduction to Internet Mail. Message format (1) Message format (2) Introduction to Internet Mail Noah Sematimba Based on Materials by Philip Hazel. Mail agents MUA = Mail User Agent Interacts directly with the end user Pine, MH, Elm, mutt, mail, Eudora, Marcel, Mailstrom,