【正文】 DNS ? P2P applications ? Socket programming with TCP ? Socket programming with UDP ? Building a Web server 2: Application Layer 47 DNS: Domain Name System People: many identifiers: ? SSN, name, passport Inter hosts, routers: ? IP address (32 bit) used for addressing datagrams ? “name”, ., used by humans Q: map between IP addresses and name ? Domain Name System: ? distributed database implemented in hierarchy of many name servers ? applicationlayer protocol host, routers, name servers to municate to resolve names (address/name translation) ? note: core Inter function, implemented as applicationlayer protocol ? plexity at work’s “edge” 2: Application Layer 48 DNS Why not centralize DNS? ? single point of failure ? traffic volume ? distant centralized database ? maintenance doesn’t scale! DNS services ? hostname to IP address translation ? host aliasing ? Canonical, alias names ? mail server aliasing ? load distribution ? replicated Web servers: set of IP addresses for one canonical name 2: Application Layer 49 Root DNS Servers DNS servers DNS servers edu DNS servers DNS servers DNS servers DNS servers DNS servers DNS servers Distributed, Hierarchical Database Client wants IP for 。 1st approx: ? client queries a root server to find DNS server ? client queries DNS server to get DNS server ? client queries DNS server to get IP address for 2: Application Layer 50 DNS: Root name servers ? contacted by local name server that can not resolve name ? root name server: ? contacts authoritative name server if name mapping not known ? gets mapping ? returns mapping to local name server 13 root name servers worldwide b USCISI Marina del Rey, CA l ICANN Los Angeles, CA e NASA Mt View, CA f Inter Software C. Palo Alto, CA (and 36 other locations) i Autonomica, Stockholm (plus 28 other locations) k RIPE London (also 16 other locations) m WIDE Tokyo (also Seoul, Paris, SF) a Verisign, Dulles, VA c Cogent, Herndon, VA (also LA) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 21 locations) 2: Application Layer 51 TLD and Authoritative Servers ?Toplevel domain (TLD) servers: ? responsible for , , , edu, etc, and all toplevel country domains uk, fr, ca, jp. ? Network Solutions maintains servers for TLD ? Educause for edu TLD ?Authoritative DNS servers: ? anization’s DNS servers, providing authoritative hostname to IP mappings for anization’s servers (., Web, mail). ? can be maintained by anization or service provider 2: Application Layer 52 Local Name Server ?does not strictly belong to hierarchy ?each ISP (residential ISP, pany, university) has one. ? also called “default name server” ?when host makes DNS query, query is sent to its local DNS server ? acts as proxy, forwards query into hierarchy 2: Application Layer 53 requesting host root DNS server local DNS server 1 2 3 4 5 6 authoritative DNS server 7 8 TLD DNS server DNS name resolution example ? Host at wants IP address for iterated query: ? contacted server replies with name of server to contact ? “I don’t know this name, but ask this server” 2: Application Layer 54 requesting host root DNS server local DNS server 1 2 4 5 6 authoritative DNS server 7 8 TLD DNS server 3 recursive query: ? puts burden of name resolution on contacted name server ? heavy load? DNS name resolution example 2: Application Layer 55 DNS: caching and updating records ? once (any) name server learns mapping, it caches mapping ? cache entries timeout (disappear) after some time ? TLD servers typically cached in local name servers ? Thus root name servers not often visited ? update/notify mechanisms under design by IETF ? RFC 2136 ? 2: Application Layer 56 DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header ? identification: 16 bit for query, reply to query uses same ? flags: ? query or reply ? recursion desired ? recursion available ? reply is authoritative 2: Application Layer 57 DNS protocol, messages Name, type fields for a query RRs in response to query records for authoritative servers additional “helpful” info that may be use