Chpt 2 2 Flashcards
Email Three major components
- User agents
- Mail servers
- Simple mail transfer protocol: SMTP
Where does the SMTP work?
Between mail servers to send email messages.
SMTP RFC
Uses TCP to reliably transfer email messages from client (mail server initiating connection)
Three phases of transfer.
Command/response interaction (like HTTP).
Three phases of transfer of SMTP RFC
SMTP handshaking (greeting)
SMTP transfer of messages
SMTP closure.
RFC 2822
Defines syntax for e-mail message itself (like HTML defines syntax for web documents).
IMAP
Internet Mail access Protocol:
Messages stored ons server, provide retrieval, deletion, folders of stored messages on server.
DNS
Domain Name System
A distributed database is implemented in a hierarchy of many name servers.
Application-layer protocol hosts, and DNS servers communicate to resolve names.
DNS Services
- Hostname to IP address translation.
- Host aliasing
- Mail server aliasing.
- Load distribution
Why not centralize DNS
- Single Point of Failure.
- Traffic volume
- Distant centralized database
- Maintenance.
DOESN’T SCALE!
Three levels of DNS
Root, Top-Level Domain, Authoritative
DNS must have ___ to function.
Internet
Who manages root DNS domains?
ICANN
Internet Corporation for Assigned Names and Numbers.
When the host makes a DNS query, it is sent to where?
Its local DNS server.
Does the local DNS server strictly belong to the hierarchy?
No.
Iterated Query
Contacted server replies with the name of the server to contact. (Tree-Model)
Recursive Query
Puts the burden of name resolution on the contracted name server. (Linear-Model)
Caching DNS Info
Once (any) name server learns mapping, it caches mapping and immediately returns a cached mapping in response to a query.
Cached entries may be out of date.
DNS Records Types
A - hostname/IP
NS - domain name
CNAME - alias names
MX - name of SMTP mail server.
DNS query and reply messages, both have __.
The same format:
Message header (ID) and Flags.
DNS Security
DDoS Attacks - bombard with servers with traffic. (or bombard TLD servers)
Spoofing Attacks: Intercept DNS queries, and return bogus replies.
Peer to Peer (P2P)
Not always on the server.
End systems directly communicate.
Self Scalability.
Peer change IP addresses. (Are intermittently connected)
