9.4. Proxying Without a Proxy Server
Some
services, such as SMTP, NNTP, and NTP, naturally support proxying.
These services are all designed so that transactions (email messages
for SMTP, Usenet news postings for NNTP, and clock settings for NTP)
move between servers, instead of going directly from a client to a
final destination server. For SMTP, the messages are forwarded
towards an email message's destination. NNTP forwards messages
to all neighbor servers. NTP provides time updates when they're
requested but supports a hierarchy of servers. With these schemes,
each intermediate server is effectively acting as a proxy for the
original sender or server.
If you examine the "Received:" headers of incoming
Internet email (these headers trace a message's path through
the network from sender to recipient), you quickly discover that very
few messages travel directly from the sender's machine to the
recipient's machine. It's far more common these days for
the message to pass through at least four machines:
- The sender's machine
- The outgoing mail gateway at the sender's site (or the
sender's Internet service provider)
- The incoming mail gateway at the recipient's site
- Finally, the recipient's machine
Each of the intermediate servers (the mail gateways) is acting as a
proxy server for the sender, even though the sender may not be
dealing with them directly.
Figure 9-3 illustrates
this situation.
Figure 9-3. Store-and-forward services (like SMTP) naturally support proxying
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9.3. Proxy Server Terminology | | 9.5. Using SOCKS for Proxying |