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From: barmar@think.com (Barry Margolin)
Newsgroups: comp.unix.wizards
Subject: Re: Wizard-level questions
Message-ID: <1991Feb3.083938.13614@Think.COM>
Date: 3 Feb 91 08:39:38 GMT
References: <120574@uunet.UU.NET> <5653@auspex.auspex.com> <120840@uunet.UU.NET>
Sender: news@Think.COM
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 22

In article <120840@uunet.UU.NET> rbj@uunet.UU.NET (Root Boy Jim) writes:
>All the network really guarantees you is the identity of the IP address,
>and port number.
...
>Actually, only the network part of the info is truly reliable.

Actually, none of it is *truly* reliable.  The sending host can put
anything in its source address field of the packet.  However, if a host on
a different network says that it's a host on your network it won't be able
to receive any replies, because you'll send your replies to the real host.
For TCP-based protocols this is generally good enough, because they require
an initial two-way handshake before they do anything (it's possible to put
data in the initial SYN packet, but I know of no protocols that make use of
this).  However, UDP-based protocols often work without requiring responses
(e.g. an NFS DeleteFile operation will be obeyed and then the response will
be attempted).

--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
{uunet,harvard}!think!barmar