Wirt Atmar wrote:
> In QCTerm, we return only two telnet errors:
>
> o Connection refused
> o Connection timed out
>
> The first error is an active error and is returned immediately. The most
> common two causes of that refusal are that (i) telnet is not running on the host
> machine (the most common form of the refusal on MPE, but virtually unheard of
> on any other system), or (ii) the telnet service is being actively blocked by a
> firewall or on the host.
>
> In either case, in a "connection refused", you touched the host and your
> connection is being actively refused.
The first is caused by an error detected by the protocol stack,
typically an ICMP unreachable message received from the destination or
something along the way, but could be other 'esoteric' errors. The
application may or may not differentiate between the various return
codes. The second is caused by the client protocol stack not receiving
anything within the allotted time.
You will receive the former case if:
* any inline packet filtering device (firewall, router, etc) is
configured to reject connections to the destination network,
host, port, or protocol [ICMP unreachable, net/host/proto/port
administratively prohibited], or
* the destination host denies your request (e.g., deny in INETDSEC.NET)
or the telnet server is not running [ICMP port unreachable].
* the destination host is literally unreachable due to a link failure
and the neighbor nearest you has registered the failure [not in a
retry/recovery effort] and returns the unreachable.
You will receive the latter case if:
* any inline packet filtering device (firewall, router, etc) is
configured to DROP connections to the destination network, host,
port, or protocol [NO ICMP unreachables returned], or
* any inline packet filtering device is configured to block the ICMP
return packets generated by the former case
Jeff
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