Date: Tue, 9 Jan 1996 22:48:10 +0100 (MET) From: grog@lemis.de (Greg Lehey) To: jkh@time.cdrom.com (Jordan K. Hubbard) Cc: hackers@freebsd.org (FreeBSD Hackers) Subject: Re: Using `ping' to diagnose network connections reasonable? Message-ID: <199601092148.WAA23824@allegro.lemis.de> In-Reply-To: <24751.821114877@time.cdrom.com> from "Jordan K. Hubbard" at Jan 8, 96 07:27:57 am
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Jordan K. Hubbard writes:
>
> I'd like to add some code to sysinstall which will attempt to
> `diagnose' a link before accepting the configuration parameters,
> catching a lot of adapter misconfiguration and incorrect data errors
> that sysinstall misses now (to fail less gracefully later). My
> question is whether or not `ping' is a reasonable way to measure
> connectivity between your host and the gateway & dns machines. Is it
> reasonable to assume that if a host supports forwarding or DNS
> queries, it will also answer pings? What if you've got pings blocked
> somehow but allow DNS traffic through? I wouldn't want to flag a host
> as `unreachable' when in fact it would have worked fine for its
> intended purpose! That would be worse than no error checking at all.
Well, here are three possibilities to consider:
1. I have an ISDN dialup connection, and I don't like people costing
me money by pinging me, so I have got my ISP to drop ICMP packets
at his end.
2. I have an PPP dialup connection, and I haven't got my ISP to drop
ICMP packets at his end. Still, setting up the connection takes
so long that the first 20 packets fall into the bit bucket.
3. I have a machine with an independent IP processor. If the main
machine fails, you can't talk to it (how about that), but you can
still ping it. This isn't made up, I really do have a machine
like that (based on a 68020, would you believe, but the IP
processor is an 80386 :-).
In each of these cases, your ping check may come to an incorrect
conclusion. Pinging is nice for an approximation, but it's no
substitute for the real thing.
Greg
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