Date: Mon, 10 Dec 2001 10:02:35 +1000 (EST) From: Colin Campbell <sgcccdc@citec.qld.gov.au> To: Jim Weeks <jim@siteplus.net> Cc: "Forrest W. Christian" <forrestc@imach.com>, Rowan Crowe <rowan@sensation.net.au>, <freebsd-isp@FreeBSD.ORG> Subject: Re: arplookup Message-ID: <Pine.BSF.4.33.0112100947210.93856-100000@guru.citec.qld.gov.au> In-Reply-To: <Pine.BSF.4.21.0112091821530.562-100000@veager.jwweeks.com>
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Hi, On Sun, 9 Dec 2001, Jim Weeks wrote: > This is a very well thought out response, and belive I understand your > example. However, this isn't representative of the problem I am having. > This particular network is supposed to be set up on class c boundaries. > In this case we will say that the ip of my machine is 192.168.2.100 with > netmask 255.255.255.0, and my default gateway is 192.168.2.1. Are you sure that's your netmask? The only way a system will ARP for a machine is if it thinks the destination is on the same network. Without "real" information we can only assume you are checking your config thoroughly. A single digit typo could cause the problem you are describing. I understand your desire top keep addresses "secret" but without the output from: ifconfig -a netstat -rn we really are flying blind. For example, if your networks are your server 192.168.2.100 and router 192.168.2.1, netmask 255.255.254.0 (even though you're "sure" it isn't) and the unreachable hosts are 192.168.3.x and 192.168.3.y then you'll see exactly what you re seeing. There's only two things that can cause what you are seeing 1) your netmask isn't what you think it is 2) you have some NAT going on If it helps, here's my understanding of how "routing" works: o a packet destined for host "X" is created o the IP stack looks at all the machine's interfaces to see if any is on the same network: for each interface get IP address of interface apply netmask to interface result is "network" address apply the current netmask to the destination result is destination "network" address o if the resulting networks are the same, then the destination is directly reachable from that interface consult kernel ARP table for destination MAC address, if not there, send an ARP request to get the MAC address for the destination IP o if no host interfaces match, consult the routing tables and find the most specific matching network, its router and the local intertface consult kernel ARP table for MAC address of router, if noth there, send ARP request for router o by now all the needed information is available for the IP and ethernet frames: source IP - from the interface packet departs source MAC - ditto destination IP - from the original request destination MAC - either from destination machine if on the same wire, or the router MAC address Colin -- Colin Campbell Unix Support/Postmaster/Hostmaster CITEC +61 7 3006 4710 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-isp" in the body of the message
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