Re: ARP packets usage

Moe Trin wrote:
On Thu, 25 Oct 2007, in the Usenet newsgroup comp.os.linux.networking, in
article <4720f999$0$22312$ba620e4c@xxxxxxxxxxxxxx>, goarilla wrote:

Moe Trin wrote:

Fairly quiet - but without knowing _what_ you are looking at, it is
difficult to say if this is normal or not. For example, if you
are looking at a DSL connection, you are not likely to see any
mono-cast traffic (traffic to/from a single IP address) that is not
directed at your host. But you will _PROBABLY_ see all _broadcast_
traffic, where the router/switch does not know if "you" are the
destination or not.

little question don't routers split up broadcast domains ?

Classic routers - your big boxes from Cisco, Foundry, and others,
that follow RFC1812 do not forward broadcasts - because the network
address ranges are different on the various interfaces. See sections
5.3.4. and 5.3.5 et.seq. for details.

1812 Requirements for IP Version 4 Routers. F. Baker, Ed.. June 1995.
(Format: TXT=415740 bytes) (Obsoletes RFC1716, RFC1009) (Updated
by RFC2644) (Status: PROPOSED STANDARD)

The "routers" normally found in the home behave differently, because
they are not routers in the classic sense. In many cases, they are
doing port/IP translating, such that you have a non-routable (RFC1918)
address on your side, and can have multiple systems that appear on
the Internet as one. In other cases, they are behaving more like
Ethernet switches, separating traffic (collision domains) between the
ISP side and your system[s]. On Monday, you asked this question in
the thread "Do MAC addresses go to internet?", and in my response
(Message-Id: <slrnfhpug6.uho.ibuprofin@xxxxxxxxxxxxxxxxx>) I suggested
trying to use a packet sniffer to see what's on your wires. Did this
not work?


i did not found any MAC adresses belonging to machines other than the ones
that should be on the LAN so i guess i'm safe. but seriously i shouldn't have to take
into account that some routers DON'T act like routers. Routers should be routers
and conform to every letter in the rfc's

and thus broadcasts from the WAN side shouldn't be forwarded to the LAN
or does that in general only occurs in the reverse (eg from LAN -> WAN) ?


this was a dump question i know NO broadcasts should be forwarded
and that direction is irrelevant, but i was phishing about the posters
idea of router functionality.

The only time a "router" should forward broadcasts (other than DHCP
requests when the router is configured as a DHCP Relay Agent - see
RFC1542 et.seq.) is when it is not acting as a classic router per
RFC1812. ARP packets are not forwarded by such routers, because the
Ethernet concept doesn't need the "end" MAC address, but it DOES need
the MAC address of the "next hop". As far as ARP is concerned, the only
time an ARP request is forwarded is in Proxy-ARP where the "router" is
attempting to make it appear that a system on a separate interface but
using the same IP range is on the local network wire. See the
"Proxy-ARP-Subnet" mini-howto

-rw-rw-r-- 1 gferg ldp 19372 Aug 28 2000 Proxy-ARP-Subnet

for additional details.

Old guy

i've seen this behaviour (eg MAC next hop) in packets but i've never had
somebody explain the reason for this so short and beautifully thanks :D
.

Relevant Pages

  • SUMMARY RE: Arp Problem - long
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  • Re: ARP packets usage
    ... Ethernet switches, separating traffic between the ... i did not found any MAC adresses belonging to machines other than the ... DON'T act like routers. ... This basically rules out moving packets over Ethernet by any other ...
    (comp.os.linux.networking)
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