Re: Problem in ARP 2.4.20 kernel

From: Chris Friesen (cfriesen@nortelnetworks.com)
Date: Mon May 19 2003 - 19:36:25 EST

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In reply to: David Ashley: "Problem in ARP 2.4.20 kernel"
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David Ashley wrote:

> The problem I ran into was the kernel's handling of ARP requests. What linux
> does is each interface receives the arp request, and every single one
> answers the request. So it becomes a race condition which response gets to
> the client, and the client will have usually an incorrect mac address/ip
> address arp entry.

This was hashed through on the list about a year back. You might try googling
next time...
This is actually standards compliant behaviour, as silly as it sounds. However,
if you want stricter arp behaviour, and are using source-based routing, the
following will fix it.

echo 1 > /proc/sys/net/ipv4/conf/all/arp_filter

Here is an example setup on 2.4.18:

# ip add
1: lo: <LOOPBACK,UP> mtu 16436 qdisc noqueue
     link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
     inet 127.0.0.1/8 brd 127.255.255.255 scope host lo
2: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 100
     link/ether 00:30:65:bf:46:ba brd ff:ff:ff:ff:ff:ff
     inet 47.129.82.58/24 brd 47.129.82.255 scope global eth0
3: eth1: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 100
     link/ether 00:50:ff:90:04:44 brd ff:ff:ff:ff:ff:ff
     inet 47.129.82.107/24 brd 47.129.82.255 scope global eth1
# ip ru
0: from all lookup local
32764: from 47.129.82.107 lookup 101
32765: from 47.129.82.58 lookup 100
32766: from all lookup main
32767: from all lookup 253
# ip ro
47.129.82.0/24 dev eth0 proto kernel scope link src 47.129.82.58
47.129.82.0/24 dev eth1 proto kernel scope link src 47.129.82.107
127.0.0.0/8 dev lo scope link
default via 47.129.82.1 dev eth0
# ip ro li table 100
47.129.82.0/24 dev eth0 scope link
default via 47.129.82.1 dev eth0
# ip ro li table 101
47.129.82.0/24 dev eth1 scope link
default via 47.129.82.1 dev eth1

With this setup, arp requests for 47.129.82.58 will only be answered by eth0,
and similarly for 47.129.82.107 and eth1.

Chris

-- Chris Friesen | MailStop: 043/33/F10 Nortel Networks | work: (613) 765-0557 3500 Carling Avenue | fax: (613) 765-2986 Nepean, ON K2H 8E9 Canada | email: cfriesen@nortelnetworks.com

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Next message: Davide Libenzi: "Re: Comparing the aio and epoll event frameworks."
Previous message: Rusty Russell: "Re: try_then_request_module"
In reply to: David Ashley: "Problem in ARP 2.4.20 kernel"
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