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SubjectProblem in ARP 2.4.20 kernel
I'm working on a mechanism to load balance across multiple 100 mbit interfaces
using the 2.4.20 linux kernel + custom server software. I have several
interfaces all on the same subnet with different IP addresses. They are
all connected to the same multi-port switch.

The intent is that a client can connect to any of the IP addresses
specifically and only burden that one. Some clients can connect to #1, others
to #2, etc.

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.

I fixed this problem by modifying net/ipv4/arp.c:
//add to top of file in the #includes
#include <linux/inetdevice.h> // (DA) 20030515 to fix arp problem
//...then later in function arp_process()
if (addr_type == RTN_LOCAL) {
n = neigh_event_ns(&arp_tbl, sha, &sip, dev);
if (n) {
+ struct in_device *ind;
int dont_send = 0;
if (IN_DEV_ARPFILTER(in_dev))
dont_send |= arp_filter(sip,tip,dev);
+// (DA) 20030515 only send arp response if dev's IP address matches
+ if((ind=__in_dev_get(dev))) {
+ struct in_ifaddr *ifa;
+ ifa=ind->ifa_list;
+ while(ifa)
+ {
+ if(ifa->ifa_address==tip) break;
+ ifa=ifa->ifa_next;
+ }
+ if(!ifa) dont_send=1;
+ }
if (!dont_send)

goto out;

Before sending the arp response, the requested IP address is checked against
the interface's configured IP address, and only if there is a match will an
ARP response be sent.

I think the the check is harmless in any case. It's not clear to me if you'd
ever want each interface answering the ARP requests, and it is clear there
is a valid reason for wanting to wire up a computer this way. Enjoy!


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