Messages in this thread | | | Date | Fri, 16 May 2003 15:02:21 -0700 | From | David Ashley <> | Subject | Problem in ARP 2.4.20 kernel |
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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) arp_send(ARPOP_REPLY,ETH_P_ARP,sip,dev,tip,sha,dev->dev_addr,sha);
neigh_release(n); } 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!
-Dave
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