nanog mailing list archives
Re: ARP is sourced from loopback address
From: William Herrin <bill () herrin us>
Date: Mon, 30 Jan 2012 18:07:15 -0500
On Mon, Jan 30, 2012 at 4:27 PM, Joe Maimon <jmaimon () ttec com> wrote:
Is this normal behavior? Whats the workaround? Why havent I run into this before? 192.168.76.1 is a HSRP address on a ring of routers transiting a private non routed vlan to the service addresses hosted on systems that have independent management interfaces.
Hi Joe, Linux frequently does Really Stupid Things with ARP. You can generally force it to do the right thing with the arp_announce, arp_ignore and arp_filter sysctl's as well as the arptables command. If I understand your problem correctly, you have a virtual IP on a loopback interface and when that virtual IP is pinged, the Linux box uses it as the source address in the arp request instead of using the correct source address for that interface. Because the source address is not valid for that LAN, the router does not respond. Workaround: vi /etc/sysctl.conf: net.ipv4.conf.all.arp_announce = 1 net.ipv4.conf.eth1.arp_announce = 1 sysctl -p This forces the box to use eth1's IP address when making an ARP request from eth1 instead of using the VIP in the source address of the IP packet (the default behavior). #arp_announce - INTEGER # Define different restriction levels for announcing the local # source IP address from IP packets in ARP requests sent on # interface: # 0 - (default) Use any local address, configured on any interface # 1 - Try to avoid local addresses that are not in the target's # subnet for this interface. # 2 - Always use the best local address for this target. # In this mode we ignore the source address in the IP packet # and try to select local address that we prefer for talks with # the target host. Regards, Bill Herrin -- William D. Herrin ................ herrin () dirtside comĀ bill () herrin us 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/> Falls Church, VA 22042-3004
Current thread:
- ARP is sourced from loopback address Joe Maimon (Jan 30)
- Re: ARP is sourced from loopback address Keegan Holley (Jan 30)
- Re: ARP is sourced from loopback address Joe Maimon (Jan 30)
- Re: ARP is sourced from loopback address Keegan Holley (Jan 31)
- Re: ARP is sourced from loopback address Joe Maimon (Jan 30)
- Re: ARP is sourced from loopback address William Herrin (Jan 30)
- Re: ARP is sourced from loopback address Joe Maimon (Jan 30)
- Re: ARP is sourced from loopback address William Herrin (Jan 30)
- Re: ARP is sourced from loopback address Ray Soucy (Jan 31)
- Re: ARP is sourced from loopback address Joe Maimon (Jan 30)
- Re: ARP is sourced from loopback address Keegan Holley (Jan 30)