force default gateway through eth0

On a CentOS 5.8 machine, I needed to keep the default gateway route through eth0, even though both eth0 and eth1 were receiving DHCP. It took a suprisingly long time to Google that in CentOS, you can simply use GATEWAYDEV=eth0 in /etc/sysconfig/network rather than mucking with /etc/sysconfig/network-scripts/, etc.

It is a little annoying that although CentOS will pick up the hostname from the first dhcp upped interface (eth0 in this case), it will set the default route through the last dhcp upped interface (eth1 in this case).

xen on centos on hp blades: slow networking

Had Xen running on CentOS on some HP 460c blades. CentOS by itself on the blades had great network performance, but the blades running with the Xen kernel had terrible performance, both in the domU and the dom0. It turns out that the issue is the particular networking driver that the CentOS/RHEL/Fedora family has for the blades' Broadcom Corporation NetXtreme II BCM57711E 10-Gigabit PCIe cards. You can see something similar on this RHEL bug.

The lazy workaround is to add "options bnx2x disable_tpa=1" to /etc/modprobe.conf on the dom0 and then restart the dom0. Like so:

# cat /etc/modprobe.conf

alias eth0 bnx2x

alias eth1 bnx2x

options bnx2x disable_tpa=1

alias scsi_hostadapter cciss

alias scsi_hostadapter1 usb-storage

# 

HP advises that you can manually reload the driver, but this did not work for me.

aliased ip not recognized by router

We had to manually assign an aliased ip to a failover host, call him jeeves, to replace a failed server, call him bertie. Assume bertie's ip is bbb.bbb.bbb.bbb.

(Note: to add an ip onto an interface, say eth0, use something like:
/sbin/ifconfig eth0:1 inet bbb.bbb.bbb.bbb netmask 255.255.248.0 broadcast bbb.bbb.ccc.255 up)

Previously, this had been all we'd had to do, but this time, we were only able to access the ip behind the router, not from hosts outside the local network.

The trick was to send a manual arp request to the router from jeeves, after co-opting bertie's ip:
/sbin/arping -c 2 -s bbb.bbb.bbb.bbb rrr.rrr.rrr.0
(where rrr.rrr.rrr.0 is the router ip)

Normally, this arp request goes to the router when you restart an interface, but aliasing on an ip does not necessarily cause your host to talk to the router. So you may need to do it manually.