Hi James,
Antonio is correct. The default address is used when the destination address is not on a subnet that is on one of your local interfaces.
Any packet destined for an address on the 192.168.6.0/24 subnet will automatically be sent with a source address of 192.168.6.1
Same with any packet destined for an address on the 216.185.71.0/24 subnet will be sent with a source address of 216.185.71.1.
The kernel uses the first address on an interface as the primary address. You can see this if you just do ifconfig ifname, you will only see the first address you assign to the interface.
Hope this helps, Steve
On 02/18/2015 12:51 PM, Antonio S. Martins Jr. wrote:
Hi James,
Simply remove the GATEWAY line from the eth0:192 interface config :D Then you'll had only one default gateway. And the source IP to all unknown addresswill be the routeable one.
Att., Antonio.----- "James B. Byrne" byrnejb@harte-lyne.ca escreveu:
De: "James B. Byrne" byrnejb@harte-lyne.ca Para: centos@centos.org Enviadas: Quarta-feira, 18 de Fevereiro de 2015 15:39:16 (GMT-0300) Auto-Detected Assunto: [CentOS] Help with routing question.
CentOS-6.6
We have a host that has multiple IPv4 addresses aliased to eth0. The primary address is 216.185.71.x and the alias is 192.168.6.x.
This host connects to devices on both netblocks without problems. Only default routing is used and it looks like this:
#ip route 192.168.6.0/24 dev eth0 proto kernel scope link src 192.168.6.x 216.185.71.0/24 dev eth0 proto kernel scope link src 216.185.71.x 169.254.0.0/16 dev eth0 scope link metric 1002 default via 192.168.6.1 dev eth0 src 192.168.6.x default via 216.185.71.1 dev eth0
When the system connects to internal systems via ssh it uses the src 216.185.71.x for devices on that netblock and 192.168.6.x for devices on the other.
The problem is that when we try to establish an ssh connection off-site to another netblock altogether the host uses 192.168.6.x as the source and the destination gets the public side IP address of our gateway router as the point of origin due to masquerading.
I have solved this by explicitly binding ssh to the public ipv4 when connecting using the --bind=216.185.71.x parameter. But I have two questions I would like to find answers for
- Why is ssh using the private IP in preference to the public IP
when connecting to off-site addresses?
- How does one configure the routing table on network startup to
specifically detail the route particular addresses are supposed to take?
For diagnosis here are the ifcfg scripts used for both interfaces:
DEVICE="eth0" BOOTPROTO="static" BROADCAST="216.185.71.255" DNS1="216.185.71.33" GATEWAY="216.185.71.1" HWADDR="38:60:77:D5:AC:D8" IPADDR="216.185.71.x" IPV6INIT="yes" IPV6_AUTOCONF="yes" NETMASK="255.255.255.0" NM_CONTROLLED="no" ONBOOT="yes" TYPE="Ethernet" UUID="0202e615-ce93-4fe1-833a-c11259afb850"
DEVICE="eth0:192" BOOTPROTO="static" BROADCAST="192.168.6.255" GATEWAY="192.168.6.1" IPADDR="192.168.6.x" NETMASK="255.255.255.0" NM_CONTROLLED="no" ONPARENT="yes" TYPE="Ethernet"
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