CentOS policy routing – why yes, it can be done!

Over the years working in LAN networking there are several situations that dictate a host/server have multiple IP addresses on the same or different, physical or logical, devices.  For instance, connecting to a private management-only network/vlan, offering connectivity to a inside network on a private NIC, etc etc.

 

This scenario often causes two somewhat annoying behaviours:

 

1) the return traffic often is sourced from the “primary” IP address of the host/server, most often the one that is on the subnet associated with the default gateway

2) a surprising number of alleged “network administrators” seem to think having multiple gateways (one for each IP address of course )  is a good idea.  Well, over the years I have come across this situation and, in every case, this has obviously  NEVER WORKED.  

 

Situation #2 can only be fixed by not mind numbingly entering multiple gateways without restraint.  As for situation #1, RedHat/CentOS and derivatives support via iproute2 the ability to make traffic rules, ensuring that IP traffic is sourced by a particular IP in cases you can define.  Great!  Multiple NIC or logical interface routing on Linux is possible! (and yes, it involves having multiple gateways, but not stupidly and blindly adding them in the routing table….)

It is very simple to implement and involves the steps below.  As an example, lets assume we created a management VLAN (VLAN4) and want to add an logical interface on a server in that VLAN to access it internally.  We will be using 10.0.10.0/28 as an inside network.

 

Step 1: Create a VLAN interface

This creates the necessary interface on VLAN4 from primary physical interface eth0:

vi /etc/sysconfig/network-scripts/ifcfg-eth0.4

DEVICE=eth0.4
BOOTPROTO=static
ONBOOT=yes
TYPE=Ethernet
IPV6INIT=no
IPADDR=10.0.10.2
NETMASK=255.255.255.240
NETWORK=10.0.10.0
VLAN=yes

Step 2: Create a iproute2 table for that management network

Edit /etc/iproute2/rt_tables to add a new entry and give it a arbitrary (unused 😉 ) name:

vi /etc/iproute2/rt_tables

#
# reserved values
#
255     local
254     main
253     default
0       unspec
#
# local
#
#1      inr.ruhep
200     MGMT

Note that between 200 and MGMT is a tab character.

 

Step 3: Create a default route for that network

vi /etc/sysconfig/network-scripts/route-eth0.4

default table MGMT via 10.0.10.1

this creates a default route for the MGMT/10.0.10.0/28 network to 10.0.10.1 which is your inside routing intelligence.

Step 4: Create routing rule for 10.0.10.2

To ensure that traffic received on 10.0.10.2 is utilizing the MGMT network only as a source address, a rule must be defined to enable this:

vi /etc/sysconfig/network-scripts/rule-eth0.4

from 10.0.10.2 table MGMT

 

and thats it!  restart your network:

/etc/rc.d/init.d/network restart

 

Using iproute2 commands, we can check that what we did works (as well as using wireshark 😉 )

 

[root@server network-scripts]# ip rule show
0:      from all lookup local
32765:  from 10.0.10.2 lookup MGMT
32766:  from all lookup main
32767:  from all lookup default
 
[root@server network-scripts]# ip route show
10.0.10.0/28 dev eth0.4  proto kernel  scope link  src 10.0.10.2
66.1.1.0/25 dev eth0  proto kernel  scope link  src 66.1.1.12
169.254.0.0/16 dev eth0  scope link  metric 1002
169.254.0.0/16 dev eth0.4  scope link  metric 1006
default via 66.1.1.125 dev eth0

 

Note: this also would work with a second physical interface, for instance to utilize a second NIC card instead of a VLAN logical interface, substitute all use of eth0.4 for eth1.