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4.5. Configuring Static Routes in ifcfg files

Static routes set using ip commands at the command prompt will be lost if the system is shutdown or restarted. To configure static routes to be persistent after a system restart, they must be placed in per-interface configuration files in the /etc/sysconfig/network-scripts/ directory. The file name should be of the format route-interface. There are two types of commands to use in the configuration files:

Static Routes Using the IP Command Arguments Format

If required in a per-interface configuration file, for example /etc/sysconfig/network-scripts/route-eth0, define a route to a default gateway on the first line. This is only required if the gateway is not set through DHCP and is not set globally in the /etc/sysconfig/network file:
default via 192.168.1.1 dev interface
where 192.168.1.1 is the IP address of the default gateway. The interface is the interface that is connected to, or can reach, the default gateway. The dev option can be omitted, it is optional. Note that this setting takes precedence over a setting in the /etc/sysconfig/network file.
If a route to a remote network is required, a static route can be specified as follows. Each line is parsed as an individual route:
10.10.10.0/24 via 192.168.1.1 [dev interface]
where 10.10.10.0/24 is the network address and prefix length of the remote or destination network. The address 192.168.1.1 is the IP address leading to the remote network. It is preferably the next hop address but the address of the exit interface will work. The next hop means the remote end of a link, for example a gateway or router. The dev option can be used to specify the exit interface interface but it is not required. Add as many static routes as required.
The following is an example of a route-interface file using the ip command arguments format. The default gateway is 192.168.0.1, interface eth0 and a leased line or WAN connection is available at 192.168.0.10. The two static routes are for reaching the 10.10.10.0/24 network and the 172.16.1.10/32 host:
default via 192.168.0.1 dev eth0
10.10.10.0/24 via 192.168.0.10 dev eth0
172.16.1.10/32 via 192.168.0.10 dev eth0
In the above example, packets going to the local 192.168.0.0/24 network will be directed out the interface attached to that network. Packets going to the 10.10.10.0/24 network and 172.16.1.10/32 host will be directed to 192.168.0.10. Packets to unknown, remote, networks will use the default gateway therefore static routes should only be configured for remote networks or hosts if the default route is not suitable. Remote in this context means any networks or hosts that are not directly attached to the system.
For IPv6 configuration, an example of a route6-interface file in ip route format:
2001:db8:1::/48 via 2001:db8::1 metric 2048
2001:db8:2::/48
Specifying an exit interface is optional. It can be useful if you want to force traffic out of a specific interface. For example, in the case of a VPN, you can force traffic to a remote network to pass through a tun0 interface even when the interface is in a different subnet to the destination network.
The ip route format can be used to specify a source address. For example:
10.10.10.0/24 via 192.168.0.10 src 192.168.0.2
To define an existing policy-based routing configuration, which specifies multiple routing tables, see Section 4.5.1, “Understanding Policy-routing”.

Important

If the default gateway is already assigned by DHCP and if the same gateway with the same metric is specified in a configuration file, an error during start-up, or when bringing up an interface, will occur. The follow error message may be shown: "RTNETLINK answers: File exists". This error may be ignored.

Static Routes Using the Network/Netmask Directives Format

You can also use the network/netmask directives format for route-interface files. The following is a template for the network/netmask format, with instructions following afterwards:
ADDRESS0=10.10.10.0
NETMASK0=255.255.255.0
GATEWAY0=192.168.1.1
  • ADDRESS0=10.10.10.0 is the network address of the remote network or host to be reached.
  • NETMASK0=255.255.255.0 is the netmask for the network address defined with ADDRESS0=10.10.10.0.
  • GATEWAY0=192.168.1.1 is the default gateway, or an IP address that can be used to reach ADDRESS0=10.10.10.0
The following is an example of a route-interface file using the network/netmask directives format. The default gateway is 192.168.0.1 but a leased line or WAN connection is available at 192.168.0.10. The two static routes are for reaching the 10.10.10.0/24 and 172.16.1.0/24 networks:
ADDRESS0=10.10.10.0
NETMASK0=255.255.255.0
GATEWAY0=192.168.0.10
ADDRESS1=172.16.1.10
NETMASK1=255.255.255.0
GATEWAY1=192.168.0.10
Subsequent static routes must be numbered sequentially, and must not skip any values. For example, ADDRESS0, ADDRESS1, ADDRESS2, and so on.
By default, forwarding packets from one interface to another, or out of the same interface, is disabled for security reasons. This prevents the system acting as a router for external traffic. If you need the system to route external traffic, such as when sharing a connection or configuring a VPN server, you will need to enable IP forwarding. See the Red Hat Enterprise Linux 7 Security Guide for more details.

4.5.1. Understanding Policy-routing

Policy-routing also known as source-routing, is a mechanism for more flexible routing configurations. Routing decisions are commonly made based on the destination IP address of a package. Policy-routing allows more flexibility to select routes based on other routing properties, such as source IP address, source port, protocol type. Routing tables stores route information about networks. They are identified by either numeric values or names, which can be configured in the /etc/iproute2/rt_tables file. The default table is identified with 254. Using policy-routing, you also need rules. Rules are used to select a routing table, based on certain properties of packets.
For initscripts, the routing table is a property of the route that can be configured through the table argument. The ip route format can be used to define an existing policy-based routing configuration, which specifies multiple routing tables:
10.10.10.0/24 via 192.168.0.10 table 1
10.10.10.0/24 via 192.168.0.10 table 2
To specify routing rules in initscripts, edit them to the /etc/sysconfig/network-scripts/rule-eth0 file for IPv4 or to the /etc/sysconfig/network-scripts/rule6-eth0 file for IPv6.
NetworkManager supports policy-routing, but rules are not supported yet. The rules must be configured by the user running a custom script. For each manual static route, a routing table can be selected:
  • ipv4.route-table for IPv4
    and
  • ipv6.route-table for IPv6.
By setting routes to a particular table, all routes from DHCP, autoconf6, DHCP6 are placed in that specific table. In addition, all routes for subnets that have already configured addresses, are placed in the corresponding routing table. For example, if you configure the 192.168.1.10/24 address, the 192.168.1.0/24 subnet is contained in ipv4.route-table.
For more details about policy-routing rules, see the ip-rule(8) man page. For routing tables, see the ip-route(8) man page.