Chapter 20. Configuring policy-based routing to define alternative routes

By default, the kernel in RHEL decides where to forward network packets based on the destination address using a routing table. Policy-based routing enables you to configure complex routing scenarios. For example, you can route packets based on various criteria, such as the source address, packet metadata, or protocol.

This section describes of how to configure policy-based routing using NetworkManager.

Note

On systems that use NetworkManager, only the nmcli utility supports setting routing rules and assigning routes to specific tables.

20.1. Routing traffic from a specific subnet to a different default gateway using NetworkManager

This section describes how to configure RHEL as a router that, by default, routes all traffic to Internet provider A using the default route. Using policy-based routing, RHEL routes traffic received from the internal workstations subnet to provider B.

The procedure assumes the following network topology:

policy based routing

Prerequisites

  • The system uses NetworkManager to configure the network, which is the default on RHEL 8.

  • The RHEL router you want to set up in the procedure has four network interfaces:

    • The enp7s0 interface is connected to the network of provider A. The gateway IP in the provider’s network is 198.51.100.2, and the network uses a /30 network mask.
    • The enp1s0 interface is connected to the network of provider B. The gateway IP in the provider’s network is 192.0.2.2, and the network uses a /30 network mask.
    • The enp8s0 interface is connected to the 10.0.0.0/24 subnet with internal workstations.
    • The enp9s0 interface is connected to the 203.0.113.0/24 subnet with the company’s servers.
  • Hosts in the internal workstations subnet use 10.0.0.1 as the default gateway. In the procedure, you assign this IP address to the enp8s0 network interface of the router.
  • Hosts in the server subnet use 203.0.113.1 as the default gateway. In the procedure, you assign this IP address to the enp9s0 network interface of the router.
  • The firewalld service is enabled and active.

Procedure

  1. Configure the network interface to provider A: 

    # nmcli connection add type ethernet con-name Provider-A ifname enp7s0 ipv4.method manual ipv4.addresses 198.51.100.1/30 ipv4.gateway 198.51.100.2 ipv4.dns 198.51.100.200 connection.zone external

    The nmcli connection add command creates a NetworkManager connection profile. The following list describes the options of the command:

    • type ethernet: Defines that the connection type is Ethernet.
    • con-name connection_name: Sets the name of the profile. Use a meaningful name to avoid confusion.
    • ifname network_device: Sets the network interface.
    • ipv4.method manual: Enables to configure a static IP address.
    • ipv4.addresses IP_address/subnet_mask: Sets the IPv4 addresses and subnet mask.
    • ipv4.gateway IP_address: Sets the default gateway address.
    • ipv4.dns IP_of_DNS_server: Sets the IPv4 address of the DNS server.
    • connection.zone firewalld_zone: Assigns the network interface to the defined firewalld zone. Note that firewalld automatically enables masquerading for interfaces assigned to the external zone.

  2. Configure the network interface to provider B: 

    # nmcli connection add type ethernet con-name Provider-B ifname enp1s0 ipv4.method manual ipv4.addresses 192.0.2.1/30 ipv4.routes "0.0.0.0/0 192.0.2.2 table=5000" connection.zone external

    This command uses the ipv4.routes parameter instead of ipv4.gateway to set the default gateway. This is required to assign the default gateway for this connection to a different routing table (5000) than the default. NetworkManager automatically creates this new routing table when the connection is activated.

  3. Configure the network interface to the internal workstations subnet: 

    # nmcli connection add type ethernet con-name Internal-Workstations ifname enp8s0 ipv4.method manual ipv4.addresses 10.0.0.1/24 ipv4.routes "10.0.0.0/24 src=192.0.2.1 table=5000" ipv4.routing-rules "priority 5 from 10.0.0.0/24 table 5000" connection.zone internal

    This command uses the ipv4.routes parameter to add a static route to the routing table with ID 5000. This static route for the 10.0.0.0/24 subnet uses the IP of the local network interface to provider B (192.0.2.1) as next hop.

    Additionally, the command uses the ipv4.routing-rules parameter to add a routing rule with priority 5 that routes traffic from the 10.0.0.0/24 subnet to table 5000. Low values have a high priority.

    Note that the syntax in the ipv4.routing-rules parameter is the same as in an ip route add command, except that ipv4.routing-rules always requires specifying a priority.

  4. Configure the network interface to the server subnet: 

    # nmcli connection add type ethernet con-name Servers ifname enp9s0 ipv4.method manual ipv4.addresses 203.0.113.1/24 connection.zone internal

Verification steps

  1. On a RHEL host in the internal workstation subnet:

    1. Install the traceroute package: 

      # yum install traceroute

    2. Use the traceroute utility to display the route to a host on the Internet: 

      # traceroute redhat.com
traceroute to redhat.com (209.132.183.105), 30 hops max, 60 byte packets
 1  10.0.0.1 (10.0.0.1)     0.337 ms  0.260 ms  0.223 ms
 2  192.0.2.1 (192.0.2.1)   0.884 ms  1.066 ms  1.248 ms
 ...

      The output of the command displays that the router sends packets over 192.0.2.1, which is the network of provider B.

  2. On a RHEL host in the server subnet:

    1. Install the traceroute package: 

      # yum install traceroute

    2. Use the traceroute utility to display the route to a host on the Internet: 

      # traceroute redhat.com
traceroute to redhat.com (209.132.183.105), 30 hops max, 60 byte packets
 1  203.0.113.1 (203.0.113.1)    2.179 ms  2.073 ms  1.944 ms
 2  198.51.100.2 (198.51.100.2)  1.868 ms  1.798 ms  1.549 ms
 ...

      The output of the command displays that the router sends packets over 198.51.100.2, which is the network of provider A.

Troubleshooting steps

On the RHEL router:

  1. Display the rule list: 

    # ip rule list
0:	from all lookup local
5:	from 10.0.0.0/24 lookup 5000
32766:	from all lookup main
32767:	from all lookup default

    By default, RHEL contains rules for the tables local, main, and default.

  2. Display the routes in table 5000: 

    # ip route list table 5000
0.0.0.0/0 via 192.0.2.2 dev enp1s0 proto static metric 100
10.0.0.0/24 dev enp8s0 proto static scope link src 192.0.2.1 metric 102

  3. Display the interfaces and firewall zones: 

    # firewall-cmd --get-active-zones
external
  interfaces: enp1s0 enp7s0
internal
  interfaces: enp8s0 enp9s0

  4. Verify that the external zone has masquerading enabled: 

    # firewall-cmd --info-zone=external
external (active)
  target: default
  icmp-block-inversion: no
  interfaces: enp1s0 enp7s0
  sources:
  services: ssh
  ports:
  protocols:
  masquerade: yes
  ...

Additional resources

  • For further details about the ipv4.* parameters you can set in the nmcli connection add command, see the IPv4 settings section in the nm-settings(5) man page.
  • For further details about the connection.* parameters you can set in the nmcli connection add command, see the Connection settings section in the nm-settings(5) man page.
  • For further details about managing NetworkManager connections using nmcli, see the Connection management commands section in the nmcli(1) man page.

20.2. Overview of configuration files involved in policy-based routing when using the legacy network scripts

If you use the legacy network scripts instead of NetworkManager to configure your network, you can also configure policy-based routing.

Note

Configuring the network using the legacy network scripts provided by the network-scripts package is deprecated in RHEL 8. Red Hat recommends that you use NetworkManager to configure policy-based routing. For an example, see Section 20.1, “Routing traffic from a specific subnet to a different default gateway using NetworkManager”.

The following configuration files are involved in policy-based routing when you use the legacy network scripts:

  • /etc/sysconfig/network-scripts/route-interface: This file defines the IPv4 routes. Use the table option to specify the routing table. For example: 

    192.0.2.0/24 via 198.51.100.1 table 1
203.0.113.0/24 via 198.51.100.2 table 2
  • /etc/sysconfig/network-scripts/route6-interface: This file defines the IPv6 routes.

  • /etc/sysconfig/network-scripts/rule-interface: This file defines the rules for IPv4 source networks for which the kernel routes traffic to specific routing tables. For example: 

    from 192.0.2.0/24 lookup 1
from 203.0.113.0/24 lookup 2
  • /etc/sysconfig/network-scripts/rule6-interface: This file defines the rules for IPv6 source networks for which the kernel routes traffic to specific routing tables.

  • /etc/iproute2/rt_tables: This file defines the mappings if you want to use names instead of numbers to refer to specific routing tables. For example: 

    1     Provider_A
2     Provider_B

Additional resources

  • For further details about IP routing, see the ip-route(8) man page.
  • For further details about routing rules, see the ip-rule(8) man page.

20.3. Routing traffic from a specific subnet to a different default gateway using the legacy network scripts

This section describes how to configure RHEL as a router that, by default, routes all traffic to internet provider A using the default route. Using policy-based routing, RHEL routes traffic received from the internal workstations subnet to provider B.

Important

Configuring the network using the legacy network scripts provided by the network-scripts package is deprecated in RHEL 8. Follow the procedure in this section only if you use the legacy network scripts instead of NetworkManager on your host. If you use NetworkManager to manage your network settings, see Routing traffic from a specific subnet to a different default gateway using NetworkManager.

The procedure assumes the following network topology:

policy based routing

Note

The legacy network scripts process configuration files in alphabetical order. Therefore, you must name the configuration files in a way that ensures that an interface, that is used in rules and routes of other interfaces, are up when a depending interface requires it. To accomplish the correct order, this procedure uses numbers in the ifcfg-*, route-*, and rules-* files.

Prerequisites

  • The NetworkManager package is not installed, or the NetworkManager service is disabled.
  • The network-scripts package is installed.

  • The RHEL router you want to set up in the procedure has four network interfaces:

    • The enp7s0 interface is connected to the network of provider A. The gateway IP in the provider’s network is 198.51.100.2, and the network uses a /30 network mask.
    • The enp1s0 interface is connected to the network of provider B. The gateway IP in the provider’s network is 192.0.2.2, and the network uses a /30 network mask.
    • The enp8s0 interface is connected to the 10.0.0.0/24 subnet with internal workstations.
    • The enp9s0 interface is connected to the 203.0.113.0/24 subnet with the company’s servers.
  • Hosts in the internal workstations subnet use 10.0.0.1 as the default gateway. In the procedure, you assign this IP address to the enp8s0 network interface of the router.
  • Hosts in the server subnet use 203.0.113.1 as the default gateway. In the procedure, you assign this IP address to the enp9s0 network interface of the router.
  • The firewalld service is enabled and active.

Procedure

  1. Add the configuration for the network interface to provider A by creating the /etc/sysconfig/network-scripts/ifcfg-1_Provider-A file with the following content: 

    TYPE=Ethernet
IPADDR=198.51.100.1
PREFIX=30
GATEWAY=198.51.100.2
DNS1=198.51.100.200
DEFROUTE=yes
NAME=1_Provider-A
DEVICE=enp7s0
ONBOOT=yes
ZONE=external

    The following list describes the parameters used in the configuration file:

    • TYPE=Ethernet: Defines that the connection type is Ethernet.
    • IPADDR=IP_address: Sets the IPv4 address.
    • PREFIX=subnet_mask: Sets the subnet mask.
    • GATEWAY=IP_address: Sets the default gateway address.
    • DNS1=IP_of_DNS_server: Sets the IPv4 address of the DNS server.
    • DEFROUTE=yes|no: Defines whether the connection is a default route or not.
    • NAME=connection_name: Sets the name of the connection profile. Use a meaningful name to avoid confusion.
    • DEVICE=network_device: Sets the network interface.
    • ONBOOT=yes: Defines that RHEL starts this connection when the system boots.
    • ZONE=firewalld_zone: Assigns the network interface to the defined firewalld zone. Note that firewalld automatically enables masquerading for interfaces assigned to the external zone.

  2. Add the configuration for the network interface to provider B:

    1. Create the /etc/sysconfig/network-scripts/ifcfg-2_Provider-B file with the following content: 

      TYPE=Ethernet
IPADDR=192.0.2.1
PREFIX=30
DEFROUTE=no
NAME=2_Provider-B
DEVICE=enp1s0
ONBOOT=yes
ZONE=external

      Note that the configuration file for this interface does not contain a default gateway setting.

    2. Assign the gateway for the 2_Provider-B connection to a separate routing table. Therefore, create the /etc/sysconfig/network-scripts/route-2_Provider-B file with the following content: 

      0.0.0.0/0 via 192.0.2.2 table 5000

      This entry assigns the gateway and traffic from all subnets routed through this gateway to table 5000.

  3. Create the configuration for the network interface to the internal workstations subnet:

    1. Create the /etc/sysconfig/network-scripts/ifcfg-3_Internal-Workstations file with the following content: 

      TYPE=Ethernet
IPADDR=10.0.0.1
PREFIX=24
DEFROUTE=no
NAME=3_Internal-Workstations
DEVICE=enp8s0
ONBOOT=yes
ZONE=internal

    2. Add the routing rule configuration for the internal workstation subnet. Therefore, create the /etc/sysconfig/network-scripts/rule-3_Internal-Workstations file with the following content: 

      pri 5 from 10.0.0.0/24 table 5000

      This configuration defines a routing rule with priority 5 that routes all traffic from the 10.0.0.0/24 subnet to table 5000. Low values have a high priority.

    3. Create the /etc/sysconfig/network-scripts/route-3_Internal-Workstations file with the following content to add a static route to the routing table with ID 5000: 

      10.0.0.0/24 via 192.0.2.1 table 5000

      This static route defines that RHEL sends traffic from the 10.0.0.0/24 subnet to the IP of the local network interface to provider B (192.0.2.1). This interface is to routing table 5000 and used as the next hop.

  4. Add the configuration for the network interface to the server subnet by creating the /etc/sysconfig/network-scripts/ifcfg-4_Servers file with the following content: 

    TYPE=Ethernet
IPADDR=203.0.113.1
PREFIX=24
DEFROUTE=no
NAME=4_Servers
DEVICE=enp9s0
ONBOOT=yes
ZONE=internal

  5. Restart the network: 

    # systemctl restart network

Verification steps

  1. On a RHEL host in the internal workstation subnet:

    1. Install the traceroute package: 

      # yum install traceroute

    2. Use the traceroute utility to display the route to a host on the internet: 

      # traceroute redhat.com
traceroute to redhat.com (209.132.183.105), 30 hops max, 60 byte packets
 1  10.0.0.1 (10.0.0.1)     0.337 ms  0.260 ms  0.223 ms
 2  192.0.2.1 (192.0.2.1)   0.884 ms  1.066 ms  1.248 ms
 ...

      The output of the command displays that the router sends packets over 192.0.2.1, which is the network of provider B.

  2. On a RHEL host in the server subnet:

    1. Install the traceroute package: 

      # yum install traceroute

    2. Use the traceroute utility to display the route to a host on the internet: 

      # traceroute redhat.com
traceroute to redhat.com (209.132.183.105), 30 hops max, 60 byte packets
 1  203.0.113.1 (203.0.113.1)    2.179 ms  2.073 ms  1.944 ms
 2  198.51.100.2 (198.51.100.2)  1.868 ms  1.798 ms  1.549 ms
 ...

      The output of the command displays that the router sends packets over 198.51.100.2, which is the network of provider A.

Troubleshooting steps

On the RHEL router:

  1. Display the rule list: 

    # ip rule list
0:      from all lookup local
5:      from 10.0.0.0/24 lookup 5000
32766:  from all lookup main
32767:  from all lookup default

    By default, RHEL contains rules for the tables local, main, and default.

  2. Display the routes in table 5000: 

    # ip route list table 5000
default via 192.0.2.2 dev enp1s0
10.0.0.0/24 via 192.0.2.1 dev enp1s0

  3. Display the interfaces and firewall zones: 

    # firewall-cmd --get-active-zones
external
  interfaces: enp1s0 enp7s0
internal
  interfaces: enp8s0 enp9s0

  4. Verify that the external zone has masquerading enabled: 

    # firewall-cmd --info-zone=external
external (active)
  target: default
  icmp-block-inversion: no
  interfaces: enp1s0 enp7s0
  sources:
  services: ssh
  ports:
  protocols:
  masquerade: yes
  ...

Additional resources