There is sometimes a desire to route on packet attributes other than the destination, or the destination and QoS bits. For example, we might want to route packets based in part on the packet source, or on the TCP port number. This kind of routing is decidedly nonstandard, though it is often available, and often an important component of traffic engineering.
This option is often known as policy-based routing, because packets are routed according to attributes specified by local administrative policy. (This term should not be confused with BGP routing policy (10.6 Border Gateway Protocol, BGP), which means something quite different.)
Policy-based routing is not used frequently, but one routing decision of this type can have far-reaching effects. If an ISP wishes to route customer voice traffic differently from customer data traffic, for example, it need only apply policy-based routing to classify traffic at the point of entry, and send the voice traffic to its own router. After that, ordinary routers on the voice path and on the separate data path can continue the forwarding without using policy-based methods.
Sometimes policy-based routing is used to mark packets for special processing; this might mean different routing further downstream or it might mean being sent along the same path as the other traffic but with preferential treatment. For two packet-marking strategies, see 20.7 Differentiated Services and 20.12 Multi-Protocol Label Switching (MPLS).
On Linux systems policy-based routing is part of the Linux Advanced Routing facility, often grouped with some advanced queuing features known as Traffic Control; the combination is referred to as LARTC.
As a simple example of what can be done, suppose a site has two links L1 and L2 to the Internet, with L1 the default route to the Internet. Perhaps L1 is faster and L2 serves more as a backup; perhaps L2 has been added to increase outbound capacity. A site may wish to route some outbound traffic via L2 for any of the following reasons:
the traffic may involve protocols deemed lower in priority (eg email)
the traffic may be real-time traffic that can benefit from reduced competition on L2
the traffic may come from lower-priority senders; eg some customers within the site may be relegated to using L2 because they are paying less
a few large-volume elephant flows may be offloaded from L1 to L2
In the first two cases, routing might be based on the destination port numbers; in the third, it might be based on the source IP address. In the fourth case, a site’s classification of its elephant flows may have accumulated over time.
Note that nothing can be done in the inbound direction unless L1 and L2 lead to the same ISP, and even there any special routing would be at the discretion of that ISP.
The trick with LARTC is to be compatible with existing routing-update protocols; this would be a problem if the kernel forwarding table simply added columns for other packet attributes that neighboring non-LARTC routers knew nothing about. Instead, the forwarding table is split up into multiple ⟨dest, next_hop⟩ (or ⟨dest, QoS, next_hop⟩) tables. One of these tables is the main table, and is the table that is updated by routing-update protocols interacting with neighbors. Before a packet is forwarded, administratively supplied rules are consulted to determine which table to apply; these rules are allowed to consult other packet attributes. The collection of tables and rules is known as the routing policy database.
As a simple example, in the situation above the main table would have an entry ⟨default, L1⟩ (more precisely, it would have the IP address of the far end of the L1 link instead of L1 itself). There would also be another table, perhaps named slow, with a single entry ⟨default, L2⟩. If a rule is created to have a packet routed using the “slow” table, then that packet will be forwarded via L2. Here is one such Linux rule, applying to traffic from host 10.0.0.17:
ip rule add from 10.0.0.17 table slow
Now suppose we want to route traffic to port 25 (the SMTP port) via L2. This is harder; Linux provides no support here for routing based on port numbers. However, we can instead use the iptables
fwmark; its value is 0 by default. The
fwmark is not actually part of the packet; it is associated with the packet only while the latter remains within the kernel.
iptables --table mangle --append PREROUTING \\ --protocol tcp --dest-port 25 --jump MARK --set-mark 1 ip rule add fwmark 1 table slow
Consult the applicable man pages for further details.
The iptables mechanism can also be used to set the appropriate QoS bits – the IPv4 DS bits (7.1 The IPv4 Header) or the IPv6 Traffic Class bits (8.1 The IPv6 Header) – so that a single standard IP forwarding table can be used, though support for the IPv4 QoS bits is limited.