Iproute2
Advanced Routing and iproute2 - the ip routing utility
iproute2.txt research document, details may be sketchy and incomplete. v0.02
ifconfig and route use iproute2 syscalls. However, the 'ip' tool is the key to access advanced iproute2 syscalls.
[ Usage examples of the ip tool ] ip link list : shows links including MAC but not IP address ip address show : more info including IP and queueing disipline ip route show : equivalent to route -n except uses iproute2
Address Resolution Protocol (ARP) resolves the hardware address of another machine on the same local network. ARP determines where a machine is at on the LAN (really it determines what the MAC is of that machine).
synonymous terminology: MAC address = location = hardware address
IP address is Layer 3 networking, Media Access Control (MAC) is Layer 2.
An IP address knows not the location of a machine, ARP does. Machines on the Internet have DNS names which resolve to IP addresses, not to be confused with knowing the location (MAC address).
When computer A wants to find computer B on the same LAN, computer A does an ARP broadcast with IP (layer 3). Computer B with matching IP answers with its MAC address. Further communication is layer 2 by MAC. The ARP entry for computer B remains in the ARP cache of computer A for a limited duration.
When computer A wants to find computer C, which is on another network across the Internet, 'A' knows the subnet is different from its own and therefore references its gateway (the router) and asks it for the location (MAC) of computer C. Through Internet routing the local router finds the remote router which is the gateway for computer C and asks it what the MAC address is for computer C. If the remote router does not know, it does an ARP broadcast on the LAN of computer C. Computer C answers telling its router its MAC address. That remote router then sends the MAC back to the local router for computer A and informs computer A of the MAC address for 'C'. Further communication is layer 2 from local machine to local router and layer 3 between routers.
synonymous terminology: arp cache = neighbor cache = neigh
[ Usage examples of ip relating to ARP ] ip neigh show : view current ARP cache table ip neigh delete X.X.X.X dev ethX : delete IP X.X.X.X from APR cache
Of the three routing tables which are part of iproute2, the traditional 'route' command only modifies the main table. The 'ip' tool can modify all three tables but modifies the main table by default.
ip route ls : shows only the main route table ip rule list : display current route rules and priority ip route list table local : shows necessary stuff in the local table ip route list table main : same as 'ip route ls' ip route flush cache : clear all route cache, do after modification
You can create your own route tables. To create a custom table:
echo 200 tablename >> /etc/iproute2/rt_tables
(above: creates a route table in rt_tables)
ip rule add from X.X.X.X table tablename
(above: source IP or computer on LAN)
ip route add default via Y.Y.Y.Y dev ethX table tablename
(above: assigns route for computer on LAN)
Example: Two Internet Providers - Multihoming
A business may have two Internet providers. To set up routing for two ISPs on linux consider this example a generic guide.
192.168.0.1 = IP of Internal network (irrelevant) on eth1 64.21.10.250 = IP of first ISP on eth0 gw 64.21.10.1 network 255.255.255.0 128.42.20.250 = IP of second ISP on eth2 gw 128.42.20.1 network 255.255.255.0
1. create two tables and set up routing
ip route add 255.255.255.0 dev eth0 src 64.21.10.250 table T1 ip route add default via 64.21.10.1 table T1 ip route add 255.255.255.0 dev eth2 src 128.42.20.250 table T2 ip route add default via 128.42.20.1 table T2
2. set up main routing table
ip route add 255.255.255.0 dev eth0 src 64.21.10.250 ip route add 255.255.255.0 dev eth2 src 128.42.20.250
3. set the preference for the default route
ip route add default via 64.21.10.1
4. routing rules for interfaces
ip rule add from 64.21.10.250 table T1 ip rule add from 128.42.20.250 table T2
- . load balancing between the two providers
ip route add default scope global nexthop via 64.21.10.1 dev eth0 weight 1 nexthop via 128.42.20.1 dev eth2 weight 1
Thu Oct 23 11:48:27 CDT 2003
