Need to solve complex network problem Post: 302170087

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Special Forums IP Networking Need to solve complex network problem Post 302170087 by bakunin on Sunday 24th of February 2008 09:30:01 AM

02-24-2008

Registered User

The "default route" may be shown as an attribute for eth0 but *every* IP host has one and exactly one default route.

A route is telling the IP stack (and, regardless of the number of interfaces, there is only one IP stack per host) that "everything directed to network <destination> is to be sent to <gateway>, <gateway> will know how to handle it". "<network>" in this regard is a bit of a stretch, it can be a "network" consisting of only one host, if the subnet mask is all bits set to 1.

A "default" route is nothing different from that, just that "destination" is not a network or a host, but "everything not covered by other rules".

For instance:

your IP networks: 192.168.1/24, 192.168.2/24 in each of them your own IP adress will be 100. There is a router on network 192.168.1, which connects the network(s) to the world and has the adress 1 in the network.

Your IP address layout:

eth0: 192.168.1.100 / 255.255.255.0
eth1: 192.168.2.100 / 255.255.255.0

Your routing table (i do that on an AIX box, so it looks a bit different from Linux):

Code:

# netstat -rn
Routing tables
Destination        Gateway           Flags   Refs     Use  If   Exp  Groups

Route Tree for Protocol Family 2 (Internet):
default            192.168.1.1       UG        9    229328 en0      -      -   =>
192.168.1/24       192.168.1.100     U         0         0 en0      -      -   =>
192.168.2/24       192.168.2.100     U         0         0 en1      -      -   =>

The IP stack will now work like that: if it has to send a packet to a destination on one of "its own" networks (192.168.1/2.x) it will do so using eth0 or eth1, depending on rule 2 or 3.

If it has to send a package to 192.168.3.100 (or any other network), it will first consult all the rules it knows (2&3), finding out that no one fits. Now it falls back on its default route, which tells it to send it faithfully to 192.168.1.1, a host who (hopefully) will know what to do with it. Now it again looks up how to find 192.168.1.1 and - bingo! - it knows from rule 2 how to reach that host.

If the router would be moved from 192.168.1.1 to 192.168.2.1 you would have to change the default route and in the same way the packet would leave the host via eth1 instead of eth0.

I hope this clears things up.

bakunin

bakunin

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systemd-networkd

SYSTEMD-NETWORKD.SERVICE(8)				     systemd-networkd.service				       SYSTEMD-NETWORKD.SERVICE(8)

NAME

       systemd-networkd.service, systemd-networkd - Network manager

SYNOPSIS

       systemd-networkd.service

       /lib/systemd/systemd-networkd

DESCRIPTION

       systemd-networkd is a system service that manages networks. It detects and configures network devices as they appear, as well as creating
       virtual network devices.

       To configure low-level link settings independently of networks, see systemd.link(5).

       systemd-networkd will create network devices based on the configuration in systemd.netdev(5) files, respecting the [Match] sections in
       those files.

       systemd-networkd will manage network addresses and routes for any link for which it finds a .network file with an appropriate [Match]
       section, see systemd.network(5). For those links, it will flush existing network addresses and routes when bringing up the device. Any
       links not matched by one of the .network files will be ignored. It is also possible to explicitly tell systemd-networkd to ignore a link by
       using Unmanaged=yes option, see systemd.network(5).

       When systemd-networkd exits, it generally leaves existing network devices and configuration intact. This makes it possible to transition
       from the initrams and to restart the service without breaking connectivity. This also means that when configuration is updated and
       systemd-networkd is restarted, netdev interfaces for which configuration was removed will not be dropped, and may need to be cleaned up
       manually.

CONFIGURATION FILES

       The configuration files are read from the files located in the system network directory /lib/systemd/network, the volatile runtime network
       directory /run/systemd/network and the local administration network directory /etc/systemd/network.

       Networks are configured in .network files, see systemd.network(5), and virtual network devices are configured in .netdev files, see
       systemd.netdev(5).

SEE ALSO

       systemd(1), systemd.link(5), systemd.network(5), systemd.netdev(5), systemd-networkd-wait-online.service(8)

systemd 237													       SYSTEMD-NETWORKD.SERVICE(8)