android: How to see the routing table and dns in adb shell

http://groups.google.com/group/android-platform/browse_thread/thread/e0a75cce49273357

For checking DNS server addresses, you can use getprop, such
as:

| # getprop net.dns1
| 192.168.32.18

or, if you'd like to know the values which dhcp server
offers:

| # getprop dhcp.tiwlan0.dns1
| 192.168.32.18

You also can see dhcp supply default gateway value:

| # getprop dhcp.tiwlan0.gateway
| 192.168.32.1

You might see all property values if you exec 'getprop'
without arguments.

If you'd like to see live routing table, the easiest(but
complicated to understand) way is to see /proc/net/route.

| # cat /proc/net/route
| Iface Destination     Gateway         Flags   RefCnt  Use     Metric  Mask            MTU     Window  IRTT
| tiwlan0       0020A8C0        00000000        0001    0       0       0       00FFFFFF        0       0       0
| tiwlan0       00000000        0120A8C0        0003    0       0       0       00000000        0       0       0

The Destination '00000000' means, of course, default route
and Gateway value '0120A8C0' means '1.32.168.192' in
decimal('192.168.32.1' in reverse), so you can know what is
default gateway address is.

linux: (link) changing network adapter priority

http://superuser.com/questions/216678/ubuntu-network-priority

In short: Network card priority is decided by calculated routing speed so you cannot manually change it easily. However, you can set an adapter never to be a default network connection by setting 'Use this connection only for resources on its network' at 'System->Preferences->Network Connections->Edit->IPv4 Setting->Routes...'.

linux: how to setup your linux as internet gateway(Ubuntu 9.10)

There are many posts about this topic however some of them are quite old.

Here's the simple instruction of doing it.

0. Prerequisites

Linux machine with 2 network adapters.

A Laptop usually have two network adapters: one for LAN, one for Wi-fi.

Below, I have two adapters connected to:

  eth1: connected to private network

  wlan1: connected to internet

1. turn on ip forward functionality

open /etc/sysctl.conf

uncomment(or change if set to 0) net.ipv4.ip_forward = 1

$sysctl -p /etc/sysctl.conf

you can check whether it is correctly set or not with

$cat /proc/sys/net/ipv4/ip_forward

1

2. setup ip forwarding & masquerading

Reminder: wlan1 is to internet, eth1 is to intranet(local network)

$iptables -t nat -A POSTROUTING -o wlan1 -j MASQUERADE

$iptables -A FORWARD -i eth1 -j ACCEPT

Simply done.

Now change gateway of your local network pc to linux machine's IP address and you can use internet from your local network pc via linux gateway.

3. troubleshooting

If you cannot access internet from your local network pc, check linux machine's routing table.

$ip ro

192.168.16.0/24 dev wlan1 proto kernel scope link src 192.168.16.21 metric 2

192.168.0.0/24 dev eth1 proto kernel scope link src 192.168.0.58 metric 1

...

default via 192.168.0.1 dev eth1

As above, if eth1(private network) is set as default route, change default route to wlan1(internet) by:

$ip ro change default via xxx.xxx.xxx.xxx dev wlan1

$ip ro flush cache

where xxx.xxx.xxx.xxx is your wlan1(internet accessible adpater) gateway.