02:17 pm, 8 Jul 07
quickly sharing a net connection
I seem to often (at least often enough to know I've done it more than a few times) need to quickly share a computer's net connection. This time it's because we had to pull the net connection from my desktop because the building was getting repainted; my laptop has wireless but my desktop doesn't. I always just barely remember how to do this each time I need to, so I may as well write it down even though it's relatively simple.
Three steps:
Three steps:
- Get the two computers speaking via IP. Pick addresses that aren't already used elsewhere. A fancy way of doing this might be
avahi-autoipd, which implements 'IPv4LL, "Dynamic Configuration of IPv4 Link-Local Addresses" (IETF RFC3927)', but that's only in Ubuntu feisty. In any case you need to tell the internet-less machine to route through the gateway one (as well as give it the upstream DNS server address). - Tell the laptop machine to forward packets routed to it:
echo 1 > /proc/sys/net/ipv4/ip_forward - Tell iptables to masquerade (NAT) packets going out the wireless interface:
iptables -t nat -A POSTROUTING -o ath0 -j MASQUERADE
echo 0 into the ip forwarding control and clear ("flush") the nat iptable: iptables -t nat -F .
Small tip: since I am never ever logged in as root unless the system is in single user mode, I prefer to use
sysctlinstead of writing into/procdirectly, because that’s straightforward to invoke withsudo:If you use redirection to write in
/proc, the shell executing the command has to run as root, so you need contortions likewhich involves quotes, and can hence be a pain.
Also, to be slightly more safe on your masquerade rule you should make sure you only masquerade connections from your internal LAN. You can add a "-i " as in: iptables -t nat -A POSTROUTING -i eth0 -o ath0 -j MASQUERADE
I've always loved how easy it is to get stuff like this running in Linux. Just the other day when I was having that problem with VMWare I just did your step 2 here and had an instant router without any further ado.
On the other hand, when I've had to do similar things to the above with Windows Server 2003 boxes at work I've almost wanted to either cry or take a hammer to the server.