Here’s two scripts you can run if you have a 3G card, and a few friends stuck in the hotel room with you who want WiFi.

First, turn your laptop into an access point. These scripts work on MacOS X running 10.5.4 or better — good luck!

I call this script ‘make-me-an-access-point.sh’:

#!/bin/sh

#Edit your /etc/rc.conf to set the option firewall_enable to YES
# Edit your /etc/rc.firewall to add lines:

/usr/sbin/natd -dynamic -interface ppp0   

sysctl -w net.inet.ip.forwarding=1

/sbin/ipfw -f flush
/sbin/ipfw add 1000 pass all from 127.0.0.1 to 127.0.0.1
/sbin/ipfw add 2000 divert natd ip from any to any via ppp0
/sbin/ipfw add 6500 pass all from any to any

ifconfig en1 192.168.1.1 up netmask 255.255.255.0 broadcast 192.168.0.255

dhcpd

and when you want this to go away, do:

laptop:bin jna$ cat undo-make-me-an-access-point.sh
#!/bin/sh
#
# undo the make-me-an-access-point script
#
sysctl -w net.inet.ip.forwarding=0

sudo /sbin/ipfw -f flush
sudo kill -9 `ps -efl |  grep dhcp | egrep -v VMware |  awk '{ print $2 }'`

echo "Now turn off the Airport, then wait a moment, and turn it back on."

The contents of my dhcpd.conf are very simple:

# dhcpd.conf
#
# Sample configuration file for ISC dhcpd
#
ddns-update-style ad-hoc;

# option definitions common to all supported networks...
option domain-name "retina.net";
option domain-name-servers 209.183.50.151;
default-lease-time 600;
max-lease-time 7200;

# If this DHCP server is the official DHCP server for the local
# network, the authoritative directive should be uncommented.
authoritative;

# Use this to send dhcp log messages to a different log file (you also
# have to hack syslog.conf to complete the redirection).
log-facility local7;

# No service will be given on this subnet, but declaring it helps the
# DHCP server to understand the network topology.

subnet 192.168.1.0 netmask 255.255.255.0 {
  range 192.168.1.100 192.168.1.200;
  option routers 192.168.1.1;
  option domain-name-servers 206.13.28.12;
  option broadcast-address  192.168.1.255;
}