Howto Setup Linux as router for ppp dial up (PSTN/ISDN) connections

This step by step howto will help you to setup Linux as router only in two (2) minutes. I'm assuming that your setup is as follows:

(A) You are using RedHat Linux, FC linux/Debian Linux (this should work with ALL Other distros as well :D )

(B) ppp0 is internet interface (connected to Modem for example) and eth0 connected to your internal lan (connected to your HUB/Switch for example)

My Linux  --> eth0
Box      --> ppp0 (comes up when you dial up into ISP)

Step # 1 Turn on ip forwarding in kernel

1) Open linux kernel configuration file:
# vi /etc/sysctl.conf

2) Add/modify following line:
net.ipv4.ip_forward = 1

Step # 2 Restart network
# /etc/init.d/network restart
OR
# service network restart

(Note first dial to ISP using wvdial or similar program )

Step # 3 Setup IP forwarding and Masquerading (to act as router), you need to use NAT option of iptables as follows:
# iptables --table nat --append POSTROUTING --out-interface ppp0 -j MASQUERADE
# iptables --append FORWARD --in-interface eth0 -j ACCEPT

Step # 4 You are done! Test it with ping or dig:
# ping your-isp.com
# dig www.nixcraft.com
# dig yahoo.com

Step # 5 Point all client to your eth0 IP address as Router/Gateway.

Step # 6 Put code described in step # 3 to script and call it from /etc/rc.local

• Tweet
• 
• 

You should follow me on twitter here or grab rss feed to keep track of new changes.

Featured Articles:

• 30 Handy Bash Shell Aliases For Linux / Unix / Mac OS X
• Top 30 Nmap Command Examples For Sys/Network Admins
• 25 PHP Security Best Practices For Sys Admins
• 20 Linux System Monitoring Tools Every SysAdmin Should Know
• 20 Linux Server Hardening Security Tips
• Linux: 20 Iptables Examples For New SysAdmins
• Top 20 OpenSSH Server Best Security Practices
• Top 20 Nginx WebServer Best Security Practices
• 20 Examples: Make Sure Unix / Linux Configuration Files Are Free From Syntax Errors
• 15 Greatest Open Source Terminal Applications Of 2012
• My 10 UNIX Command Line Mistakes
• Top 10 Open Source Web-Based Project Management Software
• Top 5 Email Client For Linux, Mac OS X, and Windows Users
• The Novice Guide To Buying A Linux Laptop