Install PoPToP Point to Point Tunneling Server:
sudo apt-get install pptpd
Edit /etc/pptpd.conf file:
sudo joe /etc/pptpd.conf
Uncomment the following lines (replace IP range if you like)
localip 192.168.0.1 remoteip 192.168.1.1-255
Save and exit.
Edit /etc/ppp/pptpd-options file
sudo joe /etc/ppp/pptpd-options
Make sure you have this:
refuse-pap refuse-chap refuse-mschap require-mschap-v2 require-mppe-128 proxyarp nodefaultroute lock nobsdcomp noipx ## you don't need IPX mtu 1490 ## may help your linux client from disconnecting mru 1490 ## may help your linux client from disconnecting
Save and exit.
Next step is to add users who can use this connection.
sudo joe /etc/ppp/chap-secrets
The file should look like this:
# Secrets for authentication using CHAP # client server secret IP addresses cviorel pptpd my_secret_password * another_user pptpd his_secret_password *
Now we need to configure IP Masquerading on the VPN server.
The purpose of IP Masquerading is to allow machines with private, non-routable IP addresses on your network to access the Internet through the machine doing the masquerading.
IP Masquerading can be achieved using custom ufw rules. This is possible because the current back-end for ufw is iptables-restore with the rules files located in /etc/ufw/*.rules. These files are a great place to add legacy iptables rules used without ufw, and rules that are more network gateway or bridge related.
The rules are split into two different files, rules that should be executed before ufw command line rules, and rules that are executed after ufw command line rules.
a) First, packet forwarding needs to be enabled in ufw. Two configuration files will need to be adjusted, in /etc/default/ufw change the DEFAULT_FORWARD_POLICY to “ACCEPT”:
Then edit /etc/ufw/sysctl.conf and uncomment:
Similarly, for IPv6 forwarding uncomment:
b) Now we will add rules to the /etc/ufw/before.rules file. The default rules only configure the filter table, and to enable masquerading the nat table will need to be configured. Add the following to the top of the file just after the header comments:
# nat Table rules *nat :POSTROUTING ACCEPT [0:0] # Forward traffic from eth1 through eth0. -A POSTROUTING -s 192.168.0.0/24 -o eth0 -j MASQUERADE # don't delete the 'COMMIT' line or these nat table rules won't be processed COMMIT
The comments are not strictly necessary, but it is considered good practice to document your configuration. Also, when modifying any of the rules files in /etc/ufw, make sure these lines are the last line for each table modified:
# don't delete the 'COMMIT' line or these rules won't be processed COMMIT
First, since we trust pptpd completely, I would accept all traffic to/from my pptpd. I added this lines at the beginning of the filter section.
-A ufw-before-input -i ppp+ -j ACCEPT -A ufw-before-output -i ppp+ -j ACCEPT
Additionally, I must forward traffic to/from my pptpd. These lines was also added after the above lines.
-A ufw-before-forward -s 192.168.0.0/24 -j ACCEPT -A ufw-before-forward -d 192.168.0.0/24 -j ACCEPT
c) Finally, disable and re-enable ufw to apply the changes:
sudo ufw disable && sudo ufw enable
IP Masquerading should now be enabled. You can also add any additional FORWARD rules to the /etc/ufw/before.rules. It is recommended that these additional rules be
added to the ufw-before-forward chain.
iptables can also be used to enable masquerading.
a) Similar to ufw, the first step is to enable IPv4 packet forwarding by editing /etc/sysctl.conf and uncomment the following line:
If you wish to enable IPv6 forwarding also uncomment:
- Next, execute the sysctl command to enable the new settings in the configuration file:
sudo sysctl -p
- IP Masquerading can now be accomplished with a single iptables rule, which may differ slightly based on your network configuration:
sudo iptables -t nat -A POSTROUTING -s 192.168.0.0/16 -o ppp0 -j MASQUERADE
The above command assumes that your private address space is 192.168.0.0/16 and that your Internet-facing device is ppp0. The syntax is broken down as follows:
- -t nat — the rule is to go into the nat table
- -A POSTROUTING — the rule is to be appended (-A) to the POSTROUTING chain
- -s 192.168.0.0/16 — the rule applies to traffic originating from the specified address space
- -o ppp0 — the rule applies to traffic scheduled to be routed through the specified network device
- -j MASQUERADE — traffic matching this rule is to “jump” (-j) to the MASQUERADE target to be manipulated as described above
b) Also, each chain in the filter table (the default table, and where most or all packet filtering occurs) has a default policy of ACCEPT, but if you are creating a firewall in addition to a gateway device, you may have set the policies to DROP or REJECT, in which case your masqueraded traffic needs to be allowed through the FORWARD chain for the above rule to work:
sudo iptables -A FORWARD -s 192.168.0.0/16 -o ppp0 -j ACCEPT sudo iptables -A FORWARD -d 192.168.0.0/16 -m state --state ESTABLISHED,RELATED -i ppp0 -j ACCEPT
The above commands will allow all connections from your local network to the Internet and all traffic related to those connections to return to the machine that initiated them.
c) If you want masquerading to be enabled on reboot, which you probably do, edit /etc/rc.local and add any commands used above. For example add the first command with no filtering:
iptables -t nat -A POSTROUTING -s 192.168.0.0/16 -o ppp0 -j MASQUERADE
Firewall logs are essential for recognizing attacks, troubleshooting your firewall rules, and noticing unusual activity on your network. You must include logging rules in your firewall for them to be generated, though, and logging rules must come before any applicable terminating rule (a rule with a target that decides the fate of the packet, such as ACCEPT, DROP, or REJECT).
If you are using ufw, you can turn on logging by entering the following in a terminal:
sudo ufw logging on
To turn logging off in ufw, simply replace on with off in the above command.
If using iptables instead of ufw, enter:
sudo iptables -A INPUT -m state --state NEW -p tcp --dport 80 -j LOG --log-prefix "NEW_HTTP_CONN: "
A request on port 80 from the local machine, then, would generate a log in dmesg that looks like this:
[4304885.870000] NEW_HTTP_CONN: IN=lo OUT= MAC=00:00:00:00:00:00:00:00:00:00:00:00:08:00 SRC=127.0.0.1 DST=127.0.0.1 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=58288 DF PROTO=TCP SPT=53981 DPT=80 WINDOW=32767 RES=0x00 SYN URGP=0
The above log will also appear in /var/log/messages, /var/log/syslog, and /var/log/kern.log. This behavior can be modified by editing /etc/syslog.conf appropriately or by installing and configuring ulogd and using the ULOG target instead of LOG. The ulogd daemon is a userspace server that listens for logging instructions from the kernel specifically for firewalls, and can log to any file you like, or even to a PostgreSQL or MySQL database. Making sense of your firewall logs can be simplified by using a log analyzing tool such as fwanalog, fwlogwatch, or lire.
NOTE: Documentation for this article is taken from https://help.ubuntu.com/8.04/serverguide/C/firewall.html.