Ubuntu – iptables port forwarding (prerouting port 80) and firewall not working together

iptableslinodenode.jsport-forwardingUbuntu

I have read many articles and answers on this topic and I have been in discussion with Linode support, but no one seems to be able to answer my exact problem.

Seems easy – I'd like to use a iptables firewall to restrict access to all ports except 22, 80 and 443. Linode has a great write up here: https://library.linode.com/securing-your-server#sph_creating-a-firewall and I used their firewall rules as is. The firewall works well.

I also want to preroute some ports, because this is a nodejs app. So I used:

sudo iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 8080
sudo iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 443 -j REDIRECT --to-port 3000

These rules work if I do not have the firewall rules. In fact, I'm using them right now, but I've had to leave the firewall down.

If I add the firewall rules, the PREROUTING stops working. If I save the active iptables rules into a file to view, both the firewall (filter rules) and the PREROUTING (nat rules) are present, but only the firewall rules work. See here:

# Generated by iptables-save v1.4.18 on Wed Mar 26 02:40:04 2014
*security
:INPUT ACCEPT [1606:135329]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [1206:144815]
COMMIT
# Completed on Wed Mar 26 02:40:04 2014
# Generated by iptables-save v1.4.18 on Wed Mar 26 02:40:04 2014
*raw
:PREROUTING ACCEPT [1620:139613]
:OUTPUT ACCEPT [1206:144815]
COMMIT
# Completed on Wed Mar 26 02:40:04 2014
# Generated by iptables-save v1.4.18 on Wed Mar 26 02:40:04 2014
*nat
:PREROUTING ACCEPT [4:248]
:INPUT ACCEPT [6:376]
:OUTPUT ACCEPT [12:728]
:POSTROUTING ACCEPT [12:728]
-A PREROUTING -p tcp -m tcp --dport 80 -j REDIRECT --to-ports 8080
-A PREROUTING -p tcp -m tcp --dport 443 -j REDIRECT --to-ports 3000
COMMIT
# Completed on Wed Mar 26 02:40:04 2014
# Generated by iptables-save v1.4.18 on Wed Mar 26 02:40:04 2014
*mangle
:PREROUTING ACCEPT [1620:139613]
:INPUT ACCEPT [1606:135329]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [1206:144815]
:POSTROUTING ACCEPT [1206:144815]
COMMIT
# Completed on Wed Mar 26 02:40:04 2014
# Generated by iptables-save v1.4.18 on Wed Mar 26 02:40:04 2014
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -i lo -j ACCEPT
-A INPUT -d 127.0.0.0/8 -j REJECT --reject-with icmp-port-unreachable
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp --dport 80 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 443 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 22 -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7
-A INPUT -j DROP
-A FORWARD -j DROP
-A OUTPUT -j ACCEPT
COMMIT
# Completed on Wed Mar 26 02:40:04 2014

If I use iptables -F, it will flush the firewall (filter) rules only and the PREROUTING will start working again. So it's definitely a conflict. The order of the rule blocks doesn't seem to matter as this (above) is the standard output by iptables regardless of the order in which I saved the rules to iptables.

To me, it seems like it's one of two things:

PREROUTING ports that also have the ACCEPT rule could mean the
prerouting is ignored
that I'm PREROUTING to a port that is technically blocked (but I thought that's where the "PRE" came in)

Anyone know?

Thanks!

Paul

Best Answer

After PREROUTING local packets go to the local filter where they get dropped (your case 2).

So you just need to allow incoming traffic to those ports in iptables:

iptables -A INPUT -p tcp -m tcp --dport 8080 -j ACCEPT

iptables -A INPUT -p tcp -m tcp --dport 3000 -j ACCEPT

Since you are redirecting http(s) traffic from 80/443 to 8080/3000 is like the latter ports are open and exposed to the Internet anyways, so there's no difference from the outside wrorld and there are no further security implications.

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Linux – iptables port forward forwarding

You're sending the traffic to 10.52.208.221. Given the config you posted, your problem is the webserver, not the firewall. Your rules look to be correct. FORWARD and INPUT are redirected to RH-Firewall-1-INPUT where your first rule is to allow all traffic. As somebody else pointed out, you could be allowing all traffic on eth1, while the world is actually coming in eth0. Secondary, you have to NAT the traffic as it goes back out to the world

iptables --table nat --append POSTROUTING --proto tcp --dport 80 --jump MASQUERADE -o OUT_INTERFACE

Your traffic originating from the router will never hit the input or forward chains, but instead traverse the output chain on to the webserver. Other systems in that subnet will similarly go directly to the webserver. Systems out on the world at large however will traverse the INPUT / FORWARD chains and need SNAT as well as DNAT so that it appears to the world to be one machine. You still cannot test from within your network. you must test from the opposite interface from the webserver. Get me your IP addresses and I'll point you to the proper configs.

Iptables – ftp tls firewalled :(

In Passive mode, when the client wants to get a file from the server or send a file to the server, the FTP server will pick a random port and send that port to the FTP client.

When you're not using encryption, a properly configured firewall (using the ip_conntrack_ftp helper kernel module, which may be what you're missing for non-TLS connections) would "listen in" on the connection and mark these connections as RELATED. With encryption the firewall can't listen in.

The quick and dirty solution to this is to configure the FTP server to choose a small range of ports for passive connections, and then allow access to all of these ports. For instance, in vsftpd:

pasv_min_port=12000
pasv_max_port=12049

Then in iptables:

iptables -A INPUT -p tcp -m tcp -i eth0 --dport 12000:12049 -j ACCEPT

Allowing anyone to access these ports opens one possible exploit: if someone were to be scanning them over and over they might get lucky and be able to "beat" the real user to the data port and grab the file. Ideally your FTP server would check and make sure the connection is coming from the same place as the original connection, but thanks to things like "FXP" (transferring files from one server to another server by convincing one to make an active connection to the other's passive data port) some servers don't check the connection by default. You should check your configuration file and see if there is an option to disable FXP, and use it. (vsftpd calls this "promiscuous" and is disabled by default.)

Best Answer

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Iptables – ftp tls firewalled :(

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