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The_Great_Sephiroth
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Joined: 03 Oct 2014
Posts: 1606
Location: Fayetteville, NC, USA
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 Posted: Wed Jan 15, 2025 7:17 pm Post subject: OpenVPN stopped passing data... |
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I have an OLD (2009) client running OpenVPN on a Linux server that has been solid for ages. Suddenly, this changed. OpenVPN connects, but we cannot access LAN resources. I BELIEVE it is due to an update that reset the firewall rules. I found an old firewall script I wrote, but it does not work. I can only reach the server itself.
LAN: 192.168.111.0/24
OpenVPN: 192.168.110.0/24
Server: 192.168.111.201
PCs: 192.168.111.101-15
Firewall Script:
| Code: |
#!/bin/bash
# Clear out everything
iptables -F
iptables -X
iptables -t nat -F
iptables -t nat -X
iptables -t mangle -F
iptables -t mangle -X
iptables -t raw -F
iptables -t raw -X
iptables -t security -F
iptables -t security -X
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT DROP
# Always allow lo unless you want to murder Linux
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT
# Setup the input table
iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -i tun0 -j ACCEPT
iptables -A INPUT -i enp3s0 -p tcp --dport ssh -j ACCEPT
iptables -A INPUT -i enp3s0 -p udp --dport openvpn -j ACCEPT
iptables -A INPUT -i enp3s0 -p udp --dport bootps:bootpc -j ACCEPT
iptables -A INPUT -i enp3s0 -p udp --dport domain -j ACCEPT
iptables -A INPUT -i enp3s0 -p tcp --dport microsoft-ds -j ACCEPT
iptables -A INPUT -i enp3s0 -p tcp --dport netbios-ssn -j ACCEPT
iptables -A INPUT -p icmp --icmp-type 8/0 -j ACCEPT
# Setup the output table
iptables -A OUTPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A OUTPUT -o tun0 -j ACCEPT
iptables -A OUTPUT -o enp3s0 -p udp --dport domain -j ACCEPT
iptables -A OUTPUT -o enp3s0 -p udp --dport bootps:bootpc -j ACCEPT
iptables -A OUTPUT -o enp3s0 -p tcp --dport http -j ACCEPT
iptables -A OUTPUT -o enp3s0 -p udp --dport ntp -j ACCEPT
iptables -A OUTPUT -o enp3s0 -p tcp --dport microsoft-ds -j ACCEPT
iptables -A OUTPUT -o enp3s0 -p tcp --dport netbios-ssn -j ACCEPT
iptables -A OUTPUT -o enp3s0 -p tcp --dport rsync -j ACCEPT
iptables -A OUTPUT -p icmp --icmp-type 8/0 -j ACCEPT
# Setup the forward table
iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -s 192.168.110.0/24 -i tun0 -o enps3s0 -j ACCEPT
iptables -A FORWARD -p icmp --icmp-type 8/0 -j ACCEPT
# Setup the postrouting table
iptables -t nat -A POSTROUTING -s 192.168.110.0/24 -o enp3s0 -j MASQUERADE
# Setup logging
# iptables -A INPUT -j LOG --log-prefix "!!! IN "
# iptables -A OUTPUT -j LOG --log-prefix "!!! OUT "
# iptables -A FORWARD -j LOG --log-prefix "!!! FWD "
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Does anybody have any ideas here?
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Hu
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Joined: 06 Mar 2007
Posts: 23012
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| Posted: Wed Jan 15, 2025 7:37 pm Post subject: |
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Is IPv4 forwarding enabled? Can you give an example of a specific connection that doesn't work?
Why is this done as a shell script with no error checking? Gentoo has the ability to save iptables rules and play them back on boot. |
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The_Great_Sephiroth
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Joined: 03 Oct 2014
Posts: 1606
Location: Fayetteville, NC, USA
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| Posted: Wed Jan 15, 2025 7:52 pm Post subject: |
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This is an OLD system, but I just checked and net.ipv4.ip_forward is indeed set to 1. I wrote this script a decade ago. It is not run any more and instead iptables are saved and loaded as you said. The script got me this far.
| Code: |
Chain INPUT (policy DROP)
target prot opt source destination
ACCEPT all -- anywhere anywhere
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
ACCEPT all -- anywhere anywhere
ACCEPT tcp -- anywhere anywhere tcp dpt:ssh
ACCEPT udp -- anywhere anywhere udp dpt:openvpn
ACCEPT udp -- anywhere anywhere udp dpts:bootps:bootpc
ACCEPT udp -- anywhere anywhere udp dpt:domain
ACCEPT tcp -- anywhere anywhere tcp dpt:microsoft-ds
ACCEPT tcp -- anywhere anywhere tcp dpt:netbios-ssn
ACCEPT icmp -- anywhere anywhere icmptype 8 code 0
Chain FORWARD (policy DROP)
target prot opt source destination
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
ACCEPT all -- 192.168.110.0/24 anywhere
ACCEPT icmp -- anywhere anywhere icmptype 8 code 0
Chain OUTPUT (policy DROP)
target prot opt source destination
ACCEPT all -- anywhere anywhere
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
ACCEPT all -- anywhere anywhere
ACCEPT udp -- anywhere anywhere udp dpt:domain
ACCEPT udp -- anywhere anywhere udp dpts:bootps:bootpc
ACCEPT tcp -- anywhere anywhere tcp dpt:http
ACCEPT udp -- anywhere anywhere udp dpt:ntp
ACCEPT tcp -- anywhere anywhere tcp dpt:microsoft-ds
ACCEPT tcp -- anywhere anywhere tcp dpt:netbios-ssn
ACCEPT tcp -- anywhere anywhere tcp dpt:rsync
ACCEPT icmp -- anywhere anywhere icmptype 8 code 0
[sv01 ~]# iptables -t nat -L
Chain PREROUTING (policy ACCEPT)
target prot opt source destination
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
Chain POSTROUTING (policy ACCEPT)
target prot opt source destination
MASQUERADE all -- 192.168.110.0/24 anywhere
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Hope that helps.
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The_Great_Sephiroth
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Location: Fayetteville, NC, USA
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| Posted: Thu Jan 16, 2025 1:15 pm Post subject: |
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I have a temporary solution. I have narrowed it down to my iptables rules. When I clear out all rules and only do the one line that masquerades, everything works. Something in my rules is not allowing SMB or RDP data (and probably more) to not go from the tunnel to the LAN. Not sure what, but it does indeed work with just the one line.
Are there any iptables gurus around that can explain to me why my rules break the VPN?
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Hu
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Joined: 06 Mar 2007
Posts: 23012
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| Posted: Thu Jan 16, 2025 2:26 pm Post subject: |
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You allowed three specific types of traffic to be forwarded, and drop everything else. If leaving the forwarding chain open makes this work, we can assume that none of those three specific rules applies to the traffic you want forwarded. Therefore:- Your traffic is not part of an existing connection. It needs to have created a connection before it can be part of one, and creation is failing.
- Your traffic is not sourced from the allowed subnet.
- Your traffic is not ICMP type 8.
You probably meant for that middle rule to work, or you previously had additional rules that are now entirely missing. Check that the traffic has the correct source address, comes in the correct network device, and would (if it were allowed) go out the correct Ethernet device.
Further, we can infer that since the MASQUERADE rule has two of those checks (source address, destination device) and is matching, that the problem must be the input network device, since the FORWARD chain should match that (but does not) and the MASQUERADE rule does not check that (and works). |
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pietinger
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Joined: 17 Oct 2006
Posts: 5315
Location: Bavaria
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The_Great_Sephiroth
Veteran
Joined: 03 Oct 2014
Posts: 1606
Location: Fayetteville, NC, USA
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| Posted: Thu Jan 16, 2025 4:01 pm Post subject: |
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I'm going to need to brush up then. Been years since I messed with iptables. The server only has one NIC (enp3s0) which is used for everything instead of the setup with a WAN NIC and a LAN NIC. Need to think it through. Thank you both for your help. It's good to see that you're still here. I disappeared for a few years but I still have Gentoo on my laptops.
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Hu
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Joined: 06 Mar 2007
Posts: 23012
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| Posted: Thu Jan 16, 2025 4:18 pm Post subject: |
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| You have one NIC, but at least one virtual interface: tun0, the virtual device that represents the traffic coming off the VPN. |
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pietinger
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Joined: 17 Oct 2006
Posts: 5315
Location: Bavaria
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| Posted: Thu Jan 16, 2025 5:04 pm Post subject: |
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You have already prepared something very useful in your script:
| Code: | # Setup logging
# iptables -A INPUT -j LOG --log-prefix “!!! IN ”
# iptables -A OUTPUT -j LOG --log-prefix “!!!! OUT ”
# iptables -A FORWARD -j LOG --log-prefix “!!!! FWD ” |
Activate it and check in your system log what is being dropped.
I do the same with a small addition so that my log is not loaded with these harmless packets that every router sends (before I log everything - like you):
| Code: | | iptables -A INPUT -p 2 -j DROP # stuff from router |
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