Re: Recommended change for the networking page in wiki

[Laine Stump <laine@xxxxxxxxx>]

On 06/08/2015 03:24 PM, BJ wrote:

Hello all,

I was told on IRC that I should come here to discuss a recommended change on the networking page in the wiki.

If you take a look at the "Basic Script" shown here: http://wiki.libvirt.org/page/Networking#Forwarding_Incoming_Connections

It does two things:

1) Create a DNAT rule in the NAT table of IPTABLES

2) Create a FORWARD rule in the FILTER table of IPTABLES

The FORWARD rule is set up as it ought to be, however, the DNAT rule has some unintended consequences. I set up a DNAT on port 80, and suddenly, I couldn't access out on port 80 anymore from my guest machine.

I'm not seeing the same results here. outbound port 80 is still allowed from my host once the rule is in place. Of course incoming port 80 isn't being redirected to the guest either, so I think I have bigger problems. (this is likely due to the fact that I haven't tried the script again since 2013 or so, and a lot has likely changed with the iptables rules added to a Fedora system since then)

However, if I changed the destination address from "anywhere" to the IP of the host machine, the problem resolved. So I change the script to as follows. (Changes are highlighted. For some reason the original script didn't work using /bin/sh, but it did with /bin/bash, so I changed that too).

I don't know for sure, but my guess is that this line:

  length=$(( ${#Host_port[@]} - 1 ))

which was added by vgerris in order to support forwarding of multiple ports, could be what's causing the incompatibility (that wasn't in the original, simpler version of the script, written by me.)

#!/bin/bash

# used some from advanced script to have multiple ports: use an equal number of guest and host ports

Guest_name=xxxxxxx

Guest_ipaddr=xxx.xxx.xxx.xx

Host_ipaddr=xxx.xxx.xxx.xx

Host_port=( '80' '443' )

Guest_port=( '80' '443' )

length=$(( ${#Host_port[@]} - 1 ))

if [ "${1}" = "${Guest_name}" ]; then

   if [ "${2}" = "stopped" -o "${2}" = "reconnect" ]; then

       for i in `seq 0 $length`; do

               iptables -t nat -D PREROUTING -d ${Host_ipaddr} -p tcp --dport ${Host_port[$i]} -j DNAT --to ${Guest_ipaddr}:${Guest_port[$i]}

               iptables -D FORWARD -d ${Guest_ipaddr}/32 -p tcp -m state --state NEW -m tcp --dport ${Guest_port[$i]} -j ACCEPT

       done

   fi

   if [ "${2}" = "start" -o "${2}" = "reconnect" ]; then

       for i in `seq 0 $length`; do

               iptables -t nat -A PREROUTING -d ${Host_ipaddr} -p tcp --dport ${Host_port[$i]} -j DNAT --to ${Guest_ipaddr}:${Guest_port[$i]}

               iptables -I FORWARD 4 -d ${Guest_ipaddr}/32 -p tcp -m state --state NEW -m tcp --dport ${Guest_port[$i]} -j ACCEPT

       done

   fi

fi

Lastly, I should note that I am using Ubuntu 14.04, both for the host and guest. 

I'm also curious as to why this is considered a hack method. It states in the wiki that "This method is a hack", but it doesn't express why.

I consider it a hack because:

1) It requires the IP address of the guest to be known before the guest is started, so either you need to guess the guest's IP (if the guest is getting its IP address via dhcp) or configured the guest IP address in multiple places.

2) It requires using a hook script, which "taints" *all* guests on this machine, rendering them "unsupported" in the eyes of some (in practice, once you see what is causing the tainting it's not such a big deal, but it does mean that an external script gets control with elevated privileges).

3) The "configuration" is just the contents of the shell script, so it can't be supported by any higher level management application - you will always need to directly modify this single shell script file.

4) If you change the configuration for a guest while that guest is running, any forwarding rules that were a part of the old config but not in the new config will be orphaned in your iptables nat table until *all* rules are flushed.

5) If anything else messes with the iptables rules, these port forwarding rules are broken. Especially on older versions of libvirt (0.9.12 and older, which is around the time that entry was written), just restarting libvirtd would break the port forwarding.

6) As both of us have experienced here, interaction with the iptables rules of the underlying system can easily result in it not working at all.


The proper way to support this would be XML added to the domain configuration, something like:

   <interface type='network'>
     <source network='default'/>
     <model type='virtio'/
     <mac address='52:54:00:11:22:33'/>
     <redirect protocol='tcp'>
       <public address='1.2.3.4' port='2200'/>
       <private port='22'/>
     </redirect>
     ...
  </interface>

(or something like that). libvirt would then auto-determine the IP address of the interface and add the necessary iptables rules (or perhaps an rinetd config line item, which may be less prone to sabotage by distro-specific default iptables setups). The inverse would be automatically done when the domain was shutdown.

Many VM Servers have similar features. I know Virtual Box does, I use the same feature there. It may not be how I would set up a production server, but doesn't make it a hack.

It's not the port forwarding itself that I consider to be a hack, it's the method that's used by that script to accomplish it. (And since I was the original author of the script and the entry in the wiki, I think I am allowed to criticize it :-)

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