Re: firewall oddities when accessing services at the far side of an IPsec N2N connection

["Peter Müller" <peter.mueller@ipfire.org>]

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Hello Michael,

> Hello Michael,
> 
> thank you for your reply.
> 
>> Hi,
>>
>>> On 25 Jan 2020, at 16:10, Peter Müller <peter.mueller(a)ipfire.org> wrote:
>>>
>>> Hello list,
>>>
>>> due to reasons, I currently work on migrating an upstream (Squid) proxy
>>> machine from HardenedBSD connected via OpenVPN to OpenBSD connected via IPsec.
>>>
>>> The latter one seems to work since the connection is stable and SSH usage
>>> over the tunnel is possible. However, using the remote Squid proxy as an
>>> upstream proxy (refer to corresponding setting in proxy.cgi) is impossible
>>> as responses are not answered from the remote side:
>>>
>>>> [root(a)maverick ~]# export http_proxy="http://10.xxx.xxx.2:3128/"
>>>> [root(a)maverick ~]# wget -vv example.com
>>>> --2020-01-25 16:58:00--  http://example.com/
>>>> Connecting to 10.xxx.xxx.2:3128... connected.
>>>> Proxy request sent, awaiting response... 
>>>> (wget stalls and eventually runs in a timeout)
>>>
>>> Oddly enough, doing the same thing on a machine within the GREEN network works:
>>>
>>>> user(a)machine:~> export http_proxy="http://10.xxx.xxx.2:3128/"
>>>> user(a)machine:~> wget -vv heise.de
>>>> --2020-01-25 16:59:26--  http://heise.de/
>>>> Verbindungsaufbau zu 10.xxx.xxx.2:3128 … verbunden.
>>>> Proxy-Anforderung gesendet, auf Antwort wird gewartet … 407 Proxy Authentication Required
>>>> 2020-01-25 16:59:26 FEHLER 407: Proxy Authentication Required.
>>> However, a SSH login _is_ possible from the firewall machine to the remote
>>> IPsec one, which makes me writing this mail as I am not sure about the behaviour's
>>> root cause.
>>>
>>> Connecting to the IPsec machine seems to require a firewall rule like this:
>>> - source: firewall (any)
>>> - use NAT: yes, source NAT enabled, new source IP address = GREEN
>>> - destination: IPsec remote machine
>>> - protocol: any
>>>
>>> If source NAT is omitted, accessing the IPsec machine is not possible via
>>> any given way (ping, SSH, Squid, ...). However, _if_ SNAT is enabled, it
>>> also affects connections made from the machine within the GREEN network.
>>
>> The NAT rule should not be necessary because we are setting the correct source in the route in the IPsec routing table.

Unfortunately, it is. If the NAT rule is not present, traffic to the remote IPsec
IP address is emitted to the internet directly and dropped a few hops later by
my ISP. :-(

This behaviour is reproducible for _all_ IPsec N2N connections running here, and
as far as I am concerned, it should not happen. ping to remote IPsec destinations
from a machine within GREEN works fine and is passed through the tunnel.

Thanks, and best regards,
Peter Müller

>>
>> Can you post what is in there?
> 
> Here you are:
>> [root(a)maverick ~]# ip route show table 220
>> [3 lines regarding other IPsec connections redacted]
>> 10.xxx.xxx.2 dev ppp0 proto static scope link src 10.xxx.xxx.1 [IP address of the GREEN interface]
> 
> I have not experimented with TRACE flags in iptables yet, but since disabling
> the IPS does not have any effect on this, I guess that is not the root cause for once. :-)
> 
> Thanks, and best regards,
> Peter Müller
> 
>>
>>> As far as I am concerned, there are two oddities:
>>> (a) Even with SNAT enabled, the firewall itself is unable to reliably establish
>>> a connection to an remote IPsec destination.
>>> (b) If SNAT is enabled for outgoing traffic generated by the firewall, it
>>> also seems to affect traffic from GREEN/... sources, while it is not configured
>>> to do so.
>>>
>>> Is there anybody who got remote upstream proxies via IPsec working?
>>> Are (a) and (b) bugs? If not: What shall I do to work around these?
>>>
>>> Thanks, and best regards,
>>> Peter Müller
>>