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 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.

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