Closed port 22 in the jail redirects to the outer system
robroy at robroygregg.com
Thu Dec 8 17:20:50 UTC 2016
Good day Bertram,
On Wed, 7 Dec 2016, Bertram Scharpf wrote:
> On Tuesday, 06. Dec 2016, 22:05:09 -0800, Robroy Gregg wrote:
>> Bertram Scharpf wrote:
>>> How can I make a port 22 request fail if an SSH server is running on the
>>> outer machine but not inside the jail?
>> If I've understood your situation correctly, the idea here's to configure
>> the host FreeBSD system's ssh daemon to associate itself only with the
>> host system's IP address.
>> By default, the ssh daemon associates itself with all IP addresses your
>> computer's configured to use (host + jails), which leads to the
>> fall-through effect you're experiencing when your jail's ssh daemon isn't
> That's exactly what I meant. I don't know why, but I always thought a
> jail should grab all requests on its IP and then look up a server
Bertram, I think you probably understand these details already, but I'll
add a few comments.
Original, non-VNET jails share networking with the host in a way which
takes some getting used to. Yet once you are used to it, its simplicity
has a lot of appeal, when compared to other virtualization styles which
have more stuff (code, bugs) sandwiched in between the guest and the host.
I'm not sure if this is always true, yet I've noticed that when a process
on the host and a process in a jail both (effectively) listen on the same
IP and port, the process in the jail will "win." Incoming traffic will go
to the jailed process. Yet if that process exits, the same incoming
traffic will then "fall through" to the host, which could obviously be a
really bad thing.
As others have mentioned, setting up a host to run jails means configuring
daemons on the host to listen only on the host's specific IP address, and
not on a catch-all 0.0.0.0 address.
The NFS server's another classic example of something that makes itself
available on all of the computer's IPs (host + jails) by default, and has
to be configured to avoid using IPs assigned to jails--otherwise, the
jails themselves will look like NFS servers, which is probably not what
anybody would want.
These configuration changes aren't necessary inside of the jails
themselves, since they're prevented from listening on, or sending traffic
from any IP address other than what they've been configured to use. If a
jailed process tries to send a packet with any IP other than its
configured IP, it'll silently be re-written to match the jail's configured
IP before being sent out.
I have noticed though, that some programs (like Samba), will log errors
when they start in a jail, unless they're explicitly configured to use
only the jail's IP address. It's as though their attempt to bind to a
catch-all 0.0.0.0 address can return an error, despite working--at least
insofar as the process binding successfully to the jail's IP (a subset of
what the process tried to bind to). So though it's much less likely to be
necessary, things may run more cleanly in the jail if they're configured
explicitly to bind to the jail's IP.
>> On the host system, edit /etc/ssh/sshd_config, and add a line like this,
>> assuming your host system's IP is 10.0.0.1.
>> ListenAddress 10.0.0.1
> I should have found this myself. Sorry for the noise.
'happy to have had a chance to reply to one of the few things I happen to
> Thank you!
You're welcome Bertram!
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