Commit 5ac82404 authored by Kevin Atkinson's avatar Kevin Atkinson
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www/doc/hw-recommend.html HWRecommend
www/doc/jail.html jail
www/doc/linkdelays.html linkdelays
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<center>
<h2>FreeBSD Jail-based Virtual Node Implementation</h2>
</center>
This page describes the changes we made to FreeBSD jails to support Emulab
virtual nodes and describes the boot time setup process for those jails.
<h3>Jail Changes</h3>
Following is a list of the features we added, and bugs we fixed in FreeBSD
jails. All of the new features are optional, controlled by sysctl MIBs
and per-jail flags. This new jail implementation is backward compatible
with the original implementation, meaning all new features are disabled
by default.
<ul>
<li> Allow a jailed process to bind to multiple IP addresses.
The default implementation of jail allows processes inside of a jail
to bind to just one IP, the IP that was specified to the jail command.
In that implementation, if a process specifies INADDR_ANY,
the kernel silently changes it to the jail IP. If however there
are other interfaces on the node, or if tunnels are being used to
construct an overlay for the experiment, it is necessary to allow
processes inside the jail to bind to those interfaces. In our
modified implementation, when the
jail is created, a list of auxiliary IPs can be specified on the
command line, telling the kernel to allow processes inside
the jail to bind to any of those IPs (including the jail IP).
When the bind happens, the kernel checks the jails list of IPs;
this applies to sockets bound for outgoing traffic, as well as
incoming traffic. Further, the set of accessible IPs determine
the list of interfaces that a jail can see so that, for example,
ifconfig inside a jail will only list the interfaces and IPs
available to the jail.
<li> Allow jails to bind to INADDR_ANY. The default behavior (and original
implementation) of jail maps INADDR_ANY to the jail's main IP address.
However, when a jail is allowed to access other IPs,
then INADDR_ANY actually means a subset of all the interfaces on
the node that the jail is allowed to use (which might also be
tunnels). There are two situations in which this matters:
<ul>
<li> A process is connecting to another address, and has
specified its local address as INADDR_ANY (which is typical).
Instead of binding the local address of packets to the jail IP,
the local address is set to the actual address of the interface
that the packet is routed out of. If there are IP aliases on the
interface, the list of aliases is searched for a match against
one of the allowed prison IPs. If there is a match, the local
address is set to that IP. Otherwise the address is set to the
main address of the interface (this is not correct; it should be
an error). This is to support multiplexing links using IP
aliases. If we were to use IP tunnels or some other form of
virtual interface, there would be no need to search the list of
aliases.
<li> A process is binding a local socket for an incoming
connection. In this case, any of the prison IPs can be the local
target of the connection, but it is not until the connection is
actually made that the address can be checked. This is done in
the pcb lookup routine. For each pcb, if the port matches and the
local address is INADDR_ANY, and the pcb was created within a
jail, then the list of the prison IPs is searched, looking for a
match. If no match is found, the pcb is skipped. This behavior
improves compatibility with existing server applications which
typically specify INADDR_ANY. If the kernel were to continue
binding INADDR_ANY sockets to the main IP address of the jail,
such applications would only be able to receive packets on the
primary jail interface.
</ul>
<li> Allow access to raw sockets. The jail is allowed to
both read and write, but is restricted from accessing the
firewall, dummynet, route, and RSVP interfaces. We also ensure
that the packet header reflects a source IP address appropriate
for the jail: INADDR_ANY is mapped to an appropriate address for
the outgoing interface and fixed addresses that are not part of
the jail set are rejected. This feature allows ping, traceroute
and gated to work in jails.
<li> Allow read-only access to BPF devices.
The interface is not put into promiscuous mode, so
the jail is not able to see all of the packets on the wire, but
only those addressed to the node. However, if the interface is
already in promiscuous mode (say, because someone outside the jail
is using tcpdump), then the jail will also be able to see any packet
that goes by. Even when not in promiscuous mode, a jail will see
all packets destined for the interface whether targeted to a
valid jail IP address or not. This could be fixed, and the
promiscuous-mode problem avoided, by augmenting the filter given
when the bpf device is setup. Allowing BPF access enables use
of tcpdump and other packet trace tools within jails.
<li> Restrict the port range to which a jail can bind. This allows
multiple jails on the same node to safely share the port space
without stepping on each other in environments where jails
cannot be assigned their own IP addresses.
Since the ultimate goal is to allow
different experiments to coexist in jails on the same node, the
port space has to be allocated globally, with the same port space
assigned to all jails across an experiment, so as not to conflict
with any other experiments. This assignment is done when the
experiment is swapped in so that swapped experiments are not
holding ranges (16 bits of port space does not go very far).
<li> Disallow FS unmounts inside a jail unless the mount was created
in the jail. This is a bug fix that prevents a jail from unmounting
a filesystem and exposing the underlying mount point to which it
likely shouldn't have access.
<li> Added per-jail flags to control various existing and new jail features.
These are in addition to sysctls which control
the global availability of a given feature. Existing features thus
controlled are: access to SYSV IPC facilities, access to routing
sockets and ability to turn on and off filesystem quotas. New
features controlled are: access to raw sockets, access to read-only
BPF and the ability to use INADDR_ANY. Additionally, there is a
new global sysctl to allow jails to be configured with multiple
IP addresses.
</ul>
<h3>Starting a FreeBSD Virtual Node</h3>
The goal for Emulab jail-based virtual nodes (hence forth known just as
"jails") is to set up an environment that is as much like the standard
Emulab node environment as possible. This makes it easy for the Emulab
infrastructure as well as for the Emulab user.
Also note that the intent is to use jails both locally (Emulab cluster
nodes) and remotely (wide-area, RON nodes), where there are
going to be different security considerations. Hence the need for
per-jail permissions bits as mentioned above. Setting up the jail is
broken into two parts; the stuff that needs to be done outside the
jail (creating the jail filesystem, setting up interfaces, tunnels,
routes, mounting shared filesystems) because the jail does not have
enough permission, and the stuff that can be done inside the jail
(creating accounts, installing software, starting programs and traffic
generators). Following is a description of those two phases.
<br>
<br>
<h4>Setting up the jail, phase one:</h4>
To set up the outer environment it is necessary to:
<ul>
<li> Create the tunnels if the experiment requested tunnels. This applies
only to wide-area nodes, not to local nodes. At the same time,
routes are setup if the user requested them (static and manual
only; we do not run gated on wide-area nodes!). At present, the
routing setup is done via the vtun config file, which specifies
external commands to run as each tap interface is configured and
torn down.
<li> Ask tmcd for the set of jail options that apply. Different users
and/or experiments might get differing levels of permission to
access the extended jail features mentioned above.
<li> Create a base filesystem for the jail, and then apply some
customizations to it. In addition to customizations based on the
permissions that tmcd said to use, there are the usual things
like setting the hostname of the jail, giving it a proper rc.conf
and resolv.conf, etc. More on this below.
<li> Mounting filesystems. Locally, we mount the /user and /proj
filesystems into the jail so that the users get the standard
directories.
<li> Start the tmcd proxy. More on this below.
</ul>
Setting up the filesystem for the jail is a long arduous process:
<ul>
<li> Create a zero-filled vnode file (currently set to 64MB) and find
a free vn device to configure. The root of the filesystem is
mounted under /var/emulab/jails/<jailname>/root.
<li> Copy in /root and /etc into the new jail filesystem so that each
jail gets to munge their own copy of it.
<li> NFS mount read-only /bin, /sbin, and /usr into the jailed
filesystem. This gives each jail shared access to the bulk of the
filesystem so that we do not have to duplicate. If the user
wishes to install their own software, they will need to do it
into /opt. This is perhaps not ideal.
<li> Mount a proc filesystem inside the jail. This gives the jail a
private view of it process world.
<li> Populate the jails /dev filesystem. The jail is not allowed to
run the mknod system call, so device entries must be created for
it.
<li> Create a pristine /var filesystem. Create stub entries for
several files in /etc including the passwd and group file.
Create a resolv.conf file that points to the outer host.
<li> Create an sshd config file and make sure X11 forwarding is off.
Also arrange for sshd to be started up (inside the jail) on its
per-jail assigned port (which is within the port range for the
jail).
<li> NFS mount (via a call in the tmcd library) all of the proj and
user directories for the experiment. Again, since the jail cannot
do NFS mounts inside, this is down outside. Clean out various
files for security (pem files, cvsup auth file, etc).
</ul>
The other complication in setting up the jailed environment involves
access to TMCD. Wide-area testbed nodes are not allowed to contact tmcd
without an ssl certificate, but we do not want to hand out per-jail
certificates that could be easily copied. My approach was to not allow
a jail to contact tmcd directly, but to instead go through a proxy
running outside the jail. This has the added benefit of ensuring that
the jail is not able to spoof another jail in another experiment.
The implementation of this was to add a <em>proxy</em> mode to the
tmcc client. Outside the jail, a tmcc proxy is started that creates a
unix domain socket, whose path is inside the jail filesystem. In other
words, the socket is named such that a tmcc client running inside the
jail sees it too. The tmcc client inside the jail connects to the tmcc
proxy running outside the jail via the unix domain socket, which
relays the request to tmcd (sanitizing the request string), and then
relays the answer back to the tmcc inside the jail. The proxy ensures
that there is no spoofing of the jail id. There are many other
alternatives for accomplishing this, but this was fairly easy to do.
<br>
<br>
<h4>Setting up the jail, phase two:</h4>
Once the jail system call has been issued, it is up to the inner
environment to finish getting it set up. Inside the jail, the first
program to run is a little program (injail) that is intended to
act like "init" in that it starts the initial shell and then waits
until it receives a signal to terminate. The easiest way to ensure
that all processes inside the jail are terminated is for injail to
send a TERM to the entire process group, and then a KILL to pick up
any stragglers. This is because kill all of the processes from outside
the jail is difficult (hard to see inside the jail), and because the
jail will not actually terminate until all the processes inside are
really dead.
<br>
<br>
The initial shell mentioned above is /etc/rc, which proceeds to do all
of the same boot time configuration that normally happens when a node
boots. The difference of course is that the jail has a heavily
constrained /etc/rc.conf that starts up just a few essential services
such as syslogd, cron, and sshd (on the specific port assigned sshd
for the jail; see above). The last part of configuration run is the
standard testbed setup, although again in a somewhat restricted
manner. Currently the following testbed mechanisms are supported
<em>within</em> the jailed environment:
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