Re: ASN support for iptables

["Peter Müller" <peter.mueller@link38.eu>]

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Hello,

> Hi,
> 
> On Mon, 2017-11-20 at 20:03 +0100, Peter Müller wrote:
> > Hello Michael,
> >   
> > > Hi,
> > > 
> > > On Sun, 2017-11-19 at 15:52 +0100, Peter Müller wrote:  
> > > > Hello development list,
> > > > 
> > > > today, I'd like to discuss whether a new feature in
> > > > the firewall engine of IPFire makes sense or not.
> > > > 
> > > > Since Core Update 90, IPFire supports GeoIP based firewall
> > > > rules, which goes beyond simple IP addresses or CIDR blocks
> > > > and makes firewalling easier.
> > > > 
> > > > The idea I had in mind is to add ASN (Autonomous System
> > > > Number) support for firewall rules, too.
> > > > 
> > > > An AS (Autonomous System) can be described as an administrative
> > > > instance on top of IP: For example, several IP blocks belong
> > > > to an AS, i.e. to the same company, university or whatever.
> > > > Although these blocks may be used for completely different purposes
> > > > in completely different countries, they share the same owner.
> > > > 
> > > > Every AS has a number (ASN) and a description (sometimes
> > > > abbreviated to ASDescr), while the number is unique.    
> > > 
> > > I think this makes sense and I would welcome that as a new feature in
> > > IPFire. It
> > > will in the end have some similar problems like the GeoIP blocker, but that
> > > is
> > > not too bad.  
> > 
> > Just for curiosity: Which ones are they?  
> 
> Apologies for nothing into depths of that. I just thought of the usual ones like
> not actually blocking what the user intends to block. Rules changing because
> prefixes might be moved around, etc.
Ah, yes, the usual suspects. :-)
> 
> It is just not 100% determinable. But indeed it is better than GeoIP.
True.
> 
> > I only see the (potential) problem of _another_ incomplete database here (see
> > below).
> > Since all necessary data for this AS stuff are more or less public, inaccurate
> > data and extensive manual research are not necessary.
> > 
> > All we need is to write some scripts that convert the RIPE, ... databases
> > to a suitable format.  
> 
> Does such a public database of the RIRs exist? I don't know of one.
Yes, they do exist, but mostly without the connection between an IP range and
the ASN (see below). At least for RIPE, I have not checked the others in detail.
> 
> For IPv4 handing out address space has slowed down a lot. And we should not use
> the BGP feed and parse that because then we might miss prefixes that are
> currently not announced - either deliberately or because of a downtime.
> 
> I know this:
>   http://ftp.ripe.net/pub/stats/ripencc/delegated-ripencc-extended-latest
> 
> But it doesn't connect the prefixes to an ASN AFAIK.
> 
> > Second, in contrast to the GeoIP database, false-positives are extremely rare
> > when dealing with AS information since they have to be technically correct. As
> > always, there are some "good" and "bad" ASNs, and a lot "grey" ones, which
> > stays
> > the same.  
> 
> We wouldn't really care about that, would we? That is up to the user to decide
> what they want to block/whitelist.
> 
> > >   
> > > > There are some scenarios in which AS based firewall rules
> > > > make sense, since AS information change less seldom than
> > > > IP ranges:
> > > > 
> > > > (a) One wants to block malicious traffic, but blocking entire
> > > > countries is too much since there are some legitimate partners,
> > > > customers, ... out there. With AS support, it is possible to
> > > > grant them access by simply permitting their AS. The rest of
> > > > the country may now safely be blocked.    
> > > 
> > > True. This might work well in some situations, but is probably quite useless
> > > when fighting against a botnet.  
> > 
> > I am not sure about this. In some cases, blocking access to some bullet proof/
> > rogue ISPs where many C&C servers are hosted improves the situation. Of
> > course,
> > if there is a simple DDoS attack from many different IPs, this will not help
> > very much.  
> > > 
> > > What would also be good is to open a port forwarding only from a certain AS.
> > > Let's call that whitelisting.  
> > 
> > Yes, that is a very good use case. :-)
> > 
> > And the combination of GeoIP and AS filtering (inbound and outbound)
> > opens up fare more possibilities...  
> 
> How would we combine this?
I meant that it opens up new possibilities for creating firewall rules (allowing
one AS and block/throttle the rest of a country, ...).
> 
> > > > (b) In some cases, IP ranges change very often, making firewall
> > > > rules very complex and hard to maintain, or the exact IP address
> > > > of a machine cannot be determined (dial-up connections). In
> > > > both cases, the AS (mostly) stays the same and allows firewall
> > > > rules without permitting access to a whole country.    
> > > 
> > > That will be the biggest challenge here. The database will need to be
> > > complete
> > > and needs regular updates. We don't really care if someone is actually
> > > announcing their prefix, but if they have one assigned, we should
> > > block/permit
> > > access.  
> > 
> > I agree with you.
> > 
> > We could simply ship an updated version with every Core Update (I
> > had that in mind for the CA list, too, so we don't get too big deltas).  
> 
> If we don't convert this into some nice binary format this will indeed become a
> huge database of probably ~10MB?! That would be too much to be included in every
> Core Update.
Is shipping database deltas an option? An maybe every 20th Core Update the
complete one so we do not have too much trouble with incremental updates.

If there is a nice small binary format, we should use it indeed. I am not very
deep into this subject. :-|

Shipping the database with every update avoids fetching them by the systems
itself, in which I see a set of issues:

(a) A download server for the database must be reachable. In some networks
with restrictive policies, this might be a problem. On the other hand, if
people install a Core Update, everybody understands that IPFire needs to
connect to some mirror servers. So why not shipping the databases via this way.

(The idea behind this is that every network connection can go wrong. A bit
ancient, I know. :-) )

(b) Error handling for database servers is problematic. In the given scenario:
What if downloading the AS database fails? Simply using the old version without
notifying is not an option. On the other hand, a notification (i.e. a warning
in the WebUI) needs to be noticed by the administrator. If they install
an update, they will see errors instantly.

(c) A download server is an unnecessary information leak. Since all systems
fetch their databases from a - hopefully redundant - server, this server sees
all IP addresses (and wget user agents, ...) belonging to firewall systems.
Although it may not be obvious at the first glance that IP x.y.z is an IPFire machine,
it is a good point for hacking tools to start from.

This problem becomes more important when talking about third party sites
(MaxMind!) since nobody knows what they are actually doing with the logs.
Even worse, if "geolite.maxmind.com" is going offline permanently, there
is no fallback server where the data can be fetched from.

I do not consider the IPFire mirrors as problematic here, since there is
redundancy, and the traffic is split up to more than one system.

(d) Integrity checking for downloaded databases is problematic. In most
cases, they are just .tar.gz-files. No HTTPS, no DNSSEC/DANE, no checksum
file, no GPG signature. Great.

And a firewall system is _relying_ on these data???

Of course, this problem needs to be fixed by mirror operators in first place.
But it is also a question of design: If one assumes that the internet in
general and the mirror servers in particular cannot be trusted, he/she/it
will provide signed (and GPG encrypted) data, as IPFire does. Again, why
not using this way?
> 
> > > > (c) Rogue ISPs (networks which are controlled/operated by professional
> > > > spammers or worse, such as the "Russian Business Network" (RBN),
> > > > which died in end-2007) sometimes run networks located in "good"
> > > > countries such as US or NL. Blocking them by GeoIP is not an
> > > > option because of many false-positives. AS based rules may help
> > > > here.    
> > > 
> > > The US is practically unblockable on the GeoIP filter, because too much is
> > > hosted in the US (at least according to the database by businesses that have
> > > their HQ there).  
> > 
> > Indeed, and there are several ASs over there the internet would be a better
> > place without. But I guess that applies to most other countries, too.  
> > > 
> > > So this would be a good extension to blocking more granular.
> > >   
> > > > Since the data behind this can be extracted from BGP feeds,
> > > > no external databases (such as MaxMind) are required.    
> > > 
> > > If we would use a BGP feed, we will only have the networks in the database
> > > that
> > > are currently announced. Wouldn't scraping the WHOIS database be better?  
> > 
> > Good idea. For example, RIPE has one here:
> > http://ftp.ripe.net/ripe/dbase/split/inet(6)num.gz  
> 
> And where is the link to the ASN?
Sorry, I used the wrong link while researching for the GeoIP stuff. Here is
the correct one: https://ftp.ripe.net/ripe/dbase/ripe.db.gz

As far as I know, the link between "as-block" and an IP range can be done
via the "mnt-by" or the "origin" tags. 
> 
> > > 
> > > Why not MaxMind? Not that I am in favour of that, but I am interested why it
> > > is
> > > not an option.  
> > 
> > Well, technically, it is an option. But since there is some trouble with
> > their GeoIP database, as mentioned somewhere else some time before, I'd
> > like to move away from them.  
> 
> What trouble?
Well, some of them are mentioned above already. Here are some more:

(i) There is no changelog or release note. Once in a month, the GeoIP database
is updated - and there is no (simple) way of telling which changes were made.

For example, the IP ranges belonging to Cloudflare were recently deleted from
this database. As a result, some websites became unreachable from today to
tomorrow (see bug #11482 for details) since the iptables module permits access
in case an IP is not listed (which is the correct behaviour).

Not sure how much went wrong here in productive networks, but it certainly
is an issue only MaxMind can fix.

(ii) The database update is performed silently. In case of troubles, this
procedure might not be obvious. On the other hand, if people install a Core
Update, they are in maintenance mode, scheduled downtime, or whatever and
are somewhat prepared for trouble.

(iii) The quality of the GeoIP database by MaxMind we use is poor.
For example, allocations to countries are incorrect (especially in case of
"A1 - Anonymous Proxies") even if the correct data is known. Much worse,
sometimes the allocations are incorrect without being obvious (anycast networks
are assigned to "US", or simply dropped in case of Cloudflare). It seems
like they are only scraping WHOIS, too.

There should be an "A3 - Anycast Networks" section for companies like Google,
similar to "EU" or "AP" which also only indicate a continent without going down
to the country level. A3 would be a step further.

Note that I am not talking about their chargeable products or the city
databases here.


Because of these issues, I'd like to build our own GeoIP database, but that
is off-topic here, and will be mentioned in another mail. ;-)
> 
> > And since we can build our own database here, I do not see why we should
> > rely on 3rd party data.  
> > >   
> > > > Unfortunately, my programming skills are too low for implementing
> > > > this feature. Thereof, if it is decided to do this, I will need
> > > > some help here. :-)    
> > > 
> > > *raises hand*  
> > 
> > Thank you very much. :-)  
> > >   
> > > > Technically, this is similar to the GeoIP firewall stuff (just
> > > > another database), so I assume most of the work done there can
> > > > just be copied.    
> > > 
> > > The GeoIP block uses an iptables extension which parses the database. We
> > > wouldn't use that here but would either build something with ipset or
> > > similar.  
> > 
> > Hmmm, not sure how well this performs: If there is only a small AS with
> > few IP ranges to parse, ipset would be sufficient.
> > 
> > However, if we have large networks (Level 3, Hurricane Electric, DTAG, ...),
> > ipset might become slow and cause many iptables rules. And if we update
> > the database (whomever created it), we need to reload the firewall rules.
> > Using a database lookup module similar to GeoIP avoids that issues here, as
> > far as I am concerned.  
> 
> I mean it is an option to build an iptables extension, but I am not sure if that
> is the best thing.
> 
> ipset is O(1) for each lookup. So it doesn't matter how large the AS is.
> iptables rules would be O(n) which is not very nice for larger ASNs, but we
> should have a look since I do not expect ASNs to announce hundreds of prefixes.
> The average might be way less and I have no idea what to expect as the worst-
> case performance. That needs investigation.
All right.

@Everybody else: Comments?

Best regards,
Peter Müller
> 
> Best,
> -Michael
> 
> > 
> > Best regards,
> > Peter Müller  
> > >   
> > > > Any thoughts on this idea?
> > > > 
> > > > Best regards,
> > > > Peter Müller    
> > > 
> > > -Michael  
> > 
> >