[strongSwan] How to block Netstat attacks from VPN users?
Noel Kuntze
noel.kuntze at thermi.consulting
Mon Oct 14 12:30:17 CEST 2019
Hello Houman,
You can do that. I wonder though why that is a problem. Are they providing a private subnet on the link of your server?
Kind regards
Noel
Am 14.10.19 um 12:03 schrieb Houman:
> Hi Noel,
>
> That makes sense, thank you.
>
> I received a followup email from our server provider (about a new netscan attempt from one of our users today).
>
> """
> We would recommend that you set up a local firewall and block outgoing traffic to the following prefixes
> https://tools.ietf.org/html/rfc1918
> > 10.0.0.0/8 <http://10.0.0.0/8>
> > 172.16.0.0/12 <http://172.16.0.0/12>
> > 192.168.0.0/16 <http://192.168.0.0/16>
> Please block this range of RFC1918 on your server.
> We would like to avoid further network abuse from your end.
> """
>
> Is this as simple as
>
> iptables -A FORWARD -d 10.0.0.0/8 <http://10.0.0.0/8> -j REJECT
> iptables -A FORWARD -d 172.16.0.0/12 <http://172.16.0.0/12> -j REJECT
> iptables -A FORWARD -d 192.168.0.0/16 <http://192.168.0.0/16> -j REJECT
>
> Or am I oversimplifying this?
>
> Many Thanks,
> Houman
>
>
> On Mon, 14 Oct 2019 at 13:02, Noel Kuntze <noel.kuntze at thermi.consulting> wrote:
>
> Hello Houman,
>
> Depends on if you have a whitelist or blacklist rule set.
>
> With the ruleset you have provided in this email, you need to accept the stuff you want. So up to 5 new connections per second.
>
> Kind regards
>
> Noel
>
> Am 14.10.19 um 10:40 schrieb Houman:
> > Hi Noel,
> >
> > Actually based on my firewall config, I think I have to DROP it instead of ACCEPT if it's over the 5/sec limit? Don't you agree?
> >
> > iptables -I FORWARD 2 -m conntrack --ctstate NEW -m hashlimit --hashlimit-name NETSCAN --hashlimit-mode srcip --hashlimit-srcmask 32 --hashlimit-above 5/s -j DROP
> >
> > So I replace *hashlimit-upto* with *hashlimit-above* following with a DROP.
> >
> > This is my current firewall settings based on your previous suggestion. If Iptables is clever enough to DROP the connection if hashlimit-upto is exceeded, it should work as well.
> >
> > # iptables-save
> > *filter
> > :INPUT DROP [6374:460035]
> > :FORWARD DROP [7119:2071794]
> > :OUTPUT ACCEPT [19665335:23255290771]
> > -A INPUT -i lo -j ACCEPT
> > -A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
> > -A INPUT -p tcp -m tcp --dport 443 -j ACCEPT
> > -A INPUT -p tcp -m tcp --dport 2022 -j ACCEPT
> > -A INPUT -p udp -m udp --dport 500 -j ACCEPT
> > -A INPUT -p udp -m udp --dport 4500 -j ACCEPT
> > -A FORWARD -s 10.10.0.0/17 <http://10.10.0.0/17> <http://10.10.0.0/17> -d 10.10.0.0/17 <http://10.10.0.0/17> <http://10.10.0.0/17> -j DROP
> > -A FORWARD -m conntrack --ctstate NEW -m hashlimit --hashlimit-upto 5/sec --hashlimit-burst 5 --hashlimit-mode srcip --hashlimit-name NETSCAN -j ACCEPT
> > -A FORWARD -m policy --dir in --pol ipsec -j ACCEPT
> > -A FORWARD -m policy --dir out --pol ipsec -j ACCEPT
> > COMMIT
> > # Completed on Mon Oct 14 08:30:14 2019
> > # Generated by iptables-save v1.6.1 on Mon Oct 14 08:30:14 2019
> > *nat
> > :PREROUTING ACCEPT [222978690:20761159044]
> > :INPUT ACCEPT [1143238:398065963]
> > :OUTPUT ACCEPT [245876:24207759]
> > :POSTROUTING ACCEPT [245876:24207759]
> > -A POSTROUTING -s 10.10.0.0/17 <http://10.10.0.0/17> <http://10.10.0.0/17> -o enp2s0 -m policy --dir out --pol ipsec -j ACCEPT
> > -A POSTROUTING -s 10.10.0.0/17 <http://10.10.0.0/17> <http://10.10.0.0/17> -o enp2s0 -j MASQUERADE
> > COMMIT
> > # Completed on Mon Oct 14 08:30:14 2019
> > # Generated by iptables-save v1.6.1 on Mon Oct 14 08:30:14 2019
> > *mangle
> > :PREROUTING ACCEPT [76920955633:50815277695359]
> > :INPUT ACCEPT [27612054762:8305407205459]
> > :FORWARD ACCEPT [49298861266:42508240159831]
> > :OUTPUT ACCEPT [34323442858:39692165780388]
> > :POSTROUTING ACCEPT [83603096494:82195502934979]
> > -A FORWARD -s 10.10.0.0/17 <http://10.10.0.0/17> <http://10.10.0.0/17> -o enp2s0 -p tcp -m policy --dir in --pol ipsec -m tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1361:1536 -j TCPMSS --set-mss 1360
> > COMMIT
> >
> > On Mon, 14 Oct 2019 at 11:14, Houman <houmie at gmail.com <mailto:houmie at gmail.com> <mailto:houmie at gmail.com <mailto:houmie at gmail.com>>> wrote:
> >
> > Hello Noel,
> >
> > Thanks for your solution, I just tried it:
> >
> > iptables -I FORWARD 2 -m conntrack --ctstate NEW -m hashlimit --hashlimit-mode srcip --hashlimit-srcmask 32 --hashlimit-upto 5/s -j ACCEPT
> >
> > But I got this error message:
> >
> > iptables v1.6.1: hashlimit: option "--hashlimit-name" must be specified
> >
> > I googled and added the missing name like this:
> >
> > iptables -I FORWARD 2 -m conntrack --ctstate NEW -m hashlimit --hashlimit-name NETSCAN --hashlimit-mode srcip --hashlimit-srcmask 32 --hashlimit-upto 5/s -j ACCEPT
> >
> > Do you agree with this approach to prevent VPN users from running Netscans?
> >
> > Many Thanks,
> > Houman
> >
> >
> > On Wed, 31 Jul 2019 at 14:51, Noel Kuntze <noel.kuntze at thermi.consulting> wrote:
> >
> > Hello Houman,
> >
> > A "netscan" attack isn't actually anything worthy of an abuse email.
> > It's not part of a benign usage pattern of a VPN service, but it itself isn't illegal or anything.
> > You can only slow down such scans by rate limiting the number of new connections using the hashlimit match module, for example.
> >
> > E.g. -A FORWARD -m conntrack --ctstate NEW -m hashlimit --hashlimit-mode srcip --hashlimit-srcmask 32 --hashlimit-upto 5/s -j ACCEPT
> >
> > Kind regards
> >
> > Noel
> >
> > Am 30.07.19 um 16:39 schrieb Houman:
> > > Sorry I mistyped. I meant Netscan.
> > >
> > > The abuse message was saying: *NetscanOutLevel: Netscan detected from xx.xx.xx.xx*
> > >
> > > This is possible though, that VPN users run a netscan and scan the ports. Am I correct?
> > >
> > > Thanks,
> > >
> > > On Tue, 30 Jul 2019 at 15:30, Thor Simon <Thor.Simon at twosigma.com <mailto:Thor.Simon at twosigma.com> <mailto:Thor.Simon at twosigma.com <mailto:Thor.Simon at twosigma.com>> <mailto:Thor.Simon at twosigma.com <mailto:Thor.Simon at twosigma.com> <mailto:Thor.Simon at twosigma.com <mailto:Thor.Simon at twosigma.com>>>> wrote:
> > >
> > > I don't think netstat does what you think it does. It is a _local_ tool. Perhaps the "abuse notification" you received is a phishing attack?
> > >
> > > Hae a look at the manual page:
> > >
> > > http://manpages.ubuntu.com/manpages/trusty/man8/netstat.8.html
> > >
> > > ________________________________
> > > From: Houman <houmie at gmail.com <mailto:houmie at gmail.com> <mailto:houmie at gmail.com <mailto:houmie at gmail.com>> <mailto:houmie at gmail.com <mailto:houmie at gmail.com> <mailto:houmie at gmail.com <mailto:houmie at gmail.com>>>>
> > > Sent: Jul 30, 2019 10:18 AM
> > > To: users at lists.strongswan.org <mailto:users at lists.strongswan.org> <mailto:users at lists.strongswan.org <mailto:users at lists.strongswan.org>> <mailto:users at lists.strongswan.org <mailto:users at lists.strongswan.org> <mailto:users at lists.strongswan.org <mailto:users at lists.strongswan.org>>>
> > > Subject: [strongSwan] How to block Netstat attacks from VPN users?
> > >
> > > Hello,
> > >
> > > I had an interesting abuse notification that someone has run a netstat through our VPN.
> > >
> > > > time protocol src_ip src_port dest_ip dest_port
> > > > ---------------------------------------------------------------------------
> > > > Tue Jul 30 13:38:01 2019 UDP 136.243.xxx.xxx 21346 => 172.20.10.17 21346
> > > > Tue Jul 30 13:38:01 2019 UDP 136.243.xxx.xxx 21346 => 172.20.10.19 21346
> > >
> > > I was wondering if there is a good way to block all VPN users from running hacker tools such as netstat (port scanning) altogether. Is there a reliable way to do that with iptables?
> > >
> > > I came across this snippet that should block port scans, but I'm not sure if that would block a VPN user after all since the VPN traffic is masqueraded.
> > >
> > > iptables -A port-scan -p tcp --tcp-flags SYN,ACK,FIN,RST RST -m limit --limit 1/s -j RETURN
> > > iptables -A port-scan -j DROP --log-level 6
> > > iptables -A specific-rule-set -p tcp --syn -j syn-flood
> > > iptables -A specific-rule-set -p tcp --tcp-flags SYN,ACK,FIN,RST RST -j port-scan
> > >
> > > Any suggestions, please?
> > > Many Thanks,
> > > Houman
> > >
> > >
> > >
> >
> > --
> > Noel Kuntze
> > IT security consultant
> >
> > GPG Key ID: 0x0739AD6C
> > Fingerprint: 3524 93BE B5F7 8E63 1372 AF2D F54E E40B 0739 AD6C
> >
> >
>
> --
> Noel Kuntze
> IT security consultant
>
> GPG Key ID: 0x0739AD6C
> Fingerprint: 3524 93BE B5F7 8E63 1372 AF2D F54E E40B 0739 AD6C
>
>
--
Noel Kuntze
IT security consultant
GPG Key ID: 0x0739AD6C
Fingerprint: 3524 93BE B5F7 8E63 1372 AF2D F54E E40B 0739 AD6C
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