Creative Commons License

Unless otherwise expressly stated, all original material of whatever nature created by and included in this weblog is licensed under a Creative Commons License.


Top 21 Security Experts to follow on Twitter in 2021

From the article on Security Boulevard.

  1. Rafay Baloch
  2. Troy Hunt
  3. Kevin Mitnick
  4. Rachel Tobac
  5. Mikko Hyppönen
  6. Katie Moussouris
  7. Bruce Schneier
  8. Brian Krebs
  9. Jeremiah Grossman
  10. Eugene Kaspersky
  11. Dan Lohemann
  12. Steve Morgan
  13. Tyler Cohen Wood
  14. Graham Cluley
  15. Theresa Payton
  16. Shira Rubinoff
  17. Eva Galperin
  18. Marcus J. Carey
  19. Jayson E Street
  20. Paul Asadoorian
  21. Adam K. Levin

06:58 | Networking | Permalink



This website now also serves a security.txt file which is a standardized way of making security contact information available. (Wikipedia)

The file is available in two locations /security.txt (the classic location) and /.well-known/security.txt (the standard location following RFC8615).

To easily add the file on all my domains, I'm using the following nginx config snippet.

location /security.txt {
	add_header Content-Type 'text/plain';
	add_header Cache-Control 'no-cache, no-store, must-revalidate';
	add_header Pragma 'no-cache';
	add_header Expires '0';
	add_header Vary '*';
	return 200 "Contact:\nExpires: Tue, 19 Jan 2038 03:14:07 +0000\nEncryption:\n";

location /.well-known/security.txt {
	add_header Content-Type 'text/plain';
	add_header Cache-Control 'no-cache, no-store, must-revalidate';
	add_header Pragma 'no-cache';
	add_header Expires '0';
	add_header Vary '*';
	return 200 "Contact:\nExpires: Tue, 19 Jan 2038 03:14:07 +0000\nEncryption:\n";

This snippet is stored in a dedicated file (/etc/nginx/conf_includes/securitytxt) and is included in the various server config blocks like this:

server {

	include /etc/nginx/conf_includes/securitytxt;

	location / {
		# rest of website

08:16 | Webdesign | Permalink


Fixing 'snmpd[19784]: error on subcontainer 'ia_addr' insert (-1)' messages

The default configuration of snmpd on Debian has debug level logging enabled and thus we end up with a constant flood of these messages in /var/log/syslog

snmpd[19784]: error on subcontainer 'ia_addr' insert (-1)

The fix is to lower the logging level, which can be accomplished like this on systems with systemd:

cp /lib/systemd/system/snmpd.service /etc/systemd/system/snmpd.service
sed -i 's/Lsd/LS6d/' /etc/systemd/system/snmpd.service
systemctl daemon-reload
systemctl restart snmpd

On systems without systemd, the logging level is set by the init script (unless explicitly configured in /etc/default/snmpd), and can be changed like this:

sed -i 's/Lsd/LS6d/g' /etc/default/snmpd
sed -i 's/Lsd/LS6d/g' /etc/init.d/snmpd
service snmpd restart

08:57 | Linux | Permalink


Embracing the future with SolNet

This was the initial state of my new SolNet fibre connection:

SolNet connection with native IPv6 but no IPv4

As I am a proponent of IPv6 this made me very happy, but unfortunately about 20% of my daily websites only offer legacy Internet (which later on I got working as well).

10:01 | Networking | Permalink

Prozentrechnen ist schwer

TrueWealth hat's nicht so mit Prozentrechnen:

6 + 33 + 3 + 12 + 47 = ?

08:20 | Misc | Permalink


Using your own router on a Sunrise fiber line

Sunrise does not like when people run their own router on a fiber line. While they do not directly forbid it, they don't provide any of the required configuration parameters which makes it quite hard to use your own router.

Below you'll find the required configuration parameters to make your own router connect with IPv4 and IPv6 on a Sunrise fiber line.
Beware though that especially the VLAN configuration might be different depending on your city, the following worked for me in Zürich.

Also, please note that I do not recommend Sunrise as Internet provider (as a matter of fact I'm on the way out of their contract and switching to SolNet).
Besides not supporting to bring your own router, they also like to make up additional early-termination fees (the contract states 100CHF early termination fees, but once you call them to initiate the process they tell you that it's gonna cost 300CHF as they decided to change their pricing structure unilaterally).

Enough of the ranting, now to the interesting part :-)

The Sunrise line has multiple VLANs to differentiate between Internet, Phone and TV services.
To receive an IPv4 address it requires a special value for the Client Identifier DHCP option.
For IPv6 6rd is employed, for which we need to know the prefix and gateway address.

The following configuration was tested with a MikroTik CRS125 router starting from the default settings.
For simplicity I've named the network interfaces according to their intended usage (eg. LAN, sunrise and 6rd).

The first step is to configure the VLAN on top of your fiber interface. In my case it was VLAN ID 131, others were also successful with VLAN ID 10.

/interface vlan add interface=sfp1-gateway name=sunrise vlan-id=131

Next let's put in place some basic firewall rules to make sure we're not exposing our LAN to the Internet once the connection comes up.

/ip firewall filter
add action=accept chain=forward connection-state=established,related
add action=accept chain=forward in-interface=LAN out-interface=sunrise
add action=drop chain=forward
add action=accept chain=input connection-state=established,related
add action=accept chain=input protocol=icmp
add action=drop chain=input in-interface=!LAN
/ip firewall nat
add action=masquerade chain=srcnat out-interface=sunrise

Now we can configure the special value for the Client Identifier DHCP option and configure the DHCP client on the VLAN interface.

/ip dhcp-client option add code=61 name=clientid-sunrise value="',Fast5360-sunrise'"
/ip dhcp-client add dhcp-options=clientid-sunrise disabled=no interface=sunrise

This should now give us IPv4 Internet connectivity. We can test this by checking that we received an IPv4 address, have an IPv4 default route and that we can ping a host in the Internet.

/ip dhcp-client print
Flags: X - disabled, I - invalid, D - dynamic
 0   sunrise         yes          yes               bound
/ip route check 1.1
     status: ok
  interface: sunrise
/ping count=1 1.1
  SEQ HOST                                     SIZE TTL TIME  STATUS
    0                                    56  59 1ms
    sent=1 received=1 packet-loss=0% min-rtt=1ms avg-rtt=1ms max-rtt=1ms

Sunrise doesn't offer native IPv6 connectivity but employs 6rd (which defines how to create a 6to4 tunnel based on the public IPv4 address, an IPv6 prefix and the tunnel gateway).

Before we setup the 6rd tunnel, it's important to put in place firewall rules for IPv6 as afterwards all devices on the local network will receive a public IPv6 address.

/ipv6 firewall filter
add action=accept chain=forward connection-state=established,related
add action=accept chain=forward in-interface=LAN
add action=drop chain=forward
add action=accept chain=input connection-state=established,related
add action=accept chain=input protocol=icmpv6
add action=drop chain=input

To setup the 6rd tunnel, I've modified an existing script with the specific parameters for Sunrise (namely the 2001:1710::/28 prefix and the tunnel gateway address).
The script creates the tunnel interface, configures an IPv6 address on the external interface, configures an IPv6 address on the internal interface (which also enables SLAAC to provide IPv6 addresses to the clients on the local network) and configures an IPv6 default route over the 6rd tunnel.

The script itself will be run via the scheduler, thus let's save it under the name 6rd-script.

:global ipv6localinterface "LAN"
:global uplinkinterface "sunrise"

:global IPv4addr [/ip address get [find interface=$uplinkinterface] address];
:global IPv4addr [:pick $IPv4addr 0 [:find $IPv4addr "/"]]
:global IPv4addr2 [:pick $IPv4addr 0 30]
:global IPv6temp [:toip6 ("1::" . $IPv4addr2)]

:global IPv4hex1 [:pick $IPv6temp 3 4]
:global IPv4hex2 [:pick $IPv6temp 4 7]
:global IPv4hex3 [:pick $IPv6temp 8 9]
:global IPv4hex4 [:pick $IPv6temp 9 12]
:global IPv6addr [("2001:171" . $IPv4hex1 . ":". $IPv4hex2 .$IPv4hex3 . ":" . $IPv4hex4 . "0::1/64")]
:global IPv6addrLoc [("2001:171" . $IPv4hex1 . ":". $IPv4hex2 . $IPv4hex3 . ":" . $IPv4hex4 . "1::1/64")]

#6to4 interface
:global 6to4id [/interface 6to4 find where name="6rd"]
:if ($6to4id!="") do={
:global 6to4addr [/interface 6to4 get $6to4id local-address]
if ($6to4addr != $IPv4addr) do={ :log warning "Updating local-address for 6to4 tunnel '6rd' from '$6to4addr' to '$IPv4addr'."; /interface 6to4 set [find name="6rd"] local-address=$IPv4addr }
} else { :log warning "Creating 6to4 interface '6rd'. "; /interface 6to4 add !keepalive local-address=$IPv4addr mtu=1480 name="6rd" remote-address= }

#ipv6 for uplink
:global IPv6addrnumber [/ipv6 address find where comment="6rd" and interface="6rd"]
:if ($IPv6addrnumber!="") do={
:global oldip ([/ipv6 address get $IPv6addrnumber address])
if ($oldip != $IPv6addr) do={ :log warning "Updating 6rd IPv6 from '$oldip' to '$IPv6addr'."; /ipv6 address set number=$IPv6addrnumber address=$IPv6addr disabled=no }
} else {:log warning "Setting up 6rd IPv6 '$IPv6addr' to '6rd'. "; /ipv6 address add address=$IPv6addr interface="6rd" comment="6rd" advertise=no }

#ipv6 for local
:global IPv6addrnumberLocal [/ipv6 address find where comment=("6rd_local") and interface=$ipv6localinterface]
:if ($IPv6addrnumberLocal!="") do={
:global oldip ([/ipv6 address get $IPv6addrnumberLocal address])
if ($oldip != $IPv6addrLoc) do={ :log warning "Updating 6rd LOCAL IPv6 from '$oldip' to '$IPv6addrLoc'."; /ipv6 address set number=$IPv6addrnumberLocal address=$IPv6addrLoc disabled=no }
} else {:log warning "Setting up 6rd LOCAL IPv6 '$IPv6addrLoc' na '$ipv6localinterface'. "; /ipv6 address add address=$IPv6addrLoc interface=$ipv6localinterface comment="6rd_local" advertise=yes }

#ipv6 route
:global routa [/ipv6 route find where dst-address="2000::/3" and gateway="6rd"]
if ($routa="") do={ :log warning "Setting IPv6 route '2000::/3' pres '6rd'. "; /ipv6 route add distance=1 dst-address="2000::/3" gateway="6rd" }

Once we've added the script we also need to create the scheduler entry to run it periodically (as it needs to re-configure the tunnel and addresses whenever the public IPv4 address changes).

/system scheduler add interval=1m name=schedule1 on-event=6rd-script

After the first run of the script we should now have IPv6 connectivity. Let's test this again by checking that we have a public IPv6 address, an IPv6 default route and can ping an IPv6 host in the Internet.

/ipv6 address print where interface=6rd and global 
Flags: X - disabled, I - invalid, D - dynamic, G - global, L - link-local 
 #    ADDRESS                                     FROM-POOL INTERFACE        ADVERTISE
 0  G ;;; 6rd
      2001:171c:6336:47b0::1/64                             6rd              no
/ipv6 route check 2600::
     status: ok
  interface: 6rd
    nexthop: 2600::
/ping count=1 2600::
  SEQ HOST                                     SIZE TTL TIME  STATUS
    0 2600::                                     56  50 118ms echo reply
    sent=1 received=1 packet-loss=0% min-rtt=118ms avg-rtt=118ms max-rtt=118ms

And that's how you can configure and validate IPv4 and IPv6 connectivity with your own router on a Sunrise fiber line despite them not liking it very much ;-)

22:26 | Networking | Permalink



Recently I added MTA-STS support to one of my domains, and it turns out that this was easier than expected.

MTA-STS is used to tell mail senders that your server supports TLS. And then you can define the policy for your server and tell them that they should only use TLS (resp. STARTTLS) when connecting to you and not fall back to unencrypted SMTP.

The way this works is with two components:

  • a special _mta-sts.<> TXT DNS entry indicating that your domain supports MTA-STS and the version number of your MTA-STS policy
  • a mta-sts.txt file served under a specific well-known URL https://mta-sts.<>/.well-known/mta-sts.txt containing your MTA-STS policy (which mx hosts it is valid for, should it be run in enforcing or testing mode, max-age etc.)

The idea is that a mail sender checks your MTA-STS policy through protected channels (DNSSEC, HTTPS) and then never sends mails to you in plaintext (similar approach as HSTS for HTTP but this time between mail servers).

To setup the MTA-STS configuration, I followed this Enable MTA-STS in 5 Minutes with NGINX guide from Yoonsik Park.

Then to check my configuration I used this MTA-STS validator (which is an opensource project available on GitHub), the classic //email/testTo: tool (MTA-STS checking needs to be explicitly enabled under 'More Options') and the free testing service provided by Hardenize.

09:47 | Networking | Permalink


15 years of o5

15 years ago this weblog received the current o5 design (or theme as it would be called nowadays).
During this time the design has aged quite well and also survived the move of the backend from a self-written PHP blog-engine to Jekyll.

Although it still works surprisingly well and presents the content nicely every day, there are some parts where better usage of contemporary technologies would be desirable.
It has no mobile version nor a responsive layout as the design was created before the now omnipresent smartphones were invented. Similar is the font-size hardcoded and not very adequate for todays retina displays. And yes, it uses the XHTML 1.0 strict standard with all its quirks and CSS tricks from 2002 (which luckily are still supported in current browsers).

Overall I'm quite happy that the o5 design has turned out to be so timeless and that I did not have to come up with a new one every other year (btw: I don't remember where the o5 name came from, likely the 5 is a reference to 2005 when it was created).

With the current Corona situation forcing me to spend more time at home again, I have the feeling that some things might change around the weblog (not quite sure what or when exactly, first I need to re-learn how websites are built in 2020 :-).

19:03 | Webdesign | Permalink


NAT Slipstreaming (NAT traversal part 2)

Compared to the previous post where intentional NAT traversal was discussed, here now comes an article about 'unintentional' (malicious) NAT traversal.

Samy Kamar describes in his NAT Slipstreaming article how a combination of TCP packet segmentation and smuggling SIP requests in HTTP, can be used to trick the NAT ALG of your router into opening arbitrary ports for inbound connections from the Internet to your computer.

The article analyses in detail the SIP ALG of the Linux netfilter stack in it's default configuration, but likely similar attacks could also be possible with ALGs of other protocols and vendors.

Important to note: the Linux SIP ALG module has two parameters (sip_direct_media and sip_direct_signalling), which restrict the IP address for which additional ports are opened to the one sending the original SIP packet. By default they are set to 1, but if any of these is set to 0 in a router's configuration, the described NAT Slipstreaming attack will not only allow to make inbound connections to your computer, but also to any other device in the local network!

22:55 | Networking | Permalink


How NAT traversal works

How NAT traversal works – is a very well written and detailed article from Dave Anderson explaining the different NAT scenarios and the tricks that can be used to establish a peer-to-peer UDP connection between machines sitting behind them.

21:08 | Networking | Permalink