If you run Postfix as a mail server to receive emails from the Internet as well as send out emails from the internal network to the Internet, you probably have the following scenario.
Emails from the Internet should be received without authentication. Other mail servers should be able to deliver emails without having to login first. They most likely don’t have an internal account they could use to log into your email server.
On the other hand, if users send emails from your internal network to the outside or to each other, they should authenticate first. At least they are using your domain name in their email address, and this privilege should only be available for legitimate users.
Postfix uses SASL to authenticate SMTP logins. SASL is able to use all sorts of backends for authentication. In the following example we use PAM to authenticate user logins. In larger corporate environments, you may want to connect SASL to your internal LDAP database.
These are the steps to install and configure saslauthd. Postfix should already be configured and running.
Install package sasl2-bin
Insert the follwing lines into your Postfix configuration /etc/postfix/main.cf smtpd_sasl_auth_enable = yes smtpd_sasl_service = smtpd smtpd_tls_auth_only = yes broken_sasl_auth_clients = yes … smtpd_recipient_restrictions = … # The order of recipient restrictions is important, so be careful where to insert the next line permit_sasl_authenticated, …
Create the following file: /etc/postfix/sasl/smtpd.conf pwcheck_method:saslauthd log_level: 10 mech_list: PLAIN LOGIN saslauthd_path: /var/spool/postfix/var/run/saslauthd/mux
Create the following directory: /var/spool/postfix/var/run/saslauthd sudo mkdir -p /var/spool/postfix/var/run/saslauthd
Add the user postfix to the sasl group so he is able to access thesocket directory: sudo usermod -aG sasl postfix
Create the following file: /etc/pam.d/smtpd auth required pam_listfile.so onerr=fail item=group sense=allow file=/etc/postfix/sasl-group.allowed @include common-auth @include common-account @include common-password @include common-session
Create the following file: /etc/postfix/sasl-group.allowed smtp
Add local user accounts that should be allowed to login via the smtpd service to the local group “smtp”: sudo usermod -aG smtp alice sudo usermod -aG smtp bob …
Edit the following lines in /etc/default/saslauthd MECHANISMS=”pam” OPTIONS=”-m /var/spool/postfix/var/run/saslauthd”
Enable and start the saslauthd service: sudo systemctl enable saslauthd sudo systemctl start saslauthd
Reload the postfix service: sudo systemctl reload postfix
Important
Integrating saslauthd withthe PAM module pam_listfile is crucial. Otherwise SASL authentication could be misused to guess passwords of all your local user accounts, including root. If you follow the steps above, only users that belong to the group “smtp” are allowed to login via the smtpd service.
The new eID functionality of the German identity card enables you to identify yourself with your real name towards government or commercial web services. It makes sure that it is really you who uses the web service, and not someone else who stole your online identity by email spoofing, SIM swapping, IMSI catcher, etc. .
In the following example, we will be using the eID to sign our PGP key. The new signature will uniquely identify the owner of the German identity card as the owner of the PGP key, which can then be used to e.g. sign and encrypt emails. That way PGP no longer relies on a web of trust, but works similar to the PKI concept of S/MIME certificates, in that the real identity of the owner of a certificate will be checked and then signed by a common public authority (CA) that everyone trusts.
A supported RFID card reader, e.g. from REINER SCT.
Operating system drivers for your card reader. In Ubuntu 20.04 drivers for all REINER SCT card readers (also called “cyberJack”) are included in the package libifd-cyberjack6. You can download Ubuntu drivers from their website too, but they didn’t work for me.
On Linux, the pcscd daemon that enables access to smart card readers.
An application called AusweisApp2 that handles authentication (PIN entry) and authorization (who wants to access what kind of information on your eID). In Ubuntu 20.04 AusweisApp2 is already included in the standard repositories (version 1.20.0). The app is also included as a snap install (newer version 1.20.2), but that didn’t work for me (for the error message see below).
First steps
Make sure you have the letter with the initial PIN for your eID at hand.
IMPORTANT: Make sure your RFID card reader is updated to the latest firmware release. With most card readers, the firmware can only be updated while you install the card reader on a Windows system.
IMPORTANT: Remove usbguard. Even after I permanently added the card reader to the list of allowed devices, pcscd could not find my card reader, or AusweisApp2 did not properly recognize my card reader and complained about missing drivers.
Install all necessary software packages and drivers for Ubuntu 20.04: pcscd pcsc-tools libifd-cyberjack6 libusb-1.0-0 libusb-1.0-0 libccid libpcsclite1 libpcsc-perl libpcsclite-dev
Test your card reader
Start the pcscd daemon in debug mode:
$ sudo pcscd -df 00000000 [140135772616640] pcscdaemon.c:347:main() pcscd set to foreground with debug send to stdout 00000086 [140135772616640] configfile.l:293:DBGetReaderListDir() Parsing conf directory: /etc/reader.conf.d 00000017 [140135772616640] configfile.l:329:DBGetReaderListDir() Skipping non regular file: .. 00000006 [140135772616640] configfile.l:369:DBGetReaderList() Parsing conf file: /etc/reader.conf.d/libccidtwin 00000029 [140135772616640] configfile.l:329:DBGetReaderListDir() Skipping non regular file: . 00000009 [140135772616640] pcscdaemon.c:663:main() pcsc-lite 1.8.26 daemon ready. 00003514 [140135772616640] hotplug_libudev.c:299:get_driver() Looking for a driver for VID: 0xABCD, PID: 0x1234, path: /dev/bus/usb/001/001 ...
Plug in your card reader.
IMPORTANT: If you use a USB card reader, plug it directly into your PC or laptop. Do not use a USB hub, as the hub may not provide enough power for the USB device. Also make sure to use the USB cable that came with the card reader. Longer cables may result in unstable connections.
In the output of the pcscd daemon (after a couple of seconds, wait for it!), you will see something like this:
99999999 [140135764219648] hotplug_libudev.c:655:HPEstablishUSBNotifications() USB Device add 00000158 [140135764219648] hotplug_libudev.c:299:get_driver() Looking for a driver for VID: 0x0C4B, PID: 0x0500, path: /dev/bus/usb/002/012 00000010 [140135764219648] hotplug_libudev.c:440:HPAddDevice() Adding USB device: REINER SCT cyberJack RFID standard 00000050 [140135764219648] readerfactory.c:1074:RFInitializeReader() Attempting startup of REINER SCT cyberJack RFID standard (1234567890) 00 00 using /usr/lib/pcsc/drivers/l ibifd-cyberjack.bundle/Contents/Linux/libifd-cyberjack.so CYBERJACK: Started 00001347 [140135764219648] readerfactory.c:950:RFBindFunctions() Loading IFD Handler 3.0 00023288 [140135764219648] readerfactory.c:391:RFAddReader() Using the pcscd polling thread
Notice that the pcscd daemon uses the driver from the package libifd-cyberjack we installed earlier. You can also check the output from the pcscd client tool:
$ pcsc_scan Using reader plug'n play mechanism Scanning present readers... 0: REINER SCT cyberJack RFID standard (1234567890) 00 00
Thu Nov 19 13:17:31 2020 Reader 0: REINER SCT cyberJack RFID standard (1234567890) 00 00 Event number: 0 Card state: Card removed,
As you can see, pcscd properly detected the card reader. Now insert your identity card into the card reader while pcsc_scan is running. The output of pcsc_scan will show something like this:
Install the application AusweisApp2 from the general Ubuntu repository. Do not install the snap app! In my case, the snap version of AusweisApp2 did not work properly. I got the following error message in my system logs:
Nov 18 17:32:03 server ausweisapp2-ce.pcscd[6911]: 07606784 readerfactory.c:1105:RFInitializeReader() Open Port 0x200000 Failed (usb:0c4b/0500:libudev:0:/dev/bus/usb/002/006) Nov 18 17:32:03 server ausweisapp2-ce.pcscd[6911]: 00000015 readerfactory.c:376:RFAddReader() REINER SCT cyberJack RFID standard (1234567890) init failed. Nov 18 17:32:03 server ausweisapp2-ce.pcscd[6911]: 00000073 hotplug_libudev.c:526:HPAddDevice() Failed adding USB device: REINER SCT cyberJack RFID standard
After you start the application, go to Start -> Settings -> USB card reader to check if the app can communicate with your card reader.
If you haven’t done so before, the app will ask you to change the initial PIN that you received by mail. You have to set your own PIN before you use any online service.
Test the authentication process
Go to Start -> Self-Authentication -> See my personal data. Here you can check the data that is stored on your eID, and also make sure that the authentication process is working properly.
Click on “Proceed to PIN entry”. On your card reader, you will need to confirm the service provider who wants to access your card, and also which information is requested from your card. Of course you also need to enter your new PIN.
Sign your PGP certificate
Go to Start -> Provider -> Other services -> Schlüsselbeglaubigung. The key signing service is provided by Governikus, the company that develops AusweisApp2.
Click on “To online application”. This will start your default web browser and open the URL https://pgp.governikus.de/pgp/ . Of course you can also enter the URL directly in your web browser. Just make sure that AusweisApp2 is running in the background.
On the website you may upload your PGP public certificate. After successful authentication by eID, you will receive an email with your certificate signed by Governikus. The signature certifies that the PGP key really belongs to you and not someone else who is impersonating you by using your email address (email spoofing) or smartphone number (SIM card swapping, IMSI catcher).
Summary
The whole eID authentication process on a website can be described as follows:
Start the pcscd daemon, either by “sudo systemctl start pcscd”, or if this doesn’t work by “sudo pcscd -f”.
Plug in your card reader. You should see a confirmation in the daemon output (or by typing “systemctl status pcscd” if you started pcscd with systemctl): “CYBERJACK: Started”
Start the application AusweisApp2.
Go to the website that requests eID authentication (“elektronischer Personalausweis”), and click on “Login”.
Your webbrowser automatically transfers control to AusweisApp2. There you should see who is requesting what kind of information from your eID.
Insert the identity card into your card reader.
In AusweisApp2, click on “Proceed to PIN entry”.
Control is transferred to your card reader. There you need to:
Confirm the service provider.
Confirm the data he wants to have access to.
Enter your PIN.
On the display of your card reader, you should see something like “Tunnel established”. AusweisApp2 shows something like “Authentication successful”. The website should automatically proceed to its regular contents, just as if you would have entered username and password.
That’s it. You can remove your identity card from the card reader.
Troubleshooting
If you see the following error message in the output of pcsc_scan, it means that pcsc_scan cannot communicate with the daemon pcscd. Make sure that the daemon is running.
SCardGetStatusChange: RPC transport error.
If AusweisApp2 does not recognize your card reader, or complains about missing drivers, try to start pcscd from the command line (“sudo pcscd -f”), and not as a background service (“sudo systemctl start pcscd”). Also make sure that you removed usbguard and did a reboot afterwards.
If the authentication process is not working, try to update the firmware of your smart card reader to the latest version. This might only work under Windows 10 during Windows driver installation for the new smart card reader device.
<p value="<amp-fit-text layout="fixed-height" min-font-size="6" max-font-size="72" height="80"><strong>Let's Encrypt</strong> provides an easy way to get free certificates not only for web servers, but also for email servers like Postfix.Let’s Encrypt provides an easy way to get free certificates not only for web servers, but also for email servers like Postfix.
<p value="<amp-fit-text layout="fixed-height" min-font-size="6" max-font-size="72" height="80">The way Let's Encrypt usually works requires you to setup a web server. Let's Encrypt sends you a challenge, and you have to prove ownership of the domain by providing a response to that challenge. You do this by placing the response in a certain <strong>URL on your web server</strong>:The way Let’s Encrypt usually works requires you to setup a web server. Let’s Encrypt sends you a challenge, and you have to prove ownership of the domain by providing a response to that challenge. You do this by placing the response in a certain URL on your web server:
<p value="<amp-fit-text layout="fixed-height" min-font-size="6" max-font-size="72" height="80"><strong>http://www.yourserver.com/.well-known/acme-challenge/</strong>FgedPYS65N3HfwmM7IWY2…http://www.yourserver.com/.well-known/acme-challenge/FgedPYS65N3HfwmM7IWY2…
<p value="<amp-fit-text layout="fixed-height" min-font-size="6" max-font-size="72" height="80">That way you prove that you are the owner of the domain "yourserver.com". But there is another even easier way to prove ownership of a domain: <strong>DNS</strong>. You place the response in a specific TXT record of your domain: <strong>_acme-challenge.www.yourserver.com</strong>That way you prove that you are the owner of the domain “yourserver.com”. But there is another even easier way to prove ownership of a domain: DNS. You place the response in a specific TXT record of your domain: _acme-challenge.www.yourserver.com
You can use your domain hosting service (GoDaddy, Whois, etc.) to create a new TXT record.
The “certbot” command line client does all the rest in just one call.
Under Debian 9 and 10, “certbot” is part of the official package repository.
You can run certbot on any Linux client. You don’t have to run it on the email server.
Example
<p value="<amp-fit-text layout="fixed-height" min-font-size="6" max-font-size="72" height="80">In this example the public hostname of your mail server is mx.yourserver.com. Therefore you have to create a TXT record called <strong>_acme-challenge.mx.yourserver.com</strong> . The value of the TXT record is in the output of certbot.In this example the public hostname of your mail server is mx.yourserver.com. Therefore you have to create a TXT record called _acme-challenge.mx.yourserver.com . The value of the TXT record is in the output of certbot.
# certbot certonly --manual --preferred-challenges dns -d mx.yourserver.com
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator manual, Installer None
Obtaining a new certificate
Performing the following challenges:
dns-01 challenge for mx.yourserver.com
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
NOTE: The IP of this machine will be publicly logged as having requested this
certificate. If you're running certbot in manual mode on a machine that is not
your server, please ensure you're okay with that.
Are you OK with your IP being logged?
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
(Y)es/(N)o: Y
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Please deploy a DNS TXT record under the name
_acme-challenge.mx.yourserver.com with the following value:
1A4RACHEISTBLUTWURST_egTVadkeiieikeieisfkfk
Before continuing, verify the record is deployed.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Press Enter to Continue
Waiting for verification...
Cleaning up challenges
IMPORTANT NOTES:
- Congratulations! Your certificate and chain have been saved at:
/etc/letsencrypt/live/mx.yourdomain.com/fullchain.pem
Your key file has been saved at:
/etc/letsencrypt/live/mx.yourdomain.com/privkey.pem
Your cert will expire on 2020-02-15. To obtain a new or tweaked
version of this certificate in the future, simply run certbot
again. To non-interactively renew *all* of your certificates, run
"certbot renew"
- If you like Certbot, please consider supporting our work by:
Donating to ISRG / Let's Encrypt: https://letsencrypt.org/donate
Donating to EFF: https://eff.org/donate-le
Probably one of the world’s most famous public VPN providers is leaking your traffic. The weirdest thing about it is, nobody noticed the traffic before.
Lessons learned: Always check no matter how good the reputation might be.
Update (2019-10) Nobody is perfect. Half a year later, there seems to be another problem with our famous VPN service. Not so much information about it out there, so here is a link in case you want to check yourself.
Update (2019-10) Oh noooo, now our famous VPN provider got hacked really bad. Private keys for OpenVPN have been revealed, but they already expired in 2018. Nevertheless, it could be that those keys were hacked in 2018 when they were still valid. All I can say: “Told ya!”
Current cryptographic algorithms will be broken within the next couple of years. The time to migrate to post-quantum cryptography is right now. Ah yes … and while you’re at it, don’t forget about crypto currency.
You need the package versions from at least Debian 10 testing for this to work. Installing specific packages from the testing branch is beyond the scope of this article, but there are many tutorials online.
Switch to legacy iptables (I did not try it with the new nftables packet filter that came with Debian 10):
sudo update-alternatives --config iptables
There are 2 choices for the alternative iptables (providing /usr/sbin/iptables).
Selection Path Priority Status
------------------------------------------------------------
0 /usr/sbin/iptables-nft 20 auto mode
* 1 /usr/sbin/iptables-legacy 10 manual mode
2 /usr/sbin/iptables-nft 20 manual mode
Press <enter> to keep the current choice[*], or type selection number: 1
Install iptables module “geoip” (from testing) and dependencies:
Make sure you have the right version (from Debian testing):
apt show xtables-addons-common
...
Version: 3.5-0.1
...
Download and build geoip database (zipped CSV file from MaxMind):
sudo -i
mkdir /usr/share/xt_geoip/
cd /usr/share/xt_geoip/
/usr/lib/xtables-addons/xt_geoip_dl
cd GeoLite2-Country-CSV_*
/usr/lib/xtables-addons/xt_geoip_build
cp *iv? ..
Check your iptables rules in INPUT chain. It should look something like this, if you already setup iptables:
# iptables --line-numbers -nL INPUT
Chain INPUT (policy DROP)
num target prot opt source destination
1 ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED
2 ACCEPT ...
3 ACCEPT ...
...
8 LOG all -- 0.0.0.0/0 0.0.0.0/0 state INVALID,NEW LOG flags 0 level 4 prefix "DROP input:"
Add iptables rule to block all incoming traffic from e.g. Prague/Czech Republic. Make sure to insert the new rule after the RELATED/ESTABLISHED rule and before any other ACCEPT rules. In this example, the rule is inserted as line number 2.
iptables -I INPUT 2 -m geoip --src-cc CZ -j DROP
In the second example we block all traffic except the one that is originating from the United States. TCP traffic is not simply dropped, but spoofed by the DELUDE target.
iptables -I INPUT 2 -m geoip ! --src-cc US -j DROP
iptables -I INPUT 2 -p tcp -m geoip ! --src-cc US -j DELUDE
Important things to note:
You have to reinstall package “xtables-addons-common” with every new kernel version because it is compiled during package installation using the current kernel source (see /usr/src/xtables-addons-*).
For more information about the DELUDE target in the second example, see “man xtables-addons”. It spoofs nmap scans and makes it harder for port scanners to scan the destination host. It is only valid for TCP traffic.
Always completely switch off your computer and lock your computer safely away, even if you just visit the bathroom. Screen saver locking or putting the laptop into sleep mode is not enough (Cold Boot Attacks). https://blog.f-secure.com/cold-boot-attacks
Install USBGuard to protect against unknown USB devices. (Note that USB IDs and serial numbers of USB devices can easily be replicated. Once an attacker knows the type of USB device you are using, and its serial number, USBGuard can easily be bypassed. That means: Never lend someone your USB stick, never accept a USB device from untrustworthy persons … which means anyone.)
Roland's Linux Blog 