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Securing Asterisk SIP PBX by simple iptables rule checking if the domain is correct

Securing Asterisk SIP PBX by simple iptables rule checking if the domain is correct published on No Comments on Securing Asterisk SIP PBX by simple iptables rule checking if the domain is correct


For some time I’ve been looking for a simple way to protect my Asterisk SIP pbx against attacks from bots, scanners which scans and trying dialing to premium numbers. Opening SIP port to the internet causes that there was no one minute without suspect requests hitting my Asterisk. The log was full of that attempts.

While analyzing this problem I noticed that bots, scanners, attackers using everywhere IP address of my server to trying break it. While my proper clients using domain of my Asterisk server. If user is using domain name in his SIP client/phone,this domain is used in further communication on SIP protocol.

Below I will show example of INVITE (INVITE is using to establish VoIP call) SIP request from user using domain name (, and from user using IP address ( of Asterisk server.

With domain:

With IP address:

Knowing that, I want to block requests to Asterisk server which are NOT contains my domain name. In this point I want to clarify that I have special subdomain for telephones. Bots, scanners, attackers are not knowing about this domain.
Blocking unwanted requests can be done by iptables rules with string matching.

Differences between rules result from different approach between TCP and UDP protocols when establishing a connection. TCP need to do three way handshake to establish connection, UDP not doing this.

After applying these rules, I did not see even one attack ๐Ÿ™‚
At the and we can check increasing iptables counters:

Strongswan IPsec on LEDE/OpenWRT with fast-classifier and shortcut-fe modules

Strongswan IPsec on LEDE/OpenWRT with fast-classifier and shortcut-fe modules published on 5 Comments on Strongswan IPsec on LEDE/OpenWRT with fast-classifier and shortcut-fe modules

I have using TP-Link TL-WDR4300 router with LEDE software. Recently, thanks to fast-classifier and shortcut-fe modules the router got a second life ๐Ÿ™‚ To my surprise after loading fast-classifier modules it can be able to pass 500Mb/s over NAT, which is absolutely great result ๐Ÿ™‚

But after that I noticed that my site-to-site IPsec tunnel, based on Strongswan stopped working properly… Ping was working, tcp connections over tunnel could be established, but after passing some tcp packets the connection freezes. I suspected a problem with MTU, but that was not it.
After unload kmod-fast-classifier and kmod-shortcut-fe tunnel was working properly. I’m started to reading source code of fast-classifier and shortcut-fe modules. I found that:

I realized that the offloading which is doing by fast-classifier and shortcut-fe is basing on conntrack table. The next thought was that conntrack is needed to realize NAT. Only connection between my LAN and internet (WAN) should be tracked and should be in conntrack table. I don’t need to track connection beetween my local nets connected through site-to-site vpn! Connections between my local nets can be realized only based on routing table. I decided to disable conntrack for my local nets and see if it solved my problem.
My network looks as follows:

Strongswan configuration: /etc/ipsec.conf (

Strongswan configuration: /etc/ipsec.conf (

File with secrets is the same on booth sides. /etc/ipsec.secrets

Excluding particular connections from conntrack can be done by iptables with raw and conntrack module – I had to install them earlier.

I excluded whole local network class – 192.168.0/0/12

Now my IPsec tunnel started passing traffic properly! ๐Ÿ™‚ And there is no connections from my localnets in conntrack! (so fast-classifier not offloading this connections).
The only issue was that I was unable to connect to LEDE router ( from my remote network behind IPsec tunnel. I resolved that by exclude IP of router from iptables rule:

I had to install iprange module before:

Now I can enjoy very fast internet connection with fast-classifier and fully working strongswan IPsec vpn connections ๐Ÿ™‚

IKEv2 with Let’s Encrypt- robust IPsec vpn solution for Windows, Android, Linux, macOS and iOS clients

IKEv2 with Let’s Encrypt- robust IPsec vpn solution for Windows, Android, Linux, macOS and iOS clients published on No Comments on IKEv2 with Let’s Encrypt- robust IPsec vpn solution for Windows, Android, Linux, macOS and iOS clients

Hello ๐Ÿ™‚

In this post I will describe how to prepare solid vpn gateway which works flawlessly with many different clients.

I choose the solution based on modern IKEv2 protocol created with Microsoft and Cisco together. In a big simplification – IKEv2 (Internet Key Exchange version 2) is responsible to set up a security association (SA) in the IPsec protocol suite.

Advantages of IKEv2 over IKEv1 protocol:

  • it tolerates interruptions, latency etc. on network connection. For example, if the connection is temporarily lost, or if a user moves a client computer from one network to another, IKEv2 automatically restores the VPN after the network connection is reestablished โ€” in transparent way to the user.
  • EAPย authentication – we can authenticate simply, by username and password
  • better dead peer/tunnel detection
  • consume less bandwidth

IKEv2 has built in client in Windows 7 and newer and on macOS and iOS systems.
For Android there is a StrongSwan client app which is working very well. In Linux we can simply use Strongswan which is one of IPsec implementation for Linux.

In setup below I will use certificate for server obtained from Let’s Encrypt. It is needed because Windows clients will not work with self signed certificate without adding our CA as trusted. My goal is that we don’t have to provide anything other then user name and password ๐Ÿ™‚

I will skip the part describing an obtaining a certificate from Let’s Encrypt. It is well documented in internet.
For server/gateway side I used Strongswan which provides support for IKEv2.

Server configuration

For setup server side we have to:
Install Strongswan:
(I’m installing it on OpenSUSE)

or if you using Debian based Linux distribution

I already have a certificate Let’s Encrypt for my domain. The typical catalog structure with the certificates from Let’s Encrypt looks as follows:

To use it in Strongswan it is necessary to create links to certificates and keys:

W need to provide Let’s Encrypt intermediate certificate:

This is very important step, I was spent a lot of hours to discover that Windows works properly only if we are providing intermediate certificate.

Now we can edit /etc/ipsec.conf
My configuration looks as follows:

After that we have to add private key (/etc/ipsec.d/private/privkey.pem) and define usernames and passwords for vpn clients.

This is the whole Strongswan configuration. To apply configuration Strongswan must be restarted:

If don’t have configured NAT/masquerade your clients will not have internet.

In some cases there is a problem with mtu/mss which can cause for example problem with opening some web pages. Strongswan documentation recommends reduce the MSS for packets transmitded through tunnel. Strongswan documentation
To reduce the mss, add a rule to iptables:

Clients configuration

In most clients it is trivial.


iPhone – iOS

Very simple configuration, analogous to iOS/iPhone

Unfortunately, manual configuration is slightly complicated. But then it works very well.
In newer releases of Linux distribution there is a gui plugin for network manager which provide easy configuration of vpn connection. But I haven’t tried it.

Strongswan configuration: /etc/ipsec.conf

password for user1 i stored in /etc/ipsec.secret:

We have to had intermediate and CA root certyficate in /etc/ipsec.d/cacerts:

root CA certificate is available to copy fromย ย DST Root CA X3
I had to copy it to a file in such way (with adding “—–BEGIN CERTIFICATE—–” and “—–END CERTIFICATE—–“:

In the default configuration, strongswan does not set the dns addresses provided by the server. This behavior can be changed by editing /etc/strongswan.d/charon/resolv.conf. I uncommented line with “file = /etc/resolv.conf”

Restart Strongswan:

And connection should be start:

We can check connection status:

And disconnect connection:

Linux box as an IPv6 router with SLAAC and DHCPv6-PD

Linux box as an IPv6 router with SLAAC and DHCPv6-PD published on No Comments on Linux box as an IPv6 router with SLAAC and DHCPv6-PD

Some time ago I replaced my Mikrotik router with linux box which is working as a router for my home network and as a server for some services.
I had to spend some time to set up IPv6 on linux in such way, that everything was working automatically and without need to configuring anything in statically way. This post will be only about IPv6 part of router configurations.
I omit IPv4 part and configurations of network interfaces, because it is well documented in internet.

My router is running on linux openSUSE leap 42.2. The configurations are the same for other distros but file paths of config files may be different.
eth0 – wan interface of the router
eth1 – lan interface of the router
To make the router working, I had to:

  • change some sysctls to obtain IPv6 address on wan interface of my router by Stateless autoconfiguration (SLAAC).
  • I used wide-dhcpv6-client client to obtain IPv6 adresses pool (DHCPv6-PD) to redistribute addresses from pool on my devices in home network.
  • Redistributing addresses is done by dnsmasq.

First step – sysctls:
Part of my /etc/sysctl.conf confgured to obtain IPv6 address from Stateless autoconfiguration (SLAAC) on wan interface – eth0:

After reboot I can see that my wan interface has public IPv6 address:

Second step – wide-dhcpv6-client:
Configuration of wide-dhcpv6-client which will be obtaining IPv6 address pool (DHCPv6-PD) for lan interface (eth1)

Unfortunately package of wide-dhcpv6-client does not provide configuration file for systemd. To start up wide-dhcpv6-client by systemd I created wide-dhcpv6.service entry:

Enable it on system startup:

After reboot I can see that my lan interface (eth1) has assignment IPv6 address with prefix:

Last step – dnsmasq:
Part of configuration of dnsmasq (/etc/dnsmasq.conf) to redistributing IPv6 addresses in home network. Dnsmasq will also work as dns cache. :

Enable dnsmasq on system startup:

After reboot, devices in home network should be able to use internet by IPv6 ๐Ÿ™‚

But now, devices in home network are avaible from outside. Each of device has own public IPv6 address which is awailable from outside (internet).
We have to secure it, by allowing only for connections which are initialized from our internal network. It can be done by ip6tables.

It’s all, described configuration works flawlessly for me for days ๐Ÿ™‚