Asterisk security

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Asterisk Security


Quick Start


Hardware Firewall


Most Asterisk boxes should be located behind a hardware firewall. Configure the firewall to block traffic from anyone that doesn't need to connect to you. Allow your VoIP provider, any remote phones/users, and others that may need to connect, but keep the restrictions as tight as possible. If you do have remote users, lock your firewall down to only allow those users to connect if possible, rather than opening it to the entire internet. If you have mobile users this may not be an option however.

Other services, such as SSH should be blocked by the hardware firewall.

Software Firewall


Implement a software firewall if possible, using Linux IP Chains. The rules should be similar to the hardware firewall, with the possible addition of additional rules to protect against attacks originating on the local LAN.

SecAst


SecAst is an intrusion detection and prevention system designed specifically to protect Asterisk phone systems against fraud. SecAst uses a variety of techniques to detect intrusion attempts, halt ongoing attacks, and prevent future attacks. SecAst is available in three editions, including a free edition. SecAst can be downloaded from www.generationd.com or checkout the wiki page SecAst (Asterisk Intrusion Detection and Prevention)

Port Knock


Port Knock can be used to provide remote access to your Asterisk machine, opening ports needed to access asterisk services only for your IP, using a predefined sequence of ports to request a temporary on-demand opening of specific ports needed for your remote/dynamic IP.

Fail2Ban


Implement Fail2Ban to prevent denial of service attacks (password guessing can cause excessive CPU utilization). See security warning regarding fail2ban - don't depend on it.

Strong Passwords


All remote users should have strong, alpha-numeric passwords. These should be long. They should NEVER be the same as the username or based on the user's extension.

Default Context


Your [default] context in extensions.conf should be empty. This context is used when other contexts might not match a peer. You should explicitly refer to a non-default context for remote SIP calls.

sip.conf configuration


In the [general] section, define:


[general]
...
context=bogus
allowguest=no
alwaysauthreject=yes
...


The context line will refer to a context (which you *must define* in extensions.conf!) that should handle "default" SIP calls. This should go to a context with nothing in it (unless you want to play with them - then, if you have the bandwidth, feel free to play a recording or such!). Obviously if you want to accept anonymous SIP calls, send them to the proper context (but *never* a context that can dial out!).

The "allowguest" line disables anonymous SIP calls to your PBX. Some SIP providers connect as a guest user, however, so this may be inappropriate for your situation. Also, if you want to accept anonymous SIP calls, this line would block them, so you wouldn't want that. But it is listed here because it is the safest configuration.

The "alwaysauthreject" line is important. This causes a hacker to get the same response from your PBX when they try to guess passwords whether or not they guessed a valid username. This also has the side-effect of making poorly written scanning scripts (the vast majority of hacker scripts seem to be poorly written) take less resources on your Asterisk box, as even if they scan a valid username, they'll think it doesn't exist.

In addition to these, verify that all peers listed in sip.conf are valid and have strong passwords.

iax.conf


(I'm hoping others will fill in this section - I block IAX)

Dialplan


Do your users need to be able to dial internationally? If not, make sure your dialplan blocks international calls (in the US, these calls start with a 011 typically, although some countries "look like" US numbers - so also block calls to area codes that don't correspond to areas you call). You may be able to request your provider also blocks international calls. If you only have SOME users that need to call internationally, place them in a different context than the rest of your users.

Logs, CDR


Review your logs and CDR at least daily. Even one day of illegitimate calls can add up to tons of money quickly.


References


Security in a complex piece of software like Asterisk is not a simple thing. Help us collect information on the subject:


What is Midcom you ask?

So Midcom is a IETF protocol Voip PBX speak to tell firewalls type boxes (like Ranch) what ports to open to allow calls through the firewall. The problem being as Voip get secure encrypted signaling firewalls will not be able to tell which ports to open to let media through the firewall. I am scared of the idea that my SIP proxy or IP PBX would be allowed to tell my firewall which ports to open. But I am not sure there is a better solution.

Articles





See also



Created by: oej, Last modification: Tue 27 of May, 2014 (14:27 UTC) by ocgltd


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