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IAX2 Security
Copyright (c) 2009 - Digium, Inc.
All Rights Reserved.
Document Version 1.0
09/03/09
Asterisk Development Team <asteriskteam@digium.com>
Table of Contents
1. Introduction 3
1.1. Overview 3
2. User Guide 3
2.1. Configuration 3
2.1.1. Quick Start 3
2.1.2. Controlled Networks 4
2.1.2.1. Full Upgrade 4
2.1.2.2. Partial Upgrade 4
2.1.3. Public Networks 4
2.1.3.1. Full Upgrade 4
2.1.3.2. Partial Upgrade 5
2.1.3.3. Guest Access 5
2.2. CLI Commands 6
2.2.1. iax2 show callnumber usage 6
2.2.2. iax2 show peer 6
3. Protocol Modification 6
3.1. Overview 6
3.2. Call Token Validation 7
3.3. Example Message Exchanges 8
3.3.1. Call Setup 8
3.3.2. Call Setup, client does not support CALLTOKEN 8
3.3.3. Call Setup, client supports CALLTOKEN, server does not 9
3.3.4. Call Setup from client that sends invalid token 9
4. Asterisk Implementation 9
4.1. CALLTOKEN IE Payload 9
1. Introduction
1.1. Overview
A change has been made to the IAX2 protocol to help mitigate denial of
service attacks. This change is referred to as call token validation. This
change affects how messages are exchanged and is not backwards compatible
for an older client connecting to an updated server, so a number of
options have been provided to disable call token validation as needed for
compatibility purposes.
In addition to call token validation, Asterisk can now also limit the
number of connections allowed per IP address to disallow one host from
preventing other hosts from making successful connections. These options
are referred to as call number limits.
For additional details about the configuration options referenced in this
document, see the sample configuration file, iax.conf.sample. For
information regarding the details of the call token validation protocol
modification, see section 3 (Protocol Modification) of this document.
2. User Guide
2.1. Configuration
2.1.1. Quick Start
We strongly recommend that administrators leave the IAX2 security
enhancements in place where possible. However, to bypass the security
enhancements completely and have Asterisk work exactly as it did before,
the following options can be specified in the [general] section of
iax.conf:
[general]
...
calltokenoptional = 0.0.0.0/0.0.0.0
maxcallnumbers = 16382
...
2.1.2. Controlled Networks
This section discusses what needs to be done for an Asterisk server on a
network where no unsolicited traffic will reach the IAX2 service.
2.1.2.1. Full Upgrade
If all IAX2 endpoints have been upgraded, then no changes to configuration
need to be made.
2.1.2.2. Partial Upgrade
If only some of the IAX2 endpoints have been upgraded, then some
configuration changes will need to be made for interoperability. Since
this is for a controlled network, the easiest thing to do is to disable
call token validation completely, as described in section 2.1.1.
2.1.3. Public Networks
This section discusses the use of the IAX2 security functionality on
public networks where it is possible to receive unsolicited IAX2 traffic.
2.1.3.1. Full Upgrade
If all IAX2 endpoints have been upgraded to support call token validation,
then no changes need to be made. However, for enhanced security, the
administrator may adjust call number limits to further reduce the
potential impact of malicious call number consumption. The following
configuration will allow known peers to consume more call numbers than
unknown source IP addresses:
[general]
; By default, restrict call number usage to a low number.
maxcallnumbers = 16
...
[callnumberlimits]
; For peers with known IP addresses, call number limits can
; be set in this section. This limit is per IP address for
; addresses that fall in the specified range.
; <IP>/<mask> = <limit>
192.168.1.0/255.255.255.0 = 1024
...
[peerA]
; Since we won't know the IP address of a dynamic peer until
; they register, a max call number limit can be set in a
; dynamic peer configuration section.
Type = peer
host = dynamic
maxcallnumbers = 1024
...
2.1.3.2. Partial Upgrade
If only some IAX2 endpoints have been upgraded, or the status of an IAX2
endpoint is unknown, then call token validation must be disabled to ensure
interoperability. To reduce the potential impact of disabling call token
validation, it should only be disabled for a specific peer or user as
needed. By using the auto option, call token validation will be changed to
required as soon as we determine that the peer supports it.
[friendA]
requirecalltoken = auto
...
Note that there are some cases where auto should not be used. For example,
if multiple peers use the same authentication details, and they have not
all upgraded to support call token validation, then the ones that do not
support it will get locked out. Once an upgraded client successfully
completes an authenticated call setup using call token validation,
Asterisk will require it from then on. In that case, it would be better to
set the requirecalltoken option to no.
2.1.3.3. Guest Access
Guest access via IAX2 requires special attention. Given the number of
existing IAX2 endpoints that do not support call token validation, most
systems that allow guest access should do so without requiring call token
validation.
[guest]
; Note that the name "guest" is special here. When the code
; tries to determine if call token validation is required, it
; will look for a user by the username specified in the
; request. Guest calls can be sent without a username. In
; that case, we will look for a defined user called "guest" to
; determine if call token validation is required or not.
type = user
requirecalltoken = no
...
Since disabling call token validation for the guest account allows a huge
hole for malicious call number consumption, an option has been provided to
segregate the call numbers consumed by connections not using call token
validation from those that do. That way, there are resources dedicated to
the more secure connections to ensure that service is not interrupted for
them.
[general]
maxcallnumbers_nonvalidated = 2048
2.2. CLI Commands
2.2.1. iax2 show callnumber usage
Usage: iax2 show callnumber usage [IP address]
Show current IP addresses which are consuming IAX2 call numbers.
2.2.2. iax2 show peer
This command will now also show the configured call number limit and
whether or not call token validation is required for this peer.
3. Protocol Modification
This section discusses the modification that has been made to the IAX2
protocol. This information would be most useful to implementors of IAX2.
3.1. Overview
The IAX2 protocol uses a call number to associate messages with which call
they belong to. The available amount of call numbers is finite as defined
by the protocol. Because of this, it is important to prevent attackers
from maliciously consuming call numbers. To achieve this, an enhancement
to the IAX2 protocol has been made which is referred to as call token
validation.
Call token validation ensures that an IAX2 connection is not coming from a
spoofed IP address. In addition to using call token validation, Asterisk
will also limit how many call numbers may be consumed by a given remote IP
address. These limits have defaults that will usually not need to be
changed, but can be modified for a specific need.
The combination of call token validation and call number limits is used to
mitigate a denial of service attack to consume all available IAX2 call
numbers. An alternative approach to securing IAX2 would be to use a
security layer on top of IAX2, such as DTLS [RFC4347] or IPsec [RFC4301].
3.2. Call Token Validation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
For this section, when the word "request" is used, it is referring to the
command that starts an IAX2 dialog.
This modification adds a new IAX2 frame type, and a new information
element be defined.
Frame Type: CALLTOKEN --- 0x28 (40)
IE: CALLTOKEN --- 0x36 (54)
When a request is initially sent, it SHOULD include the CALLTOKEN IE with
a zero-length payload to indicate that this client supports the CALLTOKEN
exchange. When a server receives this request, it MUST respond with the
IAX2 message CALLTOKEN. The CALLTOKEN message MUST be sent with a source
call number of 0, as a call number will not yet be allocated for this
call.
For the sake of backwards compatibility with clients that do not support
token validation, server implementations MAY process requests that do not
indicate CALLTOKEN support in their initial request. However, this SHOULD
NOT be the default behavior, as it gives up the security benefits gained
by CALLTOKEN validation.
After a client sends a request with an empty CALLTOKEN IE, it MUST be
prepared to receive a CALLTOKEN response, or to receive a response that
would be given in the case of a valid CALLTOKEN. This is how a client must
behave to inter operate with IAX2 server implementations that do not yet
support CALLTOKEN validation.
When an IAX2 client receives a CALLTOKEN response, it MUST send its
initial request again. This request MUST include the CALLTOKEN IE with a
copy of the value of the CALLTOKEN IE received in the CALLTOKEN response.
The IE value is an opaque value. Clients MUST be able to accept a
CALLTOKEN payload of any length, up to the maximum length allowed in an
IAX2 IE.
The value of the payload in the CALLTOKEN IE is an implementation detail.
It is left to the implementor to decide how sophisticated it should be.
However, it MUST be enough such that when the CALLTOKEN IE is sent back,
it can be used to verify that the source IP address and port number has
not been spoofed.
If a server receives a request with an invalid CALLTOKEN IE value, then it
MUST drop it and not respond.
3.3. Example Message Exchanges
3.3.1. Call Setup
Client Server
-------- --------
------------- NEW ----------->
(with empty CALLTOKEN IE)
<---------- CALLTOKEN ------------
(client must ignore
source call number
from this message)
------------- NEW ----------->
(with received token)
<---------- AUTHREQ ----------
... continue as normal ...
3.3.2. Call Setup, client does not support CALLTOKEN
Client Server
-------- --------
------------- NEW ----------->
(with no CALLTOKEN IE)
<----------- REJECT ----------
(sent without allocating
a call number)
------------- ACK ----------->
3.3.3. Call Setup, client supports CALLTOKEN, server does not
Client Server
-------- --------
------------- NEW ----------->
(with empty CALLTOKEN IE)
<----------- AUTHREQ ---------
(sent without allocating
a call number)
... continue as normal ...
3.3.4. Call Setup from client that sends invalid token
Client Server
-------- --------
------------- NEW ----------->
(with invalid CALLTOKEN IE)
... dropped ...
4. Asterisk Implementation
This section includes some additional details on the implementation of
these changes in Asterisk.
4.1. CALLTOKEN IE Payload
For Asterisk, we will encode the payload of the CALLTOKEN IE such that the
server is able to validate a received token without having to store any
information after transmitting the CALLTOKEN response. The CALLTOKEN IE
payload will contain:
* A timestamp (epoch based)
* SHA1 hash of the remote IP address and port, the timestamp, as well
some random data generated when Asterisk starts.
When a CALLTOKEN IE is received, its validity will be determined by
recalculating the SHA1 hash. If it is a valid token, the timestamp is
checked to determine if the token is expired. The token timeout will be
hard coded at 10 seconds for now. However, it may be made configurable at
some point if it seems to be a useful addition. If the server determines
that a received token is expired, it will treat it as an invalid token and
not respond to the request.
By using this method, we require no additional memory to be allocated for
a dialog, other than what is on the stack for processing the initial
request, until token validation is complete.
However, one thing to note with this CALLTOKEN IE encoding is that a token
would be considered valid by Asterisk every time a client sent it until we
considered it an expired token. However, with use of the "maxcallnumbers"
option, this is not actually a problem. It just means that an attacker
could hit their call number limit a bit quicker since they would only have
to acquire a single token per timeout period, instead of a token per
request.
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