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Sylk Pushserver

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Copyright (C) 2022 AG Projects

Sylk Pushserver was designed to act as a central dispatcher for mobile push notifications inside RTC provider infrastructures. Both the provider and the mobile application customer, in the case of a shared infrastructure, can easily audit problems related to the processing of push notifications.


  • Bibiana Rivadeneira
  • Tijmen de Mes


Sylk Pushserver is licensed under GNU General Public License version 3. Copy of the license

Deployment scenarios

Sylk Pushserver can be deployed together with WebRTC server applications or VoIP servers like SIP Proxies and PBXs. Its main purpose is to act as a central entity inside RTC provider infrastructures. Without such a component, the same functionality must be built inside multiple servers and as the number of mobile applications increases, the need for such central component becomes obvious.

Integration examples

  • OpenSIPS: config/opensips.cfg
  • SylkServer: built-in support


Sylk Pushserver can handle an arbitrary number of different combinations of push notification service and mobile applications. It and can be extended by using Python programming language to support new push notification services and applications. Sample applications are provided to handle Sylk and Linphone mobile applications for Apple and Firebase push notification services.

For each configured Apple application, the server maintains a persistent connection by using HTTP/2 over TLS 1.2 and reuses that connection for sending notifications related to the application. Latest voip functionality for iOS 13 or later is also suported.

Each outgoing connection can use its own set of credentials, X.509 certificates and urls. The connection failures are properly handled and incoming requests remained queued for later by using a timer dependent on the payload type.


All incoming and outgoing requests, including HTTP headers and bodies, can be logged for troubleshooting purposes in the system journal and in a separate log file. These logs can easily be correlated with the logs from the server that generated the request by using the call-id key.

Remote HTTP logging of the results is possible so that one or more third-parties can receive information about the individual push requests and responses for each application.


Sylk Pushserver expects a json over HTTP POST requests and translates it into a correspondent outgoing push notifications request to Apple Push Notifications or Firebase FCM servers.

Json object structure:

'app-id': 'com.agprojects.sylk-ios',
'platform': 'apple',
'token': '6688-71a883fe',
'device-id': 'accc8375125582aae062353',
'call-id': '4dbe8-7a53-42bd-95f3-9a7d43938',
'from': '',
'from_display_name': 'Alice',
'to': '',
'event': 'incoming_session'
'silent': True
'reason': None
"badge": "number"


  • app-id: str, id provided by the mobile application (e.g. mobile bundle ID)
  • platform: str, 'firebase', 'android', 'apple' or 'ios'
  • token: str, destination device token,
    • iOS device tokens are strings with 64 hexadecimal symbols
    • Android device push tokens can differ in length`.
  • device-id: str, the device that generated the token
  • call-id: str, the unique session id for each call
  • from: str, address of the caller
  • from_display_name, (mandatory)*, display name of the caller
  • to, address of the callee
  • media-type: str: 'audio', 'video', 'chat', 'sms' or 'file-transfer'
  • silent: bool: (optional, default True) True for silent notification
  • reason:str: (optional) Cancel reason
  • event: str, type of event:
    • For Sylk app must be 'incoming_session', 'incoming_conference', 'cancel' or 'message'
    • For Linphone app must be 'incoming_session'
  • badge: int: optional badge to display

The response is a json with the following structure:

'code': 'a numeric code equal to the HTTP response code',
'description': 'a detailed text description',
'data' : {}

data contains an arbitrary dictionary with a structure depending on the request type and the remote server response.


API version 2 supports storage of the push tokens in a Apache Cassandra Cluster or locally in a pickle file. The elements in the API methods are the same type and values as in API version 1. The API has the following methods:

POST /v2/tokens/{account} - Stores a token for {account}

    "app-id": "string",
    "platform": "string",
    "token": "string",
    "device-id": "string",
    "silent": true,
    "user-agent": "string"

DELETE /v2/tokens/{account} - Removes a token for {account}

    "app-id": "string",
    "device-id": "string"

POST /v2/tokens/{account}/push - Sends a push notification(s) for {account}

    "event": "string",
    "call-id": "string",
    "from": "string",
    "from-display-name": "string",
    "to": "string",
    "media-type": "string",
    "reason": "string",
    "badge": "number"

POST /v2/tokens/{account}/push/{device} - Sends a push notification for {account} and {device}

    "event": "string",
    "call-id": "string",
    "from": "string",
    "from-display-name": "string",
    "to": "string",
    "media-type": "string",
    "reason": "string",
    "badge": "number"

Sample client code

External APIs

For documentation related to the API used by Apple and Firebase push notifications services you must consult their respective websites. For reference, the following APIs were used for developing the server, but these links may change:

Apple Certificate

Go to Apple developer website

Go to Identifiers section

Select the app id

Scroll down to Push notifications

Click Configure

Generate a certificate.

Export the certificate to pk12 format from Keychain.

Convert the cartificate and private key to .pem format:

openssl pkcs12 -in Certificates.p12 -nocerts -out sylk.privateKey.pem openssl pkcs12 -in Certificates.p12 -clcerts -nokeys -out sylk.pem

Remove the passcode from the private key:

openssl rsa -in sylk.privateKey.pem -out sylk.key

Use sylk.pem and sylk.key inside applications.ini config file.


As a Debian package

Install the AG Projects debian software signing key:


sudo apt-key add agp-debian-gpg.key

Add these repository matching your distribution to /etc/apt/sources.list:

Update the list of available packages:

sudo apt-get update

sudo apt-get install sylk-pushserver

From source

The source code is managed using darcs version control tool. The darcs repository can be fetched with:

darcs clone

Alternatively, one can download a tar archive from:

Install Python dependencies:

pip3 install -r requirements.txt

python3 install

Building Debian package

Install building dependencies:

sudo apt install dh-virtualenv debhelper libsystemd-dev dh-python python3-dev python3-setuptools python3-pip

Build the package:

python sdist
cd dist
tar zxvf *.tar.gz
cd sylk_pushserver-?.?.?

To install the debian package manually:

sudo dpkg -i sylk-pushserver_1.0.0_all.deb
sudo apt --fix-broken install


There are two configurations files.

  • general.ini

Contains the general server settings.

  • applications.ini

Contains the settings for each mobile application, see config/applications.ini.sample. Chages to this file cause the server to autamtically reload it, there is no need to restart the server.

Remote logging

Remote logging is done using a POST request over HTTP with a json containg both the original request and the final response of the push notification.

'request': push_request,
'response': push_response

Where :

  • push_request is the original json payload received by this server
  • push_response is a json with the following format:
'code': code,                # http response code from PNS
'description': description,  # detail description of the response from the PNS
'push_url': push_url,        # the final URL of the outgoing push notification
'incoming_body': {...},      # the original request body received by the server
'outgoing_headers': {...}.   # the outgoing request headers sent to the PNS
'outgoing_body': {...}       # the outgoing request body sent to the PNS

The returned result should be a json with a consistent key. The key can be defined in the application.ini for each application. If the key is set then its value will be logged which can make troubleshooting easier.

Custom applications

Custom applications can be written in Python by subclassing existing template classes.

Define the directory for custom applications in general.ini file:

extra_applications_dir = /etc/sylk-pushserver/applications

Copy config/applications/ to the extra_applications_dir and overwrite its functions.

In applications.ini file set app_type for the custom applications:

`app_type` = *myapp*

Custom Push services

Custom PNS can be written in Python by subclassing existing template classes.

Define the directory for custom push services in general.ini file:

extra_pns_dir = /etc/sylk-pushserver/pns

Copy config/pns/ to the extra_pns dir and overwrite its classes.

In applications.ini file set app_type for the custom applications:

`app_platform` = *mypns*

Running the server

From the source code

./sylk-pushserver --config_dir <path-to-config-directory>

If the config_dir directory is not specified, the following paths are searched for:

  • /etc/sylk-pushserver *./config

For more command line options use -h.

Debian package

sudo systemctl start sylk-pushserver


For testing the server scripts/sylk-pushclient can be used.


The server is developed in Python 3 and was tested on Debian Buster 10.

Reporting bugs

You may report bugs to SIP Beyond VoIP mailing list


Mobile push notifications for RTC infrastructures







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