Troubleshooting Client Connectivity

Sometimes things don’t work when you first try them. This page will go through the common issues people find when deploying Velociraptor clients and the steps needed to debug them.

Velociraptor communications

To understand where things might go wrong, we first need to understand how Velociraptor clients communicate with the server. You can read a lot more details about Velociraptor’s encryption scheme and communication protocol in our Velociraptor Communications Blog post, but we will go through the most important aspects here.

Velociraptor’s internal PKI

Every Velociraptor deployments creates an internal PKI which underpins it. The configuration wizard create an internal CA with an X509 certificate and a private key. This CA is used to

  1. Create initial server certificates and any additional certificates for key rotation.

  2. CA public certificate is embedded in the client’s configuration and is used to verify server communications.

  3. The CA is used to create API keys for programmatic access. The server is then able to verify API clients.

The configuration file contains the CA’s X509 certificate in the Client.ca_certificate parameter (it is therefore embedded in the client configuration). The private key is contained in the CA.private_key parameter.

In a secure installation you should remove the CA.private_key section from the server config and keep it offline. You only need it to create new API keys using the velociraptor config api_client command, and the server does not need it in normal operations.

Messages

Clients and servers communicate by sending each other messages (which are simply protocol buffers), for example, a message may contain VQL queries or result sets. Messages are collected into a list and sent in a single POST operation in a MessageList protobuf. This protobuf is encrypted using a session key with a symmetric cipher (aes_128_cbc). The session key is chosen by the sending party and is written into an encrypted Cipher protobuf and sent along with each message.

Velociraptor Communication overview
Velociraptor Communication overview

This symmetric key is encoded in a Cipher Properties protobuf which is encrypted in turn using the receiving party’s public key and signed using the sending party’s private key.

HTTP protocol

Velociraptor uses HTTPS POST messages to deliver message sets to the server. The server in turn sends messages to the client in the body of the POST request. The client connects to one of the server URLs provided in the Client.server_urls setting in its config file.

Before the client communicates with the server, the client must verify it is actually talking with the correct server. This happens at two levels:

  • If the URL is a HTTPS URL then the TLS connection needs to be verified

  • The client will fetch the url /server.pem to receive the server’s internal certificate. This certificate must be verified by the embedded CA.

Note that this verification is essential in order to prevent the client from accidentally talking with captive portals or MITM proxies.

TLS verification

Velociraptor currently supports 2 modes for deployment via the config wizard:

  • Self signed mode uses internal CAs for the TLS certificates. The client knows it is in self signed mode if the Client.use_self_signed_ssl flag is true.

  • Proper certificates minted by Let’s encrypt.

Velociraptor verifies self signed TLS certificates using its built in CA. This essentially pins the server’s certificate inside the client — even if a MITM was able to mint another certificate (even if it was trusted by the global roots!) it would not be valid since it was not issued by Velociraptor’s internal CA which is the only CA we trust in this mode! In this way self signed mode is more secure than when using a public CA.

The Client.pinned_server_name specifies the common name of the server (or DNS name in the Server Alternate Name (SAN) field). The client verifies that the certificate is correct AND that the name is the same as the pinned name. You typically do not need to change this setting.

If the client is not in self signed mode (i.e. Client.use_self_signed_ssl is false or not present), it expects to verify TLS connections using the system’s root certificate store. In this configuration, Velociraptor may be susceptible to a MITM SSL inspection proxy, and we must rely on the internal encryption mechanism as described in the previous section to protect communications.

Velociraptor’s self signed mode requires that the certificates be issued by Velociraptor itself (the CA is pinned). It is not supported for the server to present self signed certificates generated by another CA!

In practice we find that often customer networks do contain SSL inspection proxies which are using self signed certificates. This breaks communications between client and server altogether. We typically prefer to deploy Let’s Encrypt certificates for reliability and better interoperability with such SSL interception proxies.

If you encounter an SSL interception proxy that breaks Velociraptor communications, you may look into ways to whitelist the Velociraptor server. Many proxies have mechanisms to whitelist certain domains.

Debugging client communications

Now that we have an understanding on the low level communication mechanism, let’s try to apply our understanding to debugging common deployment issues.

If the client does not appear to properly connect to the server, the first thing is to run it manually (using the velociraptor client -v command):

Running the client manually
Running the client manually

In the above example, I ran the client manually with the -v switch. I see the client starting up and immediately trying to connect to its URL (in this case https://test.velocidex-training.com/) However this fails and the client will wait for a short time before retrying to connect again.

Testing connectivity with curl
Testing connectivity with curl

A common problem here is network filtering making it impossible to reach the server. You can test this by simply running curl with the server’s URL.

Once you enable connectivity, you might encounter another problem

Captive portal interception
Captive portal interception

The Unable to parse PEM message indicates that the client is trying to fetch the server.pem file but it is not able to validate it. This often happens with captive portal type of proxies which interfere with the data transferred. It can also happen if your DNS setting point to a completely different server.

We can verify the server.pem manually by using curl (note that when using self signed mode you might need to provide curl with the -k flag to ignore the certificate errors):

Fetching the server certificate
Fetching the server certificate

Note that the server.pem is always signed by the velociraptor internal CA in all deployment modes (even with lets encrypt). You can view the certificate details by using openssl:

curl https://test.velocidex-training.comom/server.pem | openssl x509 -text

If your server certificate has expired, the client will refuse to connect to it. To reissue the server certificate simply recreate the server configuration file (after suitably backing up the previous config file):

velociraptor --config server.config.yaml config rotate_key > new_server.config.yaml

The above step was able to use the internal Velociraptor CA to reissue the server certificate (which is normally issued for 1 year), allowing us to rotate the certificate.

Currently there is no way to update the CA certificate without redeploying new clients (the CA certificate is embedded in the client config file). When generating the config file initially, the CA certificate is created with a 10 year validity.