How do you handle TLS proxy and interception for different types of traffic and applications?

TLS proxy and interception are techniques that allow a middlebox, such as a firewall, a load balancer, or a security appliance, to inspect and modify the encrypted traffic between a client and a server. This can have benefits for network performance, security, and compliance, but also introduces challenges and risks for different types of traffic and applications. In this article, you will learn how to handle TLS proxy and interception for web, email, and VPN traffic, as well as some common pitfalls and best practices to avoid them.

1 Web traffic

Web traffic is often subject to TLS proxy and interception due to the need for HTTPS to protect sensitive data and user privacy, as well as web security solutions requiring TLS proxy and interception for scanning for threats. However, if not done properly, TLS proxy and interception can break the end-to-end trust and security of HTTPS. To handle TLS proxy and interception for web traffic, you should use a trusted certificate authority (CA) to issue certificates for the proxy server, and install the CA's root certificate on the client devices. This allows clients to verify the proxy server's identity and trust its certificates. Additionally, you should configure the proxy server to respect the server's certificate validation and cipher suite preferences, as well as forward any certificate errors or warnings to the client. This maintains the security level and compatibility of the original connection. Lastly, you should monitor and audit the proxy server's logs and activities while applying appropriate policies and rules to filter and modify traffic so that you can ensure that privacy or integrity of data is not compromised, nor any relevant regulations or standards violated.

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Brian DeWilde

Senior Software Engineer @ LinkedIn | ex-Oracle, VSP, Volkswagen
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The opening section states that TLS proxies can improve network performance, but in most applications there will not be a performance improvement. If traffic is encrypted again by the TLS proxy, it has only added to the overall latency. (Through proper scaling, the TLS proxy can have no effect on throughput.) Performance gains will likely only be observed in the case of a TLS termination proxy, where unencrypted data is forwarded to the servers. If the servers and cipher algorithm (e.g. GCM) support hardware acceleration, the gains will be small and must be weighed against the added latency. Only if the servers or cipher algorithms (e.g CBC) do not support hardware acceleration can the performance gains can be very large.

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2 Email traffic

Email traffic is often subject to TLS proxy and interception, especially for inbound messages, as many email servers use SMTPS or STARTTLS to encrypt the email messages and attachments, and many email security solutions rely on TLS proxy and interception to scan for malicious content. However, TLS proxy and interception can interfere with the authentication and encryption of email messages if not done properly. To handle TLS proxy and interception for email traffic, one must use a valid certificate for the proxy server, configure the proxy server to pass through the email server’s certificate and hostname to the client, and monitor and audit the proxy server’s logs and activities. Doing so will ensure that the proxy server is not tampering with the email headers or content, as well as complying with relevant protocols and standards.

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3 VPN traffic

VPN traffic is a type of traffic that is not usually subject to TLS proxy and interception, as it is designed to bypass any network restrictions or censorship and provide a secure tunnel between the client and the VPN server. However, some network administrators may wish to use TLS proxy and interception for VPN traffic due to reasons such as bandwidth management, network visibility, or policy enforcement. If done incorrectly, TLS proxy and interception can also reduce the performance and security of VPN traffic. To properly handle TLS proxy and interception for VPN traffic, you need to use a compatible certificate for the proxy server, configure the VPN client to accept the proxy server's certificate, configure the proxy server to forward the VPN server's certificate and address to the client, support the VPN client's protocol and encryption options, monitor and audit the proxy server's logs and activities, and apply relevant policies and rules to filter and modify the traffic. This will ensure that the proxy server does not disrupt the VPN connection or expose data, while also complying with applicable laws and regulations.

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4 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How do you handle TLS proxy and interception for different types of traffic and applications?

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1 Web traffic

2 Email traffic

3 VPN traffic

4 Here’s what else to consider

Secure Sockets Layer (SSL)

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