Building a Proxy in Go

Talk I gave at the Golang meetup in Melbourne, Australia

Transcript

1. Building a Proxy in Go How to build a minimal transparent TLS proxy in Go
2. Who? • Mark Wolfe • Ops / Development at Versent • Versent is Hiring!
3. The Challenge?! • We build secure environments on AWS • These environments require a proxy to be configured • I discovered SSL Bump SSL Peek and Splice, a new feature in Squid proxy server, which we use a lot • I wanted to explore this solution and build on it • We ONLY want outgoing HTTPS to leave the network
4. SSL Peek and Slice • Looks inside Client Hello and Server Hello • Uses the Server Name Indication (SNI) attribute to perform the connection on the clients behalf • Supports transparent proxying of TLS connections • Enables blocking of connections based on: • Hostname filtering, *.github.com • Block based on other attributes in Client / Server Hello
5. TLS 1.x Client Hello 1.1.1.1 2.2.2.2 TCP 40001 <-> 443 Server Hello TLS C S
6. Proxy P Client Hello (serverName=S) Server Hello TLS + Proxy P C S TCP 40001 <-> 443 TCP 41001 <-> 443 1.1.1.1 1.1.1.254 (GW) 2.2.2.2
7. l7proxify • Small Go service • Using a bit of code I had written before with some new ideas • TCP Proxy • Peeks at the TLS Client Hello and pulls out SNI (hostname) • Connects to that host and joins all the streams
8. l7proxify • Uses a few great libraries • github.com/spf13/cobra for command flags • github.com/spf13/viper for configuration • github.com/apex/log for logging • Some code from the crypto/tls inside Go
9. –Linus Torvalds “Talk is cheap. Show me the code.”
10. l7proxify • So why use code from inside Go itself: • Already decodes TLS records, albeit internally • Battle tested • Really nice code • Note always respect the licence and ensure it is included with your project!
11. In AWS L7Proxyify GW 10.0.0.1/24 EIP (205.0.1.1) VPC Route 0.0.0.0/0 -> 10.0.0.1 ELB Web Servers 10.1.0.1/24 10.1.0.2/24 Private Subnet Public Subnet
12. Demo Time
13. In Summary • Keep it simple • Use libraries which make your software more amazing • Wireshark is a great piece of software • Understand what your servers are connecting to…
14. Questions? • Mark Wolfe <mark@wolfe.id.au> • @wolfeidau on Twitter • github.com/wolfeidau • http://www.wolfe.id.au
15. Links • https://en.wikipedia.org/wiki/ Transport_Layer_Security • http://wiki.squid-cache.org/Features/ SslPeekAndSplice • https://blogs.aws.amazon.com/security/post/ TxFRX7UFUIT2GD/How-to-Add-DNS-Filtering-to- Your-NAT-Instance-with-Squid • https://github.com/wolfeidau/l7proxify