A DTLS server/client library for Linux: sukat_dtls

Update: Fixed for Openssl 1.1.1!

Introduction

This is a quite minimalistic DTLS library utilizing openssl. Mostly the point is to describe how to use UDP-socket on Linux in a way that allows separating multiple clients to separate file descriptors. This is achieved by using SO_REUSEADDR and SO_REUSEPORT selective and having connected-sockets in addition to the main "listening" fd. A connected socket is "more precise", similar to a routing scenario, which allows separating client streams.

Note that I've developed this on OpenSSL 1.0.2q 20 Nov 2018. I can give no guarantees of it working on earlier Openssl-versions.

Building

Default build:

mkdir build && cd build
cmake ../ && make

To build unit tests, define the -Dtest=ON variable for cmake and run the make test target after make.

Test util: nc_dlts

nc_dtls is a test utility, similar to nc, for testing DTLS connections. Similarly to nc it sends anything in stdin towards the connected server / accepted client.

Example:

Hosting netcat, and showing the server side. More verbosely to show where listen socket ends up.

user@server ~/src/dtls_sukat/build $ ./nc_dtls -l -vv -c ../tests/certs/server_cert.pem -k ../tests/certs/server_key.pem
sukat_cert: load_certificate:194: Initialize ssl_ctx 0x618000000080 with certificate and private key
sukat_dtls: sukat_dtls_server_init:58: Created context 0x604000000750 for DTLS server on INADDR_ANY:0
dtls_nc: main:657: Hosting at: :::46342
sukat_dtls: sukat_dtls_accept:220: Accepted new fd 7 from ::ffff:10.0.0.126:54785
dtls_nc: nc_dtls_event_cb:173: Client connected from: ::ffff:10.0.0.126:54785
sukat_dtls: sukat_dtls_listen_step:141: Client 0x60d000000380 from ::ffff:10.0.0.126:54785 DTLS handshake finished
sukat_dtls: sukat_dtls_client_accept:366: Client 0x60d000000380 handshake finished
dtls_nc: nc_dtls_event_cb:173: Client established from: ::ffff:10.0.0.126:54785
Hello from client
Hello back from server

netcat from the client side.

client@host ~/src/sukat_dtls/build $ ./nc_dtls 10.0.0.201 46342
Hello from client
Hello back from server

Data visible in tshark:

    1 0.000000000   10.0.0.126 → 10.0.0.201   DTLS 354 Client Hello
    2 0.003301921   10.0.0.201 → 10.0.0.126   DTLSv1.2 102 Hello Verify Request
    3 0.003467836   10.0.0.126 → 10.0.0.201   DTLSv1.2 386 Client Hello
    4 0.005930788   10.0.0.201 → 10.0.0.126   DTLSv1.2 1044 Server Hello, Certificate, Server Hello Done
    5 0.006469950   10.0.0.126 → 10.0.0.201   DTLSv1.2 400 Client Key Exchange, Change Cipher Spec, Encrypted Handshake Message
    6 0.009892113   10.0.0.201 → 10.0.0.126   DTLSv1.2 308 New Session Ticket, Change Cipher Spec, Encrypted Handshake Message
    7 7.965690007   10.0.0.126 → 10.0.0.201   DTLSv1.2 97 Application Data
    8 11.136128817   10.0.0.201 → 10.0.0.126   DTLSv1.2 102 Application Data
    9 15.669861411   10.0.0.126 → 10.0.0.201   DTLSv1.2 81 Encrypted Alert
   10 15.671338815   10.0.0.201 → 10.0.0.126   DTLSv1.2 81 Encrypted Alert

Example with multiple clients. Server side:

user@server ~/src/dtls_sukat/build $ ./nc_dtls -l 10.0.0.201 9999 -c ../tests/certs/server_cert.pem -k ../tests/certs/server_key.pem
Hello from client 2
Hello from client 3
Hello from client 1
Hello from client 8
Hello from client 6
Hello from client 18
Hello from client 16
Hello from client 13
Hello from client 10
Hello from client 17
Hello from client 15

Client side:

user@client ~/src/sukat_dtls/build $ for i in $(seq 1 20); do echo "Hello from client $i" | ./nc_dtls 10.0.0.201 9999 & done
...

tshark data:

    1 0.000000000   10.0.0.126 → 10.0.0.201   DTLS 354 Client Hello
    2 0.002627401   10.0.0.201 → 10.0.0.126   DTLSv1.2 102 Hello Verify Request
    3 0.003481598   10.0.0.126 → 10.0.0.201   DTLSv1.2 386 Client Hello
    4 0.005473779   10.0.0.201 → 10.0.0.126   DTLSv1.2 1044 Server Hello, Certificate, Server Hello Done
    5 0.008328319   10.0.0.126 → 10.0.0.201   DTLS 354 Client Hello
    6 0.009062765   10.0.0.126 → 10.0.0.201   DTLSv1.2 400 Client Key Exchange, Change Cipher Spec, Encrypted Handshake Message
    7 0.009542738   10.0.0.201 → 10.0.0.126   DTLSv1.2 102 Hello Verify Request
    8 0.010213269   10.0.0.126 → 10.0.0.201   DTLS 354 Client Hello
    9 0.012405141   10.0.0.201 → 10.0.0.126   DTLSv1.2 308 New Session Ticket, Change Cipher Spec, Encrypted Handshake Message
   10 0.012448223   10.0.0.201 → 10.0.0.126   DTLSv1.2 102 Hello Verify Request
   11 0.013051056   10.0.0.126 → 10.0.0.201   DTLS 354 Client Hello
   12 0.013652453   10.0.0.126 → 10.0.0.201   DTLS 354 Client Hello
   13 0.014438361   10.0.0.126 → 10.0.0.201   DTLSv1.2 386 Client Hello
   14 0.014495594   10.0.0.201 → 10.0.0.126   DTLSv1.2 102 Hello Verify Request
   15 0.014519288   10.0.0.201 → 10.0.0.126   DTLSv1.2 102 Hello Verify Request
   16 0.014562802   10.0.0.126 → 10.0.0.201   DTLSv1.2 99 Application Data
   17 0.014621537   10.0.0.126 → 10.0.0.201   DTLSv1.2 81 Encrypted Alert
...

Shows multiple client connecting and getting properly separated. Of course not all client connect, as this is UDP, which is lossy.

Update on the lossyness. This probably won't happen on this newest build. The issue was with DTLS listen sending verify request to multiple clients in the same call. Now that it reads from a mem BIO the same problem shouldn't exists.

TODO:

DTLS and MTU: Since we're not using stream-oriented sockets, the MTU needs to be checked and acted accordingly.