openssl-dtls-custom-bio

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openssl-dtls-custom-bio

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simple dtls server client example with openssl custom-bio over udp

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# openssl-dtls-custom-bio

A simple dtls server client program implemented with openssl library.

A custom bio is used to expose underlying bio ctrl and packet transmission.

CA is used to verify server and client.

UDP server and client.

### Prerequisite

* a working c compiler and make
* openssl with header files
* readline library for client
```
apt-get install build-essential openssl libssl-dev libreadline-dev
```

### Build

```
make
```

### Run

start server with a list of addresses to listen
```
./server 127.0.0.1:1234 # listen on local loopback ipv4 port 1234
./server [::1]:1234 # listen on local loopback ipv6 port 1234
./server 127.0.0.1:1234 [::1]:1234 # listen on multiple port
./server [::]:1234 # listen on any interface both ipv4/ipv6 port 1234
```

start client with target server
```
./client 127.0.0.1:1234 # connect to local server on port 1234
```

once connected, some simple commands could be issued from client terminal
* `ping` returns `pong`
* `echo <some text>` returns `<some text>`
* `whoami` returns client's address and port seen by server
* `stats` returns a list of server currently serving clients
* `bc <some text>` broadcast `<some text>` to all clients

ctrl-d could be used to stop client

ctrl-c could be used to stop server or client

certs can be regenerated by
```
make delete-certs
make certs
```

### Bugs

The commands used in Makefile to generate certificates are not supposed to be
good practice, test use only.

A 2000 byte buffer size is hardcoded in program, an approaching sized message
would fail to be sent or received. This is program's bug, neither DTLS nor UDP
has this limitation. Although packet fragmentation should be avoided.

For simplicity, SSL timeout controls on bio were ignored. Although openssl
library would automatically handle state machine, (in this program) this is only
triggered on write or received packet events. Resulting
* a long polling receiver might fail to receive new message until next write
(requiring periodical read write to trigger state machine update)
* a client failed to notify shutdown would left on server's list forever
(missing dead peer detection)

### Pitfalls

`[::]` would listen on both ipv6 and ipv4, so it would conflict with other
addresses with same port, even ipv4 addresses.

Although the built executables could work with different version of openssl
library, it is recommended to rebuild in different environment. For example,
openssl changed the value of macro defined const `BIO_CTRL_DGRAM_SET_PEEK_MODE`
between 1.1.0f and 1.1.0g, this would "not" break ABI compatability but
definitely would let related function misbehave.

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