Hi Seth,
I think you are right to to remove exploitable ciphers.
But then there is also the (bad) need of some to be able to connect e.g. legacy systems.

I wouldn't mind so much about supporting the bad ciphers if one has to shoot (configure) himself to get them.
The manpage isn't to shy what it implies changing the default:
"Warning!  --tls-cipher is an expert feature, which - if used correcly - can improve the security of your VPN connection.  But it is also easy to unwittingly use it to carefully align a gun with your foot, or just break your connection.  Use with care!"

But as the openvpn people pointed out on the linked bug:
"AFAIK that's a list of available ciphers, not usable or default. For example, there's even SRP ciphers which can't be used in OpenVPN."

Ok, I can accept that, but the man page about --tls-cipher states only:
The default for --tls-cipher [...] "DEFAULT:!EXP:!PSK:!SRP:!kRSA" when using OpenSSL.

It seems almost impossible to get the list what really is accepted from anybody.
So I wanted to know what that "Default" might be so that we can make better decisions here.

Also a good chance to debug our openvpn guide for 16.04
https://help.ubuntu.com/lts/serverguide/openvpn.html

Eventually for debugging not used the service, but extra verbose direct calls:
sudo /usr/sbin/openvpn --verb 11 --cd /etc/openvpn --script-security 2 --config /etc/openvpn/server.conf
sudo /usr/sbin/openvpn --verb 11 --cd /etc/openvpn --script-security 2 --config /etc/openvpn/client.conf

The default of a 16.04<->16.04 connection seems to be:
Data Channel Decrypt: Cipher 'BF-CBC' initialized with 128 bit key
Data Channel Decrypt: Using 160 bit message hash 'SHA1' for HMAC authentication
Control Channel: TLSv1.2, cipher TLSv1/SSLv3 DHE-RSA-AES256-GCM-SHA384, 2048 bit RSA

Seth, would you consider that as a default "safe" - SHA1 is "only" used for HMAC authentication?

I had hopes I'd see something like an handshake exchange between client and server of really supported ciphers and picking one in the log. But there was none.

So I did a loop over all ciphers in "show-tls" to check which ones really would work.
for cipher in $(openvpn --show-tls | awk '{gsub("\\(.*\\)",""); print $0}' | grep -- '-' | grep -v -- '--' | grep '^TLS' | sort); do
echo "Testing: ${cipher}"; sudo /usr/sbin/openvpn --tls-cipher ${cipher} --verb 11 --cd /etc/openvpn --script-security 2 --config /etc/openvpn/client.conf; done

A few notes: the DH couldn't be translated to IANA names, if changing DH to DHE they were equivalent to the TLS-DHE-... so I removed them.

As expected quite some ended up in:
TLS_ERROR: BIO read tls_read_plaintext error: error:1408A0C1:SSL routines:ssl3_get_client_hello:no shared cipher
or for the SRP/PSK cases
TLS_ERROR: BIO read tls_read_plaintext error: error:140740B5:SSL routines:SSL23_CLIENT_HELLO:no ciphers available

Full log:
client: http://paste.ubuntu.com/15777811/
server: http://paste.ubuntu.com/15777817/

The reduced list of really "working" ciphers as it comes out-of-the-box thereby is:
Always TLSv1.2, cipher TLSv1/SSLv3, 2048 bit RSA 
EDH-RSA-DES-CBC3-SHA 
DHE-RSA-AES128-SHA 
DHE-RSA-AES128-SHA256 
DHE-RSA-AES128-GCM-SHA256 
DHE-RSA-AES256-SHA 
DHE-RSA-AES256-SHA256 
DHE-RSA-AES256-GCM-SHA384 
DHE-RSA-CAMELLIA128-SHA 
DHE-RSA-CAMELLIA256-SHA 
DHE-RSA-SEED-SHA 
DES-CBC3-SHA 
AES128-SHA 
AES128-SHA256 
AES128-GCM-SHA256 
AES256-SHA 
AES256-SHA256 
AES256-GCM-SHA384 
CAMELLIA128-SHA 
CAMELLIA256-SHA 
RC4-MD5 
RC4-SHA 
SEED-SHA

While at it I also checked for digests and ciphers.
for digest in $(openvpn --show-digests | awk '{gsub("\\(.*\\)",""); print $1}' | grep -- '-' | grep -v -- '--' | sort); do echo "Tes
ting: ${digest}"; sudo /usr/sbin/openvpn --auth ${digest} --verb 11 --cd /etc/openvpn --script-security 2 --config /etc/openvpn/client.conf; done

for cipher in $(openvpn --show-ciphers | awk '{print $1}' | grep -- '-' | grep -v -- '--' | sort); do echo "Testing ${cipher}"; sudo
/usr/sbin/openvpn --cipher ${cipher} --verb 11 --cd /etc/openvpn --script-security 2 --config /etc/openvpn/client.conf; done

Note: The settings for the data encryption and authentication "auth" and "cipher" must be set to the same value on the server to let it really "work" but that done all could initialize.

So the full list of HMAC digests work:
DSA-SHA 
DSA-SHA1 
DSA-SHA1-old 
RSA-MD4 
RSA-MD5 
RSA-RIPEMD160 
RSA-SHA 
RSA-SHA1 
RSA-SHA1-2 
RSA-SHA224 
RSA-SHA256 
RSA-SHA384 
RSA-SHA512 
ecdsa-with-SHA1

As well as the full list of ciphers for Data transfer:
AES-128-CBC 
AES-128-CFB 
AES-128-CFB1 
AES-128-CFB8 
AES-128-OFB 
AES-192-CBC 
AES-192-CFB 
AES-192-CFB1 
AES-192-CFB8 
AES-192-OFB 
AES-256-CBC 
AES-256-CFB 
AES-256-CFB1 
AES-256-CFB8 
AES-256-OFB 
BF-CBC 
BF-CFB 
BF-OFB 
CAMELLIA-128-CBC 
CAMELLIA-128-CFB 
CAMELLIA-128-CFB1 
CAMELLIA-128-CFB8 
CAMELLIA-128-OFB 
CAMELLIA-192-CBC 
CAMELLIA-192-CFB 
CAMELLIA-192-CFB1 
CAMELLIA-192-CFB8 
CAMELLIA-192-OFB 
CAMELLIA-256-CBC 
CAMELLIA-256-CFB 
CAMELLIA-256-CFB1 
CAMELLIA-256-CFB8 
CAMELLIA-256-OFB 
CAST5-CBC 
CAST5-CFB 
CAST5-OFB 
DES-CBC 
DES-CFB 
DES-CFB1 
DES-CFB8 
DES-EDE-CBC 
DES-EDE-CFB 
DES-EDE-OFB 
DES-EDE3-CBC 
DES-EDE3-CFB 
DES-EDE3-CFB1 
DES-EDE3-CFB8 
DES-EDE3-OFB 
DES-OFB 
DESX-CBC 
RC2-40-CBC 
RC2-64-CBC 
RC2-CBC 
RC2-CFB 
RC2-OFB 
SEED-CBC 
SEED-CFB 
SEED-OFB

So overall we now know the "effective" list includes critical ones.
TLS: RC4-MD5, RC4-SHA, EDH-RSA-DES-CBC3-SHA, DES-CBC3-SHA
Digests: Sha1 - is that also critical in this use?
Ciphers: DES*

Seth - it is your call what should be considered dangerous.
Would you be so kind and go back with that info to the OpenVPN community on the ticket you opened there?