Re: [PATCH] openvpn: Actually apply configured parameters

[Michael Tremer <michael.tremer@ipfire.org>]

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Hello Erik,

Thank you for your prompt reply.

Interesting that we have talked about this that long ago. I totally forgot about that conversation, and I could not derive any actionable items from it.

I guess I have now implemented the option to actually disable mssfix.

The last post refers to this:

  https://sourceforge.net/p/openvpn/mailman/message/13218191/

A post from 2004 which simply says what has to be done - ideally. The problem with that only is that it is incredibly slow - presumably because of implementation issues.

If VPNs are slow, people tend to stop using them. That will of course expose them to loads of security issues.

So what can we do here? I have no idea apart from adding this patch, so that the user has at least some kind of choice.

Do you think this will be backwards-compatible, or are we going to break installations if we were to accept the patch?

Best,
-Michael

> On 21 Oct 2020, at 17:22, ummeegge <ummeegge(a)ipfire.org> wrote:
> 
> Hi Michael,
> 
> Am Dienstag, den 20.10.2020, 13:28 +0000 schrieb Michael Tremer:
>> OpenVPN is an absolute mess. The behaviour of configuration
>> parameters has been changed over the time; default values have been
>> changed over time; and it looks like nobody is actually testing
>> anything any more.
>> 
>> I have been spending hours today on figuring out why OpenVPN
>> is so damn slow. On a Lightning Wire Labs IPFire Mini Appliance
>> it achieves about 100 MBit/s in the default configuration when
>> "openssl speed -evp aes-256-gcm" achieves over 3.5 GBit/s.
>> 
>> Changing any of the cryptography parameters does not change
>> anything. Throughput remains around 100 MBit/s.
>> 
>> I finally set "cipher none" and "auth none" which disables
>> encryption and authentication altogether but does not increase
>> throughput. From here on it was absolutely clear that it was
>> not a crypto issue.
>> 
>> OpenVPN tries to be smart here and does its own fragmentation.
>> This is the worst idea I have heard of all day, because that job
>> is normally done best by the OS.
>> 
>> Various settings which allow the user to "tune" this are grossly
>> ineffective - let alone it isn't even clear what I am supposed
>> to configure anywhere. Setting "fragment 1500" weirdly still
>> does not convince openvpn to generate a packet that is longer
>> than 1400 bytes. Who'd a thunk?
>> 
>> There is a number of other parameters to set the MTU or which
>> are related to it (tun-mtu, link-mtu, fragment, mssfix).
>> 
>> On top of all of this we have two "bugs" in ovpnmain.cgi which
>> are being fixed in this patch:
>> 
>> 1) mssfix can be configured by the user. However, we always
>>   enable it in openvpn. The default is on, we only add "mssfix"
>>   which simply turns it on.
>>   It is now being disabled when the user has chosen so in the
>>   web UI. I do not know if this is backwards-compatible.
>> 
>> 2) We cap the MTU (tun-mtu) at 1500 bytes when fragment is being
>>   used. So it becomes pointless that the user can this and the
>>   user is not being made aware of this when they hit the save
>>   button.
>>   This was added when we added path MTU discovery. Since that
>>   did not work and was removed, we can remove this now, too.
>> 
>> I archived a solid 500-600 MBit/s of goodput with these settings:
>> 
>> * Disable mssfix
>> * Set "fragment" to 0
>> * Set MTU to 9000
> We have had a discussion about that longer time ago -->
> https://bugzilla.ipfire.org/show_bug.cgi?id=11364#c18
> . Did not know that a MTU of 9000 is possible, sounds like jumbo
> frames.
> 
>> 
>> I am sure the MTU could be further increased to have bigger packets,
>> but I did not test how badly this will affect latency of the tunnel.
>> 
>> OpenVPN seems to only be able to handle a certain amount of packets
>> a second - no matter what. With larger packets, the throughput of
>> the tunnel increases, but latency might as well.
>> 
>> Signed-off-by: Michael Tremer <michael.tremer(a)ipfire.org>
>> Cc: Erik Kapfer <erik.kapfer(a)ipfire.org>
>> Cc: Stefan Schantl <stefan.schantl(a)ipfire.org>
>> ---
>> html/cgi-bin/ovpnmain.cgi | 29 +++++++++--------------------
>> 1 file changed, 9 insertions(+), 20 deletions(-)
>> 
>> diff --git a/html/cgi-bin/ovpnmain.cgi b/html/cgi-bin/ovpnmain.cgi
>> index e7bc505e7..e5bc45c1c 100644
>> --- a/html/cgi-bin/ovpnmain.cgi
>> +++ b/html/cgi-bin/ovpnmain.cgi
>> @@ -280,14 +280,7 @@ sub writeserverconf {
>>     print CONF "server $tempovpnsubnet[0] $tempovpnsubnet[1]\n";
>>     #print CONF "push \"route $netsettings{'GREEN_NETADDRESS'}
>> $netsettings{'GREEN_NETMASK'}\"\n";
>> 
>> -    # Check if we are using mssfix, fragment and set the corretct
>> mtu of 1500.
>> -    # If we doesn't use one of them, we can use the configured mtu
>> value.
>> -    if ($sovpnsettings{'MSSFIX'} eq 'on') 
>> -	{ print CONF "tun-mtu 1500\n"; }
>> -    elsif ($sovpnsettings{'FRAGMENT'} ne '' &&
>> $sovpnsettings{'DPROTOCOL'} ne 'tcp') 
>> -	{ print CONF "tun-mtu 1500\n"; }
>> -    else 
>> -	{ print CONF "tun-mtu $sovpnsettings{'DMTU'}\n"; }
>> +    print CONF "tun-mtu $sovpnsettings{'DMTU'}\n";
>> 
>>     if ($vpnsettings{'ROUTES_PUSH'} ne '') {
>> 		@temp = split(/\n/,$vpnsettings{'ROUTES_PUSH'});
>> @@ -320,6 +313,8 @@ sub writeserverconf {
>>     }
>>     if ($sovpnsettings{MSSFIX} eq 'on') {
>> 		print CONF "mssfix\n";
>> +    } else {
>> +		print CONF "mssfix 0\n";
>>     }
>>     if ($sovpnsettings{FRAGMENT} ne '' &&
>> $sovpnsettings{'DPROTOCOL'} ne 'tcp') {
>> 		print CONF "fragment $sovpnsettings{'FRAGMENT'}\n";
>> @@ -975,7 +970,7 @@ unless(-d
>> "${General::swroot}/ovpn/n2nconf/$cgiparams{'NAME'}"){mkdir
>> "${General
>>   if ($cgiparams{'MTU'} eq '') {$tunmtu = '1500'} else {$tunmtu =
>> $cgiparams{'MTU'}};
>>   print SERVERCONF "tun-mtu $tunmtu\n";
>>   if ($cgiparams{'FRAGMENT'} ne '') {print SERVERCONF "fragment
>> $cgiparams{'FRAGMENT'}\n";} 
>> -  if ($cgiparams{'MSSFIX'} eq 'on') {print SERVERCONF "mssfix\n";
>> }; 
>> +  if ($cgiparams{'MSSFIX'} eq 'on') {print SERVERCONF "mssfix\n"; }
>> else { print SERVERCONF "mssfix 0\n" };
>>   }
>> 
>>   print SERVERCONF "# Auth. Server\n"; 
>> @@ -1074,7 +1069,7 @@ unless(-d
>> "${General::swroot}/ovpn/n2nconf/$cgiparams{'NAME'}"){mkdir
>> "${General
>>   if ($cgiparams{'MTU'} eq '') {$tunmtu = '1500'} else {$tunmtu =
>> $cgiparams{'MTU'}};
>>   print CLIENTCONF "tun-mtu $tunmtu\n";
>>   if ($cgiparams{'FRAGMENT'} ne '') {print CLIENTCONF "fragment
>> $cgiparams{'FRAGMENT'}\n";}
>> -  if ($cgiparams{'MSSFIX'} eq 'on') {print CLIENTCONF "mssfix\n";
>> }; 
>> +  if ($cgiparams{'MSSFIX'} eq 'on') {print CLIENTCONF "mssfix\n"; }
>> else { print CLIENTCONF "mssfix 0\n" };
>>   }
>> 
>>   # Check host certificate if X509 is RFC3280 compliant.
>> @@ -2204,7 +2199,7 @@ if ($confighash{$cgiparams{'KEY'}}[3] eq
>> 'net'){
>>    if ($confighash{$cgiparams{'KEY'}}[31] eq '') {$tunmtu = '1500'}
>> else {$tunmtu = $confighash{$cgiparams{'KEY'}}[31]};
>>    print CLIENTCONF "tun-mtu $tunmtu\n";
>>    if ($confighash{$cgiparams{'KEY'}}[24] ne '') {print CLIENTCONF
>> "fragment $confighash{$cgiparams{'KEY'}}[24]\n";}
>> -   if ($confighash{$cgiparams{'KEY'}}[23] eq 'on') {print CLIENTCONF
>> "mssfix\n";}
>> +   if ($confighash{$cgiparams{'KEY'}}[23] eq 'on') {print CLIENTCONF
>> "mssfix\n";} else { print CLIENTCONF "mssfix 0\n"; }
>>    }
>>    # Check host certificate if X509 is RFC3280 compliant.
>>    # If not, old --ns-cert-type directive will be used.
>> @@ -2285,15 +2280,7 @@ else
>>     print CLIENTCONF "nobind\r\n";
>>     print CLIENTCONF "dev tun\r\n";
>>     print CLIENTCONF "proto $vpnsettings{'DPROTOCOL'}\r\n";
>> -
>> -    # Check if we are using fragment, mssfix and set MTU to 1500
>> -    # or use configured value.
>> -    if ($vpnsettings{FRAGMENT} ne '' && $vpnsettings{DPROTOCOL} ne
>> 'tcp' )
>> -	{ print CLIENTCONF "tun-mtu 1500\r\n"; }
>> -    elsif ($vpnsettings{MSSFIX} eq 'on')
>> -	{ print CLIENTCONF "tun-mtu 1500\r\n"; }
>> -    else
>> -	{ print CLIENTCONF "tun-mtu $vpnsettings{'DMTU'}\r\n"; }
>> +    print CLIENTCONF "tun-mtu $vpnsettings{'DMTU'}\r\n";
>> 
>>     if ( $vpnsettings{'ENABLED'} eq 'on'){
>>     	print CLIENTCONF "remote $vpnsettings{'VPN_IP'}
>> $vpnsettings{'DDEST_PORT'}\r\n";
>> @@ -2383,6 +2370,8 @@ else
>>     print CLIENTCONF "verify-x509-name
>> $vpnsettings{ROOTCERT_HOSTNAME} name\r\n";
>>     if ($vpnsettings{MSSFIX} eq 'on') {
>> 	print CLIENTCONF "mssfix\r\n";
>> +    } else {
>> +	print CLIENTCONF "mssfix 0\r\n";
>>     }
>>     if ($vpnsettings{FRAGMENT} ne '' && $vpnsettings{DPROTOCOL} ne
>> 'tcp' ) {
>> 	print CLIENTCONF "fragment $vpnsettings{'FRAGMENT'}\r\n";