Message too long

sunilmh

Hi,

I'm getting very frequest log mentioned below on my openvpn server.

read UDPv4 [EMSGSIZE Path-MTU=1404]: Message too long (code=90)

I gone through some of details and found mssfix has to be updated in server.conf.
Can any one help me out to understand the extact problem and what will be the impact of this on vpn connectivity?

Thanks in advance,
Sunil

janjust

this message shows up when OpenVPN detects that packets get fragment somewhere between client and server; to improve performance it is recommend to tweak some parameters, such as 'fragment' (udp mode only) and 'mssfix'; if you're running in udp mode I'd recommend to add

fragment 1400

to both client and server configs - this should improve the overall performance of the VPN link.

sunilmh

Thanks JJK,

Will try it out,

fragment 1400 &
for mssfix do i need to mention the same value as fragment ?

Thanks,
Sunil

janjust

if you don't specify a size for 'mssfix' the same value is used as for 'fragment' . So

fragment 1400
mssfix

is identical to

fragment 1400
mssfix 1400

sunilmh

I understand 'fragment' & 'mssfix' should be lower than "EMSGSIZE Path-MTU messgae value".
I hope my understanding on this is right.

eg: EMSGSIZE Path-MTU=1308

fragment 1300
mssfix 1300

janjust

that sounds right, but I'm not 100% certain ; your best bet is to manually determine the MTU size first; this be done using

ping -M do -s <size> server-IP

on linux and using

ping -f -l <size> server-IP

on windows; start with size=1200 and work your way up until the pings stop working. That's the maximum (payload) MTU size; on a regular LAN you should find size=1472 . This translates to an MTU of 1500 ; if the maximum size found is 1300 then set 'fragment' to 1300+28. YMMV

sunilmh

Do I need to add mssfix also to both server and client ?

janjust

yes you would need to add it on both sides.

sunilmh

Hi,

If we get a continuous logs for message too long on server side,

1. Does it affect the tunnel connectivity ?
2. Does the established connection gets arbupt exit ?

Thanks