WARNING: 'link-mtu' is used inconsistently

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adevi
OpenVpn Newbie
Posts: 2
Joined: Sat Jan 04, 2020 9:49 am

WARNING: 'link-mtu' is used inconsistently

Post by adevi » Sat Jan 04, 2020 9:58 am

Dear Support,

It seems to me, I can be wrong definitely, but maybe the remote MTU is 1472 and it can't not be higher (or I can't influence it). I read that it is not recommended to lower the link-mtu or tun-mtu, so I don't think lowering to 1472 should be tried. I also know that -fragment option is not applicable to the OpenVPN for Android (the version I am using is 3.1.0.(4292).

Expected Remote Options String (VER=V4): 'V4,dev-type tun,link-mtu 1550,tun-mtu 1500,
is there any other way to influence the client MTU or I am doing it wrong? I am far from expert I will be glad if someone can advise what should I do to avoid MTU mismatch.

What are the restrictions here? Why can I not influence the remote MTU to be 1500 bytes?

Recommendation, the same has been configured for both client and server:

# Custom user conf file for OpenVPN server
# Please add needed params only!

Code: Select all

tun-mtu 1500
mssfix 1300
# Custom user conf file for OpenVPN client
# Please add needed params only!

Code: Select all

tun-mtu 1500
mssfix 1300
Server config


proto udp4
port 1194
dev tun1
topology subnet
server 172.31.1.0 255.255.255.0
client-config-dir ccd
push "route 192.168.2.0 255.255.255.0"
auth SHA512
cipher AES-256-GCM
compress lzo
push "compress lzo"
push "redirect-gateway def1 bypass-dhcp"
push "dhcp-option DNS 192.168.2.1"

ca /etc/storage/openvpn/server/ca.crt
dh /etc/storage/openvpn/server/dh1024.pem
cert /etc/storage/openvpn/server/server.crt
key /etc/storage/openvpn/server/server.key
persist-key
persist-tun
user nobody
group nogroup
script-security 2
tmp-dir /tmp/openvpn
writepid /var/run/openvpn_svr.pid
client-connect ovpns.script
client-disconnect ovpns.script

### User params:

Code: Select all

tun-mtu 1500
mssfix 1300
ncp-disable
max-clients 10
client-to-client
keepalive 10 60
nice 3
verb 4
mute 10
Server log:

Code: Select all

Jan  4 10:23:23 openvpn-srv[562]: 37.76.13.221:51756 LZO compression initializing
Jan  4 10:23:23 openvpn-srv[562]: 37.76.13.221:51756 Control Channel MTU parms [ L:1622 D:1212 EF:38 EB:0 ET:0 EL:3 ]
Jan  4 10:23:23 openvpn-srv[562]: 37.76.13.221:51756 Data Channel MTU parms [ L:1622 D:1450 EF:122 EB:406 ET:0 EL:3 ]
Jan  4 10:23:23 openvpn-srv[562]: 37.76.13.221:51756 Local Options String (VER=V4): 'V4,dev-type tun,link-mtu 1550,tun-mtu 1500,proto UDPv4,comp-lzo,cipher AES-256-GCM,auth [null-digest],keysize 256,key-method 2,tls-server'
Jan  4 10:23:23 openvpn-srv[562]: 37.76.13.221:51756 Expected Remote Options String (VER=V4): 'V4,dev-type tun,link-mtu 1550,tun-mtu 1500,proto UDPv4,comp-lzo,cipher AES-256-GCM,auth [null-digest],keysize 256,key-method 2,tls-client'
Jan  4 10:23:23 openvpn-srv[562]: 37.76.13.221:51756 TLS: Initial packet from [AF_INET]37.76.13.221:51756, sid=f1aea08f 28fdc648
Jan  4 10:23:24 openvpn-srv[562]: 37.76.13.221:51756 VERIFY OK: depth=1, CN=OpenVPN CA
Jan  4 10:23:24 openvpn-srv[562]: 37.76.13.221:51756 VERIFY OK: depth=0, CN=myopenvpn
Jan  4 10:23:24 openvpn-srv[562]: 37.76.13.221:51756 peer info: IV_GUI_VER=OC30Android
Jan  4 10:23:24 openvpn-srv[562]: 37.76.13.221:51756 peer info: IV_VER=3.git::728733ae:Release
Jan  4 10:23:24 openvpn-srv[562]: 37.76.13.221:51756 peer info: IV_PLAT=android
Jan  4 10:23:24 openvpn-srv[562]: 37.76.13.221:51756 peer info: IV_NCP=2
Jan  4 10:23:24 openvpn-srv[562]: 37.76.13.221:51756 peer info: IV_TCPNL=1
Jan  4 10:23:24 openvpn-srv[562]: 37.76.13.221:51756 peer info: IV_PROTO=2
Jan  4 10:23:24 openvpn-srv[562]: 37.76.13.221:51756 peer info: IV_LZO=1
Jan  4 10:23:24 openvpn-srv[562]: 37.76.13.221:51756 peer info: IV_AUTO_SESS=1
Jan  4 10:23:24 openvpn-srv[562]: 37.76.13.221:51756 WARNING: 'link-mtu' is used inconsistently, local='link-mtu 1550', remote='link-mtu 1522'


Jan  4 10:35:30 openvpn-srv[2198]: MULTI: multi_create_instance called
Jan  4 10:35:30 openvpn-srv[2198]: 37.76.13.221:60560 Re-using SSL/TLS context
Jan  4 10:35:30 openvpn-srv[2198]: 37.76.13.221:60560 LZO compression initializing
Jan  4 10:35:30 openvpn-srv[2198]: 37.76.13.221:60560 Control Channel MTU parms [ L:1622 D:1212 EF:38 EB:0 ET:0 EL:3 ]
Jan  4 10:35:30 openvpn-srv[2198]: 37.76.13.221:60560 Data Channel MTU parms [ L:1622 D:1300 EF:122 EB:406 ET:0 EL:3 ]
Jan  4 10:35:30 openvpn-srv[2198]: 37.76.13.221:60560 Local Options String (VER=V4): 'V4,dev-type tun,link-mtu 1550,tun-mtu 1500,proto UDPv4,comp-lzo,cipher AES-256-GCM,auth [null-digest],keysize 256,key-method 2,tls-server'
Jan  4 10:35:30 openvpn-srv[2198]: 37.76.13.221:60560 Expected Remote Options String (VER=V4): 'V4,dev-type tun,link-mtu 1550,tun-mtu 1500,proto UDPv4,comp-lzo,cipher AES-256-GCM,auth [null-digest],keysize 256,key-method 2,tls-client'
Jan  4 10:35:30 openvpn-srv[2198]: 37.76.13.221:60560 TLS: Initial packet from [AF_INET]37.76.13.221:60560, sid=2ef3f18f bcdc44c3
Jan  4 10:35:31 openvpn-srv[2198]: 37.76.13.221:60560 VERIFY OK: depth=1, CN=OpenVPN CA
Jan  4 10:35:31 openvpn-srv[2198]: 37.76.13.221:60560 VERIFY OK: depth=0, CN=myopenvpn
Jan  4 10:35:31 openvpn-srv[2198]: 37.76.13.221:60560 peer info: IV_GUI_VER=OC30Android
Jan  4 10:35:31 openvpn-srv[2198]: 37.76.13.221:60560 peer info: IV_VER=3.git::728733ae:Release
Jan  4 10:35:31 openvpn-srv[2198]: 37.76.13.221:60560 peer info: IV_PLAT=android
Jan  4 10:35:31 openvpn-srv[2198]: 37.76.13.221:60560 peer info: IV_NCP=2
Jan  4 10:35:31 openvpn-srv[2198]: 37.76.13.221:60560 peer info: IV_TCPNL=1
Jan  4 10:35:31 openvpn-srv[2198]: 37.76.13.221:60560 peer info: IV_PROTO=2
Jan  4 10:35:31 openvpn-srv[2198]: 37.76.13.221:60560 peer info: IV_LZO=1
Jan  4 10:35:31 openvpn-srv[2198]: 37.76.13.221:60560 peer info: IV_AUTO_SESS=1
Jan  4 10:35:31 openvpn-srv[2198]: 37.76.13.221:60560 WARNING: 'link-mtu' is used inconsistently, local='link-mtu 1550', remote='link-mtu 1522'
Thank you in advance!
Last edited by Pippin on Sat Jan 04, 2020 1:46 pm, edited 1 time in total.
Reason: Formatting

adevi
OpenVpn Newbie
Posts: 2
Joined: Sat Jan 04, 2020 9:49 am

Re: WARNING: 'link-mtu' is used inconsistently

Post by adevi » Sat Jan 04, 2020 10:00 am

Maybe I should add 'mtu-test' to the config to find out more?

User avatar
TinCanTech
OpenVPN Protagonist
Posts: 6511
Joined: Fri Jun 03, 2016 1:17 pm

Re: WARNING: 'link-mtu' is used inconsistently

Post by TinCanTech » Sat Jan 04, 2020 2:00 pm

Unless you have real problems using your VPN you can safely ignore this warning.

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