OpenVPN error/warning "write UDPv4: No buffer space available"

[joegreat]

Dear All!

I am using a permanent OpenVPN connection via Proton VPN and linked to a guest WLAN for streaming from a neighbor country.
From time to time, I get the warning/error "write UDPv4 []: No buffer space available (fd=8,code=105)" in syslog - but I cannot relate the error with streaming issues - streaming works fine a far as I can see.

Any suggestions to fix this error or should I simply ignore it?

My research could not find a fix for it - what have I tried so far:
- searched the Internet for hints - found Linux parameters (/proc/sys/net settings) and MTU suggestion
- I fiddled around with OpenVPN parameters, but could not find out how the options tun_mtu, link_mtu, mss_fix and max_frament and fragment relate to each other
- mtu-test gave some results (see below), but I could not deploy them

Setup:
- Main router RT-AX88U Pro with Merlin v3006.102.6 & Entware 64-bit, OpenVPN, Samba and some Scripts
- 1 Gbit/250 Mbit FTH with 2.5 Gbit WAN connection to 1 and 2.5 Gbit LAN setup
- multiple guest WLANs (one for streaming over VPN)
- OpenVPN speed: 311.99 down, 234.19 up, ping 17 ms, jitter 0.3 ms (internet speed test under Adaptive QoS)

Below some snippets from Syslog:

Spoiler: Syslog excerpt

Jan  4 20:03:12 ovpn-client1[10409]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  5 02:03:34 ovpn-client1[10409]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  5 18:03:08 ovpn-client1[10409]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  5 18:03:08 ovpn-client1[10409]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  5 19:24:38 ovpn-client1[10409]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  6 06:03:17 ovpn-client1[10409]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  6 06:03:22 ovpn-client1[10409]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  6 06:03:26 ovpn-client1[10409]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  6 08:03:09 ovpn-client1[10409]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  6 08:03:09 ovpn-client1[10409]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  6 23:36:37 ovpn-client1[1345]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  7 16:03:19 ovpn-client1[1345]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  7 18:18:27 ovpn-client1[1345]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  8 22:03:09 ovpn-client1[1345]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan  9 18:03:12 ovpn-client1[1345]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan 10 05:00:07 ovpn-client1[1345]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan 10 22:03:10 ovpn-client1[1345]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan 11 08:03:27 ovpn-client1[1345]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan 11 09:36:17 ovpn-client1[1345]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan 11 12:03:32 ovpn-client1[1345]: write UDPv4 []: No buffer space available (fd=8,code=105)
Jan 11 14:03:11 ovpn-client1[1345]: write UDPv4 []: No buffer space available (fd=8,code=105)


Jan  6 14:28:12 ovpn-client1[6947]: WARNING: using --fragment and --mtu-test together may produce an inaccurate MTU test result
Jan  6 14:28:12 ovpn-client1[6947]: WARNING: if you use --mssfix and --fragment, you should use the "mtu" flag for both or none of of them.

Jan  6 14:11:40 ovpn-client1[25226]: NOTE: Empirical MTU test completed [Tried,Actual] local->remote=[1556,1556] remote->local=[1552,1552]
Jan  6 14:17:01 ovpn-client1[30083]: NOTE: Empirical MTU test completed [Tried,Actual] local->remote=[1580,1580] remote->local=[1576,1576]
Jan  6 14:23:20 ovpn-client1[2858]: NOTE: Empirical MTU test completed [Tried,Actual] local->remote=[1580,1580] remote->local=[1576,1576]
Jan  6 14:35:20 ovpn-client1[15633]: NOTE: Empirical MTU test completed [Tried,Actual] local->remote=[1524,1524] remote->local=[1520,1520]
Jan  6 14:46:04 ovpn-client1[20732]: NOTE: Empirical MTU test completed [Tried,Actual] local->remote=[1524,1524] remote->local=[1520,1520]
Jan  6 14:50:23 ovpn-client1[23611]: NOTE: Empirical MTU test completed [Tried,Actual] local->remote=[1524,1524] remote->local=[1520,1520]
Jan  6 14:56:03 ovpn-client1[27057]: NOTE: Empirical MTU test completed [Tried,Actual] local->remote=[1524,1524] remote->local=[1520,1520]

Jan  6 14:42:55 ovpn-client1[20732]: Control Channel MTU parms [ mss_fix:0 max_frag:0 tun_mtu:1250 tun_max_mtu:0 headroom:126 payload:1600 tailroom:126 ET:0 ]
Jan  6 14:42:55 ovpn-client1[20732]: Data Channel MTU parms [ mss_fix:0 max_frag:0 tun_mtu:1423 tun_max_mtu:1600 headroom:136 payload:1768 tailroom:562 ET:0 ]
Jan  6 14:42:57 ovpn-client1[20732]: Data Channel MTU parms [ mss_fix:0 max_frag:0 tun_mtu:1500 tun_max_mtu:1600 headroom:136 payload:1768 tailroom:562 ET:0 ]
Jan  6 14:47:16 ovpn-client1[23611]: Control Channel MTU parms [ mss_fix:0 max_frag:0 tun_mtu:1250 tun_max_mtu:0 headroom:126 payload:1600 tailroom:126 ET:0 ]
Jan  6 14:47:16 ovpn-client1[23611]: Data Channel MTU parms [ mss_fix:0 max_frag:0 tun_mtu:1423 tun_max_mtu:1600 headroom:136 payload:1768 tailroom:562 ET:0 ]
Jan  6 14:47:18 ovpn-client1[23611]: Data Channel MTU parms [ mss_fix:1408 max_frag:0 tun_mtu:1500 tun_max_mtu:1600 headroom:136 payload:1768 tailroom:562 ET:0 ]
Jan  6 14:52:55 ovpn-client1[27057]: Control Channel MTU parms [ mss_fix:0 max_frag:0 tun_mtu:1250 tun_max_mtu:0 headroom:126 payload:1600 tailroom:126 ET:0 ]
Jan  6 14:52:55 ovpn-client1[27057]: Data Channel MTU parms [ mss_fix:0 max_frag:0 tun_mtu:1500 tun_max_mtu:1600 headroom:136 payload:1768 tailroom:562 ET:0 ]
Jan  6 14:52:57 ovpn-client1[27057]: Data Channel MTU parms [ mss_fix:1400 max_frag:0 tun_mtu:1500 tun_max_mtu:1600 headroom:136 payload:1768 tailroom:562 ET:0 ]

 

[Twiglets]

See https://superuser.com/questions/1929824/openvpn-udp-over-4g-ppp-link-no-buffer-space-available

This hints that it could be related to MTU size as changing from MTU=576 to MTU=1500 gave a reduced number of 'write UDPv4 []: No buffer space available (fd=8,code=105)' errors.

Need to see the OpenVPN config on your end of the connection to see if there is anything that could be changed.

Cannot guarantee to fix it BUT it is worth a look !!!

 

[Twiglets]

OpenVPN manual is available:

Explains --fragment --mtu-test --mssfix etc ... See below

NETWORK CONFIGURATION​

OpenVPN 2.6 Manual

Open-source OpenVPN 2.6 user manual for VPN server and client configurations.

openvpn.net

Full Manual:

OpenVPN 2.6 Manual

Open-source OpenVPN 2.6 user manual for VPN server and client configurations.

openvpn.net

 

[RMerlin]

if using ProtonVPN it might be worth considering switching to WireGuard for better performance. Their WG implementation works very well within Asuswrt-Merlin, it's what I use for 24/7 VPN testing here on a guest network. I have 47 days of uptime on my main router, and that includes that VPN tunnel.