Standalone Debug Options

--show-gateway args (Standalone) Show current IPv4 and IPv6 default gateway and interface towards the gateway (if the protocol in question is enabled).

Valid syntax: ::

 --show-gateway
 --show-gateway IPv4-target
 --show-gateway IPv6-target

For IPv4 it looks for a 0.0.0.0/0 route, or the specified IPv4 address if the target can be parsed as an IPv4 address. For IPv6 this queries the route towards ::/128, or the specified IPv6 target address if the argument is an IPv6 address.

Adding a target is helpful for diagnostics to see if OpenVPN will do the right thing if there are more specific IPv4/IPv6 routes to a VPN server.

Advanced Expert Options

These are options only required when special tweaking is needed, often used when debugging or testing out special usage scenarios.

--hash-size args Set the size of the real address hash table to r and the virtual address table to v.

Valid syntax: ::

 hash-size r v

By default, both tables are sized at 4 times --max-clients buckets. With the default of 1024 of --max-clients this gives 4096 buckets.

--bcast-buffers n Allocate n buffers for broadcast datagrams (default :code:256).

--persist-local-ip Preserve initially resolved local IP address and port number across SIGUSR1 or --ping-restart restarts.

--persist-remote-ip Preserve most recently authenticated remote IP address and port number across :code:SIGUSR1 or --ping-restart restarts.

--rcvbuf size Set the TCP/UDP socket receive buffer size. Defaults to operating system default.

--shaper n Limit bandwidth of outgoing tunnel data to n bytes per second on the TCP/UDP port. Note that this will only work if mode is set to :code:p2p. If you want to limit the bandwidth in both directions, use this option on both peers.

OpenVPN uses the following algorithm to implement traffic shaping: Given a shaper rate of n bytes per second, after a datagram write of b bytes is queued on the TCP/UDP port, wait a minimum of (b / n) seconds before queuing the next write.

It should be noted that OpenVPN supports multiple tunnels between the same two peers, allowing you to construct full-speed and reduced bandwidth tunnels at the same time, routing low-priority data such as off-site backups over the reduced bandwidth tunnel, and other data over the full-speed tunnel.

Also note that for low bandwidth tunnels (under 1000 bytes per second), you should probably use lower MTU values as well (see above), otherwise the packet latency will grow so large as to trigger timeouts in the TLS layer and TCP connections running over the tunnel.

OpenVPN allows n to be between 100 bytes/sec and 100 Mbytes/sec.

--sndbuf size Set the TCP/UDP socket send buffer size. Defaults to operating system default.

--tcp-queue-limit n Maximum number of output packets queued before TCP (default :code:64).

When OpenVPN is tunneling data from a TUN/TAP device to a remote client over a TCP connection, it is possible that the TUN/TAP device might produce data at a faster rate than the TCP connection can support. When the number of output packets queued before sending to the TCP socket reaches this limit for a given client connection, OpenVPN will start to drop outgoing packets directed at this client.

--txqueuelen n (Linux only) Set the TX queue length on the TUN/TAP interface. Currently defaults to operating system default.

--disable-dco Disables the opportunistic use of data channel offloading if available. Without this option, OpenVPN will opportunistically use DCO mode if the config options and the running kernel supports using DCO.

Data channel offload currently requires data-ciphers to only contain AEAD ciphers (AES-GCM and Chacha20-Poly1305) and Linux with the ovpn module. The ovpn module has been integrated into the Linux kernel since 6.16 or is available as backport from https://github.com/OpenVPN/ovpn-backports.

Note that some options have no effect or cannot be used when DCO mode is enabled.

On platforms that do not support DCO disable-dco has no effect.