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| -rw-r--r-- | docs/configuration/interfaces/openvpn.md | 28 |
1 files changed, 27 insertions, 1 deletions
diff --git a/docs/configuration/interfaces/openvpn.md b/docs/configuration/interfaces/openvpn.md index 170c585d..da39fd00 100644 --- a/docs/configuration/interfaces/openvpn.md +++ b/docs/configuration/interfaces/openvpn.md @@ -189,6 +189,31 @@ designated monitor interface. * ``server``: Operates as a server in server-client mode. ``` + +```{cfgcmd} set interfaces openvpn \<interface\> mtu \<mtu\> + +**Configure the MTU of the tunnel interface.** + +If not set, OpenVPN uses its own default of 1500 bytes. With +{abbr}`DCO (Data Channel Offload)` enabled the default is 1420 bytes, because +the kernel data path does not adjust the TCP MSS and full-sized packets would +otherwise be fragmented on the underlying link. + +The value must be at least 100, and at least 1280 when IPv6 is used inside the +tunnel. It cannot be combined with an `openvpn-option` that sets +`tun-mtu`, `link-mtu` or `udp-mtu`. When such an option is present, the +1420 bytes default is not applied either. A `tun-mtu` pushed by the server +takes precedence on a client. + +Changing the MTU restarts the OpenVPN process. + +Example: + +:::{code-block} none +set interfaces openvpn vtun0 mtu 1400 +::: +``` + ### OpenVPN Data Channel Offload (DCO) OpenVPN {abbr}`DCO (Data Channel Offload)` improves the performance of @@ -225,7 +250,8 @@ enabled to avoid compatibility issues with existing clients. set interfaces openvpn vtun0 offload dco ::: This command enables {abbr}`DCO (Data Channel Offload)` and loads the required - kernel module. + kernel module. Unless configured otherwise, it also lowers the interface + MTU to 1420 bytes (see `mtu` above). ``` |