feat: add custom MTU option to OpenVPN setup #1300
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**Background / Rationale:**
- By default, OpenVPN uses an MTU of approximately 1500 bytes minus header overhead. However, this default can cause fragmentation issues in environments involving nested VPN tunnels, DOCSIS networks, NAT setups, MPLS networks, or certain ISPs and mobile carriers which impose smaller MTU sizes.
- Allowing administrators to specify an MTU that matches their actual path MTU helps prevent these issues by aligning the VPN traffic with network constraints.
**Implemented Changes:**
1. **Installation Wizard (`installQuestions()`):**
- Immediately after protocol selection (UDP/TCP), users are prompted:
```
Do you want to set a custom MTU for the VPN interface?
1) No, keep default
2) Yes, specify a custom MTU
```
- If a custom MTU is chosen, input is validated to be between 576 and 1500 bytes, ensuring compliance with standard minimum and maximum MTU sizes.
- The decision is stored using two variables:
- `CUSTOM_MTU` ("y" or "n")
- `MTU` (numeric user input if applicable)
2. **Server configuration (`installOpenVPN()`):**
- If `CUSTOM_MTU=="y"`, the script automatically appends to `/etc/openvpn/server.conf`:
```
tun-mtu <MTU>
mssfix <MTU - 40>
```
- The `mssfix` setting dynamically adjusts TCP packet sizes to account for VPN overhead (typically IP + TCP/UDP headers of ~40 bytes). This prevents fragmentation and improves throughput and stability.
3. **Client Template (`client-template.txt`):**
- If a custom MTU is selected, the script also ensures each generated client `.ovpn` configuration file includes:
```
tun-mtu <MTU>
mssfix <MTU - 40>
```
- This approach guarantees consistency between server and client MTU settings, eliminating potential mismatches.
**Benefits of this Feature:**
- **Reduced Packet Fragmentation:**
By precisely matching MTU settings to actual network conditions, fragmentation-related performance issues (delays, retransmissions, and reduced throughput) are mitigated or entirely eliminated.
- **Enhanced TCP Throughput:**
The automatic inclusion of `mssfix` optimizes TCP segment sizes, significantly improving overall VPN performance for TCP connections.
- **Greater Network Compatibility:**
Facilitates optimal operation in scenarios involving nested VPN setups (e.g., OpenVPN within WireGuard or IPsec tunnels), cellular network connections, or specific ISP-imposed MTU constraints.
- **Simplified Configuration Management:**
Custom MTU and MSS settings applied consistently across server and client configurations reduce administrative overhead and potential configuration errors.
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Background / Rationale:
Implemented Changes:
Installation Wizard (
installQuestions()):Do you want to set a custom MTU for the VPN interface? 1) No, keep default 2) Yes, specify a custom MTUCUSTOM_MTU("y" or "n")MTU(numeric user input if applicable)Server configuration (
installOpenVPN()):CUSTOM_MTU=="y", the script automatically appends to/etc/openvpn/server.conf:tun-mtu <MTU> mssfix <MTU - 40>mssfixsetting dynamically adjusts TCP packet sizes to account for VPN overhead (typically IP + TCP/UDP headers of ~40 bytes). This prevents fragmentation and improves throughput and stability.Client Template (
client-template.txt):.ovpnconfiguration file includes:tun-mtu <MTU> mssfix <MTU - 40>Benefits of this Feature:
Reduced Packet Fragmentation: By precisely matching MTU settings to actual network conditions, fragmentation-related performance issues (delays, retransmissions, and reduced throughput) are mitigated or entirely eliminated.
Enhanced TCP Throughput: The automatic inclusion of
mssfixoptimizes TCP segment sizes, significantly improving overall VPN performance for TCP connections.Greater Network Compatibility: Facilitates optimal operation in scenarios involving nested VPN setups (e.g., OpenVPN within WireGuard or IPsec tunnels), cellular network connections, or specific ISP-imposed MTU constraints.
Simplified Configuration Management: Custom MTU and MSS settings applied consistently across server and client configurations reduce administrative overhead and potential configuration errors.