OpenVPN / OpenVPN/openvpn3

link-mtu and auth are incorrect when server has ncp-disable and cipher AES-256-GCM

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Description

Both ics-openvpn with openvpn3 and OpenVPN Connect on Android shows the problem:

Nov 03 13:50:31 archlinux openvpn[18325]: 192.168.1.131:43902 peer info: IV_VER=2.5_master
Nov 03 13:50:31 archlinux openvpn[18325]: 192.168.1.131:43902 peer info: IV_PLAT=android
Nov 03 13:50:31 archlinux openvpn[18325]: 192.168.1.131:43902 peer info: IV_PROTO=2
Nov 03 13:50:31 archlinux openvpn[18325]: 192.168.1.131:43902 peer info: IV_NCP=2
Nov 03 13:50:31 archlinux openvpn[18325]: 192.168.1.131:43902 peer info: IV_LZ4=1
Nov 03 13:50:31 archlinux openvpn[18325]: 192.168.1.131:43902 peer info: IV_LZ4v2=1
Nov 03 13:50:31 archlinux openvpn[18325]: 192.168.1.131:43902 peer info: IV_LZO=1
Nov 03 13:50:31 archlinux openvpn[18325]: 192.168.1.131:43902 peer info: IV_COMP_STUB=1
Nov 03 13:50:31 archlinux openvpn[18325]: 192.168.1.131:43902 peer info: IV_COMP_STUBv2=1
Nov 03 13:50:31 archlinux openvpn[18325]: 192.168.1.131:43902 peer info: IV_TCPNL=1
Nov 03 13:50:31 archlinux openvpn[18325]: 192.168.1.131:43902 peer info: IV_GUI_VER=de.blinkt.openvpn_0.7.8
Nov 03 13:50:31 archlinux openvpn[18325]: 192.168.1.131:43902 [f5122] Peer Connection Initiated with [AF_INET]192.168.1.131:43902
Nov 03 13:50:31 archlinux openvpn[18325]: f5122/192.168.1.131:43902 MULTI_sva: pool returned IPv4=192.168.145.3, IPv6=(Not enabled)
Nov 03 13:50:45 archlinux openvpn[18325]: 192.168.1.131:52861 peer info: IV_GUI_VER=de.blinkt.openvpn_0.7.8
Nov 03 13:50:45 archlinux openvpn[18325]: 192.168.1.131:52861 peer info: IV_VER=3.2__qa:d87f5bbc04)
Nov 03 13:50:45 archlinux openvpn[18325]: 192.168.1.131:52861 peer info: IV_PLAT=android
Nov 03 13:50:45 archlinux openvpn[18325]: 192.168.1.131:52861 peer info: IV_NCP=2
Nov 03 13:50:45 archlinux openvpn[18325]: 192.168.1.131:52861 peer info: IV_TCPNL=1
Nov 03 13:50:45 archlinux openvpn[18325]: 192.168.1.131:52861 peer info: IV_PROTO=2
Nov 03 13:50:45 archlinux openvpn[18325]: 192.168.1.131:52861 peer info: IV_AUTO_SESS=1
Nov 03 13:50:45 archlinux openvpn[18325]: 192.168.1.131:52861 WARNING: 'link-mtu' is used inconsistently, local='link-mtu 1549', remote='link-mtu 1521'
Nov 03 13:50:45 archlinux openvpn[18325]: 192.168.1.131:52861 WARNING: 'auth' is used inconsistently, local='auth [null-digest]', remote='auth SHA1'
Nov 03 13:50:45 archlinux openvpn[18325]: 192.168.1.131:52861 [f5122] Peer Connection Initiated with [AF_INET]192.168.1.131:52861
Nov 03 13:50:45 archlinux openvpn[18325]: MULTI_sva: pool returned IPv4=192.168.145.3, IPv6=(Not enabled)
Nov 03 13:51:25 archlinux openvpn[18325]: 192.168.1.131:54952 peer info: IV_GUI_VER=OC30Android
Nov 03 13:51:25 archlinux openvpn[18325]: 192.168.1.131:54952 peer info: IV_VER=3.git::728733ae:Release
Nov 03 13:51:25 archlinux openvpn[18325]: 192.168.1.131:54952 peer info: IV_PLAT=android
Nov 03 13:51:25 archlinux openvpn[18325]: 192.168.1.131:54952 peer info: IV_NCP=2
Nov 03 13:51:25 archlinux openvpn[18325]: 192.168.1.131:54952 peer info: IV_TCPNL=1
Nov 03 13:51:25 archlinux openvpn[18325]: 192.168.1.131:54952 peer info: IV_PROTO=2
Nov 03 13:51:25 archlinux openvpn[18325]: 192.168.1.131:54952 peer info: IV_AUTO_SESS=1
Nov 03 13:51:25 archlinux openvpn[18325]: 192.168.1.131:54952 WARNING: 'link-mtu' is used inconsistently, local='link-mtu 1549', remote='link-mtu 1521'
Nov 03 13:51:25 archlinux openvpn[18325]: 192.168.1.131:54952 WARNING: 'auth' is used inconsistently, local='auth [null-digest]', remote='auth SHA1'
Nov 03 13:51:25 archlinux openvpn[18325]: 192.168.1.131:54952 [f5122] Peer Connection Initiated with [AF_INET]192.168.1.131:54952
Nov 03 13:51:25 archlinux openvpn[18325]: f5122/192.168.1.131:54952 MULTI_sva: pool returned IPv4=192.168.145.3, IPv6=(Not enabled)

As you can see, openvpn 2.5 (in ics-openvpn) doesn't have the same issue.

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

No source file or test is identified in the report. Start by reproducing the connection with ncp-disable and AES-256-GCM, then compare the link-mtu and auth negotiation with OpenVPN 2.5; done means the inconsistent-value warnings are resolved.

Written by the indexing model from the issue text.

Assessment

Tech stack
android, cpp
Domain
networking, security
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
28/100

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