Windows - directive dhcp-option DNS6 wrong syntax

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Clodo
OpenVPN User
Posts: 29
Joined: Mon Oct 10, 2011 11:25 pm

Windows - directive dhcp-option DNS6 wrong syntax

Postby Clodo » Mon Mar 20, 2017 4:23 pm

My OS: Microsoft Windows [Version 10.0.14393]
OpenVPN 2.4.0

My client config:
client
client
dev tun
proto udp
remote 1.2.3.4 80
nobind
persist-key
persist-tun
remote-cert-tls server
cipher AES-256-CBC
verb 3
explicit-exit-notify 5
ca "ca.crt"
cert "user.crt"
key "user.key"
tls-auth "ta.key" 1

redirect-gateway ipv6 def1 bypass-dhcp
dhcp-option DNS6 2001:4860:4860::8888


(2001:4860:4860::8888 are Google DNS primary IPv6)

Log:

Code: Select all

Mon Mar 20 16:56:56 2017 NETSH: C:\WINDOWS\system32\netsh.exe interface ipv6 set dns MyTap static 2001:4860:4860::8888 validate=no
Mon Mar 20 16:56:58 2017 ERROR: netsh command failed: returned error code 1
Mon Mar 20 16:57:02 2017 NETSH: command failed
Mon Mar 20 16:57:04 2017 Exiting due to fatal error


Issue:

The directive dhcp-option DNS6 2001:4860:4860::8888 try to launch netsh interface ipv6 set dns

set dns is the issue.
Not listed in MS docs:
https://msdn.microsoft.com/en-us/library/windows/desktop/bb736546(v=vs.85).aspx

Throw error if launched directly:

Code: Select all

C:\netsh interface ipv6 set dns MyTap 2001:4860:4860::8888 static validate=no
Element not found.


The correct syntax that works must be

Code: Select all

C:\netsh interface ipv6 set dnsservers "MyTap" static 2001:4860:4860::8888 primary


(p.s. note also that current OpenVPN 2.4 insert the Windows interface name in the shell without quoting...)

The openvpn source-code related to the issue is \openvpn-2.4.0\src\openvpn\tun.c , line 5130

Code: Select all

static void
netsh_set_dns6_servers(const struct in6_addr *addr_list,
                       const int addr_len,
                       const char *flex_name)
{
    struct gc_arena gc = gc_new();
    struct argv argv = argv_new();

    for (int i = 0; i < addr_len; ++i)
    {
        const char *fmt = (i == 0) ?
                          "%s%sc interface ipv6 set dns %s static %s"
                          : "%s%sc interface ipv6 add dns %s %s";
        argv_printf(&argv, fmt, get_win_sys_path(),
                    NETSH_PATH_SUFFIX, flex_name,
                    print_in6_addr(addr_list[i], 0, &gc));


netsh inline manual about ipv6:

Code: Select all

C:\>netsh interface ipv6 /?

The following commands are available:

Commands in this context:
6to4           - Changes to the `netsh interface ipv6 6to4' context.
?              - Displays a list of commands.
add            - Adds a configuration entry to a table.
delete         - Deletes a configuration entry from a table.
dump           - Displays a configuration script.
help           - Displays a list of commands.
isatap         - Changes to the `netsh interface ipv6 isatap' context.
reset          - Reset the IP configurations.
set            - Sets configuration information.
show           - Displays information.

The following sub-contexts are available:
 6to4 isatap

To view help for a command, type the command, followed by a space, and then
 type ?.


so, netsh inline manual about ipv6 set:

Code: Select all

C:\>netsh interface ipv6 set /?

The following commands are available:

Commands in this context:
set address    - Sets the IP address or default gateway to an interface.
set compartment - Modifies compartment configuration parameters.
set dnsservers - Sets DNS server mode and addresses.
set dynamicportrange - Modifies the range of ports used for dynamic port assignment.
set global     - Modifies global configuration general parameters.
set interface  - Modifies interface configuration parameters for IP.
set locality   - Shows locality info.
set neighbors  - Sets a neighbor address.
set prefixpolicy - Modifies prefix policy information.
set privacy    - Modifies privacy configuration parameters.
set route      - Modifies route parameters.
set subinterface - Modifies subinterface configuration parameters.
set teredo     - Sets Teredo state.


finally, netsh inline manual about ipv6 set dnsservers with examples:

Code: Select all

C:\>netsh interface ipv6 set dnsservers /?

Usage: set dnsservers [name=]<string> [source=]dhcp|static
             [[address=]<IP address>|none]
             [[register=]none|primary|both]
             [[validate=]yes|no]

Parameters:

      Tag            Value
      name         - The name or index of the interface.
      source       - One of the following values:
                     dhcp: Sets DHCP as the source for configuring DNS
                           servers for the specific interface.
                     static: Sets the source for configuring DNS servers
                             to local static configuration.
      address      - One of the following values:
                     <IP address>: An IP address for a DNS server.
                     none: Clears the list of DNS servers.
      register     - One of the following values:
                     none: Disables Dynamic DNS registration.
                     primary: Register under the primary DNS suffix only.
                     both: Register under both the primary DNS suffix, as
                           well as under the connection-specific suffix.
      validate     - Specifies whether validation of the DNS server setting
                     will be performed. The value is yes by default.

Remarks: Sets DNS server configuration to either DHCP or static mode. Only
         when source is 'static', is the 'addr' option also available for
         configuring a static list of DNS server IP addresses for the
         specified interface. If Validate switch is yes, then
         the newly set DNS server is validated.

Examples:

       set dnsservers name="Wired Ethernet Connection" source=dhcp
       set dnsservers "Wired Ethernet Connection" static fec0:0:0:ffff::1 primary


I'm missing something or i can open a bug-report?

Thanks for any feedback.

User avatar
TinCanTech
OpenVPN Protagonist
Posts: 2420
Joined: Fri Jun 03, 2016 1:17 pm

Re: Windows - directive dhcp-option DNS6 wrong syntax

Postby TinCanTech » Mon Mar 20, 2017 10:19 pm

With such hard work, You can always open a bug report 8-)

I cannot replicate this error myself but my W10 (ver 10.0.14393) may be slightly behind on updates.
netsh.exe : Version 10.0.14393.0, 90.5kb, 16/07/2016
SHA256(c:\windows\system32\netsh.exe)=
e5888e649c881e4bbbce472f6808f93b2b5564d3094995a5a08e66b2406c1607

Can you please verify how you are running openvpn.

Also, what happens if you define the TAP adapter with --dev-node MyTap ?


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