Debian Bug report logs - #888599
dhcpcd5: MTU setting ignored with static configuration

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Message #5 received at submit@bugs.debian.org (full text, mbox, reply):

From: Thomas Wismer <thomas.wismer@scs.ch>

To: Debian Bug Tracking System <submit@bugs.debian.org>

Subject: dhcpcd5: MTU setting ignored with static configuration

Date: Sat, 27 Jan 2018 16:48:05 +0100

Package: dhcpcd5
Version: 6.10.1-1
Severity: normal
Tags: upstream

Dear Maintainer,

To use a static IP configuration, e.g. the following lines can be added to /etc/dhcpcd.conf:

  interface eth0
  static ip_address=192.168.1.10/24
  static routers=192.168.1.1
  static domain_name_servers=192.168.1.1

Using these settings only, everything works as expected (no DHCP requests, simply apply config).


To specify a non-standard MTU, the following additional line is required:

  static interface_mtu=1400

The configuration file is still successfully parsed, but the MTU setting is NOT applied to the interface. Having a look at the source code shows that the "static interface_mtu" case is explicitly handled in the parser routine, so the specified value is expected to be applied during interface confiuration. However, it seems that the MTU handling is unintentionally bypassed in the static configuration use case.

Regards,
Thomas


-- System Information:
Debian Release: 9.3
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 4.9.0-5-amd64 (SMP w/1 CPU core)
Locale: LANG=de_CH.UTF-8, LC_CTYPE=de_CH.UTF-8 (charmap=UTF-8), LANGUAGE=de_CH:de (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages dhcpcd5 depends on:
ii  libc6  2.24-11+deb9u1

Versions of packages dhcpcd5 recommends:
ii  openresolv [resolvconf]  3.8.0-1

Versions of packages dhcpcd5 suggests:
pn  dhcpcd-gtk  <none>

-- Configuration Files:
/etc/dhcpcd.conf [Errno 2] Datei oder Verzeichnis nicht gefunden: '/etc/dhcpcd.conf'

-- no debconf information

Message #10 received at 888599-done@bugs.debian.org (full text, mbox, reply):

From: Scott Leggett <scott@sl.id.au>

To: 888599-done@bugs.debian.org

Subject: Re: dhcpcd5: MTU setting ignored with static configuration

Date: Fri, 8 Feb 2019 23:09:08 +1100

[Message part 1 (text/plain, inline)]

Version: 7.1.0-1

On Sat, 27 Jan 2018 16:48:05 +0100 Thomas Wismer <thomas.wismer@scs.ch> wrote:
> To specify a non-standard MTU, the following additional line is required:
> 
>   static interface_mtu=1400
> 
> The configuration file is still successfully parsed, but the MTU setting is NOT applied to the interface. Having a look at the source code shows that the "static interface_mtu" case is explicitly handled in the parser routine, so the specified value is expected to be applied during interface confiuration. However, it seems that the MTU handling is unintentionally bypassed in the static configuration use case.

Hi Thomas,

Thanks for the bug report!

I enquired upstream about this use-case - here's the response[0]:

> This is supported, but not quite how you expect.
>
> MTU is now set per route rather than for the whole interface, so you
> can have a larger MTU for IPv6 than for IPv4.
>
> Check output with `ip r`

I tested this configuration and it does indeed work in 7.1.0-1, so I'm
going to close this bug.

[0] https://roy.marples.name/archives/dhcpcd-discuss/0002304.html

-- 
Regards,
Scott Leggett.

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