Make server4 package compile on Windows

[guillaumerose]

I am working on an userland network stack based on gvisor (https://github.com/containers/gvisor-tap-vsock).
I use this library as the dhcp server. It works really great but doesn't compile on Windows.

This PR allows to start the server on Windows with a net.PacketConn directly server4.WithConn().

It also requires this PR in u-root: u-root/u-root#2019

[pmazzini]

lgtm, thanks for the PR. Can you add comments to the exported functions?

[guillaumerose]

I added comments. I was not really inspired. If you have better ideas, I take them :)

[guillaumerose]

u-root/u-root#2019 is merged and I can't update it here in the go.mod.
I tried go get -u github.com/u-root/u-root, etc.

$ go get github.com/u-root/u-root@efa3f7929d27a919286246551cb146506e734d13
go: github.com/u-root/u-root efa3f7929d27a919286246551cb146506e734d13 => v0.0.0-20210525171738-efa3f7929d27
go get: inconsistent versions:
	github.com/u-root/u-root@v0.0.0-20210525171738-efa3f7929d27 from github.com/u-root/u-root@efa3f7929d27a919286246551cb146506e734d13 requires github.com/u-root/u-root@v7.0.0+incompatible (not github.com/u-root/u-root@v0.0.0-20210525171738-efa3f7929d27 from github.com/u-root/u-root@efa3f7929d27a919286246551cb146506e734d13)

Do you have an idea?

replace github.com/u-root/u-root => github.com/u-root/u-root v0.0.0-20210525171738-efa3f7929d27

works fine.

[pmazzini]

lgtm

[codecov]

Codecov Report

Merging #428 (1d3b11e) into master (fb4eaaa) will not change coverage.
The diff coverage is n/a.

❗ Current head 1d3b11e differs from pull request most recent head 3a04e9c. Consider uploading reports for the commit 3a04e9c to get more accurate results

@@           Coverage Diff           @@
##           master     #428   +/-   ##
=======================================
  Coverage   67.42%   67.42%           
=======================================
  Files          88       88           
  Lines        3665     3665           
=======================================
  Hits         2471     2471           
  Misses       1030     1030           
  Partials      164      164           

Flag	Coverage Δ
integtests	∅ <ø> (∅)
unittests	67.42% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dhcpv4/server4/conn.go	37.03% <ø> (ø)

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[pmazzini]

Do you have an idea?

No, not really.

[pmazzini]

with regards to u-root, maybe this helps: #429

[hugelgupf]

yeah, #429 def helps. I took the windows change into the new repo as well.

[guillaumerose]

Yes #429 is great. I renamed conn to conn_unix. This PR should be good now.

[guillaumerose]

up! I think it is now good to merge, thanks

[pmazzini]

Codecov report failing after merging this PR.

https://github.com/insomniacslk/dhcp/runs/2772122685

{'detail': ErrorDetail(string='Unable to locate build via Github Actions API. Please upload with the Codecov repository upload token to resolve issue.', code='not_found')}

cc @insomniacslk