fix: cannot start container when IPv6 is disabled

[shouhei]

Problem

On systems where IPv6 is disabled (e.g. kernel boot parameter ipv6.disable=1), running a container with port mapping fails with the following error:

$ cat /proc/cmdline | tr ' ' '\n'  | grep ipv6
ipv6.disable=1
$ sudo nerdctl run -p 8080:8080 alpine:latest echo 'OK'
FATA[0000] failed to load networking flags: open /proc/net/tcp6: no such file or directory
$ sudo nerdctl run alpine:latest echo 'OK'
OK
$

Root Cause

When allocating host ports, nerdctl unconditionally reads /proc/net/tcp6 and /proc/net/udp6 to check for ports already in use. On IPv6-disabled systems, the kernel does not create these files, causing the port allocation to fail entirely.

Fix

Treat ErrNotExist for /proc/net/tcp6 and /proc/net/udp6 as a non-error, returning nil (no IPv6 ports in use) instead of propagating the error.

Result

After this fix, containers with port mapping (-p) start successfully on IPv6-disabled systems.

Testing

No unit test is added at this time, as netTCP6Stats and netUDP6Stats are defined as const, making it difficult to inject alternative paths. If the maintainers prefer, these could be changed to var to allow unit testing with a mock path.

[haytok]

I feel this error handling might be better placed in getUsedPorts, which calls procnet.ReadStatsFileData.

Since procnet.ReadStatsFileData is a general-purpose helper, handling cases like missing IPv6 files seems more like a caller-side concern. Keeping the helper focused on reading/parsing might make the responsibility clearer.

WDYT?

nerdctl/pkg/portutil/port_allocate_linux.go

Line 89 in 1567880

if err != nil {

nerdctl/pkg/portutil/port_allocate_linux.go

Line 96 in 1567880

if err != nil {

[shouhei]

Thanks for reviewing and the suggestion!

I initially thought keeping it inside ReadStatsFileData was better, since the caller has no knowledge of the actual file path being opened. However, I agree that explicitly handling the IPv6 case in getUsedPorts makes the intent clearer.

I'll update the fix like this:

   tempTCPV6Data, err := procnet.ReadStatsFileData("tcp6")
   if err != nil && !errors.Is(err, os.ErrNotExist) {
       return nil, err
   }

(Same for udp6.)

[AkihiroSuda]

Please squash the commits

[haytok]

LGTM, Thanks!