feat(vpn): path MTU discovery to find the best MTU (#2586)

internal/pmtud/errors.go

@@ -0,0 +1,29 @@
+package pmtud
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"strings"
+	"time"
+)
+
+var (
+	ErrICMPNotPermitted                            = errors.New("ICMP not permitted")
+	ErrICMPDestinationUnreachable                  = errors.New("ICMP destination unreachable")
+	ErrICMPCommunicationAdministrativelyProhibited = errors.New("communication administratively prohibited")
+	ErrICMPBodyUnsupported                         = errors.New("ICMP body type is not supported")
+)
+
+func wrapConnErr(err error, timedCtx context.Context, pingTimeout time.Duration) error { //nolint:revive
+	switch {
+	case strings.HasSuffix(err.Error(), "sendto: operation not permitted"):
+		err = fmt.Errorf("%w", ErrICMPNotPermitted)
+	case errors.Is(timedCtx.Err(), context.DeadlineExceeded):
+		err = fmt.Errorf("%w (timed out after %s)", net.ErrClosed, pingTimeout)
+	case timedCtx.Err() != nil:
+		err = timedCtx.Err()
+	}
+	return err
+}