Comment 3 for bug 1801128

For simplicity, here's the patch that fixes this bug (and is included in openssh 7.8):

diff --git a/openbsd-compat/port-net.c b/openbsd-compat/port-net.c
index 7050629c..bb535626 100644
--- a/openbsd-compat/port-net.c
+++ b/openbsd-compat/port-net.c
@@ -185,7 +185,7 @@ sys_tun_open(int tun, int mode, char **ifname)
  else
   debug("%s: %s mode %d fd %d", __func__, ifr.ifr_name, mode, fd);

- if (ifname != NULL && (*ifname = strdup(ifr.ifr_name)))
+ if (ifname != NULL && (*ifname = strdup(ifr.ifr_name)) == NULL)
   goto failed;

  return (fd);
@@ -272,7 +272,7 @@ sys_tun_open(int tun, int mode, char **ifname)
    goto failed;
  }

- if (ifname != NULL && (*ifname = strdup(ifr.ifr_name)))
+ if (ifname != NULL && (*ifname = strdup(ifr.ifr_name)) == NULL)
   goto failed;

  close(sock);