diff --git a/test/bare.test b/test/bare.test
index d680caa..ae513d4 100755
--- a/test/bare.test
+++ b/test/bare.test
@@ -5,13 +5,14 @@
 cleanup()
 {
     umount "${tmpdir}/root" || :
-    qemu-nbd -d /dev/nbd0 || :
+    qemu-nbd -d "$nbd" || :
     rm -rf "$tmpdir"
 }
 
 set -ef
 trap cleanup EXIT INT
 
+nbd=${NBD:-/dev/nbd1}
 devmgr=${DEVMGR:-proc}
 arch=${ARCH:-$(uname -m)}
 kernel=${KERNEL:-$(uname -r)}
@@ -25,7 +26,7 @@ image="${tmpdir}/root.qcow2"
 initrd="${tmpdir}/initramfs-$(uname -r)"
 
 qemu-img create -f qcow2 "$image" 1G
-qemu-nbd -c /dev/nbd0 "$image"
+qemu-nbd -c "$nbd" "$image"
 
 # o: Create MBR table.
 # n: Add new partition to table.
@@ -34,7 +35,7 @@ qemu-nbd -c /dev/nbd0 "$image"
 # newline: Use default value for first sector.
 # newline: Use default value for last sector.
 # w: Write changes and re-read partition table.
-fdisk /dev/nbd0 << EOF
+fdisk "$nbd" << EOF
 o
 n
 p
@@ -51,8 +52,8 @@ EOF
 
 mkdir -p "$root"
 
-mkfs.ext4 -L root /dev/nbd0p1
-mount /dev/nbd0p1 "$root"
+mkfs.ext4 -L root "${nbd}p1"
+mount "${nbd}p1" "$root"
 
 (
     tmpdir=$root; cd "$tmpdir"
@@ -78,7 +79,7 @@ EOF
 )
 
 umount "$root"
-qemu-nbd -d /dev/nbd0
+qemu-nbd -d "$nbd"
 
 (cd .. && ./tinyramfs -lk "$kernel" -c "$config" "$initrd")
 
@@ -101,11 +102,11 @@ fi
 
 "qemu-system-${arch}" "$@"
 
-qemu-nbd -c /dev/nbd0 "$image"
+qemu-nbd -c "$nbd" "$image"
 
 # Re-read partition table.
-fdisk /dev/nbd0 << EOF
+fdisk "$nbd" << EOF
 w
 EOF
 
-[ "$(e2label /dev/nbd0p1)" = success ]
+[ "$(e2label "${nbd}p1")" = success ]
diff --git a/test/luks.test b/test/luks.test
index 37b1db7..1b82838 100755
--- a/test/luks.test
+++ b/test/luks.test
@@ -6,7 +6,7 @@ cleanup()
 {
     umount "${tmpdir}/root" || :
     cryptsetup close "$name" || :
-    qemu-nbd -d /dev/nbd0 || :
+    qemu-nbd -d "$nbd" || :
     rm -rf "$tmpdir"
 }
 
@@ -15,6 +15,7 @@ command -v cryptsetup > /dev/null || exit 222
 set -ef
 trap cleanup EXIT INT
 
+nbd=${NBD:-/dev/nbd2}
 devmgr=${DEVMGR:-proc}
 arch=${ARCH:-$(uname -m)}
 kernel=${KERNEL:-$(uname -r)}
@@ -29,7 +30,7 @@ image="${tmpdir}/root.qcow2"
 initrd="${tmpdir}/initramfs-$(uname -r)"
 
 qemu-img create -f qcow2 "$image" 1G
-qemu-nbd -c /dev/nbd0 "$image"
+qemu-nbd -c "$nbd" "$image"
 
 # o: Create MBR table.
 # n: Add new partition to table.
@@ -38,7 +39,7 @@ qemu-nbd -c /dev/nbd0 "$image"
 # newline: Use default value for first sector.
 # newline: Use default value for last sector.
 # w: Write changes and re-read partition table.
-fdisk /dev/nbd0 << EOF
+fdisk "$nbd" << EOF
 o
 n
 p
@@ -50,13 +51,13 @@ EOF
 
 dd bs=512 count=1 if=/dev/urandom of="${tmpdir}/key"
 
-cryptsetup -qd "${tmpdir}/key" --pbkdf=pbkdf2 luksFormat /dev/nbd0p1
-cryptsetup  -d "${tmpdir}/key" open /dev/nbd0p1 "$name"
+cryptsetup -qd "${tmpdir}/key" --pbkdf=pbkdf2 luksFormat "${nbd}p1"
+cryptsetup  -d "${tmpdir}/key" open "${nbd}p1" "$name"
 
 cat > "$config" << EOF
 hooks=$devmgr,luks
 root=LABEL=root
-luks_root=UUID=$(cryptsetup luksUUID /dev/nbd0p1)
+luks_root=UUID=$(cryptsetup luksUUID "${nbd}p1")
 luks_key=${tmpdir}/key
 EOF
 
@@ -90,7 +91,7 @@ EOF
 
 umount "$root"
 cryptsetup close "$name"
-qemu-nbd -d /dev/nbd0
+qemu-nbd -d "$nbd"
 
 (cd .. && ./tinyramfs -lk "$kernel" -c "$config" "$initrd")
 
@@ -113,13 +114,13 @@ fi
 
 "qemu-system-${arch}" "$@"
 
-qemu-nbd -c /dev/nbd0 "$image"
+qemu-nbd -c "$nbd" "$image"
 
 # Re-read partition table.
-fdisk /dev/nbd0 << EOF
+fdisk "$nbd" << EOF
 w
 EOF
 
-cryptsetup -d "${tmpdir}/key" open /dev/nbd0p1 "$name"
+cryptsetup -d "${tmpdir}/key" open "${nbd}p1" "$name"
 
 [ "$(e2label "/dev/mapper/${name}")" = success ]
diff --git a/test/lvm.test b/test/lvm.test
index a02563c..d37b441 100755
--- a/test/lvm.test
+++ b/test/lvm.test
@@ -6,7 +6,7 @@ cleanup()
 {
     umount "${tmpdir}/root" || :
     vgchange -an "$vg" || :
-    qemu-nbd -d /dev/nbd0 || :
+    qemu-nbd -d "$nbd" || :
     rm -rf "$tmpdir"
 }
 
@@ -15,6 +15,7 @@ command -v lvm > /dev/null || exit 222
 set -ef
 trap cleanup EXIT INT
 
+nbd=${NBD:-/dev/nbd3}
 devmgr=${DEVMGR:-proc}
 arch=${ARCH:-$(uname -m)}
 kernel=${KERNEL:-$(uname -r)}
@@ -30,7 +31,7 @@ image="${tmpdir}/root.qcow2"
 initrd="${tmpdir}/initramfs-$(uname -r)"
 
 qemu-img create -f qcow2 "$image" 1G
-qemu-nbd -c /dev/nbd0 "$image"
+qemu-nbd -c "$nbd" "$image"
 
 # o: Create MBR table.
 # n: Add new partition to table.
@@ -39,7 +40,7 @@ qemu-nbd -c /dev/nbd0 "$image"
 # newline: Use default value for first sector.
 # newline: Use default value for last sector.
 # w: Write changes and re-read partition table.
-fdisk /dev/nbd0 << EOF
+fdisk "$nbd" << EOF
 o
 n
 p
@@ -56,7 +57,7 @@ lvm_group=$vg
 lvm_name=$lv
 EOF
 
-vgcreate "$vg" /dev/nbd0p1
+vgcreate "$vg" "${nbd}p1"
 lvcreate -l 100%FREE -n "$lv" "$vg"
 lvchange -ay "${vg}/${lv}"
 
@@ -90,7 +91,7 @@ EOF
 
 umount "$root"
 vgchange -an "$vg"
-qemu-nbd -d /dev/nbd0
+qemu-nbd -d "$nbd"
 
 (cd .. && ./tinyramfs -lk "$kernel" -c "$config" "$initrd")
 
@@ -113,10 +114,10 @@ fi
 
 "qemu-system-${arch}" "$@"
 
-qemu-nbd -c /dev/nbd0 "$image"
+qemu-nbd -c "$nbd" "$image"
 
 # Re-read partition table.
-fdisk /dev/nbd0 << EOF
+fdisk "$nbd" << EOF
 w
 EOF
 
diff --git a/test/zfs.test b/test/zfs.test
index 1b7565e..ffb199c 100755
--- a/test/zfs.test
+++ b/test/zfs.test
@@ -5,7 +5,7 @@
 cleanup()
 {
     zpool export "$pool" || :
-    qemu-nbd -d /dev/nbd0 || :
+    qemu-nbd -d "$nbd" || :
     rm -rf "$tmpdir"
 }
 
@@ -14,6 +14,7 @@ command -v zfs > /dev/null || exit 222
 set -ef
 trap cleanup EXIT INT
 
+nbd=${NBD:-/dev/nbd4}
 devmgr=${DEVMGR:-proc}
 arch=${ARCH:-$(uname -m)}
 kernel=${KERNEL:-$(uname -r)}
@@ -28,7 +29,7 @@ image="${tmpdir}/root.qcow2"
 initrd="${tmpdir}/initramfs-$(uname -r)"
 
 qemu-img create -f qcow2 "$image" 1G
-qemu-nbd -c /dev/nbd0 "$image"
+qemu-nbd -c "$nbd" "$image"
 
 # o: Create MBR table.
 # n: Add new partition to table.
@@ -37,7 +38,7 @@ qemu-nbd -c /dev/nbd0 "$image"
 # newline: Use default value for first sector.
 # newline: Use default value for last sector.
 # w: Write changes and re-read partition table.
-fdisk /dev/nbd0 << EOF
+fdisk "$nbd" << EOF
 o
 n
 p
@@ -49,7 +50,7 @@ EOF
 
 dd bs=512 count=1 if=/dev/urandom of="${tmpdir}/key"
 
-zpool create -m none "$pool" /dev/nbd0p1
+zpool create -m none "$pool" "${nbd}p1"
 zfs create \
     -o mountpoint=legacy \
     -o canmount=noauto \
@@ -96,7 +97,7 @@ EOF
 )
 
 zpool export "$pool"
-qemu-nbd -d /dev/nbd0
+qemu-nbd -d "$nbd"
 
 (cd .. && ./tinyramfs -lk "$kernel" -c "$config" "$initrd")
 
@@ -119,13 +120,13 @@ fi
 
 "qemu-system-${arch}" "$@"
 
-qemu-nbd -c /dev/nbd0 "$image"
+qemu-nbd -c "$nbd" "$image"
 
 # Re-read partition table.
-fdisk /dev/nbd0 << EOF
+fdisk "$nbd" << EOF
 w
 EOF
 
-zpool import -Nd /dev/nbd0p1 "$pool"
+zpool import -Nd "${nbd}p1" "$pool"
 
 [ "$(zfs get -Ho value success: "${pool}/root")" = true ]