libstore has support for Linux's -- I think it's only used with Linux -- network block devices.  However this code hasn't been kept up to date, so that it's mostly unusable at the moment.

<http://www.superunprivileged.org/hurd/nbd-results.txt>:

#v+
find examples of configurations where nbd works.
find examples of configurations where nbd does not work.


configuration #1:
nbd-server 2.8.3 from upstream running on GNU/Linux (2.6 kernel)
serving up a 12 MB .iso file
server is located at superunprivileged.org, port 4545
block size is 1024
client is located at daphne

steps:
$ settrans -ac nbd0 /hurd/storeio -Tnbd superunprivileged.org:4545/1024
$ settrans -ac iso /hurd/iso9660fs ./nbd0
$ cd iso
$ ls

results:
success
iso9660fs pauses for 14 seconds to come back to the prompt.
after iso9600fs returns, `iso' is owned by root.


configuration #2:
nbd-server 2.8.3 from upstream running on GNU/Linux (2.6 kernel)
serving up a 12 MB .iso file
server is located at superunprivileged.org, port 4545
block size is 4096
client is located at daphne

steps:
$ settrans -ac nbd0 /hurd/storeio -Tnbd superunprivileged.org:4545/4096
$ settrans -ac iso /hurd/iso9660fs ./nbd0
$ cd iso
$ ls

results:
failure on setting iso9660fs
settrans: /hurd/iso9660fs: (ipc/rcv) timed out
(after a one and a half minute pause)
on second try:
settrans: /hurd/iso9660fs: Translator died
(happens after a few seconds)
on third try:
settrans: /hurd/iso9660fs: Translator died
(happens after a few more seconds)
on fourth try:
settrans: /hurd/iso9660fs: Translator died
(happens after a few seconds)


configuration #3:
nbd-server 2.8.3 from upstream running on GNU/Linux (2.6 kernel)
serving up a 254 MB .iso file
server is located at superunprivileged.org, port 4545
block size is 1024
client is located at daphne

steps:
$ settrans -ac nbd0 /hurd/storeio -Tnbd superunprivileged.org:4545/1024
$ settrans -ac iso /hurd/iso9660fs ./nbd0
$ cd iso
$ ls

results:
success
iso9660fs pauses for 14 seconds to come back to the prompt.
after iso9600fs returns, `iso' is owned by root.


configuration #4:
nbd-server 2.8.3 from upstream running on GNU/Linux (2.6 kernel)
serving up a 32 MB ext2fs image (empty)
server is located at superunprivileged.org, port 4545
block size is 1024
client is located at daphne

steps:
$ settrans -ac nbd0 /hurd/storeio -Tnbd superunprivileged.org:4545/1024
$ settrans -ac iso /hurd/ext2fs ./nbd0
$ cd iso
$ ls

results:
failure
settrans: /hurd/ext2fs: Translator died
(happens after 20 seconds)
on second try:
settrans: /hurd/ext2fs: Translator died
(happens after 9 seconds)
on third try but with ext2fs.static instead:
settrans: /hurd/iso9660fs: (ipc/rcv) timed out
./nbd0: panic: main: device too small for superblock (0 bytes)

for all tests:
daemon.log on server contains "accept: Socket operation on non-socket"

configuration #5:
nbd-server 2.8.3 from upstream running on GNU/Linux (2.6 kernel)
serving up a 32 MB ext2fs image (empty)
server is located at superunprivileged.org, port 4545
block size is 4096
client is located at daphne

steps:
$ settrans -ac nbd0 /hurd/storeio -Tnbd superunprivileged.org:4545/4096
$ settrans -ac iso /hurd/ext2fs.static ./nbd0
$ cd iso
$ ls

results:
failure
ext2fs: ./nbd0: panic: main: device too small for superblock (4 bytes)
settrans: /hurd/ext2fs.static: Translator died
(happens after 20 seconds)


configuration #6:
nbd-server 2.8.3 from upstream running on GNU/Linux (2.6 kernel)
serving up a 32 MB ext2fs image (empty)
server is located at superunprivileged.org, port 4545
block size is 1024
client is located at daphne

steps:
$ settrans -ac nbd0 /hurd/storeio -Tnbd superunprivileged.org:4545/1024
$ settrans -ac fs /hurd/ext2fs-pre_ogi.static ./nbd0
$ cd fs
$ ls

results:
failure
settrans: ./ext2fs-pre_ogi.static: (ipc/rcv) timed out
try #2
failure.
timed out again.
settrans: ./ext2fs-pre_ogi.static: (ipc/rcv) timed out
#v-