I’m trying to use loginhook to download large amounts of data and I’ve been having problems which I’ve nailed down to nc exiting prematurely for no apparent reason. This is a simplified test script:
exec > /tmp/lhook.out.txt 2> /tmp/lhook.err.txt
nc -v server 4444 > /tmp/nc-test
echo "Exit value: $?"
On the server, when I run a simple nc listener like
echo "Hello world. | nc -l 4444, the transfer works correctly. But when I want to transfer a larger file, like
nc -l 4444 < /path/to/some/large.file, the client transfers only a small part (sometimes 2kiB, sometimes ~250 kiB). The exit value reported is 0.
Oh, and of course – the same script executed in a Terminal.app inside a user session works fine.
Can someone help with debugging, explain what is going on, or provide a solution?
dtruss snoop on netcat’s syscalls, and this is what I got:
157/0x4c6: write(0x1, "j,350 37376377377203304$f211F 16350&f 04 ", 0x400) = 1024 0 157/0x4c6: select(0x5, 0x7FFF53A87B40, 0x0, 0x0, 0x0) = 1 0 157/0x4c6: read(0x4, " 01u374)uf215f6203304 20 03331 01b353 35 17267 03213 27P213317377R 04271377377 ", 0x400) = 1024 0 157/0x4c6: write(0x1, " 01u374)uf215f6203304 20 03331 01b353 35 17267 03213 27P213317377R 04271377377 ", 0x400) = 1024 0 157/0x4c6: select(0x5, 0x7FFF53A87B40, 0x0, 0x0, 0x0) = 1 0 157/0x4c6: read(0x4, " ", 0x400) = 0 0 157/0x4c6: shutdown(0x4, 0x0, 0x0) = -1 Err#57 157/0x4c6: close(0x4) = 0 0 157/0x4c6: close(0x3) = 0 0 157/0x4c6: close(0x3) = -1 Err#9
I’m guessing that for some reason, the kernel is giving nc EOF instead of waiting for more data.
What an embarrassing oversight… 🙁
nc quits upon reading EOF from stdin. The client which receives the data runs the LoginHook script in a non-interactive shell, which directs
nc‘s stdin descriptor probably to
/dev/null. As soon as it reads EOF from it, it quits.
The fix is trivial: supply
-d switch to the receiving side. This prevents either version of netcat (BSD or Linux) from reading stdin, and the transfrer completes without issue.