Re: Linux inetd..

Aleph One (aleph1@DFW.NET)
Tue, 02 Dec 1997 00:50:26 -0600

On Sun, 30 Nov 1997, moOd wrote:

> Description:
> I've found that inetd on (*atleast*) Debian distribution of LiNUX crashes
> when port 13 (daytime) / port 37 (time) is "half-open scanned"..

The problem is two fold.

First, inetd is dying because it receives a SIGPIPE when it tries to write
to the socket returned by accept since it does not install a signal
handler for it. To fix install a signal handler for SIGPIPE. I belive the
latest version of NetKit already does this.

Now you may be wondering why does a write to the socket returned by
accept() generates a SIGPIPE. This bring us to the second issue. It seems
that at least under Linux 2.0.X accept will return a socket in the
received queue if it is not in the SYN_SENT or SYN_RECV state, even when
it has not gone through the ESTABLISHED state.

By doing a stealth scan on the port the socket goes from the SYN_RECV
state to the CLOSED state. When you try to read from such a socket you
get a SIGPIPE. The sematics of Linux's accept seems to be non-standard. I
wonder what else breaks by not handling SIGPIPE.

Aleph One /
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