Vulnerability Development: Re: FreeBSD listen()

From: D. J. Bernstein <djb_at_CR.YP.TO>
Date: Fri, 5 Nov 1999 23:58:43 -0000

I recently put together an online FTP reference manual, including a
thorough discussion of PORT and PASV security:

http://cr.yp.to/ftp/security.html

There's a paragraph that explains exactly why a listenonce() syscall,
rejecting all SYNs past the first, would make the difference between
insecure FTP and secure FTP, assuming secure TCP.

(This observation appeared in comp.protocols.tcp-ip in January 1997.)

David Schwartz writes:
> If you only wish to accept one connection, only call 'accept' once.

You're missing the point. Several connections may already have been
accepted by the operating system before you ever call accept(). The
desired behavior is for the client's connection to be rejected if it's
not going to be the one that you obtain from accept().

---Dan
Received on Nov 05 1999