Richard Stevens passed away before the third edition of
UNIX Network Programming, Volume 1 was published, but the
new authors, Bill Fenner and Andy Rudoff, have tried to carry
forward his intentions.  Mr. Stevens didn't include an explicit
LICENSE file in his original source tree, however some files
contained the following license text:

/*
 * Copyright (c) 1996 W. Richard Stevens.  All rights reserved.
 *
 * Permission to use or modify this software for educational or
 * for commercial purposes, and without fee, is hereby granted,
 * provided that the above copyright notice appears in connection
 * with any and all uses, with clear indication as to any
 * modifications made.  The author RESERVES the sole rights of
 * reproduction, publication and distribution and hence permission
 * to print this source code in any book, reference manual,
 * magazine, or other type of publication, including any digital
 * medium, must be granted in writing by W. Richard Stevens.
 *
 * The author makes no representations about the suitability of this
 * software for any purpose.  It is provided "as is" without express
 * or implied warranty.
 */

Some additional comments from his web site:

        [Richard Stevens wrote...]
        The code is available to anyone on the Internet and should
        compile easily on most current Unix systems.  The majority of
        the 10,000 lines of C code are functions that one can use as
        building blocks (a network programming toolchest) inside their
        own network applications.  Many of these functions help hide
        the differences between IPv4 and IPv6, and can aid the reader
        in developing protocol-independent code.