add SOCKS4A support to nc(1)'s proxy (-X) mode

SOCKS4A is a fairly obscure extension to the olde SOCKS4 protocol that
allows passing the destination as a string rather than a literal IPv4
address, which is the only thing that vanilla SOCKS4 supports.

The motivation for adding something so niche is to test the SOCKS4A
server code in ssh(1)'s dynamic forwarding (-D) support.

ok tb@

1 files changed, 8 insertions, 2 deletions

diff --git a/src/usr.bin/nc/nc.1 b/src/usr.bin/nc/nc.1
index 76b6dc018e..e7dc770fca 100644
--- a/src/usr.bin/nc/nc.1
+++ b/src/usr.bin/nc/nc.1
@@ -1,4 +1,4 @@
-.\"     $OpenBSD: nc.1,v 1.98 2024/04/01 12:40:18 deraadt Exp $
+.\"     $OpenBSD: nc.1,v 1.99 2025/05/21 08:41:35 djm Exp $
 .\"
 .\" Copyright (c) 1996 David Sacerdote
 .\" All rights reserved.
@@ -25,7 +25,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd $Mdocdate: April 1 2024 $
+.Dd $Mdocdate: May 21 2025 $
 .Dt NC 1
 .Os
 .Sh NAME
@@ -338,12 +338,18 @@ when talking to the proxy server.
 Supported protocols are
 .Cm 4
 (SOCKS v.4),
+.Cm 4A
+(SOCKS v.4A),
 .Cm 5
 (SOCKS v.5)
 and
 .Cm connect
 (HTTPS proxy).
 If the protocol is not specified, SOCKS version 5 is used.
+Note that the SOCKS v.4 protocol is very limited and can only be used when
+the destination host can be resolved to an IPv4 address.
+The other protocols pass the destination as a string to be interpreted
+by the remote proxy and do not have this limitiation.
 .It Fl x Ar proxy_address Ns Op : Ns Ar port
 Connect to
 .Ar destination