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@

3 files changed, 27 insertions, 7 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
diff --git a/src/usr.bin/nc/netcat.c b/src/usr.bin/nc/netcat.c
index 67beb10d0a..bf4a7ccf12 100644
--- a/src/usr.bin/nc/netcat.c
+++ b/src/usr.bin/nc/netcat.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: netcat.c,v 1.230 2025/05/14 08:56:41 tb Exp $ */
+/* $OpenBSD: netcat.c,v 1.231 2025/05/21 08:41:35 djm Exp $ */
 /*
  * Copyright (c) 2001 Eric Jackson <ericj@monkey.org>
  * Copyright (c) 2015 Bob Beck.  All rights reserved.
@@ -190,6 +190,8 @@ main(int argc, char *argv[])
                                 socksv = -1; /* HTTP proxy CONNECT */
                         else if (strcmp(optarg, "4") == 0)
                                 socksv = 4; /* SOCKS v.4 */
+                        else if (strcasecmp(optarg, "4A") == 0)
+                                socksv = 44; /* SOCKS v.4A */
                         else if (strcmp(optarg, "5") == 0)
                                 socksv = 5; /* SOCKS v.5 */
                         else
diff --git a/src/usr.bin/nc/socks.c b/src/usr.bin/nc/socks.c
index 7c7448c9c5..4c5ff90135 100644
--- a/src/usr.bin/nc/socks.c
+++ b/src/usr.bin/nc/socks.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: socks.c,v 1.31 2022/06/08 20:20:26 djm Exp $  */
+/*      $OpenBSD: socks.c,v 1.32 2025/05/21 08:41:35 djm Exp $  */
 
 /*
  * Copyright (c) 1999 Niklas Hallqvist.  All rights reserved.
@@ -293,7 +293,7 @@ socks_connect(const char *host, const char *port,
                 default:
                         errx(1, "connection failed, unsupported address type");
                 }
-        } else if (socksv == 4) {
+        } else if (socksv == 4 || socksv == 44) {
                 /* This will exit on lookup failure */
                 decode_addrport(host, port, (struct sockaddr *)&addr,
                     sizeof(addr), 1, 0);
@@ -302,10 +302,22 @@ socks_connect(const char *host, const char *port,
                 buf[0] = SOCKS_V4;
                 buf[1] = SOCKS_CONNECT; /* connect */
                 memcpy(buf + 2, &in4->sin_port, sizeof in4->sin_port);
-                memcpy(buf + 4, &in4->sin_addr, sizeof in4->sin_addr);
+                if (socksv == 4) {
+                        memcpy(buf + 4, &in4->sin_addr, sizeof in4->sin_addr);
+                } else {
+                        /* SOCKS4A uses addr of 0.0.0.x, and hostname later */
+                        buf[4] = buf[5] = buf[6] = 0;
+                        buf[7] = 1;
+                }
                 buf[8] = 0;     /* empty username */
                 wlen = 9;
-
+                if (socksv == 44) {
+                        /* SOCKS4A has nul-terminated hostname after user */
+                        if (strlcpy(buf + 9, host,
+                            sizeof(buf) - 9) >= sizeof(buf) - 9)
+                                errx(1, "hostname too big");
+                        wlen = 9 + strlen(host) + 1;
+                }
                 cnt = atomicio(vwrite, proxyfd, buf, wlen);
                 if (cnt != wlen)
                         err(1, "write failed (%zu/%zu)", cnt, wlen);