Example of making the first line special

Fixes #2.

README.md

@@ -1,9 +1,9 @@
 # Endlessh: an SSH tarpit
 
-Endlessh is an SSH tarpit that *very* slowly sends an endless, random
+Endlessh is an SSH tarpit [that *very* slowly sends an endless, random
-SSH banner. It keeps SSH clients locked up for hours or even days at a
+SSH banner][np]. It keeps SSH clients locked up for hours or even days
-time. The purpose is to put your real SSH server on another port and
+at a time. The purpose is to put your real SSH server on another port
-then let the script kiddies get stuck in this tarpit instead of
+and then let the script kiddies get stuck in this tarpit instead of
 bothering a real server.
 
 Since the tarpit is in the banner before any cryptographic exchange
@@ -11,6 +11,8 @@ occurs, this program doesn't depend on any cryptographic libraries. It's
 a simple, single-threaded, standalone C program. It uses `poll()` to
 trap multiple clients at a time.
 
+
+
 ## Usage
 
 Usage information is printed with `-h`.
@@ -68,3 +70,14 @@ MaxClients 4096
 #   2 = Very noisy debugging information
 LogLevel 0
 ```
+
+## Build issues
+
+RHEL 6 and CentOS 6 use a version of glibc older than 2.17 (December
+2012), and `clock_gettime(2)` is still in librt. For these systems you
+will need to link against librt:
+
+    make LDLIBS=-lrt
+
+
+[np]: https://nullprogram.com/blog/2019/03/22/

endlessh.c

@@ -1,4 +1,22 @@
-#define _POSIX_C_SOURCE 200809L
+#ifdef __FreeBSD__
+#  define _WITH_GETLINE
+/* The MSG_DONTWAIT send(2) flag is non-standard, but widely available.
+ * However, FreeBSD doesn't define this flag when using POSIX feature
+ * test macros. Normally feature test macros are required to expose
+ * POSIX functionality, though FreeBSD isn't strict about this. In a
+ * sense it's technically correct to hide a non-standard flag when
+ * asking for strict standards compliance, but this behavior makes this
+ * flag impossible to use in portable programs, at least without this
+ * sort of special case.
+ *
+ * To get the prototype for getline(3), we need either a POSIX feature
+ * test macro or use the FreeBSD-specific _WITH_GETLINE macro. Since we
+ * can't use the former, we'll have to go with the latter.
+ */
+#else
+#  define _POSIX_C_SOURCE 200809L
+#endif
+
 #include <time.h>
 #include <errno.h>
 #include <stdio.h>
@@ -23,6 +41,9 @@
 #define DEFAULT_MAX_CLIENTS       4096
 #define DEFAULT_CONFIG_FILE      "/etc/endlessh/config"
 
+#define FLAG_SPECIAL_SENT     (1 << 0)
+#define SPECIAL_MESSAGE       "Stay awhile and listen\r\n"
+
 #define XSTR(s) STR(s)
 #define STR(s) #s
 
@@ -73,6 +94,7 @@ struct client {
     struct client *next;
     int port;
     int fd;
+    int flags;
 };
 
 static struct client *
@@ -86,6 +108,7 @@ client_new(int fd, long long send_next)
         c->bytes_sent = 0;
         c->next = 0;
         c->fd = fd;
+        c->flags = 0;
 
         /* Set the smallest possible recieve buffer. This reduces local
          * resource usage and slows down the remote end.
@@ -496,7 +519,7 @@ usage(FILE *f)
     fprintf(f, "  -p INT    Listening port [" XSTR(DEFAULT_PORT) "]\n");
     fprintf(f, "  -v        Print diagnostics to standard output "
             "(repeatable)\n");
-    fprintf(f, "  -v        Print version information and exit\n");
+    fprintf(f, "  -V        Print version information and exit\n");
 }
 
 static void
@@ -712,7 +735,15 @@ main(int argc, char **argv)
 
             } else if (revents & POLLOUT) {
                 char line[256];
-                int len = randline(line, config.max_line_length, &rng);
+                int len;
+                if (!(client->flags & FLAG_SPECIAL_SENT)) {
+                    static const char special[] = SPECIAL_MESSAGE;
+                    len = sizeof(special) - 1;
+                    memcpy(line, special, len);
+                    client->flags |= FLAG_SPECIAL_SENT;
+                } else {
+                    len = randline(line, config.max_line_length, &rng);
+                }
                 for (;;) {
                     /* Don't really care if send is short */
                     ssize_t out = send(fd, line, len, MSG_DONTWAIT);