diff --git a/Recipe b/Recipe
index e065c37d..b3739b97 100644
--- a/Recipe
+++ b/Recipe
@@ -235,12 +235,12 @@ TERMINAL = terminal wcwidth ldiscucs logging tree234 minibidi
 
 # GUI front end and terminal emulator (putty, puttytel).
 GUITERM  = TERMINAL window windlg winctrls sizetip winprint winutils
-	 + wincfg sercfg winhelp winjump
+	 + wincfg sercfg winhelp winjump sessprep
 
 # Same thing on Unix.
 UXTERM   = TERMINAL uxcfg sercfg uxucs uxprint timing callback miscucs
 GTKTERM  = UXTERM gtkwin gtkcfg gtkdlg gtkfont gtkcols gtkmisc xkeysym
-	 + x11misc gtkcomm
+	 + x11misc gtkcomm sessprep
 GTKMAIN  = gtkmain cmdline
 
 # Non-SSH back ends (putty, puttytel, plink).
@@ -298,7 +298,7 @@ U_BE_NOSSH = be_nos_s uxser nocproxy
 putty    : [G] GUITERM NONSSH WINSSH W_BE_ALL WINMISC winx11 putty.res LIBS
 puttytel : [G] GUITERM NONSSH W_BE_NOSSH WINMISC puttytel.res nogss LIBS
 plink    : [C] winplink wincons NONSSH WINSSH W_BE_ALL logging WINMISC
-         + winx11 plink.res winnojmp noterm LIBS
+         + winx11 plink.res winnojmp sessprep noterm LIBS
 pscp     : [C] pscp winsftp wincons WINSSH BE_SSH SFTP wildcard WINMISC
          + pscp.res winnojmp LIBS
 psftp    : [C] psftp winsftp wincons WINSSH BE_SSH SFTP wildcard WINMISC
@@ -325,7 +325,7 @@ puttytel : [X] GTKTERM uxmisc misc ldisc settings uxsel U_BE_NOSSH
 	 + nogss GTKMAIN
 
 plink    : [U] uxplink uxcons NONSSH UXSSH U_BE_ALL logging UXMISC uxsignal
-         + ux_x11 noterm uxnogtk cmdline
+         + ux_x11 noterm uxnogtk sessprep cmdline
 
 PUTTYGEN_UNIX = sshrsag sshdssg sshprime sshdes sshbn sshmd5 version
          + sshrand uxnoise sshsha misc sshrsa sshdss uxcons uxstore uxmisc
diff --git a/putty.h b/putty.h
index ba7e123a..3ca82b62 100644
--- a/putty.h
+++ b/putty.h
@@ -1176,6 +1176,11 @@ void pinger_free(Pinger);
 int conf_launchable(Conf *conf);
 char const *conf_dest(Conf *conf);
 
+/*
+ * Exports from sessprep.c.
+ */
+void prepare_session(Conf *conf);
+
 /*
  * Exports from sercfg.c.
  */
diff --git a/sessprep.c b/sessprep.c
new file mode 100644
index 00000000..15d830d9
--- /dev/null
+++ b/sessprep.c
@@ -0,0 +1,84 @@
+/*
+ * sessprep.c: centralise some preprocessing done on Conf objects
+ * before launching them.
+ */
+
+#include "putty.h"
+
+void prepare_session(Conf *conf)
+{
+    char *hostbuf = dupstr(conf_get_str(conf, CONF_host));
+    char *host = hostbuf;
+    char *p, *q;
+
+    /*
+     * Trim leading whitespace from the hostname.
+     */
+    host += strspn(host, " \t");
+
+    /*
+     * See if host is of the form user@host, and separate out the
+     * username if so.
+     */
+    if (host[0] != '\0') {
+        /*
+         * Use strrchr, in case the _username_ in turn is of the form
+         * user@host, which has been known.
+         */
+        char *atsign = strrchr(host, '@');
+        if (atsign) {
+            *atsign = '\0';
+            conf_set_str(conf, CONF_username, host);
+            host = atsign + 1;
+        }
+    }
+
+    /*
+     * Trim a colon suffix off the hostname if it's there, and discard
+     * the text after it.
+     *
+     * The exact reason why we _ignore_ this text, rather than
+     * treating it as a port number, is unfortunately lost in the
+     * mists of history: the commit which originally introduced this
+     * change on 2001-05-06 was clear on _what_ it was doing but
+     * didn't bother to explain _why_. But I [SGT, 2017-12-03] suspect
+     * it has to do with priority order: what should a saved session
+     * do if its CONF_host contains 'server.example.com:123' and its
+     * CONF_port contains 456? If CONF_port contained the _default_
+     * port number then it might be a good guess that the colon suffix
+     * on the host name was intended to override that, but you don't
+     * really want to get into making heuristic judgments on that
+     * basis.
+     *
+     * (Then again, you could just as easily make the same argument
+     * about whether a 'user@' prefix on the host name should override
+     * CONF_username, which this code _does_ do. I don't have a good
+     * answer, sadly. Both these pieces of behaviour have been around
+     * for years and it would probably cause subtle breakage in all
+     * sorts of long-forgotten scripting to go changing things around
+     * now.)
+     *
+     * In order to protect unbracketed IPv6 address literals against
+     * this treatment, we do not make this change at all if there's
+     * _more_ than one (un-IPv6-bracketed) colon.
+     */
+    p = host_strchr(host, ':');
+    if (p && p != host_strrchr(host, ':')) {
+        *p = '\0';
+    }
+
+    /*
+     * Remove any remaining whitespace.
+     */
+    p = hostbuf;
+    q = host;
+    while (*q) {
+        if (*q != ' ' && *q != '\t')
+            *p++ = *q;
+        q++;
+    }
+    *p = '\0';
+
+    conf_set_str(conf, CONF_host, hostbuf);
+    sfree(hostbuf);
+}
diff --git a/unix/uxplink.c b/unix/uxplink.c
index 11ca7d1d..ebf14458 100644
--- a/unix/uxplink.c
+++ b/unix/uxplink.c
@@ -823,63 +823,7 @@ int main(int argc, char **argv)
 	usage();
     }
 
-    /*
-     * Muck about with the hostname in various ways.
-     */
-    {
-	char *hostbuf = dupstr(conf_get_str(conf, CONF_host));
-	char *host = hostbuf;
-	char *p, *q;
-
-	/*
-	 * Trim leading whitespace.
-	 */
-	host += strspn(host, " \t");
-
-	/*
-	 * See if host is of the form user@host, and separate out
-	 * the username if so.
-	 */
-	if (host[0] != '\0') {
-	    char *atsign = strrchr(host, '@');
-	    if (atsign) {
-		*atsign = '\0';
-		conf_set_str(conf, CONF_username, host);
-		host = atsign + 1;
-	    }
-	}
-
-        /*
-         * Trim a colon suffix off the hostname if it's there. In
-         * order to protect unbracketed IPv6 address literals
-         * against this treatment, we do not do this if there's
-         * _more_ than one colon.
-         */
-        {
-            char *c = host_strchr(host, ':');
- 
-            if (c) {
-                char *d = host_strchr(c+1, ':');
-                if (!d)
-                    *c = '\0';
-            }
-        }
-
-	/*
-	 * Remove any remaining whitespace.
-	 */
-	p = hostbuf;
-	q = host;
-	while (*q) {
-	    if (*q != ' ' && *q != '\t')
-		*p++ = *q;
-	    q++;
-	}
-	*p = '\0';
-
-	conf_set_str(conf, CONF_host, hostbuf);
-	sfree(hostbuf);
-    }
+    prepare_session(conf);
 
     /*
      * Perform command-line overrides on session configuration.
diff --git a/windows/window.c b/windows/window.c
index 30cbbdc9..1bef1daa 100644
--- a/windows/window.c
+++ b/windows/window.c
@@ -606,63 +606,7 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
 	    cleanup_exit(0);
 	}
 
-	/*
-	 * Muck about with the hostname in various ways.
-	 */
-	{
-	    char *hostbuf = dupstr(conf_get_str(conf, CONF_host));
-	    char *host = hostbuf;
-	    char *p, *q;
-
-	    /*
-	     * Trim leading whitespace.
-	     */
-	    host += strspn(host, " \t");
-
-	    /*
-	     * See if host is of the form user@host, and separate
-	     * out the username if so.
-	     */
-	    if (host[0] != '\0') {
-		char *atsign = strrchr(host, '@');
-		if (atsign) {
-		    *atsign = '\0';
-		    conf_set_str(conf, CONF_username, host);
-		    host = atsign + 1;
-		}
-	    }
-
-            /*
-             * Trim a colon suffix off the hostname if it's there. In
-             * order to protect unbracketed IPv6 address literals
-             * against this treatment, we do not do this if there's
-             * _more_ than one colon.
-             */
-            {
-                char *c = host_strchr(host, ':');
- 
-                if (c) {
-                    char *d = host_strchr(c+1, ':');
-                    if (!d)
-                        *c = '\0';
-                }
-            }
-
-	    /*
-	     * Remove any remaining whitespace.
-	     */
-	    p = hostbuf;
-	    q = host;
-	    while (*q) {
-		if (*q != ' ' && *q != '\t')
-		    *p++ = *q;
-		q++;
-	    }
-	    *p = '\0';
-
-	    conf_set_str(conf, CONF_host, hostbuf);
-	    sfree(hostbuf);
-	}
+        prepare_session(conf);
     }
 
     if (!prev) {
diff --git a/windows/winplink.c b/windows/winplink.c
index 65dd7702..959db5b0 100644
--- a/windows/winplink.c
+++ b/windows/winplink.c
@@ -498,63 +498,7 @@ int main(int argc, char **argv)
 	usage();
     }
 
-    /*
-     * Muck about with the hostname in various ways.
-     */
-    {
-	char *hostbuf = dupstr(conf_get_str(conf, CONF_host));
-	char *host = hostbuf;
-	char *p, *q;
-
-	/*
-	 * Trim leading whitespace.
-	 */
-	host += strspn(host, " \t");
-
-	/*
-	 * See if host is of the form user@host, and separate out
-	 * the username if so.
-	 */
-	if (host[0] != '\0') {
-	    char *atsign = strrchr(host, '@');
-	    if (atsign) {
-		*atsign = '\0';
-		conf_set_str(conf, CONF_username, host);
-		host = atsign + 1;
-	    }
-	}
-
-        /*
-         * Trim a colon suffix off the hostname if it's there. In
-         * order to protect unbracketed IPv6 address literals
-         * against this treatment, we do not do this if there's
-         * _more_ than one colon.
-         */
-        {
-            char *c = host_strchr(host, ':');
- 
-            if (c) {
-                char *d = host_strchr(c+1, ':');
-                if (!d)
-                    *c = '\0';
-            }
-        }
-
-	/*
-	 * Remove any remaining whitespace.
-	 */
-	p = hostbuf;
-	q = host;
-	while (*q) {
-	    if (*q != ' ' && *q != '\t')
-		*p++ = *q;
-	    q++;
-	}
-	*p = '\0';
-
-	conf_set_str(conf, CONF_host, hostbuf);
-	sfree(hostbuf);
-    }
+    prepare_session(conf);
 
     /*
      * Perform command-line overrides on session configuration.