securecrt克隆会话与sshd 的 MaxSessions

使用securecrt克隆会话时，原有会话连接的session数会自增。

例如

要使用ssh连接，sshd的MaxSessions必须>=1，默认是10。

如果把MaxSessions改成2，那么对同一个ssh连接，只能克隆1个，（克隆出来的ssh连接窗可以再克隆，但是对同一个连接窗只能克隆一个会话）

/var/log/secure中的报错

sshd[11318]: error: no more sessions

代码
新建的会话，消耗一个session计数，如果在当前会话中新建会话，就会继续消耗当前会话的会话数。
如果消耗的会话数大于设置的maxsessions，则报错。
openssh session.c

Session *
session_new(void)
{
        Session *s, *tmp;

        if (sessions_first_unused == -1) {
                if (sessions_nalloc >= options.max_sessions)
                        return NULL;
                debug2("%s: allocate (allocated %d max %d)",
                    __func__, sessions_nalloc, options.max_sessions);
                tmp = xrealloc(sessions, sessions_nalloc + 1,
                    sizeof(*sessions));
                if (tmp == NULL) {
                        error("%s: cannot allocate %d sessions",
                            __func__, sessions_nalloc + 1);
                        return NULL;
                }
                sessions = tmp;
                session_unused(sessions_nalloc++);
        }

        if (sessions_first_unused >= sessions_nalloc ||
            sessions_first_unused < 0) {
                fatal("%s: insane first_unused %d max %d nalloc %d",
                    __func__, sessions_first_unused, options.max_sessions,
                    sessions_nalloc);
        }

        s = &sessions[sessions_first_unused];
        if (s->used) {
                fatal("%s: session %d already used",
                    __func__, sessions_first_unused);
        }
        sessions_first_unused = s->next_unused;
        s->used = 1;
        s->next_unused = -1;
        debug("session_new: session %d", s->self);

        return s;
}


/*
 * Prepares for an interactive session.  This is called after the user has
 * been successfully authenticated.  During this message exchange, pseudo
 * terminals are allocated, X11, TCP/IP, and authentication agent forwardings
 * are requested, etc.
 */
static void
do_authenticated1(Authctxt *authctxt)
{
...
        s = session_new();
        if (s == NULL) {
                error("no more sessions");  //session_new失败，报错
                return;
        }
...

参考
http://unix.stackexchange.com/questions/26170/sshd-config-maxsessions-parameter
/usr/src/debug/openssh-6.4p1/sessions.c