securecrt克隆会话与sshd 的 MaxSessions

背景

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

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