我的服务器运行异常，希望您能帮助我找出问题所在。以下是事实：
- 该服务器有 2 个用户：
root和myuser - 服务器仅接受使用公钥的 ssh 连接
- 如果我从 Putty（在 Windows 上）连接到服务器，
myuser连接需要几秒钟，并且 cmd 提示符会立即出现 - 如果我从 Cygwin（在 Windows 上）连接到服务器，
myuser连接需要几秒钟，但 cmd 提示符出现需要大约 40 秒。从 PHP 脚本连接或从ssh同一网络中的另一个 Linux 服务器连接时也会发生同样的情况 - 我在 Cygwing 上使用
OpenSSH_7.2p1, OpenSSL 1.0.2gPHPphpseclib库，在其他 Linux 服务器上使用OpenSSH_5.3p1, OpenSSL 1.0.1e-fips - 尝试相同的程序会
root产生相同的结果
因此，在两种情况下（sshd 日志写入），ssh 连接都正确建立，session opened for user myuser但在一种情况下，cmd 提示符立即出现，而在另一种情况下，它花费的时间太长（60 秒）。
有什么想法可能导致问题吗？如果您需要有关日志消息或配置文件的更多信息，请在评论中告诉我。
更新 1
运行输出以下内容：strace ssh [email protected]
**lot of stuff**
18:20:10 read(3, "Dq\340\342\5\16\7+\367\366r\315\241\272\35\23\2225\344\366\332D\36\217\254$
18:20:10 ioctl(0, TIOCGWINSZ, {ws_row=60, ws_col=237, ws_xpixel=0, ws_ypixel=0}) = 0
18:20:10 getsockopt(3, SOL_TCP, TCP_NODELAY, [0], [4]) = 0
18:20:10 setsockopt(3, SOL_TCP, TCP_NODELAY, [1], 4) = 0
18:20:10 getsockname(3, {sa_family=AF_INET, sin_port=htons(40713), sin_addr=inet_addr("190.9$
18:20:10 setsockopt(3, SOL_IP, IP_TOS, [16], 4) = 0
18:20:10 select(7, [3], [3], NULL, NULL) = 1 (out [3])
18:20:10 write(3, "?32\275\177\356\212x\177\334>\352K\226\225)\225\3507\361Zu,I\7!\273Hv1\31$
18:20:10 select(7, [3], [], NULL, NULL) = 1 (in [3])
18:20:10 read(3, "\37\347\207\273\236\244\22\314\222\305\23\270\206zl\270X\225i0\30\207\343f$
18:20:10 select(7, [3 4], [], NULL, NULL) = 1 (in [3])
18:20:10 read(3, "J2\345\334\336\350\364\225<D\t\305Y\23\366\341!_\351\310\357C\252I\322|\3\$
18:20:10 select(7, [3 4], [5], NULL, NULL) = 1 (out [5])
18:20:10 write(5, "Last login: Fri Mar 4 18:19:33 "..., 58) = 58
18:20:10 select(7, [3 4], [], NULL, NULL) = 1 (in [3])
# Here is where it waits 60 seconds
18:21:10 read(3, ":\233\225\377P\216.\2u&\275\372\257\203\261\236\322aN\263\313\203\315\373\$
18:21:10 select(7, [3 4], [5], NULL, NULL) = 1 (out [5])
18:21:10 write(5, "\33[?1034h[securitybox@wordpress10"..., 41) = 41
18:21:10 select(7, [3 4], [], NULL, NULL) = 1 (in [4])
18:21:14 read(4, "e", 16384) = 1
18:21:14 select(7, [3 4], [3], NULL, NULL) = 1 (out [3])
18:21:14 write(3, ":\24\266\315J\202w+\213i\320\314\251\vc\17\230\372\344\237\341\341o\256\3$
18:21:14 select(7, [3 4], [], NULL, NULL) = 1 (in [3])
18:21:14 read(3, "eS\362U<\346\345f\245\27\21\323<\30J\20\261%1hx&\7k\323 \240\n^-\1P"..., 8$
18:21:14 select(7, [3 4], [5], NULL, NULL) = 1 (out [5])
18:21:14 write(5, "e", 1) = 1
18:21:14 select(7, [3 4], [], NULL, NULL) = 1 (in [4])
18:21:14 read(4, "x", 16384) = 1
**more stuff**
更新2
我知道，在上次登录：，当它挂起等待时，您可以按Ctrl + C，它会立即显示命令提示符。因此，看起来启动时有一个进程耗时太长了。
更新 3
我增加了服务器端的调试级别，以下是挂起和等待之前的最新输出：
Mar 4 17:15:31 thisserver.hostname sshd[2279]: debug3: PAM: opening session
Mar 4 17:15:32 thisserver.hostname sshd[2279]: pam_unix(sshd:session): session opened for user securitybox by (uid=0)
Mar 4 17:15:32 thisserver.hostname sshd[2279]: User child is on pid 2301
Mar 4 17:15:32 thisserver.hostname sshd[2279]: debug3: mm_request_receive entering
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: PAM: establishing credentials
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: permanently_set_uid: 10037/10037
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug2: set_newkeys: mode 0
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug2: set_newkeys: mode 1
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: Entering interactive session for SSH2.
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug2: fd 4 setting O_NONBLOCK
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug2: fd 6 setting O_NONBLOCK
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: server_init_dispatch_20
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: server_input_channel_open: ctype session rchan 0 win 1048576 max 16384
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: input_session_request
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: channel 0: new [server-session]
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug2: session_new: allocate (allocated 0 max 10)
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug3: session_unused: session id 0 unused
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: session_new: session 0
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: session_open: channel 0
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: session_open: session 0: link with channel 0
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: server_input_channel_open: confirm session
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: server_input_global_request: rtype [email protected] want_reply 0
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug3: Wrote 40 bytes for a total of 2701
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: server_input_channel_req: channel 0 request pty-req reply 1
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: session_by_channel: session 0 channel 0
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: session_input_channel_req: session 0 req pty-req
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: Allocating pty.
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug3: mm_request_send entering: type 26
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug3: mm_pty_allocate: waiting for MONITOR_ANS_PTY
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug3: mm_request_receive_expect entering: type 27
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug3: mm_request_receive entering
Mar 4 17:15:32 thisserver.hostname sshd[2279]: debug3: monitor_read: checking request 26
Mar 4 17:15:32 thisserver.hostname sshd[2279]: debug3: mm_answer_pty entering
Mar 4 17:15:32 thisserver.hostname sshd[2279]: debug2: session_new: allocate (allocated 0 max 10)
Mar 4 17:15:32 thisserver.hostname sshd[2279]: debug3: session_unused: session id 0 unused
Mar 4 17:15:32 thisserver.hostname sshd[2279]: debug1: session_new: session 0
Mar 4 17:15:32 thisserver.hostname sshd[2279]: debug3: ssh_selinux_setup_pty: setting TTY context on /dev/pts/6
Mar 4 17:15:32 thisserver.hostname sshd[2279]: debug3: ssh_selinux_setup_pty: done
Mar 4 17:15:32 thisserver.hostname sshd[2279]: debug3: mm_request_send entering: type 27
Mar 4 17:15:32 thisserver.hostname sshd[2279]: debug3: mm_answer_pty: tty /dev/pts/6 ptyfd 4
Mar 4 17:15:32 thisserver.hostname sshd[2279]: debug3: mm_request_receive entering
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: session_pty_req: session 0 alloc /dev/pts/6
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: server_input_channel_req: channel 0 request shell reply 1
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: session_by_channel: session 0 channel 0
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug1: session_input_channel_req: session 0 req shell
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug2: fd 3 setting TCP_NODELAY
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug2: channel 0: rfd 9 isatty
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug2: fd 9 setting O_NONBLOCK
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug3: fd 7 is O_NONBLOCK
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug3: Wrote 88 bytes for a total of 2789
Mar 4 17:15:32 thisserver.hostname sshd[2302]: debug1: Setting controlling tty using TIOCSCTTY.
Mar 4 17:15:32 thisserver.hostname sshd[2301]: debug3: Wrote 88 bytes for a total of 2877
Mar 4 17:15:32 thisserver.hostname sshd[2302]: debug3: Copy environment: SELINUX_ROLE_REQUESTED=
Mar 4 17:15:32 thisserver.hostname sshd[2302]: debug3: Copy environment: SELINUX_LEVEL_REQUESTED=
Mar 4 17:15:32 thisserver.hostname sshd[2302]: debug3: Copy environment: SELINUX_USE_CURRENT_RANGE=
Mar 4 17:15:32 thisserver.hostname sshd[2302]: debug3: channel 0: close_fds r -1 w -1 e -1
然后过了一会儿，在出现提示符的同时，打印：
Mar 4 17:16:32 thisserver.hostname sshd[2301]: debug3: Wrote 72 bytes for a total of 2949
您可以看到它耗时 60 秒。总是正好需要60秒。
这是使用 Putty 连接时同一日志的输出（请记住使用 Putty 会立即出现提示）。
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug3: PAM: opening session
Mar 4 17:44:45 thisserver.hostname sshd[5505]: pam_unix(sshd:session): session opened for user root by (uid=0)
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug2: set_newkeys: mode 0
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug2: cipher_init: set keylen (16 -> 32)
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug2: set_newkeys: mode 1
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug2: cipher_init: set keylen (16 -> 32)
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: Entering interactive session for SSH2.
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug2: fd 4 setting O_NONBLOCK
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug2: fd 5 setting O_NONBLOCK
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: server_init_dispatch_20
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: server_input_channel_open: ctype session rchan 256 win 16384 max 16384
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: input_session_request
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: channel 0: new [server-session]
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug2: session_new: allocate (allocated 0 max 10)
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug3: session_unused: session id 0 unused
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: session_new: session 0
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: session_open: channel 0
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: session_open: session 0: link with channel 0
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: server_input_channel_open: confirm session
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug3: Wrote 52 bytes for a total of 3185
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: server_input_channel_req: channel 0 request pty-req reply 1
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: session_by_channel: session 0 channel 0
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: session_input_channel_req: session 0 req pty-req
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: Allocating pty.
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: session_pty_req: session 0 alloc /dev/pts/7
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug3: ssh_selinux_setup_pty: setting TTY context on /dev/pts/7
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug3: ssh_selinux_setup_pty: done
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug3: Wrote 36 bytes for a total of 3221
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: server_input_channel_req: channel 0 request shell reply 1
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: session_by_channel: session 0 channel 0
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug1: session_input_channel_req: session 0 req shell
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug2: fd 3 setting TCP_NODELAY
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug2: channel 0: rfd 8 isatty
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug2: fd 8 setting O_NONBLOCK
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug3: fd 6 is O_NONBLOCK
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug3: Wrote 88 bytes for a total of 3309
Mar 4 17:44:45 thisserver.hostname sshd[5508]: debug1: Setting controlling tty using TIOCSCTTY.
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug3: Wrote 100 bytes for a total of 3409
Mar 4 17:44:45 thisserver.hostname sshd[5508]: debug1: PAM: reinitializing credentials
Mar 4 17:44:45 thisserver.hostname sshd[5508]: debug1: permanently_set_uid: 0/0
Mar 4 17:44:45 thisserver.hostname sshd[5508]: debug3: Copy environment: SELINUX_ROLE_REQUESTED=
Mar 4 17:44:45 thisserver.hostname sshd[5508]: debug3: Copy environment: SELINUX_LEVEL_REQUESTED=
Mar 4 17:44:45 thisserver.hostname sshd[5508]: debug3: Copy environment: SELINUX_USE_CURRENT_RANGE=
Mar 4 17:44:45 thisserver.hostname sshd[5508]: debug3: channel 0: close_fds r -1 w -1 e -1
Mar 4 17:44:45 thisserver.hostname sshd[5505]: debug3: Wrote 84 bytes for a total of 3493
更新 4
我发现该进程挂在系统调用上：
select(7, [3 4], [], NULL, NULL
这是执行连接时 ssh 程序的 pidstrace -p $PID的输出。$PID
答案1
当您遇到较长的登录延迟时，sshd 的反向 DNS 查找始终是备选方案。您可以尝试设置
UseDNS no
在 /etc/ssh/sshd.conf 中（然后重新启动 sshd）解决了该问题。
答案2
尝试提高服务器端而不是客户端的调试级别。
听起来可能是 PAM 堆栈中的某些东西占用了一段时间（可能是它导致查找速度变慢，而不是 sshd 本身）。ssh 服务器端的详细日志记录应该会告诉您哪个 pam 模块（如果有）导致速度变慢，并且通常有选项可以启用这些模块的详细日志记录，这可能会有所帮助。
答案3
利用 BeerSerc 的答案，在 UBUNTU LTS 14.05.5 上使用以下方法，在向我的 Linux 系统提供 SSH 用户名和密码凭据后，我实现了命令提示符的更快显示，并对以下更改进行了更改：
...become root using su or preface the next two (2) lines with the sudo command...
#cd /etc/ssh
#vi sshd_config
...navigate to end of file, add following text...
# GL 2017-02-25 Turn off DNS lookups to speed up the appearance
# of the command prompt after SSH username and
# password have been supplied
UseDNS no
...save file and exit editor...
#/etc/init.d/ssh restart