SCP传输至远程主机失败：client_loop: send disconnect: Broken pipe

问题背景 #

本地Fedora 36虚拟机向同主机的CentOS 7.9虚拟机执行scp -vvv foobar user@host:命令可成功，但向配置几乎一致的远程CentOS 7.9虚拟机执行该命令时失败，报错：

client_loop: send disconnect: Broken pipe
scp: Connection closed

成功场景Verbose日志 #

...
debug1: Sending subsystem: sftp
debug2: channel 0: request subsystem confirm 1
debug3: send packet: type 98
debug2: channel_input_open_confirmation: channel 0: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel 0: rcvd adjust 2097152
debug3: receive packet: type 99
debug2: channel_input_status_confirm: type 99 id 0
debug2: subsystem request accepted on channel 0
scp: debug2: Remote version: 3
scp: debug2: Server supports extension "posix-rename@openssh.com" revision 1
scp: debug2: Server supports extension "statvfs@openssh.com" revision 2
scp: debug2: Server supports extension "fstatvfs@openssh.com" revision 2
scp: debug2: Server supports extension "hardlink@openssh.com" revision 1
scp: debug2: Server supports extension "fsync@openssh.com" revision 1
scp: debug3: Sent message fd 6 T:17 I:1
scp: debug3: Recevied stat reply T:105 I:1 F:0x000f M:40700
scp: debug3: source_sftp: copying local foobar to remote ./foobar
scp: debug3: Sent dest message SSH2_FXP_OPEN I:2 P:./foobar M:0x001a
scp: debug3: Sent message SSH2_FXP_WRITE I:4 O:0 S:2
scp: debug3: SSH2_FXP_STATUS 0
scp: debug3: In write loop, ack for 4 2 bytes at 0
scp: debug3: Sent message SSH2_FXP_CLOSE I:3
scp: debug3: SSH2_FXP_STATUS 0
debug2: channel 0: read<=0 rfd 4 len 0
debug2: channel 0: read failed
debug2: chan_shutdown_read: channel 0: (i0 o0 sock -1 wfd 4 efd 6 [write])
debug2: channel 0: input open -> drain
debug2: channel 0: ibuf empty
debug2: channel 0: send eof
debug3: send packet: type 96
debug2: channel 0: input drain -> closed
debug3: receive packet: type 98
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
debug3: receive packet: type 96
debug2: channel 0: rcvd eof
debug2: channel 0: output open -> drain
debug2: channel 0: obuf empty
debug2: chan_shutdown_write: channel 0: (i3 o1 sock -1 wfd 5 efd 6 [write])
debug2: channel 0: output drain -> closed
debug3: receive packet: type 97
debug2: channel 0: rcvd close
debug3: channel 0: will not send data after close
debug2: channel 0: almost dead
debug2: channel 0: gc: notify user
debug2: channel 0: gc: user detached
debug2: channel 0: send close
debug3: send packet: type 97
debug2: channel 0: is dead
debug2: channel 0: garbage collecting
debug1: channel 0: free: client-session, nchannels 1
debug3: channel 0: status: The following connections are open:
  #0 client-session (t4 r0 i3/0 o3/0 e[write]/0 fd -1/-1/6 sock -1 cc -1)

debug3: send packet: type 1
Transferred: sent 3428, received 3156 bytes, in 0.3 seconds
Bytes per second: sent 12806.6, received 11790.4
debug1: Exit status 0

失败场景Verbose日志 #

...
debug1: Sending subsystem: sftp
debug2: channel 0: request subsystem confirm 1
debug3: send packet: type 98
debug2: channel_input_open_confirmation: channel 0: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug3: send packet: type 1
client_loop: send disconnect: Broken pipe
scp: Connection closed

已尝试的排查措施 #

• 使用-O选项切换至原始scp协议而非sftp
• 检查目标主机sshd配置中sftp-server是否可用且配置正确
• 将客户端MTU从1500降至1000
• 添加-o ServerAliveInterval=600选项
• 移除目标主机~/.ssh/authorized_keys中的客户端密钥

进一步排查与解决建议 #

1. 检查远程网络的防火墙/安全组规则 #

远程环境中，防火墙或云安全组可能在SSH会话建立后，阻断了sftp子系统的流量。对比本地虚拟机和远程主机的网络规则，确保22端口的双向流量完全放行（sftp流量在SSH隧道内，无需额外端口）。

2. 查看远程主机的sshd日志 #

登录远程CentOS 7.9主机，查看/var/log/secure日志，定位执行失败的时间段，查找是否有sshd相关的报错（比如sftp子系统启动失败、连接被强制断开的具体原因）。

3. 验证远程主机sftp-server的权限与SELinux限制 #

执行以下命令确认sftp-server的权限：

ls -l /usr/libexec/openssh/sftp-server

同时检查SELinux是否限制了sftp的运行：

ausearch -m avc -ts recent | grep sftp

如果有SELinux报错，可临时关闭SELinux测试（setenforce 0），或添加相应的SELinux规则。

4. 调整SSH窗口大小参数 #

失败日志中显示rwindow 0，可能是窗口大小协商异常导致管道中断。尝试在scp命令中添加窗口大小参数：

scp -vvv -o WindowSize=1024 foobar user@remote-host:

或在客户端~/.ssh/config中添加全局配置：

Host remote-host
    WindowSize 1024

5. 测试纯SSH会话稳定性 #

先建立SSH会话并保持：

ssh user@remote-host

然后在另一个终端执行scp命令，观察是否还会出现断开问题。如果纯SSH会话也不稳定，说明是底层网络或SSH连接的问题，而非scp/sftp本身。

6. 用rsync替代测试 #

执行rsync命令测试文件传输：

rsync -avvv foobar user@remote-host:

如果rsync能成功，说明问题出在scp/sftp的协议交互上；如果同样失败，可定位到更底层的网络或主机配置问题。

内容的提问来源于stack exchange，提问作者Francois