phc2sys: uds: sendto failed: Connection refused

Solution Verified - Updated -

Environment

  • Red Hat Enterprise Linux (RHEL) 6.5
  • linuxptp-1.3-1.el6.x86_64

Issue

  • The sever is using PTP (Precision Time Protocol) to synchronize clocks.
  • There are errors when phc2sys is run:
[root@server ~]# phc2sys -s eth2 -w
Nov 28 21:17:09 server phc2sys: [20696.522] uds: sendto failed: Connection refused
Nov 28 21:17:09 server phc2sys: [20696.522] failed to send message
Nov 28 21:17:10 server phc2sys: [20697.523] Waiting for ptp4l...
Nov 28 21:17:10 server phc2sys: [20697.523] uds: sendto failed: Connection refused
Nov 28 21:17:10 server phc2sys: [20697.523] failed to send message

Resolution

  • Check if ptp4l is running and synchronizing properly.
  • If the sender and the receiver are not in the same subnet network, either disable Reverse Path Filtering
    (sysctl rp_filter) or insert a static route to make sure the UDP multicast packet's reply goes out through
    the same interface used when it came in.

Root Cause

  • phc2sys acquires the offset value by reading it from ptp4l when -w is used.

Diagnostic Steps

  • To reproduce, stop ptp4l:
[root@ ~]# service ptp4l stop
Stopping ptp4l:                                            [  OK  ]
  • Then run phc2sys:
[root@ ~]# phc2sys -w  -s eth0 -m
phc2sys[151560.212]: uds: sendto failed: Connection refused
phc2sys[151560.212]: failed to send message
phc2sys[151561.213]: Waiting for ptp4l...
phc2sys[151561.214]: uds: sendto failed: Connection refused
phc2sys[151561.214]: failed to send message
phc2sys[151562.215]: Waiting for ptp4l...
phc2sys[151562.215]: uds: sendto failed: Connection refused
phc2sys[151562.215]: failed to send message
  • Then, enable ptp4l wait to minutes and it should work:
[root@ ~]# service ptp4l start
Starting ptp4l:                                            [  OK  ]

[root@ ~]# phc2sys -w  -s eth0 -m
phc2sys[151863.523]: Waiting for ptp4l...
phc2sys[151864.523]: sys offset      3014 s0 freq      +0 delay   4742
phc2sys[151865.523]: sys offset      -298 s1 freq  +36157 delay   4772
phc2sys[151866.524]: sys offset      7785 s2 freq  +43942 delay   4762
phc2sys[151867.524]: sys offset      8454 s2 freq  +46946 delay   4694
phc2sys[151868.524]: sys offset      9000 s2 freq  +50028 delay   4712
  • Also check if /proc/sys/net/ipv4/conf/*/rp_filter is enabled
  • if yes, then check if the packet source IP address causes its reply to go through another interface different from the one used when it was received

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