近日升级了 openwrt 的版本 23.05.4 并重新设置，发现宽带 pppoe 拨号频繁地断线重连，发生时间从几十秒到几分钟不等，已经影响到网络的正常使用，一些物联网设备也因此出错重启。

查看 openwrt 的日志发现，可能与 pppd 的这句 No response to 5 echo-requests 有关。

09:30:25 2024 daemon.info pppd[18184]: No response to 5 echo-requests
09:30:25 2024 daemon.notice pppd[18184]: Serial link appears to be disconnected.
09:30:25 2024 daemon.info pppd[18184]: Connect time 10.8 minutes.
09:30:25 2024 daemon.info pppd[18184]: Sent 598878 bytes, received 128821 bytes.
09:30:25 2024 daemon.notice ttyd[3380]: [ 09:30:25:3453] N: rops_handle_POLLIN_netlink: DELADDR
09:30:25 2024 daemon.notice ttyd[3380]: [ 09:30:25:3529] N: rops_handle_POLLIN_netlink: DELADDR
09:30:25 2024 daemon.err odhcp6c[18588]: Failed to send RS (Network unreachable)
09:30:25 2024 daemon.err odhcp6c[18588]: Failed to send RELEASE message to ff02::1:2 (Network unreachable)
09:30:25 2024 daemon.notice ttyd[3380]: [ 09:30:25:3619] N: rops_handle_POLLIN_netlink: DELADDR
09:30:25 2024 daemon.notice netifd: Interface 'wan6' is disabled
09:30:25 2024 daemon.notice netifd: Network device 'pppoe-wan' link is down
09:30:25 2024 daemon.notice netifd: Network alias 'pppoe-wan' link is down
09:30:25 2024 daemon.notice netifd: Interface 'wan6' has link connectivity loss
09:30:25 2024 daemon.notice netifd: Interface 'wan' has lost the connection
09:30:25 2024 daemon.notice netifd: wan6 (18588): Command failed: ubus call network.interface notify_proto { "action": 0, "link-up": false, "keep": false, "interface": "wan6" } (Permission denied)
09:30:26 2024 user.notice mwan3-hotplug[23451]: mwan3 hotplug on wan6 not called because interface disabled
09:30:26 2024 daemon.err odhcp6c[18588]: Failed to send SOLICIT message to ff02::1:2 (Network unreachable)
09:30:26 2024 daemon.notice netifd: Interface 'wan6' is now down
09:30:27 2024 user.notice mwan3-hotplug[24159]: mwan3 hotplug on wan not called because interface disabled
09:30:31 2024 daemon.notice pppd[18184]: Connection terminated.
09:30:31 2024 daemon.info pppd[18184]: Connect time 10.8 minutes.
09:30:31 2024 daemon.info pppd[18184]: Sent 598878 bytes, received 128821 bytes.
09:30:31 2024 daemon.notice pppd[18184]: Modem hangup
09:30:31 2024 daemon.info pppd[18184]: Exit.
09:30:31 2024 daemon.notice netifd: Interface 'wan' is now down
09:30:31 2024 daemon.notice netifd: Interface 'wan' is setting up now
09:30:31 2024 daemon.info pppd[24717]: Plugin pppoe.so loaded.
09:30:31 2024 daemon.info pppd[24717]: PPPoE plugin from pppd 2.4.9
09:30:31 2024 daemon.notice pppd[24717]: pppd 2.4.9 started by root, uid 0
09:30:37 2024 daemon.info pppd[24717]: PPP session is 46903
09:30:37 2024 daemon.warn pppd[24717]: Connected to CC:**:**:**:**:00 via interface wan.41
09:30:37 2024 kern.info kernel: [22535.653978] pppoe-wan: renamed from ppp0
09:30:37 2024 daemon.info pppd[24717]: Renamed interface ppp0 to pppoe-wan
09:30:37 2024 daemon.info pppd[24717]: Using interface pppoe-wan
09:30:37 2024 daemon.notice pppd[24717]: Connect: pppoe-wan <--> wan.41
09:30:37 2024 daemon.info pppd[24717]: CHAP authentication succeeded: Authentication Successful,Welcome!
09:30:37 2024 daemon.notice pppd[24717]: CHAP authentication succeeded
09:30:37 2024 daemon.notice pppd[24717]: peer from calling number CC:**:**:**:**:00 authorized
09:30:37 2024 daemon.notice pppd[24717]: local  LL address fe80::****:f63a
09:30:37 2024 daemon.notice pppd[24717]: remote LL address fe80::****:3100
09:30:37 2024 daemon.notice ttyd[3380]: [ 09:30:37:5595] N: rops_handle_POLLIN_netlink: DELADDR
09:30:37 2024 daemon.notice pppd[24717]: local  IP address 172.**.**.**
09:30:37 2024 daemon.notice pppd[24717]: remote IP address 172.**.**.**
09:30:37 2024 daemon.notice pppd[24717]: primary   DNS address 211.**.**.**
09:30:37 2024 daemon.notice pppd[24717]: secondary DNS address 120.**.**.**
09:30:37 2024 daemon.notice netifd: Network device 'pppoe-wan' link is up
09:30:37 2024 daemon.notice netifd: Interface 'wan6' is enabled
09:30:37 2024 daemon.notice netifd: Network alias 'pppoe-wan' link is up
09:30:37 2024 daemon.notice netifd: Interface 'wan6' has link connectivity
09:30:37 2024 daemon.notice netifd: Interface 'wan6' is setting up now
09:30:37 2024 daemon.notice netifd: Interface 'wan' is now up
09:30:38 2024 user.notice mwan3-hotplug[25420]: mwan3 hotplug on wan not called because interface disabled
09:30:39 2024 user.notice firewall: Reloading firewall due to ifup of wan (pppoe-wan)
09:30:41 2024 daemon.notice netifd: Interface 'wan6' is now up
09:30:42 2024 user.notice mwan3-hotplug[26292]: mwan3 hotplug on wan6 not called because interface disabled
09:30:42 2024 user.notice firewall: Reloading firewall due to ifup of wan6 (pppoe-wan)

找到 wan 接口高级设置中有两个可能相关的选项 LCP echo failure threshold, LCP echo interval。 结合页面上描述和网络资料，这个功能会向上级设备发送所谓 LCP echo request，上级设备回应则视为正常，上级设备不回应则视为失败，失败次数达到设定的阈值时则触发重连。例如这篇文章 https://www.chiphell.com/thread-1352411-1-1.html 就提到华硕路由器默认开启这个功能，但国内某些运营商不会对 LCP echo request 进行回应导致路由器掉线重拨。

但在 openwrt 中 LCP echo failure threshold 的默认值为 0，根据页面上的描述，即忽略错误、不开启这个功能。我在配置路由器时也没有修改此项，按理说不应该会出现这个问题。当我将 LCP echo failure threshold 修改为 0，LCP echo interval 修改为 3600。luci 没有显示可变更的内容。我只能将 LCP echo failure threshold 修改为 10，LCP echo interval 修改为 3600。然后前面说的几分钟就断线重连的这种状况就没有再出现了，wan 接口稳定运行。

出现问题的具体原因我没有兴趣细究，我只是普通用户，能用就行。