[FS#1462] odhcp6c fails to renew and rebind addresses

[LEDE Bugs]

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FS#1462 - odhcp6c fails to renew and rebind addresses
User who did this - clefru (clefru)

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Thanks Hans for the fast feedback. I attached the pcap trace you requested. Let me note however:

1. I was reading outdated sources on github. Reading the odhcp6c sources from git.openwrt.org shows that it very well can use different address than the broadcast address for the RENEW message.

2. My upstream DHCPv6 sets the "unicast" option[1] in the reply, which allows for odhcp6c to respond directly to this unicast address. However, it looks my ISP does not respond to DHCPv6 on that public address. I think that my ISP should set the link-local addr here, that it used to reply in the beginning of the conversation, and not a public routable internet address. So I think that it's my ISPs fault not to respond to RENEW.

3. What's open here for investigation is why odhcp6c keeps retrying the REBIND messages. In my log I can see:

Sun Apr  1 11:08:10 2018 daemon.notice odhcp6c[28904]: Send REBIND message (elapsed 0ms, rc 0)
Sun Apr  1 11:08:10 2018 daemon.notice odhcp6c[28904]: Got a valid reply after 312ms
Sun Apr  1 11:08:20 2018 daemon.notice odhcp6c[28904]: Send REBIND message (elapsed 10240ms, rc 1)
Sun Apr  1 11:08:22 2018 daemon.notice odhcp6c[28904]: Got a valid reply after 11590ms
Sun Apr  1 11:08:41 2018 daemon.notice odhcp6c[28904]: Send REBIND message (elapsed 30720ms, rc 2)
Sun Apr  1 11:08:44 2018 daemon.notice odhcp6c[28904]: Got a valid reply after 33687ms
Sun Apr  1 11:09:20 2018 daemon.notice odhcp6c[28904]: Send REBIND message (elapsed 69760ms, rc 3)
Sun Apr  1 11:09:22 2018 daemon.notice odhcp6c[28904]: Got a valid reply after 71977ms
Sun Apr  1 11:10:39 2018 daemon.notice odhcp6c[28904]: Send REBIND message (elapsed 148480ms, rc 4)

The DHCPv6 replies on the wire status on the wire indicate status code = 0 (success). Reading more odhcp6c source code didn't make me any wiser, as there are just too many moving parts to make an informed guess where this goes wrong. I'll probably have to compile my own odhcp6c with a bunch of instrumentation thrown in.

Unless you see something obviously wrong, feel free to ignore this ticket as the root cause is my ISP's unresponsive unicast address.

[1] https://tools.ietf.org/html/rfc3315#section-22.12
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https://bugs.openwrt.org/index.php?do=details&task_id=1462#comment4543