dhcpd doesn't log DHCPv6 encapsulated packets from relays. The solicit, request, renew, etc. data in encapsulated packets is valuable network management information, so we've patched server/dhcpv6.c to log the contents of the encapsulated packets from relays. The changes are fairly minor, just adding a call to log_packet_in() from dhcpv6_relay_forw(), and fleshing out log_packet_in() to log the encapsulated packets in a manner similar to the current logging of outgoing packets. Here are before and after log examples:

Before:

 # relayed solicit (not logged), advert, relayed request (not logged), reply
 Relay-forward message from 2607:f6d0:0:a228::1 port 547, link address 2607:f6d0:0:a228::1, peer address fe80::c7c:c368:33bd:2af5
 Picking pool address 2607:f6d0:0:a228:bad:c0ff:ee:69
 Advertise NA: address 2607:f6d0:0:a228:bad:c0ff:ee:69 to client with duid 00:01:00:01:20:b6:0d:d6:d4:61:9d:0f:45:e2 iaid = 0 valid for 900 seconds
 Sending Relay-reply to 2607:f6d0:0:a228::1 port 547
 Relay-forward message from 2607:f6d0:0:a228::1 port 547, link address 2607:f6d0:0:a228::1, peer address fe80::c7c:c368:33bd:2af5
 Reply NA: address 2607:f6d0:0:a228:bad:c0ff:ee:69 to client with duid 00:01:00:01:20:b6:0d:d6:d4:61:9d:0f:45:e2 iaid = 0 valid for 900 seconds
 Sending Relay-reply to 2607:f6d0:0:a228::1 port 547

 # relayed renew (not logged), reply
 Relay-forward message from 2607:f6d0:0:a13f::a port 547, link address 2607:f6d0:0:a13f::a, peer address fe80::5100:7e7e:5d08:4358
 Reply NA: address 2607:f6d0:0:a13f:bad:c0ff:ee:6e to client with duid 00:01:00:01:1f:37:41:82:f8:b1:56:b6:9c:a9 iaid = 1 valid for 900 seconds
 Sending Relay-reply to 2607:f6d0:0:a13f::a port 547

After:

 # relayed solicit, advert, relayed request, reply
 Received Relay-forward message from 2607:f6d0:0:a13f::a port 547, link address 2607:f6d0:0:a13f::a, peer address fe80::10f0:d2f0:fa9d:7737
 Solicit NA: from client with duid 00:01:00:01:21:1d:e4:63:78:7b:8a:ab:16:09 iaid = 0
 Picking pool address 2607:f6d0:0:a13f:bad:c0ff:ee:6c
 Advertise NA: address 2607:f6d0:0:a13f:bad:c0ff:ee:6c to client with duid 00:01:00:01:21:1d:e4:63:78:7b:8a:ab:16:09 iaid = 0 valid for 900 seconds
 Sending Relay-reply to 2607:f6d0:0:a228::1 port 547
 Received Relay-forward message from 2607:f6d0:0:a13f::a port 547, link address 2607:f6d0:0:a13f::a, peer address fe80::10f0:d2f0:fa9d:7737
 Request NA: address 2607:f6d0:0:a13f:bad:c0ff:ee:6c from client with duid 00:01:00:01:21:1d:e4:63:78:7b:8a:ab:16:09 iaid = 0
 Reply NA: address 2607:f6d0:0:a13f:bad:c0ff:ee:6c to client with duid 00:01:00:01:21:1d:e4:63:78:7b:8a:ab:16:09 iaid = 0 valid for 900 seconds
 Relay-reply to 2607:f6d0:0:a13f::a port 547

 # relayed renew, reply
 Received Relay-forward message from 2607:f6d0:0:a13f::a port 547, link address 2607:f6d0:0:a13f::a, peer address fe80::5100:7e7e:5d08:4358
 Renew NA: address 2607:f6d0:0:a13f:bad:c0ff:ee:62 from client with duid 00:01:00:01:1f:37:41:82:f8:b1:56:b6:9c:a9 iaid = 1
 Reply NA: address 2607:f6d0:0:a13f:bad:c0ff:ee:62 to client with duid 00:01:00:01:1f:37:41:82:f8:b1:56:b6:9c:a9 iaid = 1 valid for 900 seconds
 Sending Relay-reply to 2607:f6d0:0:a13f::a port 547

As you can see, the incoming encapsulated packets are now logged in the same style as the outgoing encapsulated packets. Those messages are bracketed by "Received Relay-forward" and "Sending Relay-reply." Hopefully you'll agree that's more informative and consider accepting this patch or implementing something similar.

The attached patch is relative to 4.3.6. I'm attaching the patched dhcpv6.c as well, in case patch is passé. Virtually all the changes are in log_packet_in() and I'm pretty sure it'll log a superset of what it logged before (i.e., logging should be identical to before with some new logging, and with "Received " prepended to the "Relay-forward" log message to make it consistent with the "Sending Relay-replay" message).