Extend prefix_filter to work in relay mode for RA and NDP

[lqf96]

Background

See #385 for details.

Changes

General

• Change the naming of pio_filter_* to prefix_filter_*, since the prefix_filter will no longer be used just for RA.
• Added IN6_MATCH_PREFIX_FILTER alongside ADDR_MATCH_PREFIX_FILTER to test if a single IPv6 address is in the prefix filter.

NDP

• Apply prefix_filter to neighbor solicitation from downstream in handle_solicit.
• Apply prefix_filter to neighbor and interface address updates on upstream interface in ndp_netevent_cb.

RA

• Apply prefix_filter to filter out PIO and RIO from upstream RA in forward_router_advertisement.
• Removing PIO and RIO from RA requires editing the RA message before sending it downstream. For this purpose, a utility called iov_builder is added to odhcpd to solve the problem of applying small edits to a received message before sending it again. See below for the details and usage of this tool.
• Because of the need of editing the RA message for potential PIO and RIO exclusion, and also because how iov_builder works, forward_router_advertisement is rewritten to perform option look-up and modification together in the per-interface loop, instead of doing so separately in the current implementation. The modifications for options other than PIO and RIO are kept intact.

iov_builder

• iov_builder is a utility to apply small edits to an existing message stored in the buffer.
• It ultimately generates an iovec array pointing to chunks of the original message or chunks to added data. The iovec array can then be written or sent through vector I/O.
• Use odhcpd_iov_builder_init to initalize an iov_builder. It takes a iovec array called iov_buf as well as its iov_capacity, and pointer to the original message data. It initializes the internal editing pointer current_iov_base to data, set the current chunk length current_iov_len to 0 and inclusion include_iov to false.
• Use odhcpd_iov_builder_append to add an arbitrary iovec to the builder. This is a low-level tool. If the iov_buf is full the function returns 1.
• Use odhcpd_iov_builder_advance to add a chunk from the original message to the iov_builder. This advances the internal editing pointer by chunk_len. If include_chunk is set to false, the chunk is not added to iov_buf but is instead skipped.
• Use odhcpd_iov_builder_finalize to finalize the edition in the end. You can now use iov_buf and iov_count for any vectored I/O.

Questions

• Do we need to consider the case of service reload for NDP relay? Right now in ndp_netevent_cb both addition and deletion are guarded on the prefix_filter check. But this might not work if the user changes the prefix_filter to a more specific prefix and then reload.
• The updated forward_router_advertisement function has lower efficiency and may need to allocate for more memory due to the need of editing RA message. I thought about this but couldn't come up with a better approach here. Let me know what you think about this and if there is a better solution. It might be OK since NDP relay doesn't happen a lot unless you have many devices in your network

[lqf96]

Some information about testing: I tested this patch today on my router and can confirm that prefix_filter works for NDP, and regular RA forwarding still works. However I haven't tested RA prefix_filter because in my situation there is nothing in the RA to filter. I would appreciate if anyone can test it in their environment.