Add `ip neigh show` by Gilnaa · Pull Request #21 · rust-netlink/iproute-rs

Gilnaa · 2026-05-23T15:00:37Z

Draft PR, still missing some things, but wanted to get your feedback if possible.

Write tests
- A test for device selection (dev, vrf, nomaster)
- A test for unused
- A test for proxy
- A test for statistics (-s)
Statistics use wrong units. (Need to divide by USER_HZ, which is returned by sysconf(_SC_CLK_TCK) in C)

Depends on:

impl FromStr for NeighbourState netlink-packet-route#260

gemini-code-assist

Code Review

This pull request implements the ip neighbour command, allowing users to view and filter neighbour/ARP table entries. The implementation includes support for various filters like device, VRF, and neighbour state, as well as a global statistics flag. Feedback focuses on ensuring compatibility with stable Rust by avoiding unstable features like let_chains, correcting the command's description, and improving robustness through better address parsing and logging instead of using println! or returning hard errors for malformed netlink messages.

gemini-code-assist · 2026-05-23T15:07:10Z

+            neigh.probes = None;
+        }
+
+        neighbour_info.push(neigh);


The use of let_chains (e.g., if let Some(...) && condition) is an unstable feature in Rust. Unless this project explicitly requires a nightly compiler and has enabled the feature, this will cause a compilation error on stable Rust. It is safer to use a nested if or match.

Suggested change

neigh.probes = None;

}

neighbour_info.push(neigh);

if let Some(address_filter) = address_filter {

if neigh.dst != address_filter {

continue;

}

}

gemini-code-assist · 2026-05-23T15:07:10Z

+    let (address, radix) = if let Some(address) = address.strip_prefix("0x") {
+        (address, 16)
+    } else if let Some(address) = address.strip_prefix("0b") {
+        (address, 2)


The current address parsing logic does not handle octal representation (leading zero). While hex (0x) and binary (0b) are supported, a string like 0123 will be parsed as decimal 123 instead of octal 83. It is recommended to check for a leading zero and use radix 8.

} else if address.starts_with('0') && address.len() > 1 { (address, 8) } else {

cathay4t · 2026-05-24T01:05:02Z

+            "failed" => NudFilter::Specified(NeighbourState::Failed),
+            "noarp" => NudFilter::Specified(NeighbourState::Noarp),
+            "permanent" => NudFilter::Specified(NeighbourState::Permanent),
+            "none" => NudFilter::Specified(NeighbourState::None),


Let's impl Display and From<&str> for NeightbourType in netlink-packet-route, so others can take it also.

rust-netlink/netlink-packet-route#260

cathay4t · 2026-05-24T01:10:41Z

+        }
+    }
+
+    while let Some(opt) = opts.next() {


We should use clap options here instead accepting arbitrary option here.

The iproute2 code indicate these filters can be done by set proper value in request netlink message, so kernel can do the filter for us by not returning unmatched entries.

It might looks like:

if matches.contains_id("proxy") { req.ndm.ndm_flags |= NTF_PROXY; }

Am I allowed to read their code?
I assumed there's a licensing issue and so only prodded iproute2 with strace,
but maybe I'm just making it too hard on myself.

I already set NTF_PROXY by calling NeighbourGetRequest::proxies
This is the only flag the kernel allows, and it also does not support NUD-filtering.
See https://github.com/torvalds/linux/blob/4cbfe4502e3d4bda48eb4b83dfad8d7da3b22e90/net/core/neighbour.c#L2903

To summerise, it allows for NTF_PROXY, NDA_IFINDEX, and NDA_MASTER. (and this PR uses all 3).

When you do ask for a specific nud, iproute2 read everything and discards:

levi-win11# ip neigh 20.0.0.1 dev foo lladdr aa:aa:aa:aa:aa:aa PERMANENT levi-win11# ip neigh show nud reachable levi-win11# levi-win11# strace -e sendto,recvmsg ip neigh show nud reachable sendto(3, [[{nlmsg_len=28, nlmsg_type=RTM_GETNEIGH, nlmsg_flags=NLM_F_REQUEST|NLM_F_DUMP, nlmsg_seq=1779601713, nlmsg_pid=0}, {ndm_family=AF_UNSPEC, ndm_ifindex=0, ndm_state=NUD_NONE, ndm_flags=0, ndm_type=RTN_UNSPEC}], {nlmsg_len=0, nlmsg_type=0 /* NLMSG_??? */, nlmsg_flags=0, nlmsg_seq=0, nlmsg_pid=0}], 284, 0, NULL, 0) = 284 recvmsg(3, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base=NULL, iov_len=0}], msg_iovlen=1, msg_controllen=0, msg_flags=MSG_TRUNC}, MSG_PEEK|MSG_TRUNC) = 96 recvmsg(3, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base=[[{nlmsg_len=76, nlmsg_type=RTM_NEWNEIGH, nlmsg_flags=NLM_F_MULTI, nlmsg_seq=1779601713, nlmsg_pid=2172648}, {ndm_family=AF_INET, ndm_ifindex=if_nametoindex("foo"), ndm_state=NUD_PERMANENT, ndm_flags=0, ndm_type=RTN_BROADCAST}, [[{nla_len=8, nla_type=NDA_DST}, inet_addr("20.0.0.1")], [{nla_len=10, nla_type=NDA_LLADDR}, aa:aa:aa:aa:aa:aa], [{nla_len=8, nla_type=NDA_PROBES}, 0], [{nla_len=20, nla_type=NDA_CACHEINFO}, {ndm_confirmed=5544120, ndm_used=5544120, ndm_updated=5544120, ndm_refcnt=0}]]], [{nlmsg_len=20, nlmsg_type=NLMSG_DONE, nlmsg_flags=NLM_F_MULTI, nlmsg_seq=1779601713, nlmsg_pid=2172648}, 0]], iov_len=32768}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 96

I agree that it would make sense for the kernel to filter this; I assume it never came up as a performance edge case.

I would love to use clap, but my understanding is that it does not allow for unhyphenated-values the same way iproute2 does.

In the meantime I'll extract the argument parsing and make it a bit prettier.

My bad. clap does not support this kind of arg parsing.

For license issue, I am not lawyer. But knowledge is not licensed, we read iproute2 to understand how kernel handle these filters, then use that knowledge to produce Rust code which is completely different from iproute2 C code.

I see, thank you for the clarification!

I had similar arg parsing code at https://github.com/rust-netlink/iproute-rs/blob/main/src/ip/link/add.rs#L69 . Let me know if you have better idea.

cathay4t · 2026-05-24T01:11:39Z

+    Ok(Some(cli_addr_info))
+}
+
+pub(crate) async fn handle_show(


This function contains more 100+ lines of code which is normally a sign of bad design. Try to split it into small functions.

cathay4t · 2026-05-24T01:13:02Z

USER_HZ can be retrieved by nix::unistd::sysconf(nix::unistd::SysconfVar::CLK_TCK)

Gilnaa force-pushed the neigh-show branch 2 times, most recently from 5ec9ddb to 911c737 Compare May 23, 2026 15:04

gemini-code-assist Bot reviewed May 23, 2026

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Gilnaa force-pushed the neigh-show branch from 911c737 to d9dd585 Compare May 23, 2026 15:28

cathay4t requested changes May 24, 2026

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Gilnaa force-pushed the neigh-show branch from d9dd585 to eb555cd Compare May 24, 2026 07:26

Gilnaa commented May 24, 2026

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Comment thread Cargo.toml Outdated

Gilnaa added 2 commits May 30, 2026 10:26

Add ip neigh show

1427ba5

Add neighbour tests

7266d26

Gilnaa force-pushed the neigh-show branch from eb555cd to 7266d26 Compare May 30, 2026 08:26

-            neigh.probes = None;
-        }
-        neighbour_info.push(neigh);
+        if let Some(address_filter) = address_filter {
+            if neigh.dst != address_filter {
+                continue;
+            }
+        }

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cathay4t commented May 24, 2026

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Gilnaa commented May 23, 2026 •

edited

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