Pure OCaml ARP table lookup - reads /proc/net/arp on Linux and arp -a on macOS/BSD
Squashed 'ocaml-arp/' content from commit 0343a7e git-subtree-split: 0343a7ebaa7145be267107f5d35840025ec8899f
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.ocamlformat
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.ocamlformat
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version = 0.28.1
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arp.opam
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arp.opam
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# This file is generated by dune, edit dune-project instead
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opam-version: "2.0"
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synopsis: "Pure OCaml ARP table lookup"
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description:
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"Read the system ARP cache without external dependencies. Uses /proc/net/arp on Linux and arp -a on macOS/BSD."
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depends: [
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"ocaml" {>= "4.14.0"}
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"dune" {>= "3.0" & >= "3.0"}
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"alcotest" {with-test}
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"odoc" {with-doc}
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]
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build: [
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["dune" "subst"] {dev}
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[
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"dune"
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"build"
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"-p"
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name
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"-j"
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jobs
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"@install"
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"@runtest" {with-test}
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"@doc" {with-doc}
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]
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]
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dune-project
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dune-project
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(lang dune 3.0)
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(name arp)
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(generate_opam_files true)
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(package
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(name arp)
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(synopsis "Pure OCaml ARP table lookup")
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(description
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"Read the system ARP cache without external dependencies. Uses /proc/net/arp on Linux and arp -a on macOS/BSD.")
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(depends
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(ocaml (>= 4.14.0))
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(dune (>= 3.0))
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(alcotest :with-test)))
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lib/arp.ml
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lib/arp.ml
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(*---------------------------------------------------------------------------
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Copyright (c) 2025 Thomas Gazagnaire. All rights reserved.
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SPDX-License-Identifier: MIT
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---------------------------------------------------------------------------*)
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(** ARP table lookup.
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Pure OCaml implementation:
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- Linux: reads /proc/net/arp directly
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- macOS/BSD: parses output of arp -a *)
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type entry = { ip : string; mac : string; interface : string option }
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let parse_proc_net_arp lines =
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(* /proc/net/arp format:
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IP address HW type Flags HW address Mask Device
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192.168.1.1 0x1 0x2 aa:bb:cc:dd:ee:ff * eth0 *)
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match lines with
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| [] -> []
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| _ :: data_lines ->
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List.filter_map
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(fun line ->
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let parts =
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String.split_on_char ' ' line
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|> List.filter (fun s -> String.length s > 0)
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in
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match parts with
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| ip :: _ :: flags :: mac :: _ :: iface :: _ ->
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(* flags 0x2 = complete entry, 0x0 = incomplete *)
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if flags <> "0x0" && mac <> "00:00:00:00:00:00" then
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Some { ip; mac; interface = Some iface }
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else None
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| _ -> None)
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data_lines
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let parse_arp_a_output lines =
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(* arp -a format (macOS/BSD):
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hostname (192.168.1.1) at aa:bb:cc:dd:ee:ff on en0 ifscope [ethernet] *)
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List.filter_map
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(fun line ->
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let parts =
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String.split_on_char ' ' line
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|> List.filter (fun s -> String.length s > 0)
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in
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match parts with
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| _ :: ip_paren :: _ :: mac :: _ :: iface :: _
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when String.length ip_paren > 2
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&& ip_paren.[0] = '('
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&& ip_paren.[String.length ip_paren - 1] = ')' ->
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let ip = String.sub ip_paren 1 (String.length ip_paren - 2) in
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if mac <> "(incomplete)" then Some { ip; mac; interface = Some iface }
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else None
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| _ -> None)
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lines
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let read_file path =
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try
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let ic = open_in path in
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let rec read_lines acc =
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match input_line ic with
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| line -> read_lines (line :: acc)
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| exception End_of_file ->
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close_in ic;
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List.rev acc
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in
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Some (read_lines [])
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with _ -> None
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let run_command args =
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let cmd = String.concat " " args in
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try
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let ic = Unix.open_process_in cmd in
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let rec read_lines acc =
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match input_line ic with
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| line -> read_lines (line :: acc)
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| exception End_of_file ->
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ignore (Unix.close_process_in ic);
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List.rev acc
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in
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Some (read_lines [])
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with _ -> None
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let get_table () =
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(* Try Linux /proc/net/arp first *)
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match read_file "/proc/net/arp" with
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| Some lines -> parse_proc_net_arp lines
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| None -> (
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(* Fall back to arp -a for macOS/BSD *)
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match run_command [ "arp"; "-a" ] with
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| Some lines -> parse_arp_a_output lines
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| None -> [])
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let lookup ip =
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let entries = get_table () in
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List.find_opt (fun e -> e.ip = ip) entries
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let lookup_mac ip = Option.map (fun e -> e.mac) (lookup ip)
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lib/arp.mli
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lib/arp.mli
···
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(*---------------------------------------------------------------------------
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Copyright (c) 2025 Thomas Gazagnaire. All rights reserved.
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SPDX-License-Identifier: MIT
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---------------------------------------------------------------------------*)
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(** ARP table lookup.
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Pure OCaml implementation that reads the system ARP cache:
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- Linux: reads [/proc/net/arp] directly
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- macOS/BSD: parses output of [arp -a]
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{2 Example}
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{[
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let () =
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List.iter
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(fun entry -> Printf.printf "%s -> %s\n" entry.Arp.ip entry.Arp.mac)
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(Arp.get_table ())
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]} *)
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type entry = { ip : string; mac : string; interface : string option }
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(** An ARP table entry. *)
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val get_table : unit -> entry list
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(** [get_table ()] returns all entries from the system ARP cache. *)
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val lookup : string -> entry option
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(** [lookup ip] finds the ARP entry for the given IP address. *)
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val lookup_mac : string -> string option
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(** [lookup_mac ip] returns the MAC address for the given IP, if known. *)