ip_route.rb

require "ipaddr"

class RouteTable
  def initialize(routes)
    @routes = routes
  end

  # IPAddr or nil
  # nil -> direct connected
  def next_hop_route_entry(dst_addr)
    return nil if direct_connected?(dst_addr)

    # デフォルトルートの場合
    default_routes = @routes.select { |route| route.direct_network_addr.nil? }

    # デフォルトルートのうち、メトリクス値が無いのがあった場合はそれを返す
    top_priority_route = default_routes.find { |default_routes| default_routes.metric.nil? }
    return top_priority_route unless top_priority_route.nil?

    # 最もメトリクス値が小さいデフォルトルートのエントリを返す
    default_routes.min_by do |default_route|
      default_route.metric
    end
  end

  private

  def direct_connected?(dst_addr)
    @routes.find do |route|
      next if route.direct_network_addr.nil?

      route.direct_network_addr.include?(IPAddr.new(dst_addr))
    end
  end
end

class RouteEntry
  attr_reader :direct_network_addr, :next_hop_addr, :next_hop_device, :direct_network_addr, :direct_network_device, :metric

  # ex)
  #   default via 172.31.0.1 dev eth0
  #   172.18.0.0/16 dev br-8140256d9f85 proto kernel scope link src 172.18.0.1 linkdown
  #   172.31.34.0/24 dev eth2 proto kernel scope link src 172.31.34.23
  def initialize(route_string: nil)
    @route_string = route_string
    parse_route_from_string unless @route_string.empty?
  end

  def to_s
    "direct_connected: #{@direct_connected}, next_hop_addr: #{@next_hop_addr}, " + \
    "next_hop_device: #{@next_hop_device}, direct_network_addr: #{@direct_network_addr}, " + \
    "direct_network_device: #{@direct_network_device}"
  end

  private

  def parse_route_from_string
    params = @route_string.split(" ")
    if params[0] == "default"
      @direct_connected = true
      @next_hop_addr = IPAddr.new(params[2])
      @next_hop_device = params[4]
      @direct_network_addr = nil
      @direct_network_device = nil
      @metric = params[6] # metric 値がない（最優先のルート）場合は nil
    else
      @direct_connected = false
      @next_hop_addr = nil
      @next_hop_device = nil
      @direct_network_addr = IPAddr.new(params[0])
      @direct_network_device = params[2]
      @metric = nil
    end
  end
end

output = `ip route`
routes_string = output.split("\n")
routes = routes_string.map do |route_string|
  RouteEntry.new(route_string: route_string)
end

rt = RouteTable.new(routes)
pp rt.next_hop_route_entry("8.8.8.8") #  #<RouteEntry 172.31.0.1/255.255.255.255>
pp rt.next_hop_route_entry("172.31.34.1") # nil

NextHopのアドレスを取得するスクリプト