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Simple Network Management Protocol (SNMP)
User's Guide
Version 4.16

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# 8 Agent Implementation Example

This Implementation Example section describes how an MIB can be implemented with the SNMP Development Toolkit.

The example shown can be found in the toolkit distribution.

The agent is configured with the configuration tool, using default suggestions for everything but the manager node.

### 8.1 MIB

The MIB used in this example is called EX1-MIB. It contains two objects, a variable with a name and a table with friends.

    EX1-MIB DEFINITIONS ::= BEGIN

              IMPORTS
                      RowStatus      FROM STANDARD-MIB
                      DisplayString  FROM RFC1213-MIB
                      OBJECT-TYPE    FROM RFC-1212
                      ;

              example1       OBJECT IDENTIFIER ::= { experimental 7 }

              myName OBJECT-TYPE
                  SYNTAX  DisplayString (SIZE (0..255))
                  ACCESS  read-write
                  STATUS  mandatory
                  DESCRIPTION
                          "My own name"
                  ::= { example1 1 }

              friendsTable OBJECT-TYPE
                  SYNTAX  SEQUENCE OF FriendsEntry
                  ACCESS  not-accessible
                  STATUS  mandatory
                  DESCRIPTION
                          "A list of friends."
                  ::= { example1 4 }

              friendsEntry OBJECT-TYPE
                  SYNTAX  FriendsEntry
                  ACCESS  not-accessible
                  STATUS  mandatory
                  DESCRIPTION
                          ""
                  INDEX   { fIndex }
                  ::= { friendsTable 1 }

              FriendsEntry ::=
                  SEQUENCE {
                fIndex
                          INTEGER,
                       fName
                          DisplayString,
                       fAddress
                          DisplayString,
                       fStatus
                          RowStatus              }

              fIndex OBJECT-TYPE
                  SYNTAX  INTEGER
                  ACCESS  not-accessible
                  STATUS  mandatory
                   DESCRIPTION
                          "number of friend"
                  ::= { friendsEntry 1 }

              fName OBJECT-TYPE
                  SYNTAX  DisplayString (SIZE (0..255))
                  ACCESS  read-write
                  STATUS  mandatory
                  DESCRIPTION
                          "Name of friend"
                  ::= { friendsEntry 2 }
              fAddress OBJECT-TYPE
                  SYNTAX  DisplayString (SIZE (0..255))
                  ACCESS  read-write
                  STATUS  mandatory
                  DESCRIPTION
                          "Address of friend"
                  ::= { friendsEntry 3 }
               fStatus OBJECT-TYPE
                  SYNTAX      RowStatus
                  ACCESS      read-write
                  STATUS      mandatory
                  DESCRIPTION
                          "The status of this conceptual row."
                  ::= { friendsEntry 4 }
              fTrap TRAP-TYPE
                  ENTERPRISE  example1
                  VARIABLES   { myName, fIndex }
                  DESCRIPTION
                              "This trap is sent when something happens to
             the friend specified by fIndex."
                  ::= 1
    END

### 8.2 Default Implementation

Without writing any instrumentation functions, we can compile the MIB and use the default implementation of it. Recall that MIBs imported by "EX1-MIB.mib" must be present and compiled in the current directory ("./STANDARD-MIB.bin","./RFC1213-MIB.bin") when compiling.

    unix> erl -config ./sys
    1> application:start(snmp).
    ok
    2> snmpc:compile("EX1-MIB").
    No accessfunction for 'friendsTable', using default.
    No accessfunction for 'myName', using default.
    {ok, "EX1-MIB.bin"}
    3> snmpa:load_mibs(snmp_master_agent, ["EX1-MIB"]).
    ok

This MIB is now loaded into the agent, and a manager can ask questions. As an example of this, we start another Erlang system and the simple Erlang manager in the toolkit:

    1> snmp_test_mgr:start_link([{agent,"dront.ericsson.se"},{community,"all-rights"},
     %% making it understand symbolic names: {mibs,["EX1-MIB","STANDARD-MIB"]}]).
    {ok, <0.89.0>}
    %% a get-next request with one OID.
    2> snmp_test_mgr:gn([[1,3,6,1,3,7]]).
    ok
    * Got PDU:
    [myName,0] = []
    %% A set-request (now using symbolic names for convenience)
    3> snmp_test_mgr:s([{[myName,0], "Martin"}]).
    ok
    * Got PDU:
    [myName,0] = "Martin"
    %% Try the same get-next request again
    4> snmp_test_mgr:gn([[1,3,6,1,3,7]]).
    ok
    * Got PDU:
    [myName,0] = "Martin"
    %% ... and we got the new value.
    %% you can event do row operations. How to add a row:
    5> snmp_test_mgr:s([{[fName,0], "Martin"}, {[fAddress,0],"home"}, {[fStatus,0],4}]).
     %% createAndGo
    ok
    * Got PDU:
    [fName,0] = "Martin"
    [fAddress,0] = "home"
    [fStatus,0] = 4
    6> snmp_test_mgr:gn([[myName,0]]).
    ok
    * Got PDU:
    [fName,0] = "Martin"
    7> snmp_test_mgr:gn().
    ok
    * Got PDU:
    [fAddress,0] = "home"
    8> snmp_test_mgr:gn().
    ok
    * Got PDU:
    [fStatus,0] = 1
    9>

### 8.3 Manual Implementation

The following example shows a "manual" implementation of the EX1-MIB in Erlang. In this example, the values of the objects are stored in an Erlang server. The server has a 2-tuple as loop data, where the first element is the value of variable myName, and the second is a sorted list of rows in the table friendsTable. Each row is a 4-tuple.

Note

There are more efficient ways to create tables manually, i.e. to use the module snmp_index.

#### Code

    -module(ex1).
    -author('dummy@flop.org').
    %% External exports
    -export([start/0, my_name/1, my_name/2, friends_table/3]).
    %% Internal exports
    -export([init/0]).
    -define(status_col, 4).
    -define(active, 1).
    -define(notInService, 2).
    -define(notReady, 3).
    -define(createAndGo, 4).   % Action; written, not read
    -define(createAndWait, 5). % Action; written, not read
    -define(destroy, 6).       % Action; written, not read
    start() ->
        spawn(ex1, init, []).
    %%----------------------------------------------------------------
    %% Instrumentation function for variable myName.
    %% Returns: (get) {value, Name}
    %%          (set) noError
    %%----------------------------------------------------------------
    my_name(get) ->
        ex1_server ! {self(), get_my_name},
        Name = wait_answer(),
        {value, Name}.
    my_name(set, NewName) ->
        ex1_server ! {self(), {set_my_name, NewName}},
        noError.
    %%----------------------------------------------------------------
    %% Instrumentation function for table friendsTable.
    %%----------------------------------------------------------------
    friends_table(get, RowIndex, Cols) ->
        case get_row(RowIndex) of
       {ok, Row} ->
            get_cols(Cols, Row);
       _  ->
            {noValue, noSuchInstance}
        end;
    friends_table(get_next, RowIndex, Cols) ->
        case get_next_row(RowIndex) of
       {ok, Row} ->
            get_next_cols(Cols, Row);
       _  ->
           case get_next_row([]) of
         {ok, Row} ->
             % Get next cols from first row.
             NewCols = add_one_to_cols(Cols),
             get_next_cols(NewCols, Row);
         _  ->
            end_of_table(Cols)
            end
        end;
    %%----------------------------------------------------------------
    %% If RowStatus is set, then:
    %%    *) If set to destroy, check that row does exist
    %%    *) If set to createAndGo, check that row does not exist AND
    %%         that all columns are given values.
    %%    *) Otherwise, error (for simplicity).
    %% Otherwise, row is modified; check that row exists.
    %%----------------------------------------------------------------
    friends_table(is_set_ok, RowIndex, Cols) ->
        RowExists =
       case get_row(RowIndex) of
            {ok, _Row} -> true;
           _ -> false
       end,
        case is_row_status_col_changed(Cols) of
       {true, ?destroy} when RowExists == true ->
            {noError, 0};
       {true, ?createAndGo} when RowExists == false,
                                     length(Cols) == 3 ->
            {noError, 0};
       {true, _} ->
           {inconsistentValue, ?status_col};
       false when RowExists == true ->
            {noError, 0};
       _ ->
            [{Col, _NewVal} | _Cols] = Cols,
           {inconsistentName, Col}
          end;
    friends_table(set, RowIndex, Cols) ->
        case is_row_status_col_changed(Cols) of
       {true, ?destroy} ->
            ex1_server ! {self(), {delete_row, RowIndex}};
       {true, ?createAndGo} ->
           NewRow = make_row(RowIndex, Cols),
            ex1_server ! {self(), {add_row, NewRow}};
       false ->
           {ok, Row} = get_row(RowIndex),
            NewRow = merge_rows(Row, Cols),
        ex1_server ! {self(), {delete_row, RowIndex}},
           ex1_server ! {self(), {add_row, NewRow}}
       end,
        {noError, 0}.

    %%----------------------------------------------------------------
    %% Make a list of {value, Val} of the Row and Cols list.
    %%----------------------------------------------------------------
    get_cols([Col | Cols], Row) ->
        [{value, element(Col, Row)} | get_cols(Cols, Row)];
    get_cols([], _Row) ->
        [].
    %%----------------------------------------------------------------
    %% As get_cols, but the Cols list may contain invalid column
    %% numbers. If it does, we must find the next valid column,
    %% or return endOfTable.
    %%----------------------------------------------------------------
    get_next_cols([Col | Cols], Row) when Col < 2 ->
        [{[2, element(1, Row)], element(2, Row)} |
         get_next_cols(Cols, Row)];
    get_next_cols([Col | Cols], Row) when Col > 4 ->
        [endOfTable |
         get_next_cols(Cols, Row)];
    get_next_cols([Col | Cols], Row) ->
        [{[Col, element(1, Row)], element(Col, Row)} |
         get_next_cols(Cols, Row)];
    get_next_cols([], _Row) ->
        [].
    %%----------------------------------------------------------------
    %% Make a list of endOfTable with as many elems as Cols list.
    %%----------------------------------------------------------------
    end_of_table([Col | Cols]) ->
        [endOfTable | end_of_table(Cols)];
    end_of_table([]) ->
        [].
    add_one_to_cols([Col | Cols]) ->
        [Col + 1 | add_one_to_cols(Cols)];
    add_one_to_cols([]) ->
        [].
    is_row_status_col_changed(Cols) ->
        case lists:keysearch(?status_col, 1, Cols) of
       {value, {?status_col, StatusVal}} ->
            {true, StatusVal};
       _ -> false
        end.
    get_row(RowIndex) ->
        ex1_server ! {self(), {get_row, RowIndex}},
        wait_answer().
    get_next_row(RowIndex) ->
        ex1_server ! {self(), {get_next_row, RowIndex}},
        wait_answer().
    wait_answer() ->
        receive
       {ex1_server, Answer} ->
         Answer
        end.
    %%%---------------------------------------------------------------
    %%% Server code follows
    %%%---------------------------------------------------------------
    init() ->
        register(ex1_server, self()),
        loop("", []).

    loop(MyName, Table) ->
        receive
       {From, get_my_name} ->
            From ! {ex1_server, MyName},
           loop(MyName, Table);
       {From, {set_my_name, NewName}} ->
            loop(NewName, Table);
       {From, {get_row, RowIndex}} ->
           Res = table_get_row(Table, RowIndex),
           From ! {ex1_server, Res},
           loop(MyName, Table);
       {From, {get_next_row, RowIndex}} ->
           Res = table_get_next_row(Table, RowIndex),
            From ! {ex1_server, Res},
           loop(MyName, Table);
       {From, {delete_row, RowIndex}} ->
        NewTable = table_delete_row(Table, RowIndex),
           loop(MyName, NewTable);
       {From, {add_row, NewRow}} ->
           NewTable = table_add_row(Table, NewRow),
           loop(MyName, NewTable)
        end.
    %%%---------------------------------------------------------------
    %%% Functions for table operations. The table is represented as
    %%% a list of rows.
    %%%---------------------------------------------------------------
    table_get_row([{Index, Name, Address, Status} | _], [Index]) ->
        {ok, {Index, Name, Address, Status}};
    table_get_row([H | T], RowIndex) ->
        table_get_row(T, RowIndex);
    table_get_row([], _RowIndex) ->
        no_such_row.
    table_get_next_row([Row | T], []) ->
        {ok, Row};
    table_get_next_row([Row | T], [Index | _])
    when element(1, Row) > Index ->
        {ok, Row};
    table_get_next_row([Row | T], RowIndex) ->
        table_get_next_row(T, RowIndex);
    table_get_next_row([], RowIndex) ->
        endOfTable.
    table_delete_row([{Index, _, _, _} | T], [Index]) ->
        T;
    table_delete_row([H | T], RowIndex) ->
        [H | table_delete_row(T, RowIndex)];
    table_delete_row([], _RowIndex) ->
        [].
    table_add_row([Row | T], NewRow)
      when element(1, Row) > element(1, NewRow) ->
        [NewRow, Row | T];
    table_add_row([H | T], NewRow) ->
        [H | table_add_row(T, NewRow)];
    table_add_row([], NewRow) ->
        [NewRow].
    make_row([Index], [{2, Name}, {3, Address} | _]) ->
        {Index, Name, Address, ?active}.
    merge_rows(Row, [{Col, NewVal} | T]) ->
        merge_rows(setelement(Col, Row, NewVal), T);
    merge_rows(Row, []) ->
        Row.

#### Association File

The association file EX1-MIB.funcs for the real implementation looks as follows:

    {myName, {ex1, my_name, []}}.
    {friendsTable, {ex1, friends_table, []}}.

#### Transcript

To use the real implementation, we must recompile the MIB and load it into the agent.

    1> application:start(snmp).
    ok
    2> snmpc:compile("EX1-MIB").
    {ok,"EX1-MIB.bin"}
    3> snmpa:load_mibs(snmp_master_agent, ["EX1-MIB"]).
    ok
    4> ex1:start().
    <0.115.0>
    %% Now all requests operates on this "real" implementation.
    %% The output from the manager requests will *look* exactly the
    %% same as for the default implementation.

#### Trap Sending

How to send a trap by sending the fTrap from the master agent is shown in this section. The master agent has the MIB EX1-MIB loaded, where the trap is defined. This trap specifies that two variables should be sent along with the trap, myName and fIndex. fIndex is a table column, so we must provide its value and the index for the row in the call to snmpa:send_trap/4. In the example below, we assume that the row in question is indexed by 2 (the row with fIndex 2).

we use a simple Erlang SNMP manager, which can receive traps.

    [MANAGER]
    1> snmp_test_mgr:start_link([{agent,"dront.ericsson.se"},{community,"public"}
     %% does not have write-access
    1>{mibs,["EX1-MIB","STANDARD-MIB"]}]).
    {ok, <0.100.0>}
    2> snmp_test_mgr:s([{[myName,0], "Klas"}]).
    ok
    * Got PDU:
    Received a trap:
          Generic: 4       %% authenticationFailure
       Enterprise: [iso,2,3]
         Specific: 0
       Agent addr: [123,12,12,21]
        TimeStamp: 42993
    2>
    [AGENT]
    3> snmpa:send_trap(snmp_master_agent, fTrap,"standard trap", [{fIndex,[2],2}]).
    [MANAGER]
    2>
    * Got PDU:
    Received a trap:
          Generic: 6
       Enterprise: [example1]
         Specific: 1
       Agent addr: [123,12,12,21]
        TimeStamp: 69649
    [myName,0] = "Martin"
    [fIndex,2] = 2
    2>