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=head1 NAME |
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|
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Convert::BER::XS - I<very> low level BER en-/decoding |
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|
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=head1 SYNOPSIS |
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|
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use Convert::BER::XS ':all'; |
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|
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my $ber = ber_decode $buf |
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or die "unable to decode SNMP message"; |
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|
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# The above results in a data structure consisting of (class, tag, |
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# constructed, data) tuples. Below is such a message, SNMPv1 trap |
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# with a Cisco mac change notification. |
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# Did you know that Cisco is in the news almost every week because |
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# of some backdoor password or other extremely stupid security bug? |
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|
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[ ASN_UNIVERSAL, ASN_SEQUENCE, 1, |
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[ |
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[ ASN_UNIVERSAL, ASN_INTEGER32, 0, 0 ], # snmp version 1 |
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[ ASN_UNIVERSAL, 4, 0, "public" ], # community |
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[ ASN_CONTEXT, 4, 1, # CHOICE, constructed - trap PDU |
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[ |
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[ ASN_UNIVERSAL, ASN_OBJECT_IDENTIFIER, 0, "1.3.6.1.4.1.9.9.215.2" ], # enterprise oid |
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[ ASN_APPLICATION, 0, 0, "\x0a\x00\x00\x01" ], # SNMP IpAddress, 10.0.0.1 |
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[ ASN_UNIVERSAL, ASN_INTEGER32, 0, 6 ], # generic trap |
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[ ASN_UNIVERSAL, ASN_INTEGER32, 0, 1 ], # specific trap |
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[ ASN_APPLICATION, ASN_TIMETICKS, 0, 1817903850 ], # SNMP TimeTicks |
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[ ASN_UNIVERSAL, ASN_SEQUENCE, 1, # the varbindlist |
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[ |
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[ ASN_UNIVERSAL, ASN_SEQUENCE, 1, # a single varbind, "key value" pair |
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[ |
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[ ASN_UNIVERSAL, ASN_OBJECT_IDENTIFIER, 0, "1.3.6.1.4.1.9.9.215.1.1.8.1.2.1" ], # the oid |
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[ ASN_UNIVERSAL, ASN_OCTET_STRING, 0, "...data..." # the value |
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] |
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] |
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], |
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... |
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|
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# let's decode it a bit with some helper functions |
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|
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my $msg = ber_is_seq $ber |
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or die "SNMP message does not start with a sequence"; |
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|
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ber_is $msg->[0], ASN_UNIVERSAL, ASN_INTEGER32, 0 |
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or die "SNMP message does not start with snmp version\n"; |
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|
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# message is SNMP v1 or v2c? |
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if ($msg->[0][BER_DATA] == 0 || $msg->[0][BER_DATA] == 1) { |
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|
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# message is v1 trap? |
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if (ber_is $msg->[2], ASN_CONTEXT, 4, 1) { |
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my $trap = $msg->[2][BER_DATA]; |
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|
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# check whether trap is a cisco mac notification mac changed message |
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if ( |
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(ber_is_oid $trap->[0], "1.3.6.1.4.1.9.9.215.2") # cmnInterfaceObjects |
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and (ber_is_i32 $trap->[2], 6) |
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and (ber_is_i32 $trap->[3], 1) # mac changed msg |
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) { |
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... and so on |
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|
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# finally, let's encode it again and hope it results in the same bit pattern |
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|
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my $buf = ber_encode $ber; |
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|
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=head1 DESCRIPTION |
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|
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This module implements a I<very> low level BER/DER en-/decoder. |
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|
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If is tuned for low memory and high speed, while still maintaining some |
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level of user-friendlyness. |
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|
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Currently, not much is documented, as this is an initial release to |
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reserve CPAN namespace, stay tuned for a few days. |
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|
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=head2 ASN.1/BER/DER/... BASICS |
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|
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ASN.1 is a strange language that can be sed to describe protocols and |
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data structures. It supports various mappings to JSON, XML, but most |
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importantly, to a various binary encodings such as BER, that is the topic |
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of this module, and is used in SNMP or LDAP for example. |
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|
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While ASN.1 defines a schema that is useful to interpret encoded data, |
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the BER encoding is actually somehat self-describing: you might not know |
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whether something is a string or a number or a sequence or something else, |
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but you can nevertheless decode the overall structure, even if you end up |
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with just a binary blob for the actual value. |
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|
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This works because BER values are tagged with a type and a namespace, |
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and also have a flag that says whther a value consists of subvalues (is |
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"constructed") or not (is "primitive"). |
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|
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Tags are simple integers, and ASN.1 defines a somewhat weird assortment of |
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those - for example, you have 32 bit signed integers and 16(!) different |
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string types, but there is no unsigned32 type for example. Different |
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applications work around this in different ways, for example, SNMP defines |
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application-specific Gauge32, Counter32 and Unsigned32, which are mapped |
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to two different tags: you can distinguish between Counter32 and the |
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others, but not between Gause32 and Unsigned32, without the ASN.1 schema. |
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|
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Ugh. |
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|
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=head2 DECODED BER REPRESENTATION |
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|
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This module represents every BER value as a 4-element tuple (actually an |
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array-reference): |
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|
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[CLASS, TAG, CONSTRUCTED, DATA] |
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|
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To avoid non-descriptive hardcoded array index numbers, this module |
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defines symbolic constants to access these members: C<BER_CLASS>, |
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C<BER_TAG>, C<BER_CONSTRUCTED> and C<BER_DATA>. |
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|
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Also, the first three members are integers with a little caveat: for |
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performance reasons, these are readonly and shared, so you must not modify |
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them (increment, assign to them etc.) in any way. You may modify the |
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I<DATA> member, and you may re-assign the array itself, e.g.: |
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|
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$ber = ber_decode $binbuf; |
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|
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# the following is NOT legal: |
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$ber->[BER_CLASS] = ASN_PRIVATE; # ERROR, readonly(!) |
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|
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# but all of the following are fine: |
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$ber->[BER_DATA] = "string"; |
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$ber->[BER_DATA] = [ASN_UNIVERSAL, ASN_INTEGER32, 0, 123]; |
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@$ber = (ASN_APPLICATION, SNMP_TIMETICKS, 1000); |
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|
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I<CLASS> is something like a namespace for I<TAG>s - there is the |
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C<ASN_UNIVERSAL> namespace which defines tags common to all ASN.1 |
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implementations, the C<ASN_APPLICATION> namespace which defines tags for |
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specific applications (for example, the SNMP C<Unsigned32> type is in this |
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namespace), a special-purpose context namespace (C<ASN_CONTEXT>, used e.g. |
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for C<CHOICE>) and a private namespace (C<ASN_PRIVATE>). |
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|
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The meaning of the I<TAG> depends on the namespace, and defines a |
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(partial) interpretation of the data value. For example, right now, SNMP |
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application namespace knowledge ix hardcoded into this module, so it |
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knows that SNMP C<Unsigned32> values need to be decoded into actual perl |
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integers. |
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|
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The most common tags in the C<ASN_UNIVERSAL> namespace are |
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C<ASN_INTEGER32>, C<ASN_BIT_STRING>, C<ASN_NULL>, C<ASN_OCTET_STRING>, |
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C<ASN_OBJECT_IDENTIFIER>, C<ASN_SEQUENCE>, C<ASN_SET> and |
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C<ASN_IA5_STRING>. |
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|
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The most common tags in SNMP's C<ASN_APPLICATION> namespace |
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are C<SNMP_IPADDRESS>, C<SNMP_COUNTER32>, C<SNMP_UNSIGNED32>, |
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C<SNMP_TIMETICKS>, C<SNMP_OPAQUE> and C<SNMP_COUNTER64>. |
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|
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The I<CONSTRUCTED> flag is really just a boolean - if it is false, the |
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the value is "primitive" and contains no subvalues, kind of like a |
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non-reference perl scalar. IF it is true, then the value is "constructed" |
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which just means it contains a list of subvalues which this module will |
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en-/decode as BER tuples themselves. |
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|
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The I<DATA> value is either a reference to an array of further tuples (if |
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the value is I<CONSTRUCTED>), some decoded representation of the value, |
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if this module knows how to decode it (e.g. for the integer types above) |
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or a binary string with the raw octets if this module doesn't know how to |
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interpret the namespace/tag. |
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|
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Thus, you can always decode a BER data structure and at worst you get a |
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string in place of some nice decoded value. |
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|
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See the SYNOPSIS for an example of such an encoded tuple representation. |
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|
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=head2 HELPER FUNCTIONS |
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|
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Working with a 4-tuple for every value can be annoying. Or, rather, I<is> |
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annoying. To reduce this a bit, this module defines a number of helper |
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functions, both to match BER tuples and to conmstruct BER tuples: |
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|
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=head3 MATCH HELPERS |
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|
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Thse functions accept a BER tuple as first argument and either paertially |
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or fully match it. They often come in two forms, one which exactly matches |
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a value, and one which only matches the type and returns the value. |
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|
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They do check whether valid tuples are passed in and croak otherwise. As |
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a ease-of-use exception, they usually also accept C<undef> instead of a |
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tuple reference. in which case they silently fail to match. |
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|
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=over |
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|
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=item $bool = ber_is $tuple, $class, $tag, $constructed, $data |
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|
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This takes a BER C<$tuple> and matches its elements agains the privded |
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values, all of which are optional - values that are either missing or |
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C<undef> will be ignored, the others will be matched exactly (e.g. as if |
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you used C<==> or C<eq> (for C<$data>)). |
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|
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Some examples: |
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|
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ber_is $tuple, ASN_UNIVERSAL, ASN_SEQUENCE, 1 |
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orf die "tuple is not an ASN SEQUENCE"; |
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|
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ber_is $tuple, ASN_UNIVERSAL, ASN_NULL |
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or die "tuple is not an ASN NULL value"; |
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|
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ber_is $tuple, ASN_UNIVERSAL, ASN_INTEGER32, 0, 50 |
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or die "BER integer must be 50"; |
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|
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=item $seq = ber_is_seq $tuple |
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|
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Returns the sequence members (the array of subvalues) if the C<$tuple> is |
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an ASN SEQUENCE, i.e. the C<BER_DATA> member. If the C<$tuple> is not a |
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sequence it returns C<undef>. For example, SNMP version 1/2c/3 packets all |
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consist of an outer SEQUENCE value: |
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|
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my $ber = ber_decode $snmp_data; |
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|
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my $snmp = ber_is_seq $ber |
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or die "SNMP packet invalid: does not start with SEQUENCE"; |
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|
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# now we know $snmp is a sequence, so decode the SNMP version |
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|
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my $version = ber_is_i32 $snmp->[0] |
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or die "SNMP packet invalid: does not start with version number"; |
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|
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=item $bool = ber_is_i32 $tuple, $i32 |
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|
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Returns a true value if the C<$tuple> represents an ASN INTEGER32 with |
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the value C<$i32>. |
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|
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=item $i32 = ber_is_i32 $tuple |
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|
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Returns true (and extracts the integer value) if the C<$tuple> is an ASN |
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INTEGER32. For C<0>, this function returns a special value that is 0 but |
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true. |
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|
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=item $bool = ber_is_oid $tuple, $oid_string |
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|
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Returns true if the C<$tuple> represents an ASN_OBJECT_IDENTIFIER |
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that exactly matches C$oid_string>. Exmaple: |
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|
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ber_is_oid $tuple, "1.3.6.1.4" |
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or die "oid must be 1.3.6.1.4"; |
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|
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=item $oid = ber_is_oid $tuple |
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|
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Returns true (and extracts the OID string) if the C<$tuple> is an ASN |
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OBJECT IDENTIFIER. Otherwise, it returns C<undef>. |
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|
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=back |
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|
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=head3 CONSTRUCTION HELPERS |
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|
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=over |
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|
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=item $tuple = ber_i32 $value |
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|
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Constructs a new C<ASN_INTEGER32> tuple. |
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|
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=back |
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|
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=head2 RELATIONSHIP TO L<Convert::BER> and L<Convert::ASN1> |
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|
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This module is I<not> the XS version of L<Convert::BER>, but a different |
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take at doing the same thing. I imagine this module would be a good base |
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for speeding up either of these, or write a similar module, or write your |
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own LDAP or SNMP module for example. |
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|
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=cut |
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|
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package Convert::BER::XS; |
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|
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use common::sense; |
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|
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use XSLoader (); |
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use Exporter qw(import); |
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|
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our $VERSION = 0.1; |
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|
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XSLoader::load __PACKAGE__, $VERSION; |
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|
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our %EXPORT_TAGS = ( |
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const => [qw( |
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BER_CLASS BER_TAG BER_CONSTRUCTED BER_DATA |
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|
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ASN_BOOLEAN ASN_INTEGER32 ASN_BIT_STRING ASN_OCTET_STRING ASN_NULL ASN_OBJECT_IDENTIFIER ASN_TAG_BER ASN_TAG_MASK |
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ASN_CONSTRUCTED ASN_UNIVERSAL ASN_APPLICATION ASN_CONTEXT ASN_PRIVATE ASN_CLASS_MASK ASN_CLASS_SHIFT |
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ASN_SEQUENCE |
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|
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SNMP_IPADDRESS SNMP_COUNTER32 SNMP_UNSIGNED32 SNMP_TIMETICKS SNMP_OPAQUE SNMP_COUNTER64 |
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)], |
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encode => [qw( |
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ber_decode |
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ber_is ber_is_seq ber_is_i32 ber_is_oid |
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)], |
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decode => [qw( |
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ber_encode |
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)], |
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); |
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|
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our @EXPORT_OK = map @$_, values %EXPORT_TAGS; |
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|
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$EXPORT_TAGS{all} = \@EXPORT_OK; |
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|
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1; |
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|
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=head2 BUGS / SHORTCOMINGs |
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|
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This module does have a number of SNMPisms hardcoded, such as the SNMP |
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tags for Unsigned32 and so on. More configurability is needed, and, if |
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ever implemented, will come in a form similar to how L<JSON::XS> and |
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L<CBOR::XS> respresent things, namely with an object-oriented interface. |
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|
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=head1 AUTHOR |
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|
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Marc Lehmann <schmorp@schmorp.de> |
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http://software.schmorp.de/pkg/Convert-BER-XS |
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|
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=cut |
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|
