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# Validator Configuration File Format
You can set up a `Validator` via a configuration file.
Next, we will show you how to write a configuration file.
The configuration file consists of **rules** and **trust-anchors** that will be used in validation.
**Rules** tell the validator how to validate a packet, while **trust-anchors** tell the validator which certificates are valid immediately.
Here is an example of configuration file containing two rules.
rule
{
id "Simple Rule"
for data
filter
{
type name
name /localhost/example
relation isPrefixOf
}
checker
{
type customized
sig-type rsa-sha256
key-locator
{
type name
name /ndn/edu/ucla/KEY/yingdi/ksk-1234/ID-CERT
relation equal
}
}
}
rule
{
id "Testbed Validation Rule"
for data
checker
{
type hierarchical
sig-type rsa-sha256
}
}
trust-anchor
{
type file
file-name "testbed-trust-anchor.cert"
}
* <font color='red'>**ATTENTION: The order of rules MATTERS!**</font>
A rule can be broken into two parts:
* The first part is to qualify packets to which the rule can be applied;
* The second part is to check whether further validation process is necessary.
When receiving a packet, the validator will apply rules in the configuration file one-by-one against the packet,
until finding a rule that the packet qualifies for.
And the second part of the matched rule will be used to check the validity of the packet.
If the packet cannot qualify for any rules, it is treated as an invalid packet.
Once a packet has been matched by a rule, the rest rules will not be applied against the packet.
Therefore, you should always put the most specific rule to the top, otherwise it will become useless.
In the example configuration,
the first rule indicates that all the data packets under the name prefix "/localhost/example" must be signed by a key whose certificate name is "/ndn/edu/ucla/KEY/yingdi/ksk-1234/ID-CERT".
If a packet does not have a name under prefix "/localhost/example", validator will skip the first rule and apply the second rule.
The second rule indicates that any data packets must be validated along a hierarchy.
And a certificate stored in a file "testbed-trust-anchor.cert" is valid.
## Rules in general
A rule has four types of properties: **id**, **for**, **filter**, and **checker**.
The property **id** uniquely identifies the rule in the configuration file.
As long as being unique, any name can be given to a rule, e.g., "Simple Rule", "Testbed Validation Rule".
A rule must have one and only one **id** property.
A rule is either used to validate an interest packet or a data packet.
This information is specified in the property **for**.
Only two value can be specified: **data** and **interest**.
A rule must have one and only one **for** property.
The property **filter** further constrains the packets that can be checked by the rule.
Filter property is not required in a rule, in this case, the rule will capture all the packets passed to it.
A rule may contain more than one filters, in this case, a packet can be checked by a rule only if the packet satisfies all the filters.
* <font color='red'>**ATTENTION: A packet that satisfies all the filters may not be valid**</font>.
The property **checker** defines the conditions that a matched packet must fulfill to be treated as a valid packet.
A rule must have one and only one **checker** property.
**filter** and **checker** have their own properties.
Next, we will introduce them separately.
## Filter Property
Filter has its own **type** property.
Although a rule may contain more than one filters, there is at most one filter of each type.
So far, only one type of filter is defined: **name**.
In other word, only one filter can be specified in a rule for now.
### Name Filter
There are two ways to express the conditions on name.
The first way is to specify a relationship between the packet name and a particular name.
In this case, two more properties are required: **name** and **relation**.
A packet can fulfill the condition if the **name** has a **relation* to the packet name.
Three types of **relation** has been defined: **equal**, **isPrefixOf**, **isStrictPrefixOf**.
For example, a filter
filter
{
type name
name /localhost/example
relation equal
}
shall only capture a packet with the exact name "/localhost/example".
And a filter
filter
{
type name
name /localhost/example
relation isPrefixOf
}
shall capture a packet with name "/localhost/example" or "/localhost/example/data", but cannot catch a packet with name "/localhost/another_example".
And a filter
filter
{
type name
name /localhost/example
relation isStrictPrefixOf
}
shall capture a packet with name "/localhost/example/data", but cannot catch a packet with name "/localhost/example".
The second way is to specify an [[Regex|NDN Regular Expression]] that can match the packet.
In this case, only one property **regex** is required.
For example, a filter
filter
{
type name
regex ^[^<KEY>]*<KEY><>*<ksk-.*><ID-CERT>$
}
shall capture all the identity certificates.
## Checker Property
Passing all the filters in a rule only indicates that a packet can be checked using the rule,
and it does not necessarily implies that the packet is valid.
The validity of a packet is determined by the property **checker**, which defines the conditions that a valid packet must fulfill.
Same as **filter**, **checker** has a property **type**.
We have defined three types of checkers: **customized**, and **hierarchical**, and **fixed-anchor**.
As suggested by its name, **customized** checker allows you to customize the conditions according to specific requirements.
**hierarchical** checker and **fixed-anchor** checker are pre-defined shortcuts, which specify specific trust models separately.
### Customized Checker
So far, we only allow three customized properties in a customized checker: **sig-type**, **key-locator**.
All of them are related to the `SignatureInfo` of a packet.
checker
{
type customized
sig-type ...
key-locator
{
...
}
}
The property **sig-type** specifies the acceptable signature type.
Right now two signature types have been defined: **rsa-sha256** (which is a strong signature type) and **sha256** (which is a weak signature type).
If sig-type is sha256, then **key-locator** will be ignored.
Validator will simply calculate the digest of a packet and compare it with the one in `SignatureValue`.
If sig-type is rsa-sha256, you have to further customize the checker with **key-locator**.
The property **key-locator** which specifies the conditions on `KeyLocator`.
If the **key-locator** property is specified, it requires the existence of the `KeyLocator` field in `SignatureInfo`.
Although there are more than one types of `KeyLocator` defined in the [Packet Format](http://named-data.net/doc/ndn-tlv/signature.html),
**key-locator** property only supports one type: **name**:
key-locator
{
type name
...
}
Such a key-locator property specifies the conditions on the certificate name of the signing key.
Since the conditions are about name, they can be specified in the same way as the name filter.
For example, a checker could be:
checker
{
type customized
sig-type rsa-sha256
key-locator
{
type name
name /ndn/edu/ucla/KEY/yingdi/ksk-1234/ID-CERT
relation equal
}
}
This checker property requires that the packet must have a rsa-sha256 signature generated by a key whose certificate name is "/ndn/edu/ucla/KEY/yingdi/ksk-1234/ID-CERT".
Besides the two ways to express conditions on the `KeyLocator` name (name and regex),
you can further constrain the `KeyLocator` name using the information extracted from the packet name.
This third type of condition is expressed via a property **hyper-relation**.
The **hyper-relation** property consists of three parts:
* an NDN regular expression that can extract information from packet name
* an NDN regular expression that can extract information from `KeyLocator` name
* relation from the part extracted from `KeyLocator` name to the one extracted from the packet name
For example, a checker:
checker
{
type customized
sig-type rsa-sha256
key-locator
{
type name
hyper-relation
{
k-regex ^([^<KEY>]*)<KEY>(<>*)<ksk-.*><ID-CERT>$
k-expand \1\2
h-relation relation isPrefixOf
p-regex ^(<>*)$
p-expand \1
}
}
}
requires the packet name must be under the corresponding namespace of the `KeyLocator` name.
In some cases, you can even customize checker with another property
For example:
checker
{
type customized
sig-type rsa-sha256
key-locator
{
type name
hyper-relation
{
k-regex ^([^<KEY>]*)<KEY>(<>*)<ksk-.*><ID-CERT>$
k-expand \1\2
h-relation relation isPrefixOf
p-regex ^(<>*)$
p-expand \1
}
}
}
### Hierarchical Checker
As implied by its name, hierarchical checker requires that the packet name must be under the namespace of the packet signer.
A hierarchical checker:
checker
{
type hierarchical
sig-type rsa-sha256
}
is equivalent to a customized checker:
checker
{
type customized
sig-type rsa-sha256
key-locator
{
type name
hyper-relation
{
k-regex ^([^<KEY>]*)<KEY>(<>*)<ksk-.*><ID-CERT>$
k-expand \1\2
h-relation relation isPrefixOf
p-regex ^(<>*)$
p-expand \1
}
}
}
### Fixed-Anchor Checker
In some cases, you only accept packets signed with pre-trusted certificates, i.e. "one-step validation".
Such a trust model can be expressed with **fixed-anchor** checker.
For example:
checker
{
type fixed-anchor
sig-type rsa-sha256
}
## Trust Anchors
A configuration file must contain <font color='red'>**AT LEAST ONE**</font> trust anchors.
A configuration file may contain more than one trust anchors, but the order of trust anchors does not matter.
## Example Configuration For NLSR
The trust model of NLSR is semi-hierarchical.
An example certificate signing hierarchy is:
root
|
+--------------+---------------+
site1 site2
| |
+---------+---------+ +
operator1 operator2 operator3
| | |
+-----+-----+ +----+-----+ +-----+-----+--------+
router1 router2 router3 router4 router5 router6 router7
| | | | | | |
+ + + + + + +
NLSR NSLR NSLR NSLR NSLR NSLR NSLR
However, entities name may not follow the signing hierarchy, for example:
Entity | Identity Name | Example | Certificate Name Example
-------- | ------------------------------------------------- | ------------------------------- | ------------------------------------------------
root | /\<network\> | /ndn | /ndn/KEY/ksk-1/ID-CERT/%01
site | /\<network\>/\<site\> | /ndn/edu/ucla | /ndn/edu/ucla/KEY/ksk-2/ID-CERT/%01
operator | /\<network\>/\<site\>/%C1.O.N./\<operator-id\> | /ndn/edu/ucla/%C1.O.N./op1 | /ndn/edu/ucla/%C1.O.N./op1/KEY/ksk-3/ID-CERT/%01
router | /\<network\>/\<site\>/%C1.O.R./\<router-id\> | /ndn/edu/ucla/%C1.O.R./rt1 | /ndn/edu/ucla/%C1.O.R./rt1/KEY/ksk-4/ID-CERT/%01
NLSR | /\<network\>/\<site\>/%C1.O.R./\<router-id\>/NLSR | /ndn/edu/ucla/%C1.O.R./rt1/NLSR | /ndn/edu/ucla/%C1.O.R./rt1/NLSR/KEY/ksk-5/ID-CERT/%01
Assume that a typical NLSR data name is "/ndn/edu/ucla/%C1.O.R./rt1/NLSR/LSA/LSType.1/%01".
Then, the exception of naming hierarchy is "operator-router".
So we can write a configuration file with three rules.
The first one is a customized rule that capture the normal NLSR data.
The second one is a customized rule that handles the exception case of the hierarchy (operator->router).
And the last one is a hierarchical rule that handles the normal cases of the hierarchy.
We put the NLSR data rule to the first place, because NLSR data packets are the most frequently checked.
The hierarchical exception rule is put to the second, because it is more specific than the last one.
And here is the configuration file:
rule
{
id "NSLR LSA Rule"
for data
filter
{
type name
regex ^[^<NLSR><LSA>]*<NLSR><LSA>
}
checker
{
type customized
sig-type rsa-sha256
key-locator
{
type name
hyper-relation
{
k-regex ^([^<KEY>]*)<KEY><ksk-.*><ID-CERT>$
k-expand \1
h-relation relation equal
p-regex ^([^<NLSR><LSA>]*)<NLSR><LSA><LSType\.\d><>$
p-expand \1
}
}
}
}
rule
{
id "NSLR Hierarchy Exception Rule"
for data
filter
{
type name
regex ^[^<KEY><%C1.O.R.>]*<%C1.O.R.><><KEY><ksk-.*><ID-CERT><>$
}
checker
{
type customized
sig-type rsa-sha256
key-locator
{
type name
hyper-relation
{
k-regex ^([^<KEY><%C1.O.N.>]*)<%C1.O.N.><><KEY><ksk-.*><ID-CERT>$
k-expand \1
h-relation relation equal
p-regex ^([^<KEY><%C1.O.R.>]*)<%C1.O.R.><><KEY><ksk-.*><ID-CERT><>$
p-expand \1
}
}
}
}
rule
{
id "NSLR Hierarchical Rule"
for data
filter
{
type name
regex ^([^<KEY>]*)<KEY><ksk-.*><ID-CERT><>$
}
checker
{
type hierarchical
sig-type rsa-sha256
}
}
trust-anchor
{
type file
file-name "testbed-trust-anchor.cert"
}