BIRD 3.3.0 User's Guide: Filters

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## 5. Filters

## 5.1 Introduction

BIRD contains a simple programming language. (No, it can't yet read mail :-). There are two objects in this language: filters and functions. Filters are interpreted by BIRD core when a route is being passed between protocols and routing tables. The filter language contains control structures such as if's and switches, but it allows no loops. An example of a filter using many features can be found in `filter/test.conf`.

Filter gets the route, looks at its attributes and modifies some of them if it wishes. At the end, it decides whether to pass the changed route through (using `accept`) or whether to `reject` it. A simple filter looks like this:

    filter not_too_far
    {
            int var;
            if defined( rip_metric ) then
                    var = rip_metric;
            else {
                    var = 1;
                    rip_metric = 1;
            }
            if rip_metric > 10 then
                    reject "RIP metric is too big";
            else
                    accept "ok";
    }

As you can see, a filter has a header, a list of local variables, and a body. The header consists of the `filter` keyword followed by a (unique) name of filter. The list of local variables consists of `type name;` pairs where each pair declares one local variable. The body consists of `{ statements }`. Each statement is terminated by a `;`. You can group several statements to a single compound statement by using braces (`{ statements }`) which is useful if you want to make a bigger block of code conditional.

BIRD supports functions, so that you don not have to repeat the same blocks of code over and over. Functions can have zero or more parameters and they can have local variables. If the function returns value, then you should always specify its return type. Direct recursion is possible. Function definitions look like this:

    function name() -> int
    {
            int local_variable;
            int another_variable = 5;
            return 42;
    }

    function with_parameters(int parameter) -> pair
    {
            print parameter;
            return (1, 2);
    }

Like in C programming language, variables are declared inside function body, either at the beginning, or mixed with other statements. Declarations may contain initialization. You can also declare variables in nested blocks, such variables have scope restricted to such block. There is a deprecated syntax to declare variables after the `function` line, but before the first `{`. Functions are called like in C: `name(); with_parameters(5);`. Function may return values using the `return [expr]` command. Returning a value exits from current function (this is similar to C).

Filters are defined in a way similar to functions except they cannot have explicit parameters and cannot return. They get a route table entry as an implicit parameter, it is also passed automatically to any functions called. The filter must terminate with either `accept` or `reject` statement. If there is a runtime error in filter, the route is rejected.

A nice trick to debug filters is to use `show route filter name` from the command line client. An example session might look like:

    pavel@bug:~/bird$ ./birdc -s bird.ctl
    BIRD 0.0.0 ready.
    bird> show route
    10.0.0.0/8         dev eth0 [direct1 23:21] (240)
    195.113.30.2/32    dev tunl1 [direct1 23:21] (240)
    127.0.0.0/8        dev lo [direct1 23:21] (240)
    bird> show route ?
    show route [<prefix>] [table <t>] [filter <f>] [all] [primary]...
    bird> show route filter { if 127.0.0.5 ~ net then accept; }
    127.0.0.0/8        dev lo [direct1 23:21] (240)
    bird>

## 5.2 Data types

Each variable and each value has certain type. Booleans, integers and enums are incompatible with each other (that is to prevent you from shooting oneself in the foot).

`bool`

  This is a boolean type, it can have only two values, `true` and `false`. Boolean is the only type you can use in `if` statements.

`int`

  This is a general integer type. It is an unsigned 32bit type; i.e., you can expect it to store values from 0 to 4294967295. Overflows are not checked. You can use `0x1234` syntax to write hexadecimal values.

`pair`

  This is a pair of two short integers. Each component can have values from 0 to 65535. Literals of this type are written as `(1234,5678)`. The same syntax can also be used to construct a pair from two arbitrary integer expressions (for example `(1+2,a)`).

  Operators `.asn` and `.data` can be used to extract corresponding components of a pair: `(asn, data)`.

`quad`

  This is a dotted quad of numbers used to represent router IDs (and others). Each component can have a value from 0 to 255. Literals of this type are written like IPv4 addresses.

`string`

  This is a string of characters. There are no ways to modify strings in filters. You can pass them between functions, assign them to variables of type `string`, print such variables, use standard string comparison operations (e.g. `=, !=, <, >, <=, >=`), and concatenate two strings with `append(A, B)` function or `++` operator.

  String literals are written as `"This is a string constant"`. Additionally matching (`~, !~`) operators could be used to match a string value against a shell pattern (represented also as a string).

`bytestring`

  This is a sequence of arbitrary bytes. There are no ways to modify bytestrings in filters. You can pass them between functions, assign them to variables of type `bytestring`, print such values, compare bytestings (`=, !=`), and concatenate two bytestrings with `append(A, B)` function or `++` operator.

  Bytestring literals are written as a sequence of hexadecimal digit pairs, optionally colon-separated. A bytestring specified this way must be either at least 16 bytes (32 digits) long, or prefixed by the `hex:` prefix: `01:23:45:67:89:ab:cd:ef:01:23:45:67:89:ab:cd:ef`, `0123456789abcdef0123456789abcdef`, `hex:`, `hex:12:34:56`, `hex:12345678`.

  A bytestring can be made from a hex string using `from_hex()` function. Source strings can use any number of dots, colons, hyphens and spaces as byte separators: `from_hex(" 12.34 56:78 ab-cd-ef ")`.

`ip`

  This type can hold a single IP address. The IPv4 addresses are stored as IPv4-Mapped IPv6 addresses so one data type for both of them is used. Whether the address is IPv4 or not may be checked by `.is_v4` which returns a `bool`. IP addresses are written in the standard notation (`10.20.30.40` or `fec0:3:4::1`). You can apply special operator `.mask(number)` on values of type ip. It masks out all but first `number` bits from the IP address. So `1.2.3.4.mask(8) = 1.0.0.0` is true.

`mac`

  This type can hold a single MAC address. MAC addresses are written in the usual notation (e.g. `62:68:7f:d9:c6:ec`).

`prefix`

  This type can hold a network prefix consisting of IP address, prefix length and several other values. This is the key in route tables.

  Prefixes may be of several types, which can be determined by the special operator `.type`. The type may be:

  `NET_IP4` and `NET_IP6` prefixes hold an IP prefix. The literals are written as `ipaddress/pxlen`. There are two special operators on these: `.ip` which extracts the IP address from the pair, and `.len`, which separates prefix length from the pair. So `1.2.0.0/16.len = 16` is true.

  `NET_IP6_SADR` nettype holds both destination and source IPv6 prefix. The literals are written as `ipaddress/pxlen from ipaddress/pxlen`, where the first part is the destination prefix and the second art is the source prefix. They support the same operators as IP prefixes, but just for the destination part. They also support `.src` and `.dst` operators to get respective parts of the address as separate `NET_IP6` values.

  `NET_VPN4` and `NET_VPN6` prefixes hold an IP prefix with VPN Route Distinguisher (RFC 4364). They support the same special operators as IP prefixes, and also `.rd` which extracts the Route Distinguisher. Their literals are written as `rd ipprefix`

  `NET_ROA4` and `NET_ROA6` prefixes hold an IP prefix range together with an ASN. They support the same special operators as IP prefixes, and also `.maxlen` which extracts maximal prefix length, and `.asn` which extracts the ASN.

  `NET_ASPA` prefixes hold an ASN, which can be extracted by `.asn` operator.

  `NET_FLOW4` and `NET_FLOW6` hold an IP prefix together with a flowspec rule. Filters currently do not support much flowspec parsing, only `.src` and `.dst` operators to get source and destination parts of the flowspec as separate `NET_IP4` / `NET_IP6` values.

  `NET_ETH` prefixes holds a MAC address and a VLAN ID, which can be extracted by operators `.mac` and `.vlan_id`, respectively.

  `NET_MPLS` holds a single MPLS label and its handling is currently not implemented.

  `NET_EVPN` is a complex nettype that holds reachability information for Ethernet VPN. It has several subtypes (`NET_EVPN_EAD`, `NET_EVPN_MAC`, `NET_EVPN_IMET`, `NET_EVPN_ES`), which can be determined by the operator `.evpn_type`. Fields and related operators are: Route Distinguisher (`.rd`), MAC address (`.mac`), IP address (`.ip`), EVPN tag (`.evpn_tag`), Ethernet Segment Identifier (`.evpn_esi`), router IP (`.router_ip`). Note that only fields that are a part of primary key for given route subtype can be accessed by these operators (e.g. `.evpn_esi` can be used for `NET_EVPN_ES`, but not for `NET_EVPN_MAC` or other types), remaining attributes are stored outside prefix as other route attributes. See EVPN routes for description of specific subtypes.

  `NET_NEIGHBOR` holds a single IP address and an interface index. The IP address can be accessed by operator `.ip`, while the access to the interface index is not implemented.

`rd`

  This is a route distinguisher according to RFC 4364. There are three kinds of RDs: `asn:32bit int`, `asn4:16bit int` and `IPv4 address:32bit int`

`ec`

  This is a specialized type used to represent BGP extended community values. It is essentially a 64bit value, literals of this type are usually written as `(kind, key, value)`, where `kind` is a kind of extended community (e.g. `rt` / `ro` for a route target / route origin communities), the format and possible values of `key` and `value` are usually integers, but it depends on the used kind. Similarly to pairs, ECs can be constructed using expressions for `key` and `value` parts, (e.g. `(ro, myas, 3*10)`, where `myas` is an integer variable).

`lc`

  This is a specialized type used to represent BGP large community values. It is essentially a triplet of 32bit values, where the first value is reserved for the AS number of the issuer, while meaning of remaining parts is defined by the issuer. Literals of this type are written as `(123, 456, 789)`, with any integer values. Similarly to pairs, LCs can be constructed using expressions for its parts, (e.g. `(myas, 10+20, 3*10)`, where `myas` is an integer variable).

  Operators `.asn`, `.data1`, and `.data2` can be used to extract corresponding components of LCs: `(asn, data1, data2)`.

`int|pair|quad|ip|prefix|ec|lc|rd|enum set`

  Filters recognize several types of sets. Sets are similar to strings: you can pass them around but you cannot modify them. Literals of type `int set` look like `[ 1, 2, 5..7 ]`. As you can see, both simple values and ranges are permitted in sets.

  For pair sets, expressions like `(123,*)` can be used to denote ranges (in that case `(123,0)..(123,65535)`). You can also use `(123,5..100)` for range `(123,5)..(123,100)`. You can also use `*` and `a..b` expressions in the first part of a pair, note that such expressions are translated to a set of intervals, which may be memory intensive. E.g. `(*,4..20)` is translated to `(0,4..20), (1,4..20), (2,4..20), ... (65535, 4..20)`.

  EC sets use similar expressions like pair sets, e.g. `(rt, 123, 10..20)` or `(ro, 123, *)`. Expressions requiring the translation (like `(rt, *, 3)`) are not allowed (as they usually have 4B range for ASNs).

  Also LC sets use similar expressions like pair sets. You can use ranges and wildcards, but if one field uses that, more specific (later) fields must be wildcards. E.g., `(10, 20..30, *)` or `(10, 20, 30..40)` is valid, while `(10, *, 20..30)` or `(10, 20..30, 40)` is not valid.

  You can also use named constants or compound expressions for non-prefix set values. However, it must be possible to evaluate these expressions before daemon boots. So you can use only constants inside them. Also, in case of compound expressions, they require parentheses around them. E.g.

      define one=1;
      define myas=64500;

      int set odds = [ one, (2+1), (6-one), (2*2*2-1), 9, 11 ];
      pair set ps = [ (1,one+one), (3,4)..(4,8), (5,*), (6,3..6), (7..9,*) ];
      ec set es = [ (rt, myas, *), (rt, myas+2, 0..16*16*16-1) ];

  Sets of prefixes are special: their literals does not allow ranges, but allows prefix patterns that are written as `ipaddress/pxlen{low,high}`. Prefix `ip1/len1` matches prefix pattern `ip2/len2{l,h}` if the first `min(len1, len2)` bits of `ip1` and `ip2` are identical and `l <= len1 <= h`. A valid prefix pattern has to satisfy `low <= high`, but `pxlen` is not constrained by `low` or `high`. Obviously, a prefix matches a prefix set literal if it matches any prefix pattern in the prefix set literal.

  There are also two shorthands for prefix patterns: `address/len+` is a shorthand for `address/len{len,maxlen}` (where `maxlen` is 32 for IPv4 and 128 for IPv6), that means network prefix `address/len` and all its subnets. `address/len-` is a shorthand for `address/len{0,len}`, that means network prefix `address/len` and all its supernets (network prefixes that contain it).

  For example, `[ 1.0.0.0/8, 2.0.0.0/8+, 3.0.0.0/8-, 4.0.0.0/8{16,24} ]` matches prefix `1.0.0.0/8`, all subprefixes of `2.0.0.0/8`, all superprefixes of `3.0.0.0/8` and prefixes `4.X.X.X` whose prefix length is 16 to 24. `[ 0.0.0.0/0{20,24} ]` matches all prefixes (regardless of IP address) whose prefix length is 20 to 24, `[ 1.2.3.4/32- ]` matches any prefix that contains IP address `1.2.3.4`. `1.2.0.0/16 ~ [ 1.0.0.0/8{15,17} ]` is true, but `1.0.0.0/16 ~ [ 1.0.0.0/8- ]` is false.

  Cisco-style patterns like `10.0.0.0/8 ge 16 le 24` can be expressed in BIRD as `10.0.0.0/8{16,24}`, `192.168.0.0/16 le 24` as `192.168.0.0/16{16,24}` and `192.168.0.0/16 ge 24` as `192.168.0.0/16{24,32}`.

  It is not possible to mix IPv4 and IPv6 prefixes in a prefix set. It is currently possible to mix IPv4 and IPv6 addresses in an ip set, but that behavior may change between versions without any warning; don't do it unless you are more than sure what you are doing. (Really, don't do it.)

`enum`

  Enumeration types are fixed sets of possibilities. You can't define your own variables of such type, but some route attributes are of enumeration type. Enumeration types are incompatible with each other.

`bgppath`

  BGP path is a list of autonomous system numbers. You can't write literals of this type. There are several special operators on bgppaths:

  `P.first` returns the first ASN (the neighbor ASN) in path P.

  `P.last` returns the last ASN (the source ASN) in path P.

  `P.last_nonaggregated` returns the last ASN in the non-aggregated part of the path P.

  Both `first` and `last` return zero if there is no appropriate ASN, for example if the path contains an AS set element as the first (or the last) part. If the path ends with an AS set, `last_nonaggregated` may be used to get last ASN before any AS set.

  `P.len` returns the length of path P.

  `P.empty` makes the path P empty. Can't be used as a value, always modifies the object.

  `P.prepend(A)` prepends ASN A to path P and returns the result.

  `P.delete(A)` deletes all instances of ASN A from from path P and returns the result. A may also be an integer set, in that case the operator deletes all ASNs from path P that are also members of set A.

  `P.filter(A)` deletes all ASNs from path P that are not members of integer set A, and returns the result. I.e., `filter` do the same as `delete` with inverted set A.

  Methods `prepend`, `delete` and `filter` keep the original object intact as long as you use the result in any way. You can also write e.g. `P.prepend(A);` as a standalone statement. This variant does modify the original object with the result of the operation.

`bgpmask`

  BGP masks are patterns used for BGP path matching (using `path ~ [= 2 3 5 * =]` syntax). The masks resemble wildcard patterns as used by UNIX shells. Autonomous system numbers match themselves, `*` matches any (even empty) sequence of arbitrary AS numbers and `?` matches one arbitrary AS number. For example, if `bgp_path` is 4 3 2 1, then: `bgp_path ~ [= * 4 3 * =]` is true, but `bgp_path ~ [= * 4 5 * =]` is false. There is also `+` operator which matches one or multiple instances of previous expression, e.g. `[= 1 2+ 3 =]` matches both path 1 2 3 and path 1 2 2 2 3, but not 1 3 nor 1 2 4 3. Note that while `*` and `?` are wildcard-style operators, `+` is regex-style operator.

  BGP mask expressions can also contain integer expressions enclosed in parenthesis and integer variables, for example `[= * 4 (1+2) a =]`. You can also use ranges (e.g. `[= * 3..5 2 100..200 * =]`) and sets (e.g. `[= 1 2 [3, 5, 7] * =]`).

`clist`

  Clist is similar to a set, except that unlike other sets, it can be modified. The type is used for community list (a set of pairs) and for cluster list (a set of quads). There exist no literals of this type. There are special operators on clists:

  `C.len` returns the length of clist C.

  `C.empty` makes the list C empty. Can't be used as a value, always modifies the object.

  `C.add(P)` adds pair (or quad) P to clist C and returns the result. If item P is already in clist C, it does nothing. P may also be a clist, in that case all its members are added; i.e., it works as clist union.

  `C.delete(P)` deletes pair (or quad) P from clist C and returns the result. If clist C does not contain item P, it does nothing. P may also be a pair (or quad) set, in that case the operator deletes all items from clist C that are also members of set P. Moreover, P may also be a clist, which works analogously; i.e., it works as clist difference.

  `C.filter(P)` deletes all items from clist C that are not members of pair (or quad) set P, and returns the result. I.e., `filter` do the same as `delete` with inverted set P. P may also be a clist, which works analogously; i.e., it works as clist intersection.

  Methods `add`, `delete` and `filter` keep the original object intact as long as you use the result in any way. You can also write e.g. `P.add(A);` as a standalone statement. This variant does modify the original object with the result of the operation.

  `C.min` returns the minimum element of clist C.

  `C.max` returns the maximum element of clist C.

  Operators `.min`, `.max` can be used together with `filter` to extract the community from the specific subset of communities (e.g. localpref or prepend) without the need to check every possible value (e.g. `filter(bgp_community, [(23456, 1000..1099)]).min`).

`eclist`

  Eclist is a data type used for BGP extended community lists. Eclists are very similar to clists, but they are sets of ECs instead of pairs. The same operations (like `add`, `delete` or `~` and `!~` membership operators) can be used to modify or test eclists, with ECs instead of pairs as arguments.

`lclist`

  Lclist is a data type used for BGP large community lists. Like eclists, lclists are very similar to clists, but they are sets of LCs instead of pairs. The same operations (like `add`, `delete` or `~` and `!~` membership operators) can be used to modify or test lclists, with LCs instead of pairs as arguments.

## 5.3 Operators

The filter language supports common integer operators `(+,-,*,/)`, parentheses `(a*(b+c))`, comparison `(a=b, a!=b, a<b, a>=b)`.

Logical operations include unary not (`!`), and (`&&`), and or (`||`).

Strings and bytestrings can be concatenated with `++` operator.

Special operators include (`~`, `!~`) for "is (not) element of a set" operation - it can be used on:

- element and set of elements of the same type (returning true if element is contained in the given set)
- two strings (returning true if the first string matches a shell-like pattern stored in the second string)
- IP and prefix (returning true if IP is within the range defined by that prefix)
- prefix and prefix (returning true if the first prefix is more specific than the second one)
- bgppath and bgpmask (returning true if the path matches the mask)
- number and bgppath (returning true if the number is in the path)
- bgppath and int (number) set (returning true if any ASN from the path is in the set)
- pair/quad and clist (returning true if the pair/quad is element of the clist)
- clist and pair/quad set (returning true if there is an element of the clist that is also a member of the pair/quad set).

There are also operators related to RPKI infrastructure used to run RFC 6483 route origin validation and (draft) AS path validation.

- `roa_check(table)` checks the current route in the specified ROA table and returns `ROA_UNKNOWN`, `ROA_INVALID` or `ROA_VALID`, if the validation result is unknown, invalid, or valid, respectively. The result is valid if there is a matching ROA, it is invalid if there is either matching ROA with a different ASN, or any covering ROA with shorter maximal prefix length.
- `roa_check(table, prefix, asn)` is an explicit version of the ROA check if the user for whatever reason needs to check a different prefix or different ASN than the default one. The equivalent call of the short variant is `roa_check(table, net, bgp_path.last)` and it is faster to call the short variant.
- `aspa_check_downstream(table)` checks the current route in the specified ASPA table and returns `ASPA_UNKNOWN`, `ASPA_INVALID`, or `ASPA_VALID` if the validation result is unknown, invalid, or valid, respectively. The result is valid if there is a full coverage of matching ASPA records according to the Algorithm for Downstream Paths by the (draft). This operator is not present if BGP is not compiled in.
- `aspa_check_upstream(table)` checks the current route in the specified ASPA table as the former operator, but it applies the (stricter) Algorithm for Upstream Paths by the (draft). This operator is not present if BGP is not compiled in.
- `aspa_check(table, path, is_upstream)` is an explicit version of the former two ASPA check operators. The equivalent of `aspa_check_downstream` is `aspa_check(table, bgp_path, false)` and for `aspa_check_upstream` it is `aspa_check(table, bgp_path, true)`. Note: the ASPA check does not include the local ASN in the AS path. Also, `ASPA_INVALID` is returned for an empty AS path or for AS path containing `CONFED_SET` or `CONFED_SEQUENCE` blocks, as the (draft) stipulates.

The following example checks for ROA and ASPA on routes from a customer:

    roa6 table r6;
    aspa table at;
    attribute int valid_roa;
    attribute int valid_aspa;

    filter customer_check {
      case roa_check(r6) {
        ROA_INVALID: reject "Invalid ROA";
        ROA_VALID: valid_roa = 1;
      }

      case aspa_check_upstream(at) {
        ASPA_INVALID: reject "Invalid ASPA";
        ASPA_VALID: valid_aspa = 1;
      }

      accept;
    }

## 5.4 Control structures

Filters support several control structures: conditions, for loops and case switches.

Syntax of a condition is: `if boolean expression then commandT; else commandF;` and you can use `{ command1; command2; ... }` instead of either command. The `else` clause may be omitted. If the `boolean expression` is true, commandT is executed, otherwise commandF is executed.

For loops allow to iterate over elements in compound data like BGP paths or community lists. The syntax is: `for [ type ] variable in expr do command;` and you can also use compound command like in conditions. The expression is evaluated to a compound data, then for each element from such data the command is executed with the item assigned to the variable. A variable may be an existing one (when just name is used) or a locally defined (when type and name is used). In both cases, it must have the same type as elements.

The `case` is similar to case from Pascal. Syntax is `case expr { else: | set_body_expr /: statement ; [... ] }`. The expression after `case` can be of any type that could be a member of a set, while the set_body_expr before `:` can be anything (constants, intervals, expressions) that could be a part of a set literal. One exception is prefix type, which can be used in sets bud not in `case` structure. Multiple commands must be grouped by `{}`. If `expr` matches one of the `:` clauses, the statement or block after it is executed. If `expr` matches neither of the `:` clauses, the statement or block after `else:` is executed.

Here is example that uses `if` and `case` structures:

    if 1234 = i then printn "."; else {
            print "not 1234";
            print "You need {} around multiple commands";
    }

    for int asn in bgp_path do {
            printn "ASN: ", asn;
            if asn < 65536 then print " (2B)"; else print " (4B)";
    }

    case arg1 {
            2: { print "two"; print "Multiple commands must brace themselves."; }
            3 .. 5: print "three to five";
            else: print "something else";
    }

## 5.5 Route attributes

A filter is implicitly passed a route, and it can access its attributes just like it accesses variables. There are common route attributes, protocol-specific route attributes and custom route attributes. Most common attributes are mandatory (always defined), while remaining are optional. Attempts to access undefined attribute result in a runtime error; you can check if an attribute is defined by using the `defined( attribute )` operator. One notable exception to this rule are attributes of bgppath and *clist types, where undefined value is regarded as empty bgppath/*clist for most purposes.

Attributes can be defined by just setting them in filters. Custom attributes have to be first declared by attribute global option. You can also undefine optional attribute back to non-existence by using the `unset( attribute )` operator.

Common route attributes are:

`prefix net`

  The network prefix or anything else the route is talking about. The primary key of the routing table. Read-only. (See the chapter about routes.)

`int preference`

  Preference of the route.

`ip from`

  The router which the route has originated from.

`ip gw`

  Next hop packets routed using this route should be forwarded to.

`string proto`

  The name of the protocol which the route has been imported from. Read-only.

`enum source`

  what protocol has told me about this route. Possible values: `RTS_STATIC`, `RTS_INHERIT`, `RTS_DEVICE`, `RTS_RIP`, `RTS_OSPF`, `RTS_OSPF_IA`, `RTS_OSPF_EXT1`, `RTS_OSPF_EXT2`, `RTS_BGP`, `RTS_PIPE`, `RTS_BABEL`.

`enum dest`

  Type of destination the packets should be sent to (`RTD_ROUTER` for forwarding to a neighboring router, `RTD_DEVICE` for routing to a directly-connected network, `RTD_MULTIPATH` for multipath destinations, `RTD_BLACKHOLE` for packets to be silently discarded, `RTD_UNREACHABLE`, `RTD_PROHIBIT` for packets that should be returned with ICMP host unreachable / ICMP administratively prohibited messages). Can be changed, but only to `RTD_BLACKHOLE`, `RTD_UNREACHABLE` or `RTD_PROHIBIT`.

`string ifname`

  Name of the outgoing interface. Sink routes (like blackhole, unreachable or prohibit) and multipath routes have no interface associated with them, so `ifname` returns an empty string for such routes. Setting it would also change route to a direct one (remove gateway).

`int ifindex`

  Index of the outgoing interface. System wide index of the interface. May be used for interface matching, however indexes might change on interface creation/removal. Zero is returned for routes with undefined outgoing interfaces. Read-only.

`int weight`

  Multipath weight of route next hops. Valid values are 1-256. Reading returns the weight of the first next hop, setting it sets weights of all next hops to the specified value. Therefore, this attribute is not much useful for manipulating individual next hops of an ECMP route, but can be used in BGP multipath setup to set weights of individual routes that are merged to one ECMP route during export to the Kernel protocol (with active marge paths option).

`int gw_mpls`

  Outgoing MPLS label attached to route (i.e., incoming MPLS label on the next hop router for this label-switched path). Reading returns the label value and setting it sets it to the start of the label stack. Setting implicit-NULL label (3) disables the MPLS label stack. Only the first next hop and only one label in the label stack supported right now. This is experimental option, will be likely changed in the future to handle full MPLS label stack.

`int igp_metric`

  The optional attribute that can be used to specify a distance to the network for routes that do not have a native protocol metric attribute (like `ospf_metric1` for OSPF routes). It is used to fill in local_metric attribute when recursive nexthops are resolved, so that BGP can compare internal distances to boundary routers (see below).

`int local_metric`

  Attribute storing the route's metric to the resolved recursive nexthop, mainly used by BGP best route selection. Also known as "interior cost" in RFC 4271. It's the copy of the underlying protocol's metric, e.g. `ospf_metric1`, or the generic `igp_metric`. For recursive nexthops, setting the `local_metric` attribute in the import filters is futile, as the nexthop resolver overwrites it. Note: In versions 3.0.x and 3.1.x, the `igp_metric` attribute was confusingly used for this purpose.

`int mpls_label`

  Local MPLS label attached to the route. This attribute is produced by MPLS-aware protocols for labeled routes. It can also be set in import filters to assign static labels, but that also requires static MPLS label policy.

`enum mpls_policy`

  For MPLS-aware protocols, this attribute defines which MPLS label policy will be used for the route. It can be set in import filters to change it on per-route basis. Valid values are `MPLS_POLICY_NONE` (no label), `MPLS_POLICY_STATIC` (static label), `MPLS_POLICY_PREFIX` (per-prefix label), `MPLS_POLICY_AGGREGATE` (aggregated label), and `MPLS_POLICY_VRF` (per-VRF label). See MPLS label policy for details.

`int mpls_class`

  When MPLS label policy is set to `aggregate`, it may be useful to apply more fine-grained aggregation than just one based on next hops. When routes have different value of this attribute, they will not be aggregated under one local label even if they have the same next hops.

Protocol-specific route attributes are described in the corresponding protocol sections.

## 5.6 Other statements

The following statements are available:

`variable = expr`

  Set variable (or route attribute) to a given value.

`accept|reject [ expr ]`

  Accept or reject the route, possibly printing `expr`.

`return expr`

  Return `expr` from the current function, the function ends at this point.

`print|printn expr [, expr...]`

  Prints given expressions; useful mainly while debugging filters. The `printn` variant does not terminate the line.

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