UDPRewriter

UDPRewriter Element Documentation

NAME

UDPRewriter — Click element; rewrites TCP/UDP packets' addresses and ports

SYNOPSIS

UDPRewriter(INPUTSPEC1, ..., INPUTSPECn [, keywords])

DESCRIPTION

Rewrites the source address, source port, destination address, and/or destination port on UDP packets, along with their checksums. IPRewriter implements the functionality of a network address/port translator (NAPT). See also IPAddrRewriter and IPAddrPairRewriter, which implement Basic NAT, and TCPRewriter, which implements NAPT plus sequence number changes for TCP packets.

Despite its name, UDPRewriter will validly rewrite both TCP and UDP. However, in most uses, any given UDPRewriter will see packets of only one protocol.

UDPRewriter maintains a mapping table that records how packets are rewritten. The mapping table is indexed by flow identifier, the quadruple of source address, source port, destination address, and destination port. Each mapping contains a new flow identifier and an output port. Input packets with the indexed flow identifier are rewritten to use the new flow identifier, then emitted on the output port. A mapping is written as follows:

    (SA, SP, DA, DP) => (SA', SP', DA', DP') [OUTPUT]

When UDPRewriter receives a packet, it first looks up that packet in the mapping table by flow identifier. If the table contains a mapping for the input packet, then the packet is rewritten according to the mapping and emitted on the specified output port (but see the CONSTRAIN_FLOW keyword argument). If there was no mapping, the packet is handled by the INPUTSPEC corresponding to the input port on which the packet arrived. (There are as many input ports as INPUTSPECs.) Most INPUTSPECs install new mappings, so that future packets from the same TCP or UDP flow are handled by the mapping table rather than some INPUTSPEC. The six forms of INPUTSPEC handle input packets as follows:

'drop' or 'discard' — Discards input packets.
'pass OUTPUT' — Sends input packets to output port OUTPUT. No mappings are installed.
'keep FOUTPUT ROUTPUT' — Installs mappings that preserve the input packet's flow ID. Specifically, given an input packet with flow ID (SA, SP, DA, DP, PROTO), two mappings are installed:
```
      (SA, SP, DA, DP, PROTO) => (SA, SP, DA, DP) [FOUTPUT]
      (DA, DP, SA, SP, PROTO) => (DA, DP, SA, SP) [ROUTPUT]
```
Thus, the input packet is emitted on output port FOUTPUT unchanged, and packets from the reply flow are emitted on output port ROUTPUT unchanged.
'pattern SADDR SPORT DADDR DPORT FOUTPUT ROUTPUT' — Creates a mapping according to the given pattern, 'SADDR SPORT DADDR DPORT'. Any pattern field may be a dash '-', in which case the packet's corresponding field is left unchanged. For instance, the pattern '1.0.0.1 20 - -' will rewrite input packets' source address and port, but leave its destination address and port unchanged. SPORT may be a port range 'L-H'; UDPRewriter will choose a source port in that range so that the resulting mappings don't conflict with any existing mappings. The input packet's source port is preferred, if it is available; otherwise, a random port is chosen. If no source port is available, the packet is dropped. To allocate source ports sequentially (which can make testing easier), append a pound sign to the range, as in '1024-65535#'. To choose a random port rather than preferring the source, append a '?'.

Say a packet with flow ID (SA, SP, DA, DP, PROTO) is received, and the corresponding new flow ID is (SA', SP', DA', DP'). Then two mappings are installed:
```
      (SA, SP, DA, DP, PROTO) => (SA', SP', DA', DP') [FOUTPUT]
      (DA', DP', SA', SP', PROTO) => (DA, DP, SA, SP) [ROUTPUT]
```
Thus, the input packet is rewritten and sent to FOUTPUT, and packets from the reply flow are rewritten to look like part of the original flow and sent to ROUTPUT.
'pattern PATNAME FOUTPUT ROUTPUT' — Like 'pattern' above, but refers to named patterns defined by an IPRewriterPatterns element.
'ELEMENTNAME' — Creates mappings according to instructions from the element ELEMENTNAME. This element must implement the IPMapper interface. One example mapper is RoundRobinIPMapper.

UDPRewriter has no mappings when first initialized.

Input packets must have their IP header annotations set. Non-TCP and UDP packets, and second and subsequent fragments, are dropped unless they arrive on a 'pass' input port. UDPRewriter changes IP packet data and, optionally, destination IP address annotations; see the DST_ANNO keyword argument below.

Keyword arguments are:

TIMEOUT time — Time out connections every time seconds. Default is 5 minutes.
STREAMING_TIMEOUT time — Timeout streaming connections every time seconds. A "streaming" connection, in contrast to an "RPC-like" connection, comprises at least 3 packets and at least one packet in each direction. Default is the TIMEOUT setting.
GUARANTEE time — Preserve each connection mapping for at least time seconds after each successfully processed packet. Defaults to 5 seconds. Incoming flows are dropped if a UDPRewriter's mapping table is full of guaranteed flows.
REAP_INTERVAL time — Reap timed-out connections every time seconds. Default is 15 minutes.
MAPPING_CAPACITY capacity — Set the maximum number of mappings this rewriter can hold to capacity. Capacity can either be an integer or the name of another rewriter-like element, in which case this element will share the other element's capacity.
DST_ANNO — Boolean. If true, then set the destination IP address annotation on passing packets to the rewritten destination address. Default is true.

ELEMENT HANDLERS

table (read-only) — Returns a human-readable description of the UDPRewriter's current mapping table.

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