API Reference

The “net” object

class switchyard.switchy_real.PyLLNet(devlist, name=None)[source]

A class that represents a collection of network devices on which packets can be received and sent.

An object of this class is passed into the main function of a user’s Switchyard program. Using methods on this object, a user can send/receive packets and query the device for what interfaces are available and how they are configured.


Given an IP address, return the interface that ‘owns’ this address


Given a MAC address, return the interface that ‘owns’ this address


Given a device name, return the corresponding interface object


Return a list of interfaces incident on this node/router. Each item in the list is an Interface object, each of which includes name, ethaddr, ipaddr, and netmask attributes.


Alias for interface_by_ipaddr


Alias for interface_by_macaddr


Alias for interface_by_name


Alias for interfaces() method.

recv_packet(timeout=None, timestamp=False)[source]

Receive packets from any device on which one is available. Blocks until it receives a packet, unless a timeout value >=0 is given. Raises Shutdown exception when device(s) are shut down (i.e., on a SIGINT to the process). Raises NoPackets when there are no packets that can be read.

Returns a tuple of length 2 or 3, depending on whether the timestamp is desired.

  • device: network device name on which packet was received as a string
  • timestamp: floating point value of time at which packet was received (optionally returned; only if timestamp=True)
  • packet: Switchyard Packet object.
send_packet(dev, packet)[source]

Send a Switchyard Packet object on the given device (string name of device).

Raises SwitchyException if packet object isn’t valid, or device name isn’t recognized.


Should be called by Switchyard user code when a network object is being shut down. (This method cleans up internal threads and network interaction objects.)

Packet parsing and construction

class switchyard.lib.packet.Packet(raw=None, first_header=None)[source]

Base class for packet headers.

The Packet class acts as a container for packet headers. The + and += operators are defined for use with the Packet class to add on headers (to the end of the packet). Indexing can also be done with Packet objects to access individual header objects. Indexes may be integers (from 0 up to, but not including, the number of packet headers), or indexes may also be packet header class names. Exceptions are raised for invaliding indexing of either kind.

>>> p = Packet()
>>> p += Ethernet()
>>> p[0]
<switchyard.lib.packet.ethernet.Ethernet object at 0x10632bb08>
>>> p[Ethernet]
<switchyard.lib.packet.ethernet.Ethernet object at 0x10632bb08>
>>> str(p)
'Ethernet 00:00:00:00:00:00->00:00:00:00:00:00 IP'
>>> str(p[0])
'Ethernet 00:00:00:00:00:00->00:00:00:00:00:00 IP'
>>> str(p[Ethernet])
'Ethernet 00:00:00:00:00:00->00:00:00:00:00:00 IP'

Add a PacketHeaderBase derived class object, or a raw bytes object as the next “header” item in this packet. Note that ‘header’ may be a slight misnomer since the last portion of a packet is considered application payload and not a header per se.


Alias for add_header

static from_bytes(raw, first_header)[source]

Create a new packet by parsing the contents of a bytestring

get_header(hdrclass, returnval=None)[source]

Return the first header object that is of class hdrclass, or None if the header class isn’t found.

get_header_index(hdrclass, startidx=0)[source]

Return the first index of the header class hdrclass starting at startidx (default=0), or -1 if the header class isn’t found in the list of headers.


Return a list of packet header names in this packet.

insert_header(idx, ph)[source]

Insert a PacketHeaderBase-derived object at index idx the list of headers. Any headers previously in the Packet from index idx:len(ph) are shifted to make room for the new packet.


Return the number of headers in the packet.


Insert a PacketHeader object at the beginning of this packet (i.e., as the first header of the packet).


Return the packed length of this header


Returns serialized bytes object representing all headers/ payloads in this packet

To delete/remove a header, you can use the del operator as if the packet object is a Python list:

>>> del p[0] # delete/remove first header in packet

You can assign new header objects to a packet by integer index, but not by packet header class index:

>>> p[0] = Ethernet() # assign a new Ethernet header to index 0

Header classes

In this section, detailed documentation for all packet header classes is given. For each header class, there are three common methods that may be useful and which are not documented below for clarity:

  • size(): returns the number of bytes that the header would consist of when serialized to wire format
  • to_bytes(): returns the serialized (wire format) representation of the packet as a byte string
  • from_bytes(b): parses a byte string representing this packet header and constructs the various header fields from the raw bytes

Ethernet header

class switchyard.lib.packet.Ethernet(**kwargs)[source]

Represents an Ethernet header with fields src (source Ethernet address), dst (destination Ethernet address), and ethertype (type of header to come in the packet after the Ethernet header). All valid ethertypes are defined below.

class switchyard.lib.packet.common.EtherType[source]
IP = 0x0800
IPv4 = 0x0800
ARP = 0x0806
x8021Q = 0x8100
IPv6 = 0x86dd
SLOW = 0x8809
MPLS = 0x8847
x8021AD = 0x88a8
LLDP = 0x88cc
x8021AH = 0x88e7
IEEE8023 = 0x05dc

The EtherType class is derived from the built-in Python Enumerated class type. Note that some values start with ‘x’ since they must start with an alphabetic character to be valid in the enum.

By default, the Ethernet header addresses are all zeroes (“00:00:00:00:00:00”), and the ethertype is IPv4. Here is an example of creating an Ethernet header and setting the header fields to non-default values:

>>> e = Ethernet()
>>> e.src = "de:ad:00:00:be:ef"
>>> e.dst = "ff:ff:ff:ff:ff:ff"
>>> e.ethertype = EtherType.ARP

ARP (address resolution protocol) header

class switchyard.lib.packet.Arp(**kwargs)[source]
class switchyard.lib.packet.common.ArpOperation[source]
Request = 1
Reply = 2

The Arp class is used for constructing ARP (address resolution protocol) requests and replies. The hardwaretype property defaults to Ethernet, so you don’t need to set that when an Arp object is instantiated. The operation can be set using the enumerated type ArpOperation, as indicated above. The remaining fields hold either EthAddr or IPv4Address objects, and can be initialized using string representations of Ethernet or IPv4 addresses as appropriate. Below is an example of creating an ARP request. You can assume in the example that the senders Ethernet and IPv4 addresses are srchw and srcip, respectively. You can also assume that the IPv4 address for which we are requesting the Ethernet address is targetip.

ether = Ethernet()
ether.src = srchw
ether.dst = 'ff:ff:ff:ff:ff:ff'
ether.ethertype = EtherType.ARP
arp = Arp()
arp.operation = ArpOperation.Request
arp.senderhwaddr = srchw
arp.senderprotoaddr = srcip
arp.targethwaddr = 'ff:ff:ff:ff:ff:ff'
arp.targetprotoaddr = targetip
arppacket = ether + arp

IP version 4 header

class switchyard.lib.packet.IPv4(**kwargs)[source]

Represents an IP version 4 packet header. All properties relate to specific fields in the header and can be inspected and/or modified.

Note that the field named “hl” (“h-ell”) stands for “header length”. It is the size of the header in 4-octet quantities. It is a read-only property (cannot be set).

Note also that some IPv4 header option classes are available in Switchyard, but are currently undocumented.


Deprecated property. Use dst instead.


Deprecated property. Use src instead.

class switchyard.lib.packet.common.IPProtocol[source]
ICMP = 1
TCP = 6
UDP = 17

The IPProtocol class derives from the Python 3-builtin Enumerated class type. There are other protocol numbers defined. See switchyard.lib.packet.common for all defined values.

A just-constructed IPv4 header defaults to having all zeroes for the source and destination addresses (‘’) and the protocol number defaults to ICMP. An example of creating an IPv4 header and setting various fields is shown below:

>>> ip = IPv4()
>>> ip.srcip = ''
>>> ip.dstip = ''
>>> ip.protocol = IPProtocol.UDP
>>> ip.ttl = 64

UDP (user datagram protocol) header

class switchyard.lib.packet.UDP(**kwargs)[source]

The UDP header contains just source and destination port fields.


To construct a packet that includes an UDP header as well as some application data, the same pattern of packet construction can be followed:

>>> p = Ethernet() + IPv4() + UDP()
>>> p[1].protocol = IPProtocol.UDP
>>> p[2].srcport = 4444
>>> p[2].dstport = 5555
>>> p += b'These are some application data bytes'
>>> print (p)
Ethernet 00:00:00:00:00:00->00:00:00:00:00:00 IP | IPv4> UDP | UDP 4444->5555 | RawPacketContents (37 bytes) b'These are '...

Note that we didn’t set the IP addresses or Ethernet addresses above, but did set the IP protocol to correctly match the next header (UDP). Adding a payload to a packet is as simple as tacking on a Python bytes object. You can also construct a RawPacketContents header, which is just a packet header class that wraps a set of raw bytes.

TCP (transmission control protocol) header

class switchyard.lib.packet.TCP(**kwargs)[source]

Represents a TCP header. Includes properties to access/modify TCP header fields.


Setting TCP header flags can be done by assigning 1 to any of the mnemonic flag properties:

>>> t = TCP()
>>> t.SYN = 1

To check whether a flag has been set, you can simply inspect the the flag value:

>>> if t.SYN:
>>> ...

ICMP (Internet control message protocol) header

class switchyard.lib.packet.ICMP(**kwargs)[source]

A mother class for all ICMP message types. It holds a reference to another object that contains the specific ICMP data (icmpdata), given a particular ICMP type. Just setting the icmptype causes the data object to change (the change happens automatically when you set the icmptype). The icmpcode field will also change, but it only changes to some valid code given the new icmptype.

Represents an ICMP packet header.

class switchyard.lib.packet.common.ICMPType[source]
EchoReply = 0
DestinationUnreachable = 3
SourceQuench = 4
Redirect = 5
EchoRequest = 8
TimeExceeded = 11

The icmptype and icmpcode header fields determine the value stored in the icmpdata property. When the icmptype is set to a new value, the icmpdata field is automatically set to the correct object.

>>> i = ICMP()
>>> print (i)
ICMP EchoRequest 0 0 (0 data bytes)
>>> i.icmptype = ICMPType.TimeExceeded
>>> print (i)
ICMP TimeExceeded:TTLExpired 0 bytes of raw payload (b'') OrigDgramLen: 0
>>> i.icmpcode
<ICMPCodeTimeExceeded.TTLExpired: 0>
>>> i.icmpdata
<switchyard.lib.packet.icmp.ICMPTimeExceeded object at 0x10d3a3308>

Notice above that when the icmptype changes, other contents in the ICMP header object change appropriately.

To access and/or modify the payload (i.e., data) that comes after the ICMP header, use icmpdata.data. This object is a raw bytes object and can be accessed and or set. For example, with many ICMP error messages, up to the first 28 bytes of the “dead” packet should be included, starting with the IPv4 header. To do that, you must set the icmpdata.data attribute with the byte-level representation of the IP header data you want to include, as follows:

>>> i.icmpdata.data
>>> i.icmpdata.data = pkt.to_bytes()[:28]
>>> i.icmpdata.origdgramlen = len(pkt)
>>> print (i)
ICMP TimeExceeded:TTLExpired 28 bytes of raw payload (b'E\x00\x00\x14\x00\x00\x00\x00\x00\x01') OrigDgramLen: 42

In the above code segment, pkt should be a Packet object that just contains the IPv4 header and any subsequent headers and data. It must not include an Ethernet header. If you need to strip an Ethernet header, you can get its index (pkt.get_header_index(Ethernet)), then remove the header by index (del pkt[index]).

Notice that above, the to_bytes method returns the byte-level representation of the IP header we’re including as the payload. The to_bytes method can be called on any packet header, or on an packet object (in which case all packet headers will be byte-serialized).

To set the icmpcode, a dictionary called ICMPTypeCodeMap is defined in switchyard.lib.packet. Keys in the dictionary are of type ICMPType, and values for each key is another enumerated type indicating the valid codes for the given type.

>>> from switchyard.lib.packet import *
>>> ICMPTypeCodeMap[ICMPType.DestinationUnreachable]
<enum 'DestinationUnreachable'>

Just getting the dictionary value isn’t particularly helpful, but if you coerce the enum to a list, you can see all valid values:

>>> list(ICMPTypeCodeMap[ICMPType.DestinationUnreachable])
[ <DestinationUnreachable.ProtocolUnreachable: 2>,
  <DestinationUnreachable.SourceHostIsolated: 8>,
  <DestinationUnreachable.FragmentationRequiredDFSet: 4>,
  <DestinationUnreachable.HostUnreachable: 1>,
  <DestinationUnreachable.DestinationNetworkUnknown: 6>,
  <DestinationUnreachable.NetworkUnreachableForTOS: 11>,
  <DestinationUnreachable.HostAdministrativelyProhibited: 10>,
  <DestinationUnreachable.DestinationHostUnknown: 7>,
  <DestinationUnreachable.HostPrecedenceViolation: 14>,
  <DestinationUnreachable.PrecedenceCutoffInEffect: 15>,
  <DestinationUnreachable.NetworkAdministrativelyProhibited: 9>,
  <DestinationUnreachable.NetworkUnreachable: 0>,
  <DestinationUnreachable.SourceRouteFailed: 5>,
  <DestinationUnreachable.PortUnreachable: 3>,
  <DestinationUnreachable.CommunicationAdministrativelyProhibited: 13>,
  <DestinationUnreachable.HostUnreachableForTOS: 12> ]

Another example, but with the much simpler EchoRequest:

>>> list(ICMPTypeCodeMap[ICMPType.EchoRequest])
[<EchoRequest.EchoRequest: 0>]

If you try to set the icmpcode to an invalid value, an exception will be raised:

>>> i = ICMP()
>>> i.icmptype = ICMPType.DestinationUnreachable
>>> i.icmpcode = 44
Traceback (most recent call last):

You can either (validly) set the code using an integer, or a valid enumerated type value:

>>> i.icmpcode = 2
>>> print(i)
ICMP DestinationUnreachable:ProtocolUnreachable 0 bytes of raw payload (b'') NextHopMTU: 0
>>> i.icmpcode = ICMPTypeCodeMap[i.icmptype].HostUnreachable
>>> print (i)
ICMP DestinationUnreachable:HostUnreachable 0 bytes of raw payload (b'') NextHopMTU: 0

Below are shown the ICMP data classes, as well as any properties that can be inspected and/or modified on them.

class switchyard.lib.packet.ICMPEchoReply[source]
class switchyard.lib.packet.ICMPDestinationUnreachable[source]
class switchyard.lib.packet.ICMPSourceQuench[source]
class switchyard.lib.packet.ICMPRedirect[source]
class switchyard.lib.packet.ICMPEchoRequest[source]
class switchyard.lib.packet.ICMPTimeExceeded[source]

Utility functions

exception switchyard.lib.common.NoPackets[source]

Exception that is raised in user Switchyard program when the recv_packet() method is called on the net object and there are no packets available.

exception switchyard.lib.common.Shutdown[source]

Exception that is raised in user Switchyard program when the framework is being shut down.


Convenience function for debugging message.


Convenience function for failure message.


Convenience function for info message.


Convenience function for warning message.


Invoke the interactive debugger. Can be used anywhere within a Switchyard program.