UDP matches
This section describes matches that will only work together with UDP packets. These matches are implicitly loaded when you specify the --protocol UDP match and will be available after this specification. Note that UDP packets are not connection oriented, and hence there is no such thing as different flags to set in the packet to give data on what the datagram is supposed to do, such as open or closing a connection, or if they are just simply supposed to send data. UDP packets do not require any kind of acknowledgment either. If they are lost, they are simply lost (Not taking ICMP error messaging etc into account). This means that there are quite a lot less matches to work with on a UDP packet than there is on TCP packets. Note that the state machine will work on all kinds of packets even though UDP or ICMP packets are counted as connectionless protocols. The state machine works pretty much the same on UDP packets as on TCP packets.
Table 10-3. UDP matches
| Match | --sport, --source-port |
| Kernel | 2.3, 2.4, 2.5 and 2.6 |
| Example | iptables -A INPUT -p udp --sport 53 |
| Explanation | This match works exactly the same as its TCP counterpart. It is used to perform matches on packets based on their source UDP ports. It has support for port ranges, single ports and port inversions with the same syntax. To specify a UDP port range, you could use 22:80 which would match UDP ports 22 through 80. If the first value is omitted, port 0 is assumed. If the last port is omitted, port 65535 is assumed. If the high port comes before the low port, the ports switch place with each other automatically. Single UDP port matches look as in the example above. To invert the port match, add a ! sign, --source-port ! 53. This would match all ports but port 53. The match can understand service names, as long as they are available in the /etc/services file. Note that this match does not handle multiple separated ports and port ranges. For more information about this, look at the multiport match extension. |
| Match | --dport, --destination-port |
| Kernel | 2.3, 2.4, 2.5 and 2.6 |
| Example | iptables -A INPUT -p udp --dport 53 |
| Explanation | The same goes for this match as for --source-port above. It is exactly the same as for the equivalent TCP match, but here it applies to UDP packets. It matches packets based on their UDP destination port. The match handles port ranges, single ports and inversions. To match a single port you use, for example, --destination-port 53, to invert this you would use --destination-port ! 53. The first would match all UDP packets going to port 53 while the second would match packets but those going to the destination port 53. To specify a port range, you would, for example, use --destination-port 9:19. This example would match all packets destined for UDP port 9 through 19. If the first port is omitted, port 0 is assumed. If the second port is omitted, port 65535 is assumed. If the high port is placed before the low port, they automatically switch place, so the low port winds up before the high port. Note that this match does not handle multiple ports and port ranges. For more information about this, look at the multiport match extension. |
ICMP matches
These are the ICMP matches. These packets are even more ephemeral, that is to say short lived, than UDP packets, in the sense that they are connectionless. The ICMP protocol is mainly used for error reporting and for connection controlling and suchlike. ICMP is not a protocol subordinated to the IP protocol, but more of a protocol that augments the IP protocol and helps in handling errors. The headers of ICMP packets are very similar to those of the IP headers, but differ in a number of ways. The main feature of this protocol is the type header, that tells us what the packet is for. One example is, if we try to access an unaccessible IP address, we would normally get an ICMP host unreachable in return. For a complete listing of ICMP types, see the ICMP types appendix. There is only one ICMP specific match available for ICMP packets, and hopefully this should suffice. This match is implicitly loaded when we use the --protocol ICMP match and we get access to it automatically. Note that all the generic matches can also be used, so that among other things we can match on the source and destination addresses.