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The iplookup router

The `iplookup' router was written to fulfil a specific requirement in Cambridge. For this reason, it is not included in the binary of Exim by default. If you want to include it, then you must set


in your `Local/Makefile' configuration file.

The `iplookup' router routes an address by sending it over a TCP or UDP connection to one or more specific hosts. The host can then return the same or a different address -- in effect rewriting the recipient address in the message's envelope. If this process fails, the address can be passed on to other routers, or delivery can be deferred.

Background, for those that are interested: We have an Oracle database of all Cambridge users, and one of the bits of data it maintains for each user is where to send mail addressed to `<user>@cam.ac.uk'. The MX records for `cam.ac.uk' point to a central machine that has a large alias list that is abstracted from the database. Mail from outside is switched by this system, and originally internal mail was also done this way. However, this resulted in a fair number of messages travelling from some of our larger systems to the switch and back again. The Oracle machine now runs a UDP service that can be called by the `iplookup' router in Exim to find out where `<user>@cam.ac.uk' addresses really have to go; this saves passing through the central switch, and in many cases saves doing any remote delivery at all.

Since `iplookup' is just a rewriting router, a transport must not be specified for it.

hosts (iplookup) option

Option: hosts
Type: string
Default: unset

This option must be supplied. Its value is a colon-separated list of host names. The hosts are looked up using `gethostbyname()' and are tried in order until one responds to the query.

optional (iplookup) option

Option: optional
Type: boolean
Default: false

If `optional' is true, then if no response is obtained from any host, the address is passed on to the next router. If `optional' is false, delivery to this address is deferred.

protocol (iplookup) option

Option: protocol
Type: string
Default: "udp"

This option can be set to `udp' or `tcp' to specify which of the two protocols is to be used.

query (iplookup) option

Option: query
Type: string
Default: "${local_part}@${domain} ${local_part}@${domain}"

This defines the content of the query that is sent to the remote hosts. The repetition serves as a way of checking that a response is to the correct query in the default case (see `response_pattern' below).

reroute (iplookup) option

Option: reroute
Type: string
Default: unset

If this option is not set, the rerouted address is precisely the byte string returned by the remote host, up to the first white space, if any. If set, the string is expanded to form the rerouted address. It can include parts matched in the response by `response_pattern' by means of numeric variables such as `$1', `$2', etc. The variable `$0' refers to the entire input string, whether or not a pattern is in use. In all cases, the rerouted address must end up in the form <local_part>@<domain>.

response_pattern (iplookup) option

Option: response_pattern
Type: string
Default: unset

This option can be set to a regular expression that is applied to the string returned from the remote host. If the pattern does not match the response, the router fails. If `response_pattern' is not set, no checking of the response is done, unless the query was defaulted, in which case there is a check that the text returned after the first white space is the original address. This checks that the answer that has been received is in response to the correct question. For example, if the response is just a new domain, the following could be used:

response_pattern = "^([^@]+)$"
reroute = "${local_part}@${1}"

service (iplookup) option

Option: service
Type: integer
Default: 0

This option must be supplied. It specifies the port number for the TCP or UDP call.

timeout (iplookup) option

Option: timeout
Type: time
Default: 5s

This specifies the amount of time to wait for a response from the remote machine. The same timeout is used for the `connect()' function for a TCP call. It does not apply to UDP.

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