# Is it possible to evaluate a field in runtime?
**URL:** https://discuss.elastic.co/t/is-it-possible-to-evaluate-a-field-in-runtime/41178
**Category:** Beats
**Created:** [February 8, 2016, 12:02pm UTC](https://discuss.elastic.co/t/is-it-possible-to-evaluate-a-field-in-runtime/41178 "2016-02-08T12:02:58Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Karol\_Stojek](https://discuss.elastic.co/u/Karol_Stojek)
#### Post date: [February 8, 2016, 12:02pm UTC](https://discuss.elastic.co/t/is-it-possible-to-evaluate-a-field-in-runtime/41178/1 "2016-02-08T12:02:58Z")
Hello,
I'm preparing configuration files for my servers.
Currently there is a hostname sent by filebeats. But I'd like to add another field - with IP.
I thought the easiest would be to set the env. variable and evaluate it during the startup of filebeat, but I don't think filebeat supports it.
So is it possible to do it somehow, or I just need to modify the config file on every server by hand and set a proper IP value as a custom field?
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### Author:  [@andrewkroh](https://discuss.elastic.co/u/andrewkroh)
#### Post date: [February 8, 2016, 3:43pm UTC](https://discuss.elastic.co/t/is-it-possible-to-evaluate-a-field-in-runtime/41178/2 "2016-02-08T15:43:09Z")
We have implemented environment variable replacement in config files but it hasn't been released yet. [https://github.com/elastic/beats/pull/715](https://github.com/elastic/beats/pull/715)
It looks like someone requested the ability to have the Logstash lumberjack input add the client IP address to the event in [https://github.com/logstash-plugins/logstash-input-lumberjack/pull/3](https://github.com/logstash-plugins/logstash-input-lumberjack/pull/3) and even submitted a PR that never merged. logstash-input-lumberjack was the predecessor of logstash-input-beats.
[logstash-input-beats](https://github.com/logstash-plugins/logstash-input-beats) seems like the right place to have this feature since it will have the client IP address. If a server was multi-homed it might be hard from Filebeat to determine what the correct IP address is to add to the event. You could open a ticket for this feature in logstash-input-beats if you like. And a PR probably wouldn't be too hard either since someone already did the work for logstash-input-lumberjack.
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### Author:  [@system](https://discuss.elastic.co/u/system)
#### Post date: [July 5, 2017, 9:55pm UTC](https://discuss.elastic.co/t/is-it-possible-to-evaluate-a-field-in-runtime/41178/3 "2017-07-05T21:55:57Z")