diff --git a/api.js b/api.js
index 55966dc..87b1ff6 100644
--- a/api.js
+++ b/api.js
@@ -9,6 +9,7 @@ import {
 	createDbHafas as createHafas,
 	defaults as dbHafasDefaults,
 } from 'db-hafas'
+import {createWriteStream} from 'node:fs'
 import {createHafasRestApi} from 'hafas-rest-api'
 import createHealthCheck from 'hafas-client-health-check'
 import Redis from 'ioredis'
@@ -42,6 +43,21 @@ if (process.env.RANDOM_LOCAL_ADDRESSES_RANGE) {
 	}
 }
 
+if (process.env.HAFAS_REQ_RES_LOG_FILE) {
+	const hafasLogPath = process.env.HAFAS_REQ_RES_LOG_FILE
+	const hafasLog = createWriteStream(hafasLogPath, {flags: 'a'}) // append-only
+	hafasLog.on('error', (err) => console.error('hafasLog error', err))
+
+	customDbProfile.logRequest = (ctx, req, reqId) => {
+		console.error(reqId, 'req', req.body + '') // todo: remove
+		hafasLog.write(JSON.stringify([reqId, 'req', req.body + '']) + '\n')
+	}
+	customDbProfile.logResponse = (ctx, res, body, reqId) => {
+		console.error(reqId, 'res', body + '') // todo: remove
+		hafasLog.write(JSON.stringify([reqId, 'res', body + '']) + '\n')
+	}
+}
+
 // todo: use process.env.HAFAS_USER_AGENT if defined
 let hafas = createHafas(pkg.name, {
 	profile: customDbProfile,