PostgreSQL client unable to resolve hostnames in Docker container

Question

I'm trying to use Docker Compose to create two containers: one containing PostgreSQL (actually PostGIS), and the other containing a web app that queries PostgreSQL.

My docker-compose.yaml is:

version: '3.1'

services:
  db:
    build: 
      context: .
      dockerfile: Dockerfile-db
    restart: always
    ports:
      - 5432:5432
  web:
    build:
      context: .
      dockerfile: Dockerfile-web
      args:
        PGHOSTADDR: db
    restart: always
    ports:
      - 80:80
    depends_on:
      - db

The application Dockerfile is:

FROM php:apache
ARG PGHOSTADDR
ENV PGHOSTADDR=$PGHOSTADDR
COPY . /var/www/html/
RUN apt-get update && mkdir -p /usr/share/man/man1 /usr/share/man/man7 && apt-get install -y postgresql-client strace ltrace iputils-ping bind9-host dnsutils
CMD until psql -c '\q' ; do sleep 1 ; done ; /usr/sbin/apache2

The PostgreSQL Dockerfile is just:

FROM mdillon/postgis

The PostgreSQL container starts and appears to work perfectly - I can query it from the host, and I can query it from the application container so long as I use the IP address of the PostgreSQL container.

However, 'psql' running in the app container is unable to reach PostgreSQL:

$ docker-compose up -d && echo MARK && docker-compose logs --follow web
WARNING: The Docker Engine you're using is running in swarm mode.

Compose does not use swarm mode to deploy services to multiple nodes in a swarm. All containers will be scheduled on the current node.

To deploy your application across the swarm, use `docker stack deploy`.

Starting myapp_db_1 ... 
Starting myapp_db_1 ... done
Starting myapp_web_1 ... 
Starting myapp_web_1 ... done
MARK
Attaching to myapp_web_1
web_1  | psql: could not translate host name "db" to address: Name or service not known
web_1  | psql: could not translate host name "db" to address: Name or service not known
web_1  | psql: could not translate host name "db" to address: Name or service not known
web_1  | psql: could not translate host name "db" to address: Name or service not known

Usually, 'psql' outputs this diagnostic when it fails to resolve the hostname given. However hostname resolution seems to be working fine in the app container:

$ docker exec -it myapp_web_1 bash
root@3a11fb7a22a1:/var/www/html# dig db

; <<>> DiG 9.10.3-P4-Debian <<>> db
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 21082
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;db.                IN  A

;; ANSWER SECTION:
db.         600 IN  A   172.22.0.2

;; Query time: 0 msec
;; SERVER: 127.0.0.11#53(127.0.0.11)
;; WHEN: Fri Dec 29 04:18:32 UTC 2017
;; MSG SIZE  rcvd: 38

root@3a11fb7a22a1:/var/www/html# ping -c 2 db
PING db (172.22.0.2) 56(84) bytes of data.
64 bytes from myapp_db_1.myapp_default (172.22.0.2): icmp_seq=1 ttl=64 time=0.036 ms
64 bytes from myapp_db_1.myapp_default (172.22.0.2): icmp_seq=2 ttl=64 time=0.045 ms

--- db ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1020ms
rtt min/avg/max/mdev = 0.036/0.040/0.045/0.007 ms
root@3a11fb7a22a1:/var/www/html#

Also, the PostgreSQL client and server appear to be working perfectly apart from this hostname-related problem:

root@3a11fb7a22a1:/var/www/html# unset PGHOSTADDR
root@3a11fb7a22a1:/var/www/html# psql -h 172.22.0.2 -U postgres -c 'SELECT 1;'
 ?column? 
----------
        1
(1 row)

root@3a11fb7a22a1:/var/www/html#

Why isn't 'psql' resolving 'db' to 172.22.0.2, as 'dig' is able to do, and hence working?

AnotherSmellyGeek · Accepted Answer · 2017-12-29 05:30:47Z

2

I was setting PGHOSTADDR, which needs to be a dotted-decimal IP address, whereas I should have been setting PGHOST, which can be a hostname, as is clearly explained in the PostgreSQL documentation.

Unsetting PGHOSTADDR and instead setting PGHOST makes 'psql' work as expected:

root@3a11fb7a22a1:/var/www/html# psql -U postgres -c 'SELECT 1;'
psql: could not translate host name "db" to address: Name or service not known
root@3a11fb7a22a1:/var/www/html# unset PGHOSTADDR
root@3a11fb7a22a1:/var/www/html# PGHOST=db
root@3a11fb7a22a1:/var/www/html# export PGHOST
root@3a11fb7a22a1:/var/www/html# psql -U postgres -c 'SELECT 1;'
 ?column? 
----------
        1
(1 row)

root@3a11fb7a22a1:/var/www/html#

Another failure to RTFM.

answered Dec 29, 2017 at 5:30

AnotherSmellyGeek

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1 Comment

AnotherSmellyGeek Over a year ago

I find it unnecessarily confusing and redundant to have both PGHOST and PGHOSTADDR. I should submit a patch to make both of them accept either a hostname or an IP address.

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