[possible BUG] Reverse DNS lookup occuring even when using `--nodns none`

[AlexWinder]

I'm using the Docker environment to generate an SSL report for a local Docker container - one which is not publicly accessible.

docker run --rm -it ghcr.io/testssl/testssl.sh --nodns none https://172.18.254.2

However, when it does this, it still attempts (and halts) at trying to run an rDNS request. Below is the output with debug level 6:

$ docker run --rm -it ghcr.io/testssl/testssl.sh --nodns none --debug 6 https://172.18.254.2
do_allciphers          = false
do_vulnerabilities     = true
do_beast               = true
do_lucky13             = true
do_breach              = true
do_ccs_injection       = true
do_ticketbleed         = true
do_cipher_per_proto    = false
do_crime               = true
do_freak               = true
do_logjam              = true
do_drown               = true
do_header              = true
do_heartbleed          = true
do_mx_all_ips          = false
do_fs                  = true
do_protocols           = true
do_rc4                 = true
do_starttls_injection  = true
do_grease              = false
do_opossum             = true
do_robot               = true
do_renego              = true
do_cipherlists         = true
do_server_defaults     = true
do_server_preference   = true
do_ssl_poodle          = true
do_tls_fallback_scsv   = true
do_winshock            = true
do_sweet32             = true
do_client_simulation   = true
do_cipher_match        = false
do_tls_sockets         = false
do_mass_testing        = false
do_display_only        = false
do_rating              = true
URI:                   : https://172.18.254.2

#####################################################################
  testssl.sh version 3.3dev from https://testssl.sh/dev/

  This program is free software. Distribution and modification under
  GPLv2 permitted. USAGE w/o ANY WARRANTY. USE IT AT YOUR OWN RISK!

  Please file bugs @ https://testssl.sh/bugs/
#####################################################################

  Using OpenSSL 1.0.2-bad (Mar 28 2025)  [~179 ciphers]
  on 0768cbc70c82:/home/testssl/bin/openssl.Linux.x86_64
  built: Fri Mar 28 16:54:51 2025, platform: linux-x86_64
  Using bash 5.2.37

172.18.254.2:443
URL_PATH: /
Testing all IPv4 addresses (port 443): 172.18.254.2
--------------------------------------------------------------------------------------------------------------------------
 Start 2026-03-30 14:09:11        -->> 172.18.254.2:443 (172.18.254.2) <<--

 rDNS (172.18.254.2):    (instructed to skip DNS queries)

This IP address will never resolve because it's local to my own environment.

I'm unsure if this is a bug, or if this is due to my own bad interpretation of the documentation.

Please advise.

Thanks!

[drwetter]

Hi @AlexWinder ,

I rather suspect a network issue here. I am pretty sure it does no rDNS / PTR request here -- see (instructed to skip DNS queries) . Code is

determine_rdns() {
     local saved_openssl_conf="$OPENSSL_CONF"
     local nodeip="" rdns="" line=""

     [[ "$NODNS" == none ]] && rDNS="(instructed to skip DNS queries)" && return 0        # No DNS lookups at all
     [[ "$NODNS" == min ]] && rDNS="(instructed to minimize DNS queries)" && return 0     # PTR records were not asked for
[..]

Thus closing. If you believe this is wrong, let me know.

[AlexWinder]

Thanks @drwetter

There's a possibility of a network issue, though I'm not 100% certain. My use case is a Docker container running Nginx. I have it running locally on my machine as a development environment and it has a self-signed certificate.

I'm not certain that it was the rDNS stage that is the issue, just that this is the final stage that was printed to the screen when it appeared to hang.

Is there perhaps a debug mode which shows what else is happening, then I can do some more digging to test if it's perhaps halting somewhere else? I found the --debug 6 mode, but it didn't seem to print out very much in the way of debug messages.

Thanks!

[drwetter]

You probably have to exec into the container and then do like bash -x testssl.sh --nodns none --debug 6 https://172.18.254.2 . I thought e.g. set -x ; export SHELLOPTS ; docker run --rm -it ghcr.io/testssl/testssl.sh --nodns none https://172.18.254.2 docker would inherit this but it does not.

But easier would be to just press ^C when running. In my case I get a result like this

rDNS (172.18.254.2):    (instructed to skip DNS queries)^C/usr/local/bin/testssl.sh: connect: Interrupted system call
/usr/local/bin/testssl.sh: line 12040: /dev/tcp/172.18.254.2/443: Interrupted system call

Check whether it's the same for you.

[AlexWinder]

Hi @drwetter

I've found the issue.

I was executing docker run --rm -it ghcr.io/testssl/testssl.sh --nodns none https://172.18.254.2 from my host, but I've just done another test and found what the issue was. I needed to mount the --network within the testssl container:

docker run --network dev-network --rm -it ghcr.io/testssl/testssl.sh --nodns none https://172.18.254.2

This now works.

So perhaps the issue from before was instead that it couldn't reach TCP 443 altogether? But either way, this is resolved. Thanks for your guidance.