Invalid wifi hostname

[leifnel]

The hostname sent in the dhcp request contains an underscore; ESP_123456

This causes DDNS updates to fail, as underscore is not legal in hostname (except as first character)

dhcpd[32702]: Unable to add forward map from ESP_123456.lan. to 192.168.1.152: REFUSED

Solution: Just omit the underscore.

[leifnel]

This is a workaround:

` String hostname = WiFi.hostname();
  hostname[3]='-';
  WiFi.hostname(hostname);
  WiFi.begin(ssid, password);`

This changes the hostname to ESP-123456, which is a valid hostname.

[devyte]

@leifnel when you opened the issue, you were presented with a template requesting specific info. You ignored it its entirety. Please edit your post and fill it in.
Are you talking about an example sketch, your own app, or something that is hardcoded in the libs?

[devyte]

@leifnel Will you be providing the info requested in the issue template? At the very least an example sketch is needed.
Also, I can't find anywhere in the core code where the hostname is set to ESP_somenum. The only place I see that does this is the example sketch here:
libraries/ESP8266mDNS/examples/mDNS-SD_Extended/mDNS-SD_Extended.ino
Is that what you are referring to?

[leifnel]

The relevant code is simply

#include <ESP8266WiFi.h>
void setup() {
  WiFi.mode(WIFI_STA);
  WiFi.begin("ssid", "password");
}

The hostname is in the format ESP_123456, where 123456 is the last 6 characters of the macaddress.
When the device is connecting to a wifi, where the dhcp-server tries to update the bind9 dns-server with the hostname, the ESP_123456 name is denied, because underscore is not a valid character in hostnames.

[igrr]

This looks like an SDK bug, but we can work around it somewhere in WiFi.begin.

[Makuna]

A better place would be in ESP8266WiFiGenericClass::enableSTA() as it is common to the at least three paths to configure and setup WiFi.

[d-a-v]

This recently came to my attention (I had searched for this before, it was too hidden):

1. A "name" (Net, Host, Gateway, or Domain name) is a text string up
   to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus
   sign (-), and period (.).  Note that periods are only allowed when
   they serve to delimit components of "domain style names". (See
   RFC-921, "Domain Name System Implementation Schedule", for
   background).  No blank or space characters are permitted as part of a
   name. No distinction is made between upper and lower case.  The first
   character must be an alpha character.  The last character must not be
   a minus sign or period.  A host which serves as a GATEWAY should have
   "-GATEWAY" or "-GW" as part of its name.  Hosts which do not serve as
   Internet gateways should not use "-GATEWAY" and "-GW" as part of
   their names. A host which is a TAC should have "-TAC" as the last
   part of its host name, if it is a DoD host.  Single character names
   or nicknames are not allowed.

RFCs:
Internet host table specification (1985)
ref-ed from Requirements for Internet Hosts -- Application and Support (1989, section 2.1 Host Names and Numbers)

Hostnames are set in lwIP interfaces (netif, one for STA, one for AP, at least).
They become visible in the network on a DHCP request (STA as a client). That's why hostname must be set up very early in the sketch (a DHCP request is silently done shortly after reset in persistence mode).

For that reason, a fixing check should be done in lwip2 (when setting up network interfaces), and also in WiFi.hostname(name) by a common function in the core that would replace offending chars / fix hostname validity.

[leifnel]

An (unrelated) comment on this: some nameservers (or GUI's) incorrectly prevent entries, which are not hostnames, to have names like _sip.example.org.

[devyte]

Reference:
espressif/ESP8266_NONOS_SDK#208