sms

just send sms

built in under 6h :)

Usage

curl -L "https://sms.problem.li/send?key=YOUR_KEY&to=0791234567&msg=Hello"
curl -L "https://sms.problem.li/up"  # Health check

Limits

• Per key: 7/min, 1000/day
• Per recipient: 7/min, 40/day
• Global: 5000/day
• Roaming and premium numbers are blocked
• max 70 Characters per Message
• ONLY GSM7 Characters

Architecture

Internet → Caddy (443/HTTPS) → Flask (8080) → Serial → SIM7600E-H 4G HAT → SMS ↑ sms.problem.li → CNAME → sms-problemli.duckdns.org → Raspberry Pi IP

Deployment

On Raspberry Pi 4:

1. mount the SIM7600E-H 4G HAT, connect the antenna to MAIN, set jumpers A+B to UART mode

2. set project

   PROJECT=$(hostname)

3. create Installation directory

   sudo mkdir /opt/$PROJECT && sudo chown -cR pi /opt/$PROJECT

4. clone code

   sudo apt install git
   cd /opt
   sudo git clone https://github.com/blemli/sms $PROJECT
   cd $PROJECT

5. add at least one api key

   cp keys.dic.example keys.dic
   key=$(python3 -c "import secrets; print(secrets.token_hex(32))")
   op item create $key
   echo "problemli $key">>keys.dic

   [!IMPORTANT]

   Make sure to add every key to 1password

6. add duckdns token to .env

   sudo apt install vim
   echo "DUCK_TOKEN=xxxx">>.env

7. run setup

   ./setup/setup.sh

Caution

Make sure roaming and premium numbers are blocked on the provider level

Maintenance

SIM7600 Cheat Sheet

If the hat makes problems you can debug directly with:

minicom -D /dev/serial0

Command	Description	Expected Response
AT	Basic test	OK
AT+CPIN?	SIM status	READY
AT+CSQ	Signal strength	10-31 good, 99 = none
AT+CREG?	Network registration	0,1 or 0,5 = registered
AT+COPS?	Carrier name	e.g. spusu
AT+CMGF=1	Set SMS text mode	OK
AT+CMGS="+41..."	Send SMS, then type msg, end with Ctrl+Z	+CMGS: <id>

Exit minicom: Ctrl+A then X

DuckDNS Logs

journalctl -t duckdns        # all logs
journalctl -t duckdns -f     # follow live

Update Code

setup/update.sh

Add API-Key

name=xyz
key=$(python3 -c "import secrets; print(secrets.token_hex(32))")
echo "$name $key">>keys.dic
sudo service sms restart

Important

make sure to save the key in 1password and send it only via kpaste

Reset Rate Limits

Rate limits are stored in memory. Restart the service to reset all limits:

sudo service sms restart

improvements

• test what happens if provider rejects message
• allow for 160 chars now that all the weird characters are blocked?
• test foreign numbers in swiss network (should work)
• replace swiss.dic with prefixes.dic to make more universal
• remove blacklist since its blocked at provider level
• factor out all the modem code...
• don't lose the rate limits on service restart
• add json/headers support
• add bruno tests
• Load Balancing with multiple pis :)
• rebrand to smsli
• add dictionary to volume
• strip spaces from numbers?