diff --git a/README.md b/README.md
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+# FediBlockHole
+
+A tool for keeping a Mastodon instance blocklist synchronised with remote lists.
+
+## Features
+
+ - Import and export block lists from CSV files.
+ - Read a block list from a remote instance (if a token is configured)
+
+## Installing
+
+Instance admins who want to use this tool will need to add an Application at
+`https://<instance-domain>/settings/applications/` they can authorise with an
+OAuth token. For each instance you connect to, add this token to the config file.
+
+### Reading remote instance blocklists
+
+To read admin blocks from a remote instance, you'll need to ask the instance admin to add a new Application at `https://<instance-domain>/settings/applications/` and then tell you the access token.
+
+The application needs the `admin:read:domain_blocks` OAuth scope, but unfortunately this
+scope isn't available in the current application screen (v4.0.2 of Mastodon at
+time of writing). There is a way to do it with scopes, but it's really
+dangerous, so I'm not going to tell you what it is here.
+
+A better way is to ask the instance admin to connect to the PostgreSQL database
+and add the scope there, like this:
+
+```
+UPDATE oauth_access_tokens
+    SET scopes='admin:read:domain_blocks'
+    WHERE token='<your_app_token>';
+```
+
+When that's done, FediBlockHole should be able to use its token to authorise
+adding or updating domain blocks via the API.
+
+### Writing instance blocklists
+
+To write domain blocks into an instance requires both the
+`admin:read:domain_blocks` and `admin:write:domain_blocks` OAuth scopes. The
+`read` scope is used to read the current list of domain blocks so we update ones
+that already exist, rather than trying to add all new ones and clutter up the
+instance.
+
+Again, there's no way to do this (yet) on the application admin
+screen so we need to ask our destination admins to update the application
+permissions similar to reading domain blocks:
+
+```
+UPDATE oauth_access_tokens
+    SET scopes='admin:read:domain_blocks admin:write:domain_blocks'
+    WHERE token='<your_app_token>';
+```
+
+When that's done, FediBlockHole should be able to use its token to authorise
+adding or updating domain blocks via the API.
+
+## Configuring
+
+Once you have your applications and tokens and scopes set up, create a
+configuration file for FediBlockHole to use. You can put it anywhere and use the
+`-c <configfile>` commandline parameter to tell FediBlockHole where it is.
+
+Or you can use the default location of `/etc/default/fediblockhole.conf.toml`.
+
+As the filename suggests, FediBlockHole uses TOML syntax.
+
+There are 2 key sections:
+
+ 1. `blocklist_instance_sources`: A list of instances to read blocklists from
+ 1. `blocklist_instance_destinations`: A list of instances to write blocklists to
+
+Each is a list of dictionaries of the form:
+```
+{ domain = '<domain_name>', token = '<BearerToken>' }
+```
+
+The `domain` is the fully-qualified domain name of the API host for an instance
+you want to read or write domain blocks to/from. The `BearerToken` is the OAuth
+token for the application that's configured in the instance to allow you to
+read/write domain blocks, as discussed above.
+
+## Using the tool
+
+Once you've configured the tool, run it like this:
+
+```
+fediblock_sync.py -c <configfile_path>
+```
+
+If you put the config file in `/etc/default/fediblockhole.conf.toml` you don't need to pass the config file path.
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diff --git a/bin/fediblock_sync.py b/bin/fediblock_sync.py
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+#!/usr/bin/python3
+# Export and import blocklists via API
+
+import argparse
+import toml
+import csv
+import requests
+import json
+import csv
+import time
+
+import logging
+logging.basicConfig(level=logging.DEBUG,
+                    format='%(asctime)s %(levelname)s %(message)s')
+import pprint
+
+log = logging.getLogger('fediblock_sync')
+
+CONFIGFILE = "/home/mastodon/etc/admin.conf"
+
+def sync_blocklists(conf: dict):
+    """Sync instance blocklists from remote sources.
+    """
+    # Build a dict of blocklists we retrieve from remote sources.
+    # We will merge these later using a merge algorithm we choose.
+
+    blocklists = {}
+    # Fetch blocklists from URLs
+    # for listurl in conf.blocklist_csv_sources:
+    #     blocklists[listurl] = {}
+    #     response = requests.get(url)
+    #     log.debug(f"Fetched blocklist CSV file: {response.content}")
+
+    # Fetch blocklists from remote instances
+    for blocklist_src in conf['blocklist_instance_sources']:
+        domain = blocklist_src['domain']
+        token = blocklist_src['token']
+        blocklists[domain] = fetch_instance_blocklist(token, domain)
+
+    # Merge blocklists into an update dict
+    merged = merge_blocklists(blocklists)
+
+    # log.debug(f"Merged blocklist ready:\n")
+    # pprint.pp(merged)
+
+    # Push the blocklist to destination instances
+    for dest in conf['blocklist_instance_destinations']:
+        domain = dest['domain']
+        token = dest['token']
+        push_blocklist(token, domain, merged.values())
+
+def merge_blocklists(blocklists: dict, mergeplan: str='max') -> dict:
+    """Merge fetched remote blocklists into a bulk update
+
+    @param mergeplan: An optional method of merging overlapping block definitions
+        'max' (the default) uses the highest severity block found
+        'min' uses the lowest severity block found
+    """
+    # log.debug(f"Merging blocklists {blocklists} ...")
+    # Remote blocklists may have conflicting overlaps. We need to
+    # decide whether or not to override earlier entries with later
+    # ones or not. How to choose which entry is 'correct'?
+    merged = {}
+
+    for key, blist in blocklists.items():
+        log.debug(f"Adding blocks from {key} ...")
+        for blockdef in blist:
+            # log.debug(f"Checking blockdef {blockdef} ...")
+            domain = blockdef['domain']
+            if domain in merged:
+                blockdata = merged[domain]
+
+                # If the public or private comment is different,
+                # append it to the existing comment, joined with a newline
+                if blockdef['public_comment'] != blockdata['public_comment'] and blockdata['public_comment'] != '':
+                    blockdata['public_comment'] = '\n'.join([blockdef['public_comment'], blockdata['public_comment']])
+
+                if blockdef['private_comment'] != blockdata['private_comment'] and blockdata['private_comment'] != '':
+                    blockdata['private_comment'] = '\n'.join([blockdef['private_comment'], blockdata['private_comment']])
+
+                # How do we override an earlier block definition?
+                if mergeplan in ['max', None]:
+                    # Use the highest block level found (the default)
+                    if blockdef['severity'] == 'suspend':
+                        blockdata['severity'] = 'suspend'
+
+                    if blockdef['reject_media'] == True:
+                        blockdata['reject_media'] = True
+
+                    if blockdef['reject_reports'] == True:
+                        blockdata['reject_reports'] = True
+
+                elif mergeplan in ['min']:
+                    # Use the lowest block level found
+                    if blockdef['severity'] == 'silence':
+                        blockdata['severity'] = 'silence'
+
+                    if blockdef['reject_media'] == False:
+                        blockdata['reject_media'] = False
+
+                    if blockdef['reject_reports'] == False:
+                        blockdata['reject_reports'] = False
+
+                else:
+                    raise NotImplementedError(f"Mergeplan '{mergeplan}' not implemented.")
+
+            else:
+                # New block
+                blockdata = {
+                    'domain': blockdef['domain'],
+                    # Default to Silence if nothing is specified
+                    'severity': blockdef.get('severity', 'silence'),
+                    'public_comment': blockdef['public_comment'],
+                    'private_comment': blockdef['private_comment'],
+                    'reject_media': blockdef.get('reject_media', False),
+                    'reject_reports': blockdef.get('reject_reports', False),
+                    'obfuscate': blockdef.get('obfuscate', False),
+                }
+            merged[domain] = blockdata
+
+    return merged
+
+def fetch_instance_blocklist(token: str, host: str) -> list:
+    """Fetch existing block list from server
+    """
+    api_path = "/api/v1/admin/domain_blocks"
+
+    url = f"https://{host}{api_path}"
+
+    domain_blocks = []
+    link = True
+
+    while link:
+        response = requests.get(url, headers={'Authorization': f"Bearer {token}"})
+        if response.status_code != 200:
+            log.error(f"Cannot fetch remote blocklist: {response.content}")
+            raise ValueError("Unable to fetch domain block list: %s", response)
+        domain_blocks.extend(json.loads(response.content))
+        
+        # Parse the link header to find the next url to fetch
+        # This is a weird and janky way of doing pagination but
+        # hey nothing we can do about it we just have to deal
+        link = response.headers['Link']
+        pagination = link.split(', ')
+        if len(pagination) != 2:
+            link = None
+            break
+        else:
+            next = pagination[0]
+            prev = pagination[1]
+        
+            urlstring, rel = next.split('; ')
+            url = urlstring.strip('<').rstrip('>')
+
+    log.debug(f"Found {len(domain_blocks)} existing domain blocks.")
+    return domain_blocks
+
+def export_blocklist(token: str, host: str, outfile: str):
+    """Export current server blocklist to a csv file"""
+    blocklist = fetch_instance_blocklist(token, host)
+    fieldnames = ['id', 'domain', 'severity', 'reject_media', 'reject_reports', 'private_comment', 'public_comment', 'obfuscate']
+
+    blocklist = sorted(blocklist, key=lambda x: int(x['id']))
+
+    with open(outfile, "w") as fp:
+        writer = csv.DictWriter(fp, fieldnames, extrasaction='ignore')
+        writer.writeheader()
+        writer.writerows(blocklist)
+
+def delete_blocklist(token: str, host: str, blockfile: str):
+    """Delete domain blocks listed in blockfile"""
+    with open(blockfile) as fp:
+        reader = csv.DictReader(fp)
+        for row in reader:
+            domain = row['domain']
+            id = row['id']
+            log.debug(f"Deleting {domain} (id: {id}) from blocklist...")
+            delete_block(token, host, id)
+
+def delete_block(token: str, host: str, id: int):
+    """Remove a domain block"""
+    log.debug(f"Removing domain block {id} at {host}...")
+    api_path = "/api/v1/admin/domain_blocks/"
+
+    url = f"https://{host}{api_path}{id}"
+
+    response = requests.delete(url,
+        headers={'Authorization': f"Bearer {token}"}
+    )
+    if response.status_code != 200:
+        if response.status_code == 404:
+            log.warn(f"No such domain block: {id}")
+            return
+
+        raise ValueError(f"Something went wrong: {response.status_code}: {response.content}")
+
+def update_known_block(token: str, host: str, blockdict: dict):
+    """Update an existing domain block with information in blockdict"""
+    api_path = "/api/v1/admin/domain_blocks/"
+
+    id = blockdict['id']
+    blockdata = blockdict.copy()
+    del blockdata['id']
+
+    url = f"https://{host}{api_path}{id}"
+
+    response = requests.put(url,
+        headers={'Authorization': f"Bearer {token}"},
+        data=blockdata
+    )
+    if response.status_code != 200:
+        raise ValueError(f"Something went wrong: {response.status_code}: {response.content}")
+
+def add_block(token: str, host: str, blockdata: dict):
+    """Block a domain on Mastodon host
+    """
+    log.debug(f"Blocking domain {blockdata['domain']} at {host}...")
+    api_path = "/api/v1/admin/domain_blocks"
+
+    url = f"https://{host}{api_path}"
+
+    response = requests.post(url,
+        headers={'Authorization': f"Bearer {token}"},
+        data=blockdata
+    )
+    if response.status_code != 200:
+        raise ValueError(f"Something went wrong: {response.status_code}: {response.content}")
+
+def push_blocklist(token: str, host: str, blocklist: list[dict]):
+    """Push a blocklist to a remote instance.
+    
+    Merging the blocklist with the existing list the instance has,
+    updating existing entries if they exist.
+
+    @param token: The Bearer token for OAUTH API authentication
+    @param host: The instance host, FQDN or IP
+    @param blocklist: A list of block definitions. They must include the domain.
+    """
+    # Fetch the existing blocklist from the instance
+    serverblocks = fetch_instance_blocklist(token, host)
+
+    # Convert serverblocks to a dictionary keyed by domain name
+    knownblocks = {row['domain']: row for row in serverblocks}
+
+    for row in blocklist:
+        # log.debug(f"Importing definition: {row}")
+
+        if 'id' in row: del row['id']
+
+        try:
+            blockdict = knownblocks[row['domain']]
+            log.info(f"Block already exists for {row['domain']}, merging data...")
+
+            # Check if anything is actually different and needs updating
+            change_needed = False
+            for key in [
+                'severity',
+                'public_comment',
+                'private_comment',
+                'reject_media',
+                'reject_reports',
+                'obfuscate',
+                ]:
+                try:
+                    if blockdict[key] != knownblocks[key]:
+                        change_needed = True
+                        break
+                except KeyError:
+                    break
+            
+            if change_needed:
+                log.debug(f"Change detected. Updating domain block for {row['domain']}")
+                blockdict.update(row)
+                update_known_block(token, host, blockdict)
+                # add a pause here so we don't melt the instance
+                time.sleep(1)
+
+            else:
+                log.info("No differences detected. Not updating.")
+
+        except KeyError:
+            # domain doesn't have an entry, so we need to create one
+            blockdata = {
+                'domain': row['domain'],
+                # Default to Silence if nothing is specified
+                'severity': row.get('severity', 'silence'),
+                'public_comment': row['public_comment'],
+                'private_comment': row['private_comment'],
+                'reject_media': row.get('reject_media', False),
+                'reject_reports': row.get('reject_reports', False),
+                'obfuscate': row.get('obfuscate', False),
+            }
+            log.info(f"Adding new block for {blockdata['domain']}...")
+            add_block(token, host, blockdata)
+            # add a pause here so we don't melt the instance
+            time.sleep(1)
+
+def load_config(configfile: str):
+    """Augment commandline arguments with config file parameters
+    
+    Config file is expected to be in TOML format
+    """
+    conf = toml.load(configfile)
+    return conf
+
+def save_intermediate(blocklist: list, source: str, filedir: str):
+    """Save a blocklist we've downloaded from a remote source
+
+    Save a local copy of the remote blocklist after downloading it.
+    """
+
+
+if __name__ == '__main__':
+
+    ap = argparse.ArgumentParser(description="Bulk blocklist tool",
+                                 formatter_class=argparse.ArgumentDefaultsHelpFormatter)
+    ap.add_argument('-c', '--config', default='/etc/default/fediblockhole.conf.toml', help="Config file")
+
+    ap.add_argument('--loglevel', choices=['debug', 'info', 'warning', 'error', 'critical'], help="Set log output level.")
+
+    args = ap.parse_args()
+    if args.loglevel is not None:
+        levelname = args.loglevel.upper()
+        log.setLevel(getattr(logging, levelname))
+
+    # Load the configuration file
+    conf = load_config(args.config)
+    
+    # Do the work of syncing
+    sync_blocklists(conf)
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diff --git a/etc/sample.fediblockhole.conf.toml b/etc/sample.fediblockhole.conf.toml
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+# List of instances to read blocklists from, 
+# with the Bearer token authorised by the instance
+blocklist_instance_sources = [
+    {
+        domain = 'eigenmagic.net',
+        token = '<a_token_with_read_auth>'
+    },
+    {
+        domain = 'jorts.horse',
+        token = '<a_different_token>'
+    },
+]
+
+# List of instances to write blocklist to
+blocklist_instance_destinations = [
+    {
+        domain = 'eigenmagic.net',
+        token = '<read_write_token>'
+    },
+]
\ No newline at end of file
diff --git a/requirements.txt b/requirements.txt
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+++ b/requirements.txt
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+requests
+toml
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