diff --git a/bin/irslackd.js b/bin/irslackd.js
index ea9e97d..c120c01 100755
--- a/bin/irslackd.js
+++ b/bin/irslackd.js
@@ -10,6 +10,7 @@ const DEFAULT_PORT = 6697;
 const DEFAULT_TLS_PKEY = os.homedir() + '/.irslackd/pkey.pem';
 const DEFAULT_TLS_CERT = os.homedir() + '/.irslackd/cert.pem';
 const DEFAULT_RTM_CLIENT_LOG_LEVEL = 'info';
+const DEFAULT_LINE_LEN = 4096;
 
 const opt = require('node-getopt').create([
   [ 'h', 'help',            'Show this help' ],
@@ -19,6 +20,7 @@ const opt = require('node-getopt').create([
   [ 'c', 'cert=PATH',       'Set TLS cert path (default: ' + DEFAULT_TLS_CERT + ')' ],
   [ 'L', 'rtmLogLvl=LEVEL', 'Set RTM Client log level (default: ' + DEFAULT_RTM_CLIENT_LOG_LEVEL + ')' ],
   [ 'i', 'insecure',        'Do not use TLS encryption (not recommended)' ],
+  [ 'l', 'lineLen',         'Set RPL_ISUPPORT LINELEN (default: ' + DEFAULT_LINE_LEN + ')' ],
 ]).bindHelp().parseSystem();
 
 new irslackd.Irslackd({
@@ -29,4 +31,5 @@ new irslackd.Irslackd({
     cert: fs.readFileSync(opt.options.cert || DEFAULT_TLS_CERT),
   },
   rtmClientLogLevel: opt.options.rtmLogLvl || DEFAULT_RTM_CLIENT_LOG_LEVEL,
+  lineLen: opt.options.lineLen || DEFAULT_LINE_LEN,
 }).listen();
diff --git a/lib/ircd.js b/lib/ircd.js
index ddef2cd..a73ad55 100644
--- a/lib/ircd.js
+++ b/lib/ircd.js
@@ -6,9 +6,10 @@ const util = require('util');
 const EventEmitter = require('events');
 
 class Ircd extends EventEmitter {
-  constructor(tlsOpts) {
+  constructor(config) {
     super();
-    this.tlsOpts = tlsOpts;
+    this.tlsOpts = config.tlsOpts;
+    this.lineLen = config.lineLen;
   }
   listen(port, host) {
     const socketFunc = this.onConnect.bind(this);
@@ -62,6 +63,8 @@ class Ircd extends EventEmitter {
     console.log('irc_out', line);
     line += '\r\n';
     if (line.length > 512) {
+      this.emit('ircError', socket, 'Long line (' + line.length + ' bytes) (clients without LINELEN support may reject): ' + line);
+    } else if (line.length > this.lineLen) {
       this.emit('ircError', socket, 'Long line (' + line.length + ' bytes) (clients may reject): ' + line);
     }
     return socket.write(line);
diff --git a/lib/irslackd.js b/lib/irslackd.js
index 5cc1927..f8e7db7 100644
--- a/lib/irslackd.js
+++ b/lib/irslackd.js
@@ -20,7 +20,7 @@ class Irslackd {
   }
   listen() {
     const self = this;
-    self.ircd = self.getNewIrcd(self.config.tlsOpts);
+    self.ircd = self.getNewIrcd(self.config);
     const requireInit = true;
     const noRequireInit = false;
     new Map([
@@ -509,8 +509,13 @@ class Irslackd {
     }
   }
   async onSlackReady(ircUser, event) {
-    // Send MOTD
+    // RPL_WELCOME
     this.ircd.write(ircUser.socket, 'irslackd', '001', [ ircUser.ircNick, 'irslackd' ]);
+
+    // RPL_ISUPPORT
+    this.ircd.write(ircUser.socket, 'irslackd', '005', [ ircUser.ircNick, 'LINELEN=' + this.config.lineLen, 'are supported by this server' ]);
+
+    // RPL_ENDOFMOTD
     this.ircd.write(ircUser.socket, 'irslackd', '376', [ ircUser.ircNick, 'End of MOTD' ]);
 
     // set user presence to auto instead of away
@@ -1331,8 +1336,8 @@ class Irslackd {
     await ircUser.slackWeb.apiCallOrThrow('users.setPresence', { presence: status });
   }
   // Dependency injectors
-  getNewIrcd(tlsOpts) {
-    return new ircd.Ircd(tlsOpts);
+  getNewIrcd(config) {
+    return new ircd.Ircd(config);
   }
   getNewSlackWebClient(token, cookie = null) {
     const params = cookie ? { headers: { Cookie: cookie }} : {};
diff --git a/tests/mocks.js b/tests/mocks.js
index c27dad8..546df50 100644
--- a/tests/mocks.js
+++ b/tests/mocks.js
@@ -132,6 +132,7 @@ async function connectOneIrcClient(t, prefs = []) {
     ircSocket.expect(':irslackd 353 test_slack_user = #test_chan_1 :test_slack_user test_slack_user test_slack_fooo test_slack_barr');
     ircSocket.expect(':irslackd 366 test_slack_user #test_chan_1 :End of /NAMES list');
     ircSocket.expect(':irslackd 001 test_slack_user irslackd');
+    ircSocket.expect(':irslackd 005 test_slack_user LINELEN=4096 :are supported by this server');
     ircSocket.expect(':irslackd 376 test_slack_user :End of MOTD');
   };
 
@@ -143,8 +144,9 @@ async function connectOneIrcClient(t, prefs = []) {
       key: 'key',
       cert: 'cert',
     },
+    lineLen: 4096,
   });
-  daemon.getNewIrcd           = (tlsOpts) => { return new MockIrcd(tlsOpts);     };
+  daemon.getNewIrcd           = (config) =>  { return new MockIrcd(config);      };
   daemon.getNewSlackRtmClient = (token)   => { return new MockSlackRtmClient(t); };
   daemon.getNewSlackWebClient = (token)   => {
     const slackWeb = new MockSlackWebClient(t);
@@ -190,7 +192,7 @@ async function connectOneIrcClient(t, prefs = []) {
     end: () => ircSocket.end(),
   };
 }
-connectOneIrcClient.planCount = 21;
+connectOneIrcClient.planCount = 22;
 
 exports.MockSlackWebClient = MockSlackWebClient;
 exports.MockSlackRtmClient = MockSlackRtmClient;