My Logon gets no reply and the connection closes. Why?

The acceptor did not find a session for your CompIDs, or refused the connection before reading the Logon (IP allow-list, TLS, wrong port). These cases rarely produce a Logout, so ask the counterparty to check their logs at the time of your attempt.

What HeartBtInt (108) should I send?

The value agreed with the counterparty, commonly 30 seconds. Some acceptors refuse a Logon whose interval differs from their configuration.

Logon, heartbeats and logout

FIX Logon rejected: Logout with text, or disconnect after Logon

The acceptor answered your Logon with a Logout (35=5) whose Text (58) gives the reason, or it closed the TCP connection without answering. The second case is harder: the reason is only in the acceptor's logs.

What it means

A Logon is validated in steps: the TCP connection is accepted (IP allow-list, TLS), the CompID pair is matched to a configured session, then the Logon content is checked: sequence number, credentials, heartbeat interval and session hours.

Failures in the first two steps usually end in a silent disconnect. Failures in the last step usually get a Logout with Text (58). On FIXT 1.1 sessions, the Logout can also carry SessionStatus (1409), with codes such as 5 (invalid username or password), 7 (logons not allowed at this time) and 8 (password expired).

Common causes

CompIDs not configured on the acceptor, or swapped (SenderCompID (49) and TargetCompID (56)).
Source IP not on the counterparty's allow-list, or TLS expected and plain TCP used.
Wrong Username (553) or Password (554) (FIX 4.3 and later), or credentials expected in a venue-specific field.
Sequence number too low. See the MsgSeqNum too low page.
BeginString different from the session's FIX version; QuickFIX answers with "Incorrect BeginString".
Logon outside the configured session hours.
Another connection is already logged on with the same session.
On FIXT 1.1, DefaultApplVerID (1137) missing from the Logon.

How to fix it

On the initiator (the side logging on)

Read the Logout text first. If there is none, check CompIDs, IP, port and TLS with the counterparty.
Check HeartBtInt (108) matches what was agreed, and that you are inside the session hours.
Make sure only one instance of your engine runs per session.

On the acceptor

Send a Logout with a specific Text (58) for every refusal you can name. A silent disconnect costs both sides time.
Log refused connections with source IP and CompIDs.

Example

A FIX 4.4 Logon with Username and Password, refused with a Logout.

1. Buy side: Logon

8=FIX.4.4|9=115|35=A|49=BUYSIDE|56=SELLSIDE|34=1|52=20261020-06:59:58.000|98=0|108=30|141=Y|553=buyside_trading|554=wrong-password|10=131|

2. Sell side: Logout with the reason

8=FIX.4.4|9=90|35=5|49=SELLSIDE|56=BUYSIDE|34=1|52=20261020-06:59:58.010|58=Invalid username or password|10=201|

SOH (0x01) is shown as |. BodyLength (9) and CheckSum (10) are calculated as if every | were SOH.

Open this example in the parser

Questions

My Logon gets no reply and the connection closes. Why?: The acceptor did not find a session for your CompIDs, or refused the connection before reading the Logon (IP allow-list, TLS, wrong port). These cases rarely produce a Logout, so ask the counterparty to check their logs at the time of your attempt.
What HeartBtInt (108) should I send?: The value agreed with the counterparty, commonly 30 seconds. Some acceptors refuse a Logon whose interval differs from their configuration.

Reference

Related errors

All FIX errors · FIX guides