BareBone SIP Server

A minimal Kamailio instance that answers local OPTIONS, record-routes dialog-forming requests, and relays everything else with t_relay(). There is no registrar, no NAT rewrite and no media proxy — the destination is taken from the request URI host, rewritten onto port 9, and forwarded.

The routing script is kamailio.cfg.

Call flow

An initial INVITE is checked, record-routed and relayed. In-dialog ACK and BYE follow the Record-Route set, so they come back through this server rather than going end to end.

sequenceDiagram
    autonumber
    participant UAC as Caller
    participant K as Kamailio
    participant UAS as Callee

    UAC->>K: INVITE
    Note over K: REQINIT<br/>maxfwd and sanity
    Note over K: no To-tag so skip WITHINDLG
    Note over K: record_route on INVITE
    Note over K: $du = sip:$rd:9
    K->>UAS: INVITE (t_relay)
    Note over K: branch MANAGE_BRANCH
    UAS-->>K: 180 Ringing
    Note over K: onreply MANAGE_REPLY
    K-->>UAC: 180 Ringing
    UAS-->>K: 200 OK
    K-->>UAC: 200 OK
    UAC->>K: ACK
    Note over K: WITHINDLG loose_route
    K->>UAS: ACK
    UAC->>K: BYE
    K->>UAS: BYE
    UAS-->>K: 200 OK
    K-->>UAC: 200 OK

What each route does

Route	When it runs
`REQINIT`	Every request. Drops known scanners, enforces Max-Forwards, answers a local OPTIONS with `200 Keepalive`, and rejects malformed SIP.
`WITHINDLG`	Requests that already have a To-tag. Follows the Record-Route set with `loose_route()`, record-routes in-dialog NOTIFY, and relays a stateful ACK after a failure.
`RELAY`	Arms `MANAGE_BRANCH`, `MANAGE_REPLY` and `MANAGE_FAILURE`, then calls `t_relay()`.
`MANAGE_BRANCH`	Each outgoing branch, logging the new `$ru`.
`MANAGE_REPLY`	Each incoming reply.
`MANAGE_FAILURE`	Negative final replies, unless the transaction was cancelled.

A request with no user part in the R-URI is answered locally with 484 Address Incomplete and is never relayed.

OPTIONS keepalive

When the request is OPTIONS, the R-URI is this server, and there is no user part, Kamailio answers itself instead of proxying:

sequenceDiagram
    autonumber
    participant UA as User agent
    participant K as Kamailio

    UA->>K: OPTIONS sip:server
    Note over K: REQINIT uri==myself and empty $rU
    K-->>UA: 200 Keepalive

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